lebe0004-00 ups doble conversion.pdf

Double Conversion UPS

®

A P P L I C A T I O N A N D I N S T A L L A T I O N G U I D E

Contents

Key factors in UPS installations .....................................................1

Introduction................................................................................1

Growing needs for high-quality and high-availability power..........................................1

Using this guide.........................................................................1

Structure of this document...........................................1

UPS in electrical installations ...................................................2

Component functions and parameters........................2

Sources of information in setting up installation specifications........................................3

Basic notions on installations with UPS..................................5

Supply systems with UPS...................................................5

Purpose of UPS .............................................................5

Types of UPS..................................................................5

Double-conversion static UPS......................................5

The Operating Principle (Figure 2)...............................5

Power quality of UPS ..........................................................6

Power quality of double-conversion UPS ...................6

Voltage quality for linear loads ....................................6

Voltage quality for non-linear loads ............................7

UPS power availability ......................................................10

What is meant by availability? ...................................10

How can availability be improved? ............................11

Key factors to the availability of installations with UPS.......................................................................11

Selection of the configuration ..........................................13

Prerequisite step in establishing installationspecifications ...............................................................13

Power calculations...................................................................14

Elements required for power calculations ......................14

Installation considerations..........................................14

Power of a UPS............................................................14

UPS percent load.........................................................16

UPS efficiency..............................................................16

Ratings of single-UPS configurations..............................17

Single-UPS configurations .........................................17

Power levels under steady-state conditions .............17

Power levels under transient conditions ...................18

Ratings of parallel-UPS configurations ...........................21

Parallel-UPS configurations........................................21

Power levels in redundant parallel configurations..............................................................22

Control of upstream harmonics .............................................24

UPS and upstream harmonics .........................................24

Role of the input rectifier ............................................24

Standard rectifiers.......................................................24

PFC-type transitor-based controlled active rectifiers ............................................................24

Filtering of upstream harmonics for UPS with Graetz bridge rectifiers ...............................25

Goals of harmonic filtering.........................................25

Types of harmonics filters ..........................................26

Filtering and parallel connection ...............................26

Combination of LC filters and generator...................26

Selection of a filter ............................................................28

Selection parameters for a filter ................................28

Comparison table of solutions ...................................28

System earthing arrangements ..............................................31

Background information on system earthingarrangements.....................................................................31

Protection of persons against electrical contact.......31

Types of system earthing arrangements (SEA) ........32

System earthing arrangements (SEA) .......................33

Comparison of system earthing arrangements (SEA).............................................................................35

Applications in UPS installations .....................................36

Specific aspects in systems with UPS.......................36

Protection against direct contact................................36

Protection against indirect contact ............................36

Types of systems for UPS...........................................37

Protection .................................................................................39

Protection using circuit breakers......................................39

Trip units ......................................................................39

Discrimination, cascading, current limiting ..............41

Selection of circuit breakers .............................................42

Rating ...........................................................................42

Breaking capacity ........................................................42

Ir and Im thresholds ....................................................42

Special case of generator short-circuits ....................43

Example .......................................................................43

Calculation of CB1 and CB2 ratings and breaking capacities ..............................................44

Characteristics of the most power circuit breaker CB3 possible ..................................................47

Cables .......................................................................................49

Selection of cable sizes .....................................................49

Cable temperature rise and voltage drops................49

Temperature rise .........................................................49

Voltage drops...............................................................49

Special case for neutral conductors ..........................51

Calculation example....................................................51

Example of an installation ................................................51

Energy storage.........................................................................52

Storage technologies ........................................................52

Energy storage in UPS................................................52

Available technologies ................................................52

Comparison of technologies ......................................52

Selection of a battery ........................................................54

Types of batteries ........................................................54

Backup time .................................................................54

Service life....................................................................54

Comparison between types of batteries ...................55

Battery monitoring ............................................................55

Battery monitoring on UPS ........................................55

Detection and prevention of battery failure for UPSs ...........................................................55

Human-machine interface and communication....................56

Human-machine interface (HMI) ......................................56

General characteristics................................................56

Example .......................................................................56

Communication .................................................................56

High availability for critical applications requires commun icating protection equipment.......56

Solutions ......................................................................57

Preliminary work......................................................................58

Installation considerations................................................58

Dimensions ..................................................................58

Ventilation, air-conditioning .......................................58

IP degree of protection and noise level.....................59

Battery room ......................................................................59

Battery installation method ........................................59

Battery room features .................................................59

Selection of possible configurations ...........................................62

Types of possible configurations............................................62

Basic diagrams ..................................................................62

Single source ...............................................................62

Multi-source.................................................................62

UPS configurations ...........................................................62

Single UPS ...................................................................62

Parallel UPS .................................................................62

Parallel connection with redundancy ........................64

Redundant distribution with an STS..........................65

Selection table and corresponding ranges............................66

Criteria for comparison .....................................................66

Availability ...................................................................66

Maintainability.............................................................66

Upgradeability .............................................................66

Discrimination and non-propagation of faults..........66

Installation operation and management ...................66

Diagram no. 1. Single UPS......................................................68

Diagram no. 2. Active redundancy with two integrated parallel UPS units ..................................69

Diagram no. 3. Active redundancy with integrated parallel UPS units and external maintenance bypass..........70

Diagram no. 4. Isolated redundancy with two UPS units ....71

Diagram no. 5. Active redundancy with parallel units and centralised static-switch cubicle (SSC)..................72

Diagram no. 6. Active redundancy with parallel UPS units and total isolation, single busbar .........................73

Diagram no. 7. Active redundancy with parallel UPS units and total isolation, double busbar........................74

Diagram no. 8. Active redundancy with parallel UPS units, double SSC and total isolation, single busbar............75

Diagram no. 9. Active redundancy with parallel UPS units, double SSC and total isolation, double busbar ..........77

Diagram no. 10. Isolated redundancy N + 1 ..........................79

Diagram no. 11. Redundant distribution with STS................81

Diagram no. 12 . Active redundancy with parallel UPS and a common battery....................................................83

Elimination of harmonics in installations....................................85

Harmonics ................................................................................85

Definition, origin and types of harmonics.......................85

Harmonics....................................................................85

Non-linear loads are the cause ..................................85

Linear and non-linear loads........................................86

Types of harmonics and specific aspects of zero-sequence harmonics ...................................................87

Characteristic harmonic values ........................................89

Rms value of harmonics .............................................89

Total rms current .........................................................89

Individual harmonics ..................................................89

Voltage and current harmonic distortion ..................90

Crest factor...................................................................90

Spectrum of the harmonic current ............................91

Power factor.................................................................91

Power............................................................................91

Non-linear load............................................................92

Effects of harmonics..........................................................92

Loss of apparent power ..............................................92

Temperature rise in cables .........................................92

Current in the neutral ..................................................93

Self-polluting loads .....................................................93

Risk of capacitor breakdown ......................................94

Derating of transformers ............................................95

Risk of disturbing generators .....................................96

Losses in asynchronous motors ................................96

Effects on other equipment ........................................96

Effect on recent UPS systems ....................................96

Conclusion ...................................................................96

Elimination of harmonics........................................................97

Strategies against harmonics...........................................97

Living with harmonics.......................................................97

Oversizing of equipment ............................................97

Solutions to eliminate harmonics ....................................98

Passive filters...............................................................98

Active filters / active harmonic conditioners ............98

Active harmonic conditioners ..............................................100

Active harmonic conditioners ........................................100

Characteristics ...........................................................100

Advantages of active harmonic conditioning.........100

Operating principle....................................................101

Operating modes .......................................................101

Installation modes .....................................................102

Position in the installation ........................................104

Position of current transformers upstream or downstream ..........................................................105

Advantages ................................................................107

Procedure for implementing active conditioning .........108

Conclusion on active conditioning...........................108

New installations .......................................................108

Existing installations .................................................108

Methodology..............................................................109

1. Site audit.................................................................109

2. Determination of the most suitable solution......110

3. System installation and checks............................110

Theoretical review........................................................................111

Supplying sensitive loads......................................................111

Types of electrical disturbances ......................................111

Origins of disturbances..............................................111

Types of disturbances................................................112

UPS..........................................................................................114

UPS....................................................................................114

Components of a UPS ...............................................114

UPS applications ..............................................................116

Types of UPS ..........................................................................118

Static or rotary UPS .........................................................118

Static or rotary UPS solutions ..................................118

Comparison ................................................................119

Static solution.............................................................119

Types of static UPS..........................................................120

Standards...................................................................120

UPS operating in passive-standby mode................121

UPS operating in line-interactive mode ..................122

Double-conversion UPS............................................123

Conclusion .................................................................125

UPS components and operation ..........................................126

Components of a UPS.....................................................126

General diagram of a UPS........................................126

Power sources and UPS inputs ................................127

Components of a UPS...............................................127

Main characteristics of UPS components......................130

AC input power..........................................................130

Rectifier/charger.........................................................130

Battery (* energy storage means)............................131

Inverter .......................................................................133

Output voltage Un.....................................................134

Summary diagram for main characteristics..................136

Normal AC input: .......................................................136

Bypass AC input:........................................................136

Rectifier/charger:........................................................136

Battery: .......................................................................136

Inverter: ......................................................................136

UPS operating modes .....................................................137

Normal mode (on utility power, see figure 76) .......137

Backup mode (on battery power, see figure 76) .............................................................137

Bypass mode (on static-bypass line, see figure 77) .............................................................138

Maintenance mode (on maintenance bypass, see figure 77) ...............................................139

UPS configurations..........................................................140

Parallel UPS with redundancy..................................140

Electromagnetic compatibility (EMC) ..................................142

Electromagnetic disturbances ........................................142

Electromagnetic disturbances..................................142

Examples....................................................................142

EMC standards and recommendations .........................143

Disturbances ..............................................................143

Measured values .......................................................143

UPS standards........................................................................145

Scope and observance of standards..............................145

Scope of standards....................................................145

Observance of standards and certification..............145

CE marking.................................................................145

Main standards governing UPS .....................................146

Safety................................................................................146

Electrical environment, harmonics andelectromagnetic compatibility (EMC) ......................146

Quality ........................................................................146

Ecological environment ............................................146

Acoustic noise............................................................146

Tables on harmonic-compatibility levels.................147

Energy storage.......................................................................148

Possible technologies......................................................148

Energy storage in UPS..............................................148

Batteries............................................................................148

The battery solution ..................................................148

Types of industrial batteries .....................................149

Installation modes .....................................................149

Constraints on batteries ..................................................150

Atmospheric constraints...........................................150

Access ........................................................................150

Main battery parameters ..........................................150

Recharge mode..........................................................151

Battery management.................................................151

UPS / generator-set combination .........................................153

Use of a generator...........................................................153

Long backup times ....................................................153

UPS / generator-set compatibility ............................154

Review of inrush currents.........................................155

Motors ........................................................................155

LV/LV transformers ....................................................155

Computer loads ...............................................................156

Harmonics ..............................................................................157

Harmonics ........................................................................157

Origin of harmonics ..................................................157

Consequences of harmonics ....................................157

Precautions ................................................................158

Characteristic harmonic values ......................................159

Current values............................................................159

Example .....................................................................161

Voltage values............................................................161

Power values..............................................................162

Non-linear loads and PWM technology ...............................164

Non-linear load performance of UPS using PWM technology ...................................................164

Importance of the UPS output impedance..............164

UPS operating principle............................................165

PWM inverters ...........................................................167

Comparison of different sources....................................169

Output impedance of various sources ....................169

Conclusion .................................................................169

Free-frequency chopping ................................................170

Free-frequency chopping..........................................170

PFC Rectifiers .........................................................................172

Standard and PFC rectifiers ............................................172

Standard rectifiers.....................................................172

“Clean” PFC (Power Factor Correction) rectifier.....172

PFC rectifiers..............................................................172

Implementation .........................................................173

Glossary and bibliography ..........................................................176

Glossary..................................................................................176

Bibliography...........................................................................186

Standards...................................................................186

Information contained in this publication may be considered confidential.Discretion is recommended when distributing. Materials and specifications are subject to change without notice.

CAT, CATERPILLAR, their respective logos, “Caterpillar Yellow,” the “PowerEdge” trade dress as well as corporate and product identity used herein,are trademarks of Caterpillar and may not be used without permission.

ForewordThis section of the Application and Installation Guide generally describesCaterpillar Double Conversion UPS. Additional engine systems, componentsand dynamics are addressed in other Application and Installation Guides.

Engine-specific information and data are available from a variety of sources.Refer to LEBW4950 and the Introduction section (LEBW4951) for additionalreferences.

IntroductionGrowing needs for high-qualityand high-availability power

Problems related to the quality andavailability of electrical power havebecome vitally important due to thekey role of computers and electronicsin the development of many criticalapplications.

Disturbances in distribution systems(micro-outages, outages, voltage sags,etc.) can result in major losses orsafety hazards in a number of activitiessuch as:

• Sensitive process industrieswhere a malfunction in thecontrol/monitoring systems canresult in production losses.

• Airports and hospitals wherefaulty operation of equipment canrepresent a serious danger.

• Information and communicationtechnologies where the necess arylevel of reliability and depend abilityis even higher. Data centers requirehigh-quality, “no-break” power24/365, year after year and withouthalts for maintenance.

UPS protection systems are now anintegral part of the value chain of manycompanies. Their level of availabilityand power quality have a direct effecton the service continuity of operations.Productivity, the quality of products andservices, the competitiveness of thecompany and site security depend onthe smooth operation of the UPS.Failure is not an option.

©2010 CaterpillarAll rights reserved. Page 1

Battery UPS Application and Installation Guide

Using this guideStructure of this document

Finding information

Information may be located in thegeneral contents at the start of the guide.

Sections

1. Key factors in UPS installations

presents the role of UPS in electricalinstallations and indicates the main

parameters that must be taken intoaccount. The remainder of thesection guides you through theselection process for a solution bydetermining the main elements ofan installation with a UPS.

2. Selection of the UPS configuration

presents a number of practicalexamples in view of selecting aconfiguration, from a simple, single-UPS unit through to installationsoffering exception ally high levels of availability.

3. Elimination of harmonics in

installations presents solutions toeliminate harmonic currents ininstallations.

4. Theoretical review providesbackground technical information

for devices and notions mentionedin other parts of the guide.

Finally, to facilitate the preparation of projects:

5. Glossary and bibliography definesthe main terms used in this guideand provides a list of standards anddocuments dealing with topicsrelated to UPS.

Key factors in UPS installations

©2010 CaterpillarPage 2 All rights reserved.

Application and Installation Guide Battery UPS

UPS in electrical installationsComponent functions and parameters

Figure 1: Functions of the components in installations with UPS..



Sources of information in settingup installation specifications

The diagram on the previous pageprovides a general overview of thecomponents and various parametersin installations with UPS.

Table 1 indicates:

• The order in which the subjectsare presented in this section,

• The choices that must be made,

• The purpose of each decisionwith the indication of the pages concerning the relevant elementsin this section,

• Where additional information oneach subject may be found in the other sections of thisdesign guide.

Table 1..

Choices Purpose See Additional information See

Mono ormultisourcearchitecture andconfiguration ofUPS sources

Determine the installationarchitecture and UPSconfiguration best suited toyour requirements in terms ofenergy availability, upgrades,operation and budget.

Sec. 2

Examples and comparisonof 12 typical installations,from single-UPS units tohigh-availabilityarchitectures.

Page 67

Supplying sensitive loads. Page 111

UPS configurations. Page 140

Engine generator sets. Page 153

UPS power rating

Determine the rating of theUPS unit or parallel units (forredundancy or capacity)required, taking into accountthe distribution system andload characteristics.

Pages 17-21

UPS make-up andoperation. Page 14

Control ofupstreamharmonics

Reduce voltage distortion onthe upstream busbars toacceptable levels, dependingon the power sources likely tosupply the UPS system.

Page 24

Elimination of harmonicsin installations. Sec. 3

Harmonics. Page 157

System earthingarrangements

Ensure installation compliancewith applicable standards forthe protection of life andproperty and correct operationof devices. Which systemearthing arrangements arerequired for whichapplications?

Page 31

Upstream anddownstreamprotection usingcircuit breakers

Determine the breakingcapacity and the ratings of thecircuit breakers upstream anddownstream of the UPS, solveany discrimination problems.

Page 39

Connections

Limit voltage drops andtemperature rise in the cables,as well as harmonic distortionat the load inputs.

Page 49

Battery

Operation on battery power(backup time) must last longenough to meet userrequirements.

Page 54 Energy-storage solutionsand batteries. Page 148

CommunicationDefine UPS communicationwith the electrical andcomputer environment.

Page 56

Preliminary work(if any)

Construction work andventilation must be planned,notably if there is a specialbattery room.

Page 58

StandardsBe aware of the mainapplicable UPS standards.

Electromagneticcompatibility. Page 142





Basic notions on installations with UPS

Rotary UPS (with rotating mechanicalparts, e.g. flywheels) are not includedin the standards and remain marginalon the market.

Types of UPS, see page 118 “Typesof static UPS”.

Double-conversion static UPS

This is the market leading technologyused in high-power installations dueto their unique advantages over theother technologies.

• Complete regeneration of thepower supplied at the output,

• Total isolation of the load fromthe distribution system and itsdisturbances,

• No-break transfer (whereapplicable) to a bypass line.

The Operating Principle (Figure 2)

• During normal operation, arectifier/charger turns the AC-input power into DC power tosupply an inverter and floatcharge the stored energy source,

• The inverter completelyregenerates a sinusoidal signal,turning the DC power back intoAC power that is free of alldisturbances and within strictamplitude and frequencytolerances,

• If the AC-input power fails, thestored energy source supplies thepower required by the inverter fora specified backup time

• A static bypass can transfer theload without a break in the supplyof power to a bypass line to

Supply systems with UPSPurpose of UPS

First launched in the 1970s, UPSimportance has grown in step with thedevelopment of digital technologies.

UPS are electrical devices that arepositioned between the distributionsystem and sensitive loads. Theysupply power that is much morereliable than the distribution systemand corresponds to the needs ofsensitive loads in terms of quality andavailability.

Types of UPS

The term UPS covers products withapparent power ratings from a fewhundred VA up to several MVA,implementing different technologies.

That is why standard IEC 62040-3 andits European equivalent ENV 62040-3define three standard types(topologies) of UPS.

UPS technologies include:

• Passive standby,

• Line interactive,

• Double conversion.

For the low power ratings (< 2 kVA),the three technologies coexist. Forhigher ratings, the industry leadingtechnology is double conversion withline interactive being used primarilywhere efficiency is a concern for thecustomer.

Power quality of UPSPower quality of double-conversion UPS

By design, double-conversion solid-state UPS supply to the connectedloads a sinusoidal signal that is:

• High quality because it iscontinuously regenerated andregulated (amplitude ± 1%,frequency ± 0.5%),

• Free of all disturbances from thedistribution system (due to thedouble conversion) and inparticular from micro-outagesand outages (due to the battery).

This level of quality must be ensured,whatever the type of load.

Voltage quality for linear loads

What is a linear load?

A linear load supplied with a sinusoidalvoltage draws a sinusoidal currenthaving the same frequency as thevoltage. The current may be displaced(angle φ) with respect to the voltage(figure 3).

Examples of linear loads

Many loads are linear, includingstandard light bulbs, heating units,resistive loads, motors, transformers,etc. They do not contain any activeelectronic components, only resistors(R), inductors (L) and capacitors (C).

UPS and linear loads

For this type of load, the UPS outputsignal is very high quality, i.e. thevoltage and current are perfectlysinusoidal, 50 or 60 Hz.



Figure 2: Double-conversion static UPS..

continue supplying the load ifneed be due to an internal fault,short circuit downstream, ormaintenance. This “fault-tolerant”design makes it possible tocontinue supplying power to theload in “downgraded mode” (thepower does not transit theinverter) during the time requiredto re-establish normal conditions.

Double-conversion UPS, see page126 “Components and operation”.



a frequency that is a multiple of the fundamental and whichdefines the harmonic order (e.g.the third order harmonic has afrequency 3 x 50 Hz [or 60 Hz]and the fifth order harmonic hasa frequency 5 x 50 Hz [or 60 Hz]).

The harmonic currents are caused bythe presence of power-electroniccomponents (e.g. diodes, SCRs,IGBTs) which switch the input current.

Examples of non-linear loads

Non-linear loads include all those thathave a switch-mode power supply attheir input to supply the electronics(e.g. computers, variable-speeddrives, etc.).

Figure 4: The current drawn by non-linear loads is distorted by the harmonics..

Voltage quality for non-linearloads

What is a non-linear load?

A non-linear (or distorting) loadsupplied with a sinusoidal voltagedraws periodic current that has thesame frequency as the voltage but isnot sinusoidal.

The current drawn by the load is, infact, the combination (figure 4) of:

• A sinusoidal current called thefundamental, at the 50 or 60 Hzfrequency,

• Harmonics, which are sinusoidalcurrents with an amplitude lessthan that of the fundamental, but

Figure 3: Voltage and current for linear loads..

Voltage and current harmonic

distortion

Non-linear loads cause both currentand voltage harmonics. This isbecause for each current harmonic,there is a voltage harmonic with thesame frequency. The 50 Hz (or 60 Hz)sinusoidal voltage of the UPS istherefore distorted by the harmonics.

The distortion of a sine wave ispresented as a percentage:

THD* % = total distortion = rms value of all the harmonic krms value of the fundamental

* Total Harmonic Distortion.

The following values are defined:

• TDHU % for the voltage, basedon the voltage harmonics,

• TDHI % for the current, based onthe current harmonics (figure 5).

The higher the harmonic content, thegreater the distortion.

Practically speaking, the distortion inthe current drawn by the load is muchhigher (THDI approximately 30%) thanthat of the voltage at the input (THDUapproximately 5%).



Hk% = distortion of harmonic k = rms value of harmonic k

rms value of the fundamental

Harmonic spectrum of the current

drawn by a non-linear load

The harmonic analysis of a non-linearcurrent consists in determining (figure 5):

• the harmonic orders present inthe current,

• the relative importance of eachorder, measured as thepercentage of the order.



Limiting the distortion of the output

voltage

Due to the free-frequency choppingtechnique employed, the impedance atthe output of a double conversion UPSis very low, whatever the frequency(i.e. whatever the harmonic order). Thistechnique virtually eliminates alldistortion in the output voltage whensupplying non-linear loads. The qualityof the output voltage is thus constant,even for non-linear loads.

Practically speaking, installationdesigners must:

• check UPS output values for non-linear loads and, in particular,make sure that the announcedlevel of distortion, measured forstandardised non-linear loads asper standard IEC 62040-3, is verylow (THDU < 2 to 3%),

• limit the length (impedance) ofthe output cables supplying theloads.

UPS performance for non-linearloads, see page 164.

Non-linear loads, see the section“Elimination of harmonics ininstallations” and page 85“Harmonics”.

UPS and non-linear loads

Harmonics affect the sinusoidalvoltage at the UPS output. Excessivedistortion can disturb the linear loadsconnected in parallel on the output,notably by increasing the current theydraw (temperature rise).

To maintain the quality of the UPSoutput voltage, it is necessary to limitits distortion (THDU), i.e. limit thecurrent harmonics that producevoltage distortion.

In particular, it is necessary that theimpedance (at the UPS output and inthe cables supplying the load) remainlow.

.Figure 5: Example of the harmonic spectrum of the current drawn by a non-linear load..



The availability of the energy suppliedby an electrical installationcorresponds to a statisticalmeasurement (in the form of apercentage) of its operating time.

The MTBF and MTTR values arecalculated or measured (on the basisof sufficiently long observations) forthe components. They can then beused to determine the availability ofthe installation over the period.

What are the factors contributing

to availability?

Availability depends on the MTBF andthe MTTR.

• Availability would be equal to100% if the MTTR is equal to zero(instantaneous repair) or if theMTBF is infinite (operation withno breakdowns). This isstatistically impossible;

• Practically speaking, the lowerthe MTTR and the higher theMTBF, the greater the availability.

From “3 nines” to “6 nines”

The critical nature of manyapplications has created the need formuch higher levels of availability forelectrical power.

• The “traditional” economy usespower from the public utility. Anaverage-quality distributionsystem with HV backup offers99.9% availability (3 nines), whichcorresponds to eight hours ofnon-availability per year.

• Sensitive loads require anelectrical supply capable ofproviding 99.99% availability (4 nines), which corresponds to50 minutes of non-availabilityper year.

UPS power availabilityWhat is meant by availability?

Availability of an electrical installation

Availability is the probability that theinstallation will be capable ofsupplying energy with the level ofquality required by the supplied loads.

It is expressed as a percentage.

Availability (%) = (1- MTTR) x 100MTBF

The MTTR is the mean time to repairthe supply system following a failure(including the time to detect the causeof the failure, repair it and start thesystem up again).

The MTBF is the mean time betweenfailures, i.e. the time the supplysystem is capable of ensuring correctoperation of the loads.

Example

An availability of 99.9% (called theenines) corresponds to a 99.9% chancethat the system will effectively carryout the required functions at any giventime. The difference between thisprobability and 1 (i.e. 1 - 0.999 = 0.001)indicates the level of non-availability(i.e. one chance out of 1000 that thesystem will not carry out the requiredfunctions at any given time).

What is the practical signification

of availability?

Down-time costs for criticalapplications are very high.

These applications must obviouslyremain in operation as long aspossible.

The same is true for their electricalsupply.



.Figure 6: Evolution in the level of availability required by applications...

How can availability beimproved?

To improve availability, it is necessaryto reduce the MTTR and increase the

MTBF.

Reduce the MTTR

Real-time fault detection, analysis byexperts to ensure a precise diagnosisand rapid repair all contribute toreducing the MTTR.

These efforts depend on the keyfactors listed next.

Key factors to the availability ofinstallations with UPS

A few years ago, most installationswere made up of single-UPS units,and the number of parallel systemswas small. The applications requiringthis type of installation still exist.

However, the shift toward highavailability requires use ofconfigurations offering redundancy at

a number of levels in the installation(see figure 7).

• The computer and commun -ication equipment in data centresrequires 99.9999% availability (6 nines), which corresponds to30 seconds of non-availabilityper year. This level is the meansto ensure, without risk of majorfinancial loss, operation ofinfrastructures 24/365, withoutshutdown for maintenance. It is astep toward a continuous supply.

Upgradeability

It must be possible to upgrade theinstallation over time, taking intoaccount both the need to expand theinstallation gradually and operatingrequirements.

Discrimination and non-propagation

of faults

It must be possible to limit faults to assmall a part of the installation aspossible, while enabling servicingwithout stopping operations.

Installation operation and

management

Make operations easier by providingthe means to anticipate events viainstallation supervision andmanagement systems.



This trend has led designers,depending on the criticality of theloads and the operating requirements,to take into account some or all of thekey factors listed below.

Reliability and availability

Propose a configuration correspon -ding to the level of availabilityrequired by the load, comprisingcomponents with proven levels ofreliability and backed up by a suitablelevel of service quality.

Maintainability

Ensure easy maintenance of theequipment under safe conditions forpersonnel and without interruptingoperation.

Figure 7: The required levels of availability have resulted in the.use of redundancy on a number of levels in the installation.



Selection of theconfigurationPrerequisite step in establishinginstallation specifications

The selection of a configurationdetermines the level of availabilitythat will be created for the load. It alsodetermines the possible solutions formost of the factors listed previously.

The configuration may be single ormulti-source, with single or parallelUPS units and with or withoutredundancy.

Selection of the configuration is theinitial step in establishing installationspecifications. To assist in making theright decision, section 2 is entirelydevoted to this subject. It compares thevarious configurations in terms ofavailability, protection of the loads,maintainability, upgradeability and cost.

Configuration selection based ontypical installations corresponding to different levels of availability.

Power calculations



• Disable automatic transfer(except for internal faults), whilemaintaining the possibility ofmanual transfers (e.g. formaintenance).

Power of a UPS

Rated power of a UPS

This rating, indicated in thecatalogues, is in the output power. Itis indicated as an apparent power Sn

in kVA, with the corresponding active

power Pn in kW, for a:

• Linear load,

• Load with a cos φ = 0.8.

However, last-generation UPS can supply loads with a cos φ = 0.9 leading.

Calculation of the rated power

Pn (kW) = 0.8 Sn (kVA) rated activepower

This calculation depends on theoutput voltage of the UPS and thecurrent drawn by the load, where:

Sn (kVA) = UnIn √3 in three-phasesystems

Sn (kVA) = VnIn in single-phasesystems

For a three-phase UPS, U and I arerms line values, for a single-phaseUPS, V is a phase-to-neutral voltage,where:

Un = phase-to-phase voltage

Vn = phase-to-neutral voltage

Un = Vn √3

For example, if Un = 400 volts, Vn = 230 volts.

Elements required forpower calculationsInstallation considerations

Type of load supplied

Linear loads (cos φ) or non-linearloads (power factor).

These characteristics determine thepower factor at the UPS output.

Maximum power drawn by the load

under steady-state conditions

For a load, this is the power rating. If a number of loads are connected in parallel on the UPS output, it isnecessary to calculate the total loadwhen all the loads operate at the sametime. Otherwise, it is necessary to usediversity to calculate the mostunfavourable operation in terms of thepower drawn.

In-rush currents under transient

conditions or for a short-circuit

downstream

The overload capacity of a UPSsystem depends on the time theoverload lasts. If this time limit isexceeded, the UPS transfers the loadto the Bypass AC input, if its voltagecharacteristics are within tolerances.In this case, the load is no longerprotected against disturbances on the distribution system.

Depending on the quality of theBypass AC power, it is possible to:

• Use the Bypass AC input tohandle current spikes due toswitching of devices ordownstream short-circuits. Thisavoids oversizing the system;

Table 2..

Linear loads

Three-phase Single-phase

Sinusoidal voltageu(t) = U √2 sin ωt between phases v(t) = V √2 sin ωt phase to neutral

U = V √3

Displaced sinusoidalcurrent

i(t) = I √2 sin (ωt - φ) phase current

Current crest factor √2

Apparent power S (kVA) = UI √3 cos φ S (kVA) = VI

Active power P (kW) = UI √3 cos φ = S (kVA) cos φ P (kW) = VI cos φ = S (kVA) cos φ

Reactive power Q (kvar) = UI √3 sin φ = S (kVA) sin φ Q (kvar) = VI sin φ = S (kVA) sin φ

S = √P2 + Q2

Non-linear loads

Sinusoidal voltageThe regulated UPS voltageremains sinusoidal (low THDU),whatever the type of load.

u(t) = U √2 sin ωt between phases v(t) = V √2 sin ωt phase to neutral

U = V √3

Current with harmonics

i(t) = i1(t) + ∑ihk(t) total phase current

i1(t) = I1 √2 sin (ωt - φ1) fundamental current

ik(t) = Ihk √2 sin (kωt - φk) k-order harmonic

I = √I12 + I22 + I32 + I42 + .... rms value of the total current

Fc = peak current value / rms value Current crest factor

THDI = √I12 + I22 + I32 + I42 + .... Current total harmonic distortion

I1

Apparent power S (kVA) = UI √3 S (kVA) = VI

Active power P (kW) = λ UI √3 = λ S (kVA) P (kW) = λ VI = λ S (kVA)

Power factorλ = P(kW)

S(kVA)



• The corresponding rms values U and I,

• ω = angular frequency = 2 π f

where f is the frequency (50 or 60 Hz),

• φ = displacement between thevoltage and the current undersinusoidal conditions.

Power and type of load

Table 2 presents the equations linkingthe power, voltage and current,depending on the type of load (linearor non-linear).

The following symbols are used:

• Instantaneous voltage u(t) andcurrent i(t) values,



The installation designer musttherefore pay attention to two aspectsof efficiency.

Recommendation 1: check the

efficiency for non-linear loads

The presence of non-linear loadstends to reduce the power factor tovalues below 0.8. It is thereforenecessary to check the efficiency valuefor standardised non-linear loads. Thischeck is recommended by standardsIEC 62040-3 / EN 62040-3.

Recommendation 2: check the

efficiency at the planned percent

load

Manufacturers generally indicate theefficiency at full rated load. However,its value may drop if the percent loadis lower (1). Attention must thereforebe paid to UPS operating in an active-redundancy configuration, where theunits share the total load and oftenoperate at 50% of their full rated load,or less.

(1) A UPS is optimised to operate atfull rated load. Even though losses areat their maximum at full rated load,the efficiency is also at its maximum.In a standard UPS, losses are notproportional to the percent load andthe efficiency drops sharply when thepercent load drops. This is because apart of the losses is constant and therelative percentage of this partincreases when the load decreases. To obtain high efficiency at low loadlevels, the constant losses must bevery low.

UPS efficiency, see page 133.

UPS percent load

This is the percentage of the ratedpower that is effectively drawn by theload.

Load (%) = Sload (kVA)Sn (kVA)

Recommendation: take into

account growth in loads

It is advised to leave a margin (excesspower) when setting the rated power,particularly if a site expansion isplanned. In this case, make sure thepercent load on the UPS is stillacceptable after the expansion.

UPS efficiency

This factor determines the powerdrawn by the UPS on the upstreamdistribution system, i.e. theconsumption. It may be calculated as:

η (%) = PUPSoutput (kW)PUPSinput (kW)

For a given power rating, a high levelof efficiency:

• Reduces power bills,

• Reduces heat losses and,consequently, ventilationrequirements.

It is possible to calculate the efficiencyat full rated load, i.e. with a 100% load.

ηn (%) = Pn (kW)

PUPSinput (kW)

The rated active power of the UPS isobtained by multiplying the ratedapparent power Sn (kVA) by 0.8 (if λ >0.8) or by λ (if λ< 0.8).

The efficiency can vary significantlydepending on the percent load andthe type of load.

Figure 8: Single double conversion static UPS unit and example of an overload curve...

Power levels under steady-stateconditions

A UPS is sized using the apparentrated output power Sn (kVA) and anoutput power factor of 0.8. Theseconditions correspond to an activerated power of Pn (kW) = 0.8 Sn (kVA).

In real-life situations, a UPS supplies anumber of loads with an overall powerfactor λ that is often not 0.8 due to thepresence of non-linear loads andmeans to improve the power factor;

• If λ ≥ 0.8, the UPS is still limitedto Pn (kW),

• If λ < 0.8, the UPS is limited to Sn (kW) < Pn (kW).

Consequently, selection of the powerrating in kVA must take into accountthe active power supplied to theloads.

The active power is determined byfollowing the following four steps.

1. Apparent and active power drawn

by the loads

The first step is to evaluate the power

requirements of the load.

Table 3 must be drawn up for the kloads to be supplied.



Bypass AC input. If transfer is notpossible, many UPS current limit foroverloads greater than the maximumvalue (e.g. 2.33 In peak for onesecond, which corresponds to amaximum sine wave with an rmsvalue of 2.33 / √2 = 1.65 In). Beyondone second, the UPS shuts down.

A set of disconnection switches isavailable to isolate the UPS formaintenance in complete safety.

Ratings of single-UPSconfigurationsSingle-UPS configurations

These configurations comprise asingle, double-conversion UPS unit(see figure 8). The overload capacity atthe UPS output is indicated by adiagram.

In the event of an internal fault or anoverload exceeding UPS capacity, thesystem automatically transfers to the

Table 3..

Load Apparent rated power (kVA) Input power factor λ (or cos φ) Active rated power (kW)

Load 1 S1 λ1 P1 = λ1 S1

Load 2 S2 λ2 P2 = λ2 S2

…

Load i Si λi Pi = λi Si

…

Load k Sk λk Pk = λk Sk

Total S λ P = λ S

(1) S is not the sum of Si(2) λ must be measured

or calculated (3) P = λ S = ∑ λi Si

(1) S is not the sum of Si because:• it would be necessary to calculate the vectoral sum if all loads were linear, using the angles of the different cos φ,• some of the loads are not linear.

(2) λ must be measured on site or evaluated on the basis of past experience.(3) P = λ S = ∑ λi Si because the active power is added (no displacement).



power required, otherwiseselect the next highest rating.

• If λ < 0.8, the power supplied bythe UPS is sufficient because Pn

(kW) > λ Sn (kVA), i.e. theselection is correct.

4. Percent load

The fourth step is a check to ensurethat the percent load is acceptable

now and in the future, given thedesired operating conditions.

The percent load is:

Load = S / Sn(kVA).

It must be sufficient to cover anyincreases in the load or if there areplans to expand the system to becomeredundant.

Power levels under transientconditions

Load in-rush currents

It is necessary to know the in-rushcurrent of each load and the durationof the transient conditions. If anumber of loads risk being turned onat the same time, it is necessary tosum the in-rush currents.

2. Rated apparent power of the UPS

(Sn)

The second step is to select a UPSwith an apparent-power ratingsufficient to cover the load

requirements (in kVA).

Under the given conditions, thesuitable rated apparent power for theUPS is:

Sn (kVA) > S. where S = P / λ.

In the UPS range, select the UPS witha rated power Sn (kVA) just above S. Ifreserve power is required and theselected rating is too close to S, selectthe next highest rating.

3. Check on the active power

The third step is a check to ensure thatthe selected power rating can cover

the load requirements in kW underthe stipulated operating conditions.

For the selected rating, the UPS willsupply the rated active power:

Pn (kW) = 0.8 Sn (kVA)

• If ≥ 0.8, make sure that Pn (kW) > P, i.e. that the UPScan supply the additional

Figure 9: Example of an installation...



Example

The following example is simply toillustrate the point and does notcorrespond to a real situation. Thepurpose is to indicate the requiredsteps. The installation is made up ofthree 400 V three-phase loadsconnected in parallel:

• Computer system - S1 = 4 x 10kVA (4 identical 10 kVA loads), λ= 0.6 for all the loads, in-rushcurrent 8 In over four periods 50Hz (80 ms) for each load,

• Variable-speed drive - S2 = 20kVA, λ = 0.7, in-rush current 4 Inover five periods (100 ms),

• Isolation transformer - S3 = 20kVA, λ = cos φ = 0.8, in-rushcurrent 10 In over six periods(120 ms).

Necessary checks

It is then necessary to check that theplanned UPS power rating can handlethe in-rush currents. Note that theUPS can operate for a few periods incurrent-limiting mode (e.g. 2.33 In forone second for some manufactures. Ifthe UPS cannot handle the in-rushcurrents, it is necessary to decidewhether it is acceptable to transfer tothe Bypass AC input when thetransient conditions occur. If transferis not acceptable, it is necessary toincrease the power rating.

Review of in-rush currents, seepage 159.


S = 54 / 0.68 = 79.4 kVA

An 80 kVA rating would not besufficient, i.e. a 100 kVA rating shouldbe selected or higher if a siteextension is planned.


• The UPS can supply the loads100 x 0.68 = 68 kW > 54 kW.

4 . Check on the percent load and

rated current

• The percent load is, therefore,79.4 / 100 = 79.4%.

• Rated current of the UPS - Sn (kVA) = UI √3 , i.e. I = 100 / (400 x 1.732) = 144 A.

In-rush currents under transient

conditions

The loads should be started up oneafter the other to avoid combining thein-rush currents. It is necessary tocheck that the UPS can handle the in-rush currents.

The rated currents are calculated as S(kVA) = UI √3 , i.e.:

• Computer system - In = 10 / (400 x 1.732) = 14.4 A, i.e. 8 In ≈115 A for 80 ms,

• Variable-speed drive - In = 20 /(400 x 1.732) = 28.8 A, i.e. 4 In ≈115 A for 100 ms,

• Transformer - In = 20 / (400 x 1.732) = 28.8 A, i.e. 10 In =288 A for 120 ms,

• A 100 kVA UPS with an overloadcapacity of 120%, i.e. 151 A x 1.2 = 173 A for 1 minute and150%, i.e. 151 A x 1.5 = 216 A for1 minute,

• Operation in current-limitingmode at 2.33 In, i.e. 335 A for onesecond.

If the four computer loads (10 kVAeach) are started one after the other,the 20% overload capacity of the UPS is sufficient (173 A -1mn > 115 A -80 ms).



Table 4..

LoadRated apparent power

(kVA)Input power factor

Rated active power(kW)

Computer system 40 0.8* 32*

Variable-speed drive 20 0.7 14

LV/LV transformer 20 0.8 16

Total Sλ = 0.68 measured or

estimatedP = 54 kW

* average of new top of the range systems with power factor 0.9 and older equipment with power factor between 0.7 and 0.8.

Power levels under steady-state

conditions


by the loads

Table 4 should be drawn up.

Figure 10: UPS system with parallel-connected units and a static-switch cubicle (SSC)..



comprises an automatic bypassand a maintenance bypass thatare common for a number ofparallel units without bypasses(see figure 10).

True modular parallel systems are alsoavailable, made up of dedicated andredundant modules-power, intelligence,battery and bypass, all engineered intoa design that is easily and efficientlyserviceable. Power modules can beeasily added as demand grows or ashigher levels of availability are required.

There are two types of parallelconfigurations:

• Without redundancy: all the UPSunits are required to supply theload. Failure of one unit meansthe entire system shuts down(not recommended);

• With redundancy N + 1, N + 2,

etc.: the number of UPS unitsrequired for the load is equal toN. All the UPS units (N + 1, N + 2,etc.) share the load. If one UPSunit shuts down, the remainingunits (at least equal in number toN) continue to share the load.

Typical configurations andcharacteristics, see section 2.

If the four loads are startedsimultaneously, the in-rush currentwould be 4 x 115 = 460 A > 335 A. Thesystem would current limit for 80 ms.

For the variable-speed drive, the over -load capacity is sufficient. For theisolation transformer (288 A for 120 ms),the overload capacity is again sufficient.

Ratings of parallel-UPSconfigurationsParallel-UPS configurations

Purpose of parallel connection

Parallel connection of a number ofidentical units is the means to:

• Increase the power rating,

• Establish redundancy that in -creases the MTBF and availability:

Types of parallel connection

Two types of UPS units can beconnected in parallel.

• Integrated parallel UPS units:

each UPS unit includes an auto -matic bypass and a manualmaintenance bypass. The manualbypass may be common to theentire system (in an externalcubicle);

• Parallel UPS units with an SSC:

the static-switch cubicle



Select in the UPS range the powerrating Sn (kVA) just above S/N. Ifreserve power is required or theselected rating is too close to S, selectthe next highest rating.


For the selected rating, the UPS willsupply the active rated power

Pn (kW) = 0.8 Sn (kVA)

• If λ ≥ 0.8, make sure that Pn (kW)> P, i.e. that the UPS can supplythe additional power required,otherwise select the next highestrating.

• If λ < 0.8, the power supplied bythe UPS is sufficient because Pn

(kW) > λ Sn (kVA), i.e. theselection is correct.

4. Percent load

With redundancy, the UPS units sharethe load according to the equation

S / (N+K).

The percent load for each unit whenthere is redundancy is therefore:

TL = S / (N + k) Sn (kVA).

In a non-redundant system, it iscalculated as:

TL = S / N Sn (kVA).

It must be sufficient to cover anyincreases in the load.

Example

This example will use the results fromthe last example, and we will supposethat the loads are critical, i.e.redundancy is required.

• The total load is 54 kW with anoverall power factor for all theloads of 0.68, i.e. S = 54 / 0.68 = 79.4 kVA;

Power levels in redundantparallel configurations

In a redundant parallel configurationmade up of identical units, the unitsshare the load. The power rating ofeach unit does not depend on the levelof redundancy, but must be calculatedto continue supplying the load even ifredundancy is completely lost.

Active redundancy:

• Improves availability,

• Increases the overload capacity,

• Reduces the percent load oneach UPS unit.

The power level is determined byfollowing the same four steps as for asingle-UPS configuration.


by the loads

The same type of table is used as thatfor a single UPS (see page 20).

The result is the apparent power Sthat must be supplied to the load.


units (Sn) in the configuration

Consider a level of redundancy N + K

(e.g. 2 + 1), which means:

- N units (e.g. 2) are required tosupply the load,

- K units (e.g. 1 extra unit) ensureredundancy.

Each UPS unit must be sized to enablethe system as a whole to operate with -out redundancy, i.e. with N operationalunits and K units shut down.

In this case, the N units must eachhave an apparent power rating Sn

(kVA) such that:

Sn(kVA) > S / N.



• This is the case because 2 x 50 x0.68 = 68 kW > 54 kW;

• During operation, the percentload will be:

- with redundancy, i.e. with 3UPS units sharing the load:79.4 / 3 x 50 = 52.9%;

- without redundancy, i.e. withonly 2 UPS units sharing theload: 79.4 / 2 x 50 = 79.4%.

• If 2+1 redundancy is used, twounits must be capable of supply -ing the load. Each must will haveto supply S / 2 = 79.4 / 2 = 39.7kVA;

• A 40 kVA rating would not besufficient, i.e. a 50 kVA ratingshould be selected or higher if asite extension is planned;

• If redundancy is not available,the two UPS units must becapable of supplying the load;

Control of upstream harmonics

Figure 11: Input rectifier and harmonics..



Harmonics are controlled by using

a filter (see figure 11).

PFC-type transitor-basedcontrolled active rectifiers

These transistor-based active rectifiershave a regulation system that adjuststhe input voltage and current to areference sine wave. This techniqueensures an input voltage and currentthat are:

• Perfectly sinusoidal, i.e. free ofharmonics,

• In phase, i.e. with a power factorclose to 1.

With this type of rectifier, no filters are

required.

Clean transitor-based rectifiers.

UPS and upstreamharmonicsRole of the input rectifier

UPS units draw power from the ACdistribution system via a rectifier/charger. With respect to the upstreamsystem, the rectifier is a non-linearload that causes harmonics. In termsof harmonics, there are two types ofrectifiers.

Standard rectifiers

These are three-phase rectifiersincorporating SCRs and using a six-phase bridge (Graetz bridge) withstandard chopping of the current.

This type of bridge draws harmoniccurrents with orders of n = 6 k ± 1(where k is a whole number), mainlyH5 and H7, and to a lesser degree H11and H13.



High power factor at the rectifier input

The goal is to increase the inputpower factor (generally to a levelhigher than 0.94). This reduces theconsumption of kVA and avoidsoversizing the sources.

Installation complying with standards

The goal is to comply with standardsconcerning harmonic disturbancesand with the recommendations issuedby power utilities.

• Standards on harmonicdisturbances (see table 5):

- IEC 61000-3-2 / EN 61000-3-2for devices with an inputcurrent ≤ 16 A/ph,

- IEC 61000-3-4 / EN 61000-3-4for devices with an inputcurrent > 16 A/ph.

• Standards and recommendationson the quality of distributionsystems, notably:

- IEC 61000-3-5 / EN 61000-3-5,

- EN 50160 (Europe),

- IEEE 519-2 (United States),

- ASE 3600 (Switzerland),

- G5/3 (U.K.), etc.

Standards on harmonics, see “UPSstandards” in page 145.

Table 5. Example of harmonic-currentlimitations as per guide IEC 61000-3-4/ EN 61000-3-4 for devices with aninput current > 16 A/ph (stage 1,simplified connection).

Filtering of upstreamharmonics for UPS withGraetz bridge rectifiersGoals of harmonic filtering

This section concerns only a UPS withconventional Graetz bridge rectifiers.

A “clean” upstream system

The goal is to ensure a level of voltagedistortion (THDU) on the busbarssupplying the UPS that is compatiblewith the other connected loads.

The UTE recommends limiting theTHDU to:

• 5% when the source is agenerator,

• 3% when the source is atransformer to take into account1 to 2% of THDU which mayalready be present on the HVdistribution system.

This recommendation may differ foreach country.

Practically speaking, solutions forvoltage distortion (THDU) must beimplemented in a manner specific tothe country where the installation islocated.

Easy combination with an engine

generator set

The goal is to make possible aUPS/engine generator setcombination with no risk of increasingthe level of harmonics when the loadis transferred to the generator. Thisrisk exists because the generator has a source impedance lower than that of a transformer, which increases theeffects of harmonics.



Filtering and parallel connection

When a number of UPS units areconnected in parallel and dependingon the type of filter used, it is possibleto install:

• an individual filter on each UPSunit,

• a common filter for the entireparallel configuration.

The goal is to achieve a balancebetween cost and effectiveness, takinginto account the acceptable levels ofharmonic distortion.

The comparison tables for the varioussolutions (page 29) are helpful inmaking a selection.

Combination of LC filters andgenerator

The generator can supply onlyrelatively low capacitive currents (10 to 30% of In). When an LC filter isinstalled, the main difficulty lies in thegradual start-up of the rectifier ongenerator power, when active poweris equal to zero and the generatorsupplies only the capacitive currentfor the filter. Consequently, the use ofLC filters must be correctly analysedto ensure that operation complies withmanufacturer specifications. Below isa method for selection of LC filters,using as an example a generatorderating curve, similar to thoseprovided by manufacturers.

Table 5..

Types of harmonics filters

Harmonics filters eliminate certainorders or all orders, depending ontheir technology. The following typesare available.

Passive LC filters:

• Non-compensated,

• Compensated,

• Non-compensated with contactor.

Double-bridge rectifier

Phase-shift filter

THM active filter (Active 12-pulse

technology)

Harmonic % of H1 (fundamental)

H3 21.6%

H5 10.7%

H7 7.2%

H9 3.8%

H11 3.1%

H13 2.0%

H15 0.7%

H17 1.2%

H19 1.1%

H21 ≤ 0.6%

H23 0.9%

H25 0.8%

H27 ≤ 0.6%

H29 0.7%

H31 0.7%

≥ H33 ≤ 0.6%

Even orders ≤ 0.6% or ≤ 8/n (n even order)

F: operating point at the rated load, with -out a filter or with a phase-shift filter.

Example

Consider a non-compensated filterwith a 300 kVA generator and a 200 kVA UPS.

The power rating of the rectifier,taking 87% as the efficiency value (1 / 0.87 = 1.15), is 1.15 times that ofthe inverter, i.e. 200 x 1.15 = 230 kVA.

The capacitive current of the non-compensated filter is 230 x 30% (1) =69 kVA.

The reactive power that the generatorcan handle (point A) is 300 x 0.3 = 90 kVA.

The filter is therefore compatible withthe generator.

(1) The value of 30% has beendetermined experimentally.



The curve in figure 12, provided asone example among many, shows thepower derating as a function of theoperating point, for a given generator.For a purely capacitive load λ = 0), thepower available is equal to only 30%of the rated power (point A). If weassume an apparent power ratingsuch that Pn generator = Pn rectifier,the meaning of points A, B, C, D, E and F is the following:

A: reactive power corresponding tothe capacitive current of a non-compensated filter,

B: reactive power corresponding tothe capacitive current of acompensated filter,

C: operating point at start-up with anon-compensated filter withcontactor,

D: operating point at the rated loadwith a non-compensated filter,

E: operating point at the rated loadwith a compensated filter,

Figure 12: Derating curve for a generator, as a function of the installation power factor..



Efficiency

Consumption of the filters can slightlymodify the efficiency of the install-ation as a whole.

Flexibility for set-up and upgrades

Filters are generally specific to a UPSand may be factory-mounted orinstalled after installation. Theconditioner provides overallelimination of harmonics and greatflexibility in the configuration.

Dimensions

It is necessary to check whether thefilter can be installed in the UPScabinet or in a second cabinet.

Cost

It impacts on the effectiveness of thefilter and must be weighed against theadvantages obtained.

Compliance with standards

It is necessary to determinecompliance with standards, inparticular IEC 61000-3-4, in terms ofthe individual harmonic levelsindicated in the texts.

Comparison table of solutions

The following tables list the elementsfor comparison, with a generalcomment on use of each type ofsolution.

Table 6 presents individual solutionsfor single-UPS configurations. Thesesolutions may also be used in parallelconfigurations.

Table 7 presents overall solutions forentire configurations.

Selection of a filterSelection parameters for a filter

Overall effectiveness - reduction in

distortion (THDI and THDU)

The effectiveness depends on theharmonic orders filtered and thedegree to which they are attenuated oreliminated. It is measured by the THDIat the rectifier input. The impact on theTHDI determines the level of theTHDU. It is necessary to check theperformance at the planned percentload, given that many UPS systemsoperate at percent loads between 50and 75%.

Improvement in the power factor λThe filter improves the power factor(generally to a level higher than 0.92).

Compatibility with an engine

generator set

It is also necessary to check theperformance with the plannedsource(s), either a transformer or anengine generator set. This is becausethe generator has an outputimpedance lower than that of atransformer, which increases theeffects of harmonics.

Suitable for parallel-UPS

configurations

Depending on the type of filter, it ispossible to install one on each UPSunit or set up a single filter for overallelimination of harmonics.



Table 6: Comparison of individual harmonic-filtering solutions..

Type of filterCriterion

LC non-compensated

LCcompensated

LC withcontactor

Double ridge Built-in THM

DiagramUPS

AC input

C L

Load

UPS

AC input

C L

Load

UPS

AC input

C L

Load

Rectifier Rectifier

Inverter

AC input

Load

AC input

Load

THM

UPS

Figure A Figure B Figure C Figure D Figure E

Reduction indistortionTHDI at 100% loadTHDI at 50% load

7 to 8%10%

7 to 8%10%

7 to 8%10%

10%15%

4%5%

Harmonics eliminated

H5, H7 H5, H7 H5, H7 H5, H7, H17, H19 H2 to H25

Power factorλ at 100% loadλ at 50% load

0.951

0.951

0.951

0.850.8

0.940.94

Compatibility with generator

* ** ** ** ***

Efficiency of filter *** *** *** * **

Flexibility,upgradeability

* * * * ***

Cost *** *** *** * **

Dimensions *** *** *** * ***

Connection in parallel with UPS

* * * * **

UPS UPS UPS

UPS UPS UPS UPS Inverter

Rectifier Rectifier

Inverter

Rectifier Rectifier

Figure F Figure G Figure H Figure I Figure J

Compliance withguide IEC 61000-3-4

no no no no yes

General comment

Solution suitablefor installationswithout an enginegenerator set.

Solution suitablefor installationswith an enginegenerator set. The addedinductor loadreduces thecapacitive powerthat must besupplied by theengine-generatorset.

Solution suitablefor installationscomprising anengine generatorset with a powerrating lower thanthat of the UPS. TheLC line is switchedin by the contactorat a preset valuecorresponding to an inverter percentload that isacceptable for theengine generatorset.

Solution suitablefor installationswith gensets

Solution suited to sensitiveinstallations orwith changing loadlevels. The mosteffective and themost flexiblesolution. Does notdepend on thepercent load or the type ofupstream source.

*** Excellent ** Good *Sufficient



Table 7: Comparison of overall solutions..

Type of filterCriterion

SineWave Phase-shift filter

Diagram

SW

AC input

Load

AC input

Load

AC input

Load

AC input

Load

Figure AA Figure BB Figure CC Figure DD

Reduction indistortionTHDI at 100% loadTHDI at 50% load

4%5%

< 10%35% with 1 UPS

shut down

< 5%19% with 1 UPS

shut down

< 4%12% with

1 UPS shut down

Harmonicseliminated

H2 to H25

Power factorλ at 100% loadλ at 50% load

0.951

0.80.8

Compatibility with generator

*** **

Efficiency of filter *** **

Flexibility,upgradeability

*** *

Cost *** ***

Dimensions *** *

Compliance withguide IEC 61000-3-4

yes yes

General comment

Solution suited to sensitiveinstallations or with changingload levels. The most effectiveand the most flexible solution.Does not depend on thepercent load or the type ofupstream source.

Solution cannot be modified. Suited to installations with more thantwo parallel-connected UPS units.

*** Excellent ** Good * Sufficient

System earthing arrangements



exposed conductive parts (ECP) apotential that may be sufficient tocause a dangerous current to flowthrough the body of the person incontact with the exposed conductiveparts (see figure 13).

This protection includes the pointslisted below.

• Mandatory earthing of allexposed conductive parts (ECP)that may be accessed by theuser.

The protective conductor is used forconnection to the earth. It must neverbe interrupted (no breaking devices onthe protective conductor).

The interconnection and earthingtechniques for the exposed conductiveparts (ECP) determine the system

earthing arrangement (SEA) for theinstallation.

• Disconnection of the supply

when the potential of the ECPsrisks reaching dangerous levels.Interruption is carried out by aprotection device that dependson the selected system earthingarrangement (SEA). It oftenrequires residual-current devices

(RCD) because the insulation-fault currents are generally toolow to be detected by standardovercurrent protection devices.

Background information on system earthingarrangementsProtection of persons againstelectrical contact

International standards require thatelectrical installations implement twotypes of protection of persons againstthe dangers of electrical currents.

Protection against direct contacts

The purpose of this form of protectionis to avoid “direct” contact betweenpersons and intentionally live parts

(see figure 13).

It includes the points listed below.

• Isolation of live parts usingbarriers or enclosures offering adegree of protection at leastequal to IP2X or IPXXB.

• Opening of the enclosure (doors,racks, etc.) must be possible onlyusing a key or a tool, orfollowing de-energising of thelive parts or automaticinstallation of a screen.

• Connection of the metalenclosure to a protective

conductor.

Protection against indirect contacts

and system earthing arrangements

The purpose of this form of protectionis to avoid “indirect” contact betweenpersons and exposed conductive

parts (ECP) that have become live

accidentally due to an insulation fault.The fault current creates in the

Table 8..

First letter Second letter Third letter (for TN)

Connection of the neutral Connection of the ECPs Type of protective conductor

T = earthed neutral T = exposed conductive partsearthed

C = Common neutral andprotective conductor (PEN)

I = isolated neutral N = exposed conductive partsconnected to the neutral

S = Separate neutral (N) andprotective conductor (PE)

IT, TT or TN systems TN-C orTN-S



Types of system earthingarrangements (SEA)

There are three types of systemearthing arrangements (SEA):

• Isolated neutral (IT),

• Earthed neutral (TT),

• Exposed conductive partsconnected to the neutral (TN

with TN-C and TN-S). The firsttwo letters indicate how theneutral and the ECPs of the loadsare connected.

Figure 13: Direct and indirect contacts..



Figure 14: IT system..

Earthed neutral (TT)

Figure 15: TT system..

System earthing arrangements(SEA)

Isolated neutral (IT)



Exposed conductive parts connected

to the neutral (TN)

Figure 16: TN-S system (the basic principle is identical for the TN-C system)..



Comparison of system earthing arrangements (SEA)

Table 9..

Type of SEA IT (isolated neutral) TT (earthed

neutral) TN-S (ECP to

neutral) TN-C (ECP to

neutral)

Operation

• Signalling of firstinsulation fault.

• Location andelimination of thefirst fault.

• Disconnection forthe second fault.

• Disconnection for the firstinsulation fault.

• Disconnection for the firstinsulation faultoccurs.

• Separate neutral(N) and protectiveconductor (PE).

• Disconnection for the firstinsulation fault.

• Common neutraland protectiveconductor (PEN).

Protection of persons

• Interconnection andearthing of ECPs.

• First fault:- very low current,- monitoring/indication

by an IMD.• Second fault:- potentially

dangerous current,- interruption by

overcurrentprotection devices(e.g. circuit breaker).

• Earthing of ECPscombined withuse of residual-current devices(RCD).

• First fault:- leakage current

is dangerous, but too low to bedetected by theovercurrentprotection devices,

- detection by theRCDs combinedwith breakingdevices.

• Interconnectionand earthing ofECPs and neutralimperative.

• First fault:- fault current,- interruption

by overcurrentprotection devices(e.g. circuitbreaker).

• Interconnectionand earthing ofECPs and neutralimperative.

• First fault:- fault current,- interruption

by overcurrentprotection devices(e.g. circuitbreaker).

Specificequipment

Insulation-monitoringdevice (IMD) andfault-locating device.

Residual-currentdevices (RCD).

For long distances,RCDs must be used.

Advantages anddisadvantagesEMC

• Solution offering thebest continuity ofservice (the firstfault is signalled).

• Requires competentsurveillancepersonnel (locationof the first fault).

• High EMCperformance, verylow currents in theearth cable.

• Easiest solutionto design andinstall.

• Mandatory use of RCDs.

• Different earthelectrodes(distant sources).

• Highly sensitiveto lightningstrikes.

• High installationcosts for highpower ratings.

• Difficult to design(calculation of theloop impedances).

• Flow of high faultcurrents.

• High EMCperformance, lowcurrent in the PEduring normaloperation.

• Reducedinstallation costs(one lessconductor).

• Difficult to design(calculation of theloop impedances).

• Flow of high faultcurrents.

• Low EMCperformance, highcurrents in thePEN (connectionsbetween ECPs).

Use

• Installationsrequiring continuityof service, e.g.hospitals, airports,industrial processes,ships.

• Installations andpremises wherethere is a risk of fireor explosion, i.e.mines, etc.

• Commercial and residentialpremises, publiclighting, schools,etc.

• Large commercialpremises, tallbuildings, etc.

• Industries withoutcontinuousprocesses (IT system).

• Supply ofcomputer systems.

• Large commercialpremises, tallbuildings, etc.

• Industries withoutcontinuousprocesses (IT system).

• Supply ofcomputersystems.



downstream system, either the sameor a different one, depends on itscompatibility with sensitive loads.

Table 9 provides the necessaryelements to compare the variousstandardised system earthingarrangements.

Caution, local regulations mayprohibit certain types of systemearthing arrangements.

Selection of the breaking devices

Above and beyond the inter -connection and earthing of theexposed conductive parts incompliance with a standardisedsystem earthing arrangement, theprotection of persons must beensured by breaking devices selectedaccording to the system earthingarrangement. These devices mustcause tripping of the overcurrentprotection devices in the event of aninsulation fault.

Tripping may:

• be directly provoked by suitablesettings on the overprotectiondevices (circuit breakers, fuses),

• or require (mandatory for the ITsystem) use of residual-currentdevices (RCD) that may or maynot be built into the circuitbreaker.

The RCDs are required to detect theinsulation-fault currents that are oftentoo low to trip standard overcurrentprotection devices.

Check local requirementsconcerning the safety of electricalinstallations.

Applications in UPSinstallationsSpecific aspects in systems with UPS

Implementation of the afore -mentioned protection systems in installations comprising a UPSrequires a number of precautions for a number of reasons:

• The UPS plays two roles:

- a load for the upstream system,

- a power source for thedownstream system,

• When the battery is not installedin a cabinet, an insulation faulton the DC system can lead to theflow of a residual DC component.This component can disturboperation of certain protectiondevices, notably RCDs used forthe protection of persons.

Protection against direct contact

When the battery is not installed in acabinet (generally in a special room),the measures presented at the end ofthis section should be implemented.

Protection against indirectcontact

Selection of a system earthing

arrangement

A basic protection measure requiredby the standards is the creation of astandardised system earthingarrangement both upstream anddownstream of the UPS. The twosystems can be the same or differentif certain precautions are taken. In anexisting installation to which the UPSis added, the upstream system isalready determined. Selection of the

Figure 17: Standard diagrams..



UPS are increasingly designedwithout transformers, offeringadvantages in terms of weight, sizeand efficiency. Transformerlesstechnology also makes it possible tomodulate the voltage for improvedadapatation to all types of loads, inparticular nonlinear loads withharmonics.

Transformerless technology has animpact on the use of system earthingarrangements. For more informationsee White Paper - WP 98: “TheElimination of Isolation Transformersin Data Center Power Systems”).

Many different cases may beencountered depending on theupstream and downstream earthingarrangements and the type of UPS.Your Caterpillar representative has a complete set of diagrams for allsystem earthing arrangements andUPS ranges concerned.

The ranges are designed with isola -tion transformers. All the other rangesuse transformless technology with theneutral recreated electronically.

The following pages show someexamples, contact your Caterpillarrepresentative to obtain the applicablediagram.

Types of systems for UPS

The possible systems depend on:

• The existing or selected systemupstream of the UPS,

• The system downstream of theUPS for which selection may bedetermined by:- reuse of the same system as

upstream,- the presence of isolation

transformers upstream ordownstream which make itpossible to change the systemearthing arrangement,

- the loads (e.g. computersystems require a TN-C or TN-s system),

- the organisation of thedownstream distributionsystem, with static transferswitches (STS),

• Certain requirements imposed bystandards, e.g. the protectiveconductor PE or PEN must neverbe interrupted to ensure flow ofthe fault current. A TN-C system(non-interrupted PEN) can beinstalled upstream of a TN-Ssystem (separate N and PEconductors), but not the contrary.



Identical systems upstream

and downstream

Figure 18: A few examples with the same system upstream and downstream..

Different systems upstream

and downstream

Figure 19: A few examples with different systems downstream..

Table 10..

Protection Symb. Definition Availability

Overload protection(thermal or long delay) (1) Ir Overload current setting. All trip units.

Long delay (2) tr Applies a long tripping delay(e.g. for motor starting).

Electronic trip units (e.g. Micrologic 2, 5, 6).

Short-circuit protection(magnetic or short delay) (3) Im or Isd

Short-circuit current setting. On electronic trip units, Isd is afunction of Ir (generally 2 to 10 Ir).

All trip units.

Short delay (4) tm or tsdApplies a short tripping delay(e.g. for time discrimination withdownstream circuit breaker).

Electronic trip units (e.g. Micrologic 5, 6).

Short-circuit protection,instantaneous trip (5) Ii

Instantaneous short-circuitsetting. Depends exclusively ontrip-unit rating (e.g. protection ofstatic switches).

Electronic trip units (e.g. Micrologic 5, 6).

(1) Ir is the thermal protection threshold (sometimes written Ith) of thermal-magnetic trip units or the long-delay protectionthreshold of electronic trip units. These thresholds are defined by an inverse time curve that depends on the selected setting.

(2) tr is the time delay of the long-delay thermal protection for a given value of Ir.

(3) Im is the magnetic threshold of thermal-magnetic trip units and Isd the short-delay threshold of electronic trip units.

(4) tm is the time delay (adjustable or fixed) of the magnetic protection of thermal-magnetic trip units and tsd the time delay(generally adjustable) of the short-delay protection of electronic trip units.

(5) Ii is the instantaneous tripping threshold.

Protection



Comparison

Thermal-magnetic trip units aresimple and inexpensive.

Electronic trip units offer more preciseand comprehensive settings for betteradaptation to installations and theirrequirements.

Table 10 sums up the characteristics of both types of trip units for circuit -breakers from 1 to 630 A and shouldenable you to solve most of theproblems commonly encountered(from 1 to 400 kVA).

Figure 20 presents the characteristiccurves for the trip units.

Protection using circuitbreakersThe protection system for installationswith UPS units presented here willimplement circuit breakers. Below is a presentation of the maincharacteristics of circuit breakers and their trip units.

Trip units

Technology

There are two types of trip units:

• Thermal-magnetic,

• Electronic.

Construction

• Built-in (thermal-magnetic only),

• Interchangeable.



Figure 20: Circuit-breaker time/current curves (Icu is the ultimate breaking capacity)..

Table 11..

Discrimination Concerns Principle

Current discrimination All types of trip units.

The fault current is lower than theupstream threshold setting. Ir upstream > Ir downstream and Im or Isd upstream > Im or Isddownstream.

Time discrimination Electronic trip units only (e.g. Micrologic). Delays upstream tripping by the long-

time (Ir) and short-time (Im or Isd) delay.

Energy discrimination Compact NSX and NS.

Arc pressure upstream is not sufficientto trip the upstream circuit breaker, butit is sufficient to trip the downstreamcircuit breaker.

Zone-selectiveinterlocking

Compact NSX 100 to Masterpact withMicrologic trip units.

Delays upstream tripping if the short-circuit is also detected downstream. Apilot wire connects the upstream anddownstream trip units.



Current limiting

When a high fault current hits thecircuit breaker, the breaker contactsseparate under the electrodynamicforces, an arc is created and itsresistance limits the shortcircuitenergy.

Cascading

When a short-circuit occurs down -stream of the installation (see figure 21),the fault current also flows throughthe upstream circuit breaker whichcurrent limits, thus attenuating thecurrent applied to the downstreamcircuit breaker. The breaking capacityof the latter is thus reinforced.

Discrimination, cascading,current limiting

Discrimination

Discrimination results from correctcircuit-breaker selection and settingsuch that, if a fault occurs, it trips onlythe first upstream circuit breaker.

Discrimination thus limits the part ofthe installation affected by the fault toa strict minimum. There are a numberof types of discrimination summed upin table 11 and illustrated in figure 20.

Table 12..

Type of downstream circuitIr upstream / Ir

downstream ratioIm upstream / Imdownstream ratio

Im upstream / Imdownstream ratio

Downstream trip unit all types magnetic electronic

Distribution > 1.6 > 2 > 1.5

Asynchronous motor > 3 > 2 > 1.5



Breaking capacity

The breaking capacity must beselected just above the short-circuitcurrent that can occur at the point ofinstallation.

Ir and Im thresholds

Table 12 indicates how to determinethe Ir and Im thresholds to ensurediscrimination, depending on theupstream and downstream trip units.

Remark: Time discrimination must beimplemented by qualified personnelbecause time delays before trippingincrease the thermal stress (I2t)downstream (cables, semiconductors,etc.). Caution is required if tripping ofCB2 is delayed using the Im thresholdtime delay.

Energy discrimination does notdepend on the trip unit, only on thecircuit breaker.

Ir and Im thresholds depending on the

upstream and downstream trip units

Figure 21: Upstream/downstream.discrimination and cascading.

Selection of circuitbreakersRating

The selected rating (rated current) forthe circuit breaker must be the onejust above the rated current of theprotected downstream cable.

Figure 23: Example of an installation..



Example

Consider the example used todetermine the UPS power rating (page 17) with a number of parallel-connected 400 V three-phase loads,namely:

• Computer system - S1 = 4 x 10kVA, λ = 0.6, in-rush current 8 Inover four periods (80 ms),

• Variable-speed drive - S2 = 20kVA, λ = 0.7, in-rush current 4 Inover five periods (100 ms),

• Isolation transformer - S3 = 20kVA UPS was selected, λ = 0.8,in-rush current 10 In over sixperiods (120 ms).

The three loads represent 54 kW witha power factor of 0.68.

On page 20, 100 kVA UPS wasselected, I = 100 / (400 x √3) = 144 A.

Special case of generator short-circuits

Figure 22 shows the reaction of agenerator to a short-circuit.

To avoid any uncertainty concerningthe type of excitation, we will trip atthe first peak (3 to 5 In as per X"d)using the Im protection settingwithout a time delay.

Figure 22: Generator during a short-circuit..

Figure 24: Calculation of short-circuit current for CB1 and CB2..

It is necessary to calculate the resistances and reactances upstream of CB1 and CB2 in figure 23.



Calculation of CB1 and CB2ratings and breaking capacities

The breaking capacity depends on theshort-circuit currents downstream ofCB1 and CB2 at the level of the mainlow-voltage switchboard (MLVS). Mostoften, this upstream short-circuit valueis supplied by the utility. It can also becalculated. It is necessary to determinethe sum R of the resistances upstreamand the sum X of the reactancesupstream of the considered point.

The three-phase short-circuit current is calculated as:

Isc 3-ph =U

√3 √R2 + X2

U is the phase-to-phase no-loadvoltage (load voltage + 3 to 5%).

R = Σ Rupstream and X = Σ Xupstream

In this example, we simply indicatethe general method with a number of simplifications to shorten thecalculations.

The goal is to select circuit breakersCB1 and CB2, and the most powerfulcircuit breaker CB3 compatible withdiscrimination requirements, giventhat the upstream installation includesthe following:

• 20 kV / 400 V transformer with a power rating of 630 kVA,

• 400 V engine generator set with a power rating of 400 kVA,

• Transformer to MLVS link, fivemeters of aluminium cable 4 x 240 mm2 per phase,

• Busbars to circuit breaker link,four meters using three copperbars 400 mm2 per phase.



Distribution system upstream of the transformer

• Psc = upstream short-circuit power = 500 MVA = 500 x 106 VA

• U20 = phase-to-phase no-load voltage on the transformer secondary winding = 400 V, + 3%, i.e. 410 V

• Rup = resistance upstream ≈ 15% Xup, negligible given Xup

• Xup = reactance upstream with respect to transformer secondary winding

Xup = U202

=4102

= 0.288 mΩPsc 500 x 106

Rup ≈ 0 and Xup = 0.33 mΩ.

Transformer

• Sn = rated apparent power 630 kVA

• In = rated current = 630 / U √3 = 630 103 / (400 x √3) = 909 A

• Usc = transformer short-circuit voltage = 4%

• Pcu = transformer copper losses in VA

Rtr = transformer resistance = Pcu

≈ 20% Xtr, negligible given Ztr3 In2

Xtr ≈ Ztr = transformer impedance = U202

x Usc = 4102 x 0.04 / 630 103 = 10.7 mΩSn

Rtr ≈ 0 and Xtr = 10.7 mΩ.

Cables linking the transformer to the MLVS

• Length 5 meters

• Cross-section 240 mm²

• ρ = resistivity at the normal temperature of the conductors copper: ρ = 22.5 mΩ.mm2/m, aluminium: ρ = 36 mΩ.mm2/m

• Xc = conductor reactance (typically 0.08 mΩ/m) = 0.08 x 5 = 0.4 mΩ

Rc = cable resistance (copper) = ρL

= 22.5 x 5 / (4 x 240) = 0.12 mΩS

Rc = 0.12 mΩ and Xc = 0.4 mΩ.

General circuit breaker

Typical values

Rd ≈ 0 et Xd = 0.15 mΩ.

Busbars

• Xb = busbar reactance (typically 0.15 mΩ/m) = 0.15 x 4 = 0.6 mΩ

• Rb = busbar resistance = ρ L / S= 22.5 x 4 / (3 x 400) = 0.075 mΩ (negligible)

Rb ≈ 0 and Xb = 0.6 mΩ.

Transformer Isc at the level of CB1 and CB2

• R = Total upstream resistance = 0.12 mΩ

• X = Total upstream reactance = 0.33 + 10.7 + 0.4 + 0.15 + 0.6 =12.18 mΩ

R can be neglected, given X.

Isc 3-ph =U

≈U 410 = 19.4 kA

√3 √R2 + X2 √3 X √3 x 12.18 x 10-3

Note: A rough estimate is provided by the short-circuit current on the transformerterminals, assuming that the upstream short-circuit power is infinite.

ISCT = on transformer terminals = In / Usc = 20 In = 20 x 909 = 18.2 kA

Generator Isc at the level of CB1 and CB2

• Rated apparent power of the generator = 400 kVA

• Rated current of the generator = 400 / U √3 = 400 103 / (400 x √3) = 577 A

• X"d = short-circuit voltage of the generator = 10%

It is decided to trip at 5 In (figure 22).

ISCG = on the generator terminals = 5 In = 5 x 577 = 2.9 kA

Continuous current of CB1

This is the current at the UPS input. It is necessary to multiply the UPS rating by1.2 to take into account the efficiency, i.e. 120 kVA.

Iinput = 120 / U √3 = 120 103 / (400 x √3) = 173 A

Continuous current of CB2

This is the continuous current of the loads supplied via the bypass, i.e. 54 kWwith a power factor of 0.68 for an apparent power S = 54 / 0.68 = 67.5 kVA.

Iload = 67.5 / U √3 = 120 103 / (400 x √3) = 97 A

Energising current of the largest load

The loads must be energised at different times. The highest inrush current is thatof the 20 kVA transformer, i.e. In = 28.8 A and 10 In = 288 A - 120 ms.

Calculation of the maximum static-switch current

This is the short-circuit current at the level of CB3, which is practically that of CB2.

Selection parameters

Table 13 sums up the various values calculated.





Table 13..

Characteristics of the most power circuit breaker CB3 possible

Figure 25: Calculation of the short-circuit current at CB3..

Parameter Value

Transformer short-circuit current 19.4 kA

Generator short-circuit current 2.9 kA

Rectifier current (UPS input) 173 A

Continuous load current downstream of the UPS 97 A

Energizing current of the largest load 288 A - 120 ms

Maximum static-switch current 19.4 kA

Characteristics of CB1 and CB2

Characteristic D1 D2

Breaking capacity > 19.4 kA, i.e. 25 kA > 19.4 kA, i.e. 25 kA

Continuous current > 173 A, i.e. 200 A > 97 A, i.e. 125 A

Ir threshold > 173 A +20% > 97 A + 20%

Im threshold > 173 A + 20% and< 2.9 kA - 20%

> 288 A +20% and< 2.9 kA - 20%

20% represents here the typical tolerance range of circuit-breaker settings.



Operation with bypass power

• Breaking capacity

The highest short-circuit currentdownstream of CB3 is virtuallythat of CB2 because it is assumedthat the outgoing circuits arenear the UPS.

Consequently, the breakingcapacity of CB3 is also 25 kA.

• The rating is determined by thelargest load, i.e. the 4 x 10 kVA of the computer system with acontinuous current of:

Iload = 40 / U √3 = 40 103 / (400 x √3) = 57 A

A 60 A device should be selected.

• Settings

A majority of the loads is of thedistribution type, i.e. the Irthreshold of CB3 must be lessthan 97 A / 1.6, i.e. < 61 A.

The Im threshold must be lessthan 1847 / 2, i.e. < 900 A.

Operation without bypass power

In this case, the short-circuited UPSlimits its current to 2.33 In for onesecond.

Voltage drops

Maximum values

The maximum permissible voltagedrops are:

• 3% for AC circuits (50 or 60 Hz),

• 1% for DC circuits.

Selection tables

Table 14 indicates the voltage drop inpercent for a circuit made up of 100meters of copper cable. To calculatethe voltage drop in a circuit with alength L, multiply the value in thetable by L/100.

If the voltage drop exceeds 3% on athree-phase circuit or 1% on a DCcircuit, increase the cross section ofthe conductors until the value iswithin tolerances.

Voltage drop for 100-meter cables

• Sph - the cross section of theconductors,

• In - rated current of the protectiondevices on the circuit.



Selection of cable sizesCable temperature rise andvoltage drops

The cross section of cables depends on:

• Permissible temperature rise,

• Permissible voltage drop.

For a given load, each of theseparameters results in a minimumpermissible cross section. The largerof the two must be used.

When routing cables, care must betaken to maintain the requireddistances between control circuits andpower circuits, to avoid anydisturbances caused by HF currents.

Temperature rise

Permissible temperature rise in cablesis limited by the withstand capacity ofcable insulation.

Temperature rise in cables depends on:

• The type of core (Cu or Al),

• The installation method,

• The number of touching cables.

Standards stipulate, for each type ofcable, the maximum permissiblecurrent.

Cables



Table 14..

Three-phase circuit (copper conductors)

50-60 Hz - 400 V three-phase, cos φ = 0.8, balanced 3-ph + N system

Sph (mm2) 10 16 25 35 50 70 95 120 150 185 240 300

In (A) 10 0.9

16 1.2

20 1.6 1.1

25 2.0 1.3 0.9

32 2.6 1.7 1.1

40 3.3 2.1 1.4 1.0

50 4.1 2.6 1.7 1.3 1.0

63 5.1 3.3 2.2 1.6 1.2 0.9

70 5.7 3.7 2.4 1.7 1.3 1.0 0.8

80 6.5 4.2 2.7 2.1 1.5 1.2 0.9 0.7

100 8.2 5.3 3.4 2.6 2.0 2.0 1.1 0.9 0.8

125 6.6 4.3 3.2 2.4 2.4 1.4 1.1 1.0 0.8

160 5.5 4.3 3.2 3.2 1.8 1.5 1.2 1.1 0.9

200 5.3 3.9 3.9 2.2 1.8 1.6 1.3 1.2 0.9

250 4.9 4.9 2.8 2.3 1.9 1.7 1.4 1.2

320 3.5 2.9 2.5 2.1 1.9 1.5

400 4.4 3.6 3.1 2.7 2.3 1.9

500 4.5 3.9 3.4 2.9 2.4

600 4.9 4.2 3.6 3.0

800 5.3 4.4 3.8

1000 6.5 4.7

For a three-phase 230 V circuit, multiply the result by √3.For a single-phase 208/230 V circuit, multiply the result by 2.

DC Circuit (Copper Conductors)

Sph (mm2) 25 35 50 70 95 120 150 185 240 300

In (A) 100 5.1 3.6 2.6 1.9 1.3 1.0 0.8 0.7 0.5 0.4

125 4.5 3.2 2.3 1.6 1.3 1.0 0.8 0.6 0.5

160 4.0 2.9 2.2 1.6 1.2 1.1 0.6 0.7

200 3.6 2.7 2.2 1.6 1.3 1.0 0.8

250 3.3 2.7 2.2 1.7 1.3 1.0

320 3.4 2.7 2.1 1.6 1.3

400 3.4 2.8 2.1 1.6

500 3.4 2.6 2.1

600 4.3 3.3 2.7

800 4.2 3.4

1000 5.3 4.2

1250 5.3



Example of an installation

Figure 26: Connection of cables..

Special case for neutralconductors

In three-phase systems, the third-order harmonics (and their multiples)of single phase loads add up in theneutral conductor (sum of the currentson the three phases).

For this reason, the following rule is applied - neutral cross section =

1.5 x phase cross section.

Calculation example

Consider a 70-meter 400 V three-phasecircuit, with copper conductors and arated current of 600 A.

Standard IEC 60364 indicates,depending on the installation methodand the load, a minimum crosssection. We shall assume that theminimum cross section is 95 mm2.

It is first necessary to check that thevoltage drop does not exceed 3%.

The table for three-phase circuitsindicates, for a 600 A current flowingin a 300 mm2 cable, a voltage drop of3% for 100 meters of cable, i.e. for 70meters:

3 x 70/100 = 2.1%, less than the 3%limit.

A identical calculation can be run for a DC current of 1000 A in a 10-metercable with a cross section of 240 mm².The voltage drop for 100 meters is5.3%, i.e. for ten meters:

5.3 x 10/100 = 0.53%, less than the 1%limit.

Available technologies

The various technologies currentlyavailable are the following:

• Batteries:- sealed lead-acid,- vented lead-acid,- nickel cadmium,

• Ultracapacitors,

• Flywheels:

- traditional units turning at low speeds (1500 rmp) andcombined with enginegenerator sets,

- medium-speed (7000 rpm) orhigh-speed (30 to 100 000 rpm)units.

Comparison of technologies

Batteries are by far the mostcommonly employed solution today.

They are the dominant solution due to low cost, proven effectiveness andstorage capacity, but nonethelesshave a number of disadvantages interms of size, maintenance and theenvironment.

Ultracapacitors do not yet offer thenecessary performance levels.

Flywheels operating at high speedsconstitute a possible technology interms of their power ratings (40 to 500 kW), for short backup times (12 seconds to 1 minute).

Figure 28 shows the fields ofapplication for the differenttechnologies.



Storage technologiesEnergy storage in UPS

A UPS requires an energy-storagesystem to supply the inverter withpower if utility power fails or is nolonger within tolerances.

The stored energy must have thefollowing characteristics:

• Electricity that is immediatelyavailable to ride through micro-breaks, short voltage drops andutility outages,

• Sufficient power level to supplythe entire load, i.e. a ratingequivalent to that of the UPSsystem itself,

• Backup time, generally about tenminutes, suited to the needs ofthe loads and to any othersources available (e.g. an enginegenerator set for long backuptimes).

Figure 27: Simplified diagram of a.UPS with backup energy storage.

Energy storage



Figure 28: Characteristics in terms of power ratings and backup times..

Table 15 compares the different solutions in terms of their capacity to meet the energy-storage requirements of static UPS.

Table 15,.

Criteria for comparison Technology

Sealed lead-acid batteries

Vented lead-acid batteries

Ni/Cadbatteries

Ultracapacitors Flywheels

Power **** **** **** * ***

Backup time

***5 minutes up

to severalhours

****5 minutes up

to severalhours

*5 minutes up

to severaldozen

minutes

*a few seconds

**a few dozen

seconds

Purchase price****low

***low to

medium

**medium

*high

*high

Implementation /installation / start-up

Requires a specialroom

***no

**yes

*yes

****no

**no

Temperature * * ** **** ***

Service life ** ** *** **** ***

Footprint ** ** ** **** ***

MaintenanceFrequency / time

required

**medium

**medium

*high

****none

***low

Maturity of thetechnology for UPS

**** **** **** ** ***

**** Excellent *** Good ** Fair * Poor



The following general rules apply.

• Computer systems

Battery backup time must besufficient to cover file-saving andsystem-shutdown proceduresrequired to ensure a controlledshutdown of the computersystem. Generally speaking, thecomputer department determinesthe necessary backup time,depending on its specificrequirements.

• Industrial processes

The backup-time calculationshould take into account theeconomic cost incurred by aninterruption in the process andthe time required to restart.

• Applications requiring long

backup times

An engine generator set can backup a battery if long outagesoccur, thus avoiding the need forvery large batteries. Generallyspeaking, use of an enginegenerator set becomes feasiblefor backup times greater than 30 minutes to one hour. Thecombination must be carefullystudied to optimise the generatorrating and ensure correctoperation.

Combination with an enginegenerator set, see page 153“Engine generator set”.

Service life

Battery manufactures providebatteries with service lives of 5 or 10years or longer.

Battery service life, see page 150.

Selection of a batteryTypes of batteries

The batteries most frequently used ina UPS are:

• Sealed lead-acid, also called gas-recombination batteries,

• Vented lead-acid,

• Nickel cadmium.

Lithium-polymer batteries are currentlybeing studied for use in also. Solutionsusing this technology should beavailable in two to three years.

Types of batteries, see page 149“Energy storage - Types ofbatteries”.

Selection of a battery depends on thefollowing factors:

• Operating conditions andrequirements (special room,battery cabinet, racks, etc.),

• Required backup time,

• Cost considerations.

Backup time

Manufactures typically offer:

• Standard backup times of 5, 10,15 or 30 minutes,

• Custom backup times that canreach a number of hours.

Selection depends on:

• The average duration of power-system failures,

• Any available sources offeringlong backup times (enginegenerator set, etc.),

• The type of application.



• Limitation of the battery current,

• Continuous evaluation ofavailable power taking intoaccount the battery age, thetemperature and the percent load,

• Forecast of battery service life,

• Periodic, automatic tests on thebattery, including a check on thebattery circuit, an open-circuittest, a partial-discharge test, etc.

DigiBat, see page 151 “BatteryManagement”.

Environment sensor unit

Battery operating parameters andparticularly the temperature affectbattery life. The Environment Sensor,easy to install and combined with aNetwork Management card(SNMP/Web), makes possiblemonitoring of temperature/humidityand the status of two contacts viaSNMP or the web. It also initiatesequipment shutdown if necessary.

Detection and prevention ofbattery failure for UPSs

In spite of the advantages of sealedlead-acid batteries, over time, allbatteries will fail due to aging. Withoutrigorous monitoring, the true integrityand capacity of a battery remainsunknown.

Battery-monitoring techniques have amajor impact on reliability and can beused to define the best strategy forreplacement, resulting in a better levelof protection.

Comparison between types of batteries

Sealed lead-acid batteries (gas-

recombination)

These are the most commonly usedbatteries for the following reasons:

• No maintenance,

• Easy implementation,

• Installation in all types of rooms(computer rooms, technicalrooms not specifically intendedfor batteries, etc.).

Vented batteries

This type of battery (lead-acid orNi/Cad) offers certain advantages:

• Long service life,

• Long backup times,

• High power ratings.

Vented batteries must be installed inspecial rooms complying with preciseregulations (see page 58 “Preliminarywork”) and require appropriatemaintenance.

Battery monitoringBattery monitoring on UPS

DigiBatTM

The DigiBatTM battery-monitoringsystem is an assembly of hardwareand software, which offers thefollowing functions:

• Automatic entry of batteryparameters,

• Optimised battery service life,

• Protection against excessivedischarges,

• Regulation of the battery floatingvoltage depending on thetemperature,

Human-machine interface(HMI)General characteristics

The human-machine interface on theUPS must be user-friendly, easy to useand multi-lingual (adjustable to theuser's language).

It is generally made up of a mimicpanel, a status and control panel, andan alphanumeric display. A password-protected personalisation menu maybe available for entry of installationparameters and access to detailedinformation.

Example

The HMI typically offers the followingfunctions:

On and Off buttons:

• Delayed to avoid erroneousoperations,

• With an option for a remote EPO(emergency power off),

• Independent with respect to therest of the display.

Status LEDs that clearly identify:

• Normal operation (load protected),

• Downgraded operating mode(malfunction),

• Dangerous situations for the load(load not protected),

• Operation on battery power.

Alarms:

• Alarm buzzer and buzzer resetbutton,

• Battery shutdown warning,

• General alarm,

• Battery fault.



A screen providing:

• Access to measurements:- input power (voltage, current,

frequency),- battery (voltage, charge and

discharge currents, remainingbackup time, temperature),

- inverter output (phase-to-neutral voltage, current,frequency, active and apparentpower, crest factor),

• Access to history logs:- log containing time-stamped

events,- curves and bargraphs of the

measured values.

CommunicationHigh availability for criticalapplications requires commun -icating protection equipment

The UPS system, essential formission-critical equipment, mustinclude communication features thatkeep operators continously informed,wherever they may be, of any risk ofcompromising the operating securityof the system so that they can takeimmediate action.

To ensure power availability, the UPScommunication features provide thefollowing four essential functions:

Supervision / monitoring of allinstalled UPS via software.

Notification via the network and the Internet.

Controlled shutdown (local orremote, automatic or manual) ofprotected applications.

Human-machine interface and communication



• Modbus – Jbus card (RS232 and

RS485)

- monitoring

• Relay card (Contacts)

- indications

Solutions

Communication cards

• Network management card

(Ethernet)

- web monitoring- email notification- SNMP MIB and Traps- server protection with Network

Shutdown Module- supervision with Enterprise

Power Manager or ISX Central- environment monitoring with

Environment Sensor (T°, H%,Inputs)

Ventilation, air-conditioning

Ventilation requirements

UPS are designed to operate within agiven temperature range (typically 0to 40°C) that is sufficient for mostoperating conditions withoutmodifications.

However, UPS and their auxiliaryequipment produce heat losses thatcan, if no steps are taken, increase thetemperature of a poorly ventilatedroom.

What is more, the service life of abattery is heavily dependent on theambient temperature. The service life isoptimal for temperatures between 15°C and 25° C. This factor must be takeninto account if the battery is installed inthe same room as the UPS.

A further consideration is the fact thata UPS may be installed in the sameroom as computer equipment whichoften has more severe requirementsconcerning operating-temperatureranges.

Selecting a type of ventilation

For all the above reasons, a minimumamount of ventilation is required, andwhere applicable air-conditioning, toavoid any risk of excessive tempera -ture rise in the room due to the heatlosses.

Ventilation can be by:

• Natural convection,

• Forced exchange by a ventilationsystem,

• Installation of an air-conditioningunit.

Preliminary work

Installation considerationsThe main elements that must be takeninto account for the UPS installationare the following:

• Plans for site modifications, anypreliminary work (notably for abattery room), taking into account:- the dimensions of equipment,- operating and maintenance

conditions (accessibility,clearances, etc.),

- temperature conditions thatmust be respected,

- safety considerations,- applicable standards and

regulations,

• Ventilation or air-conditioning of rooms,

• Creation of a battery room.

Dimensions

Layout of UPS cabinets and enclosuresshould be based on precise plans.

For each range:

• The dimensions and weights of:- UPS and centralised-bypass

cabinets;- battery cabinets,- any auxiliary cabinets

(autotransformers,transformers, filters, etc.),

• Minimum clearances required forcabinets and enclosures toensure optimal ventilation andsufficient access.



Battery installation method

The criteria determining the battery-installation method are the following:

• Available floor space,

• The weight that the floor canhandle (kg/m2),

• Ease of access and maintenance.

The following three methods are used.

Battery installed directly on floor

This is the most simple arrangement.However, a large battery room isrequired, given:

• The large amount of floor spaceoccupied by the battery,

• The insulated flooring (duckboard), which is mandatory if the voltage exceeds 150 volts.

Battery on racks

The battery cells are installed on anumber of different levels, off the floor.

When determining the height betweeneach rack, it is necessary to take intoaccount the space required to checkbattery levels and fill the battery cellseasily. A minimum height of 450 mmis recommended.

Battery on tiers

This installation method is similar to the preceding. It is the mostconvenient method for checkingbattery levels.

Battery room features

Whatever the installation methodselected, the battery installation must comply with the followingrequirements (the numbers indicatethe elements shown in figure 29).



Selection depends on:

• The heat losses that must beevacuated,

• The size of the room.

The thermal characteristics of a UPSmay be used to calculate ventilationneeds. They mention for each range:

• The heat losses of cabinets andany filters installed,

• The volume of air output by aventilation system.

IP degree of protection andnoise level

Degree of protection (IP)

A UPS must operate in an environmentthat is compatible with their degree ofprotection (IP 20 for a UPS fromCaterpillar), defined by standard IEC60529/EN 60529. The presence of dust,water and corrosive substances mustbe avoided.

Noise level

A UPS must produce a low level ofnoise, suited to the room where theyare installed. Measurement conditionsfor the level of noise indicated by themanufacturer must comply with stan -dard ISO 3746 (measurement of noise).

Battery roomWhere possible and if desired, thebattery should be installed in a cabinet.

Battery-cabinet dimensions areindicated for each UPS range,depending on the rated power.

However, for very high-power UPS,batteries are generally installed inspecial rooms (electrical room).

Batteries must be installed in compli -ance with international standards, localregulations and standard IEC 60364.



Layout of cells (3)

Layout must inhibit simultaneouscontact with two bare parts presentinga voltage greater than or equal to 150 V.If the condition listed before cannot bemet, terminal shields must be installedand connections must be made usinginsulated cables.

Service flooring (4)

If the voltage exceeds 150 V, specialflooring is required. It must offer surefooting, be insulated from the floorand offer at least one meter ofwalkway around the battery.

Battery connection (5)

Connections must be as short apossible.

Battery-protection circuit breaker (6)

The circuit breaker is generallyinstalled in a wall-mounted enclosure.

Fire-fighting equipment (7)

Authorized fire extinguishers includepower, CO2 or sand.

Safety equipment (8)

The safety equipment must includeprotective glasses, gloves and asource of water.

Inspection equipment (9)

• Hydrometer,

• Filling device,

• Thermometer.

Sensors (10)

• Hydrogen detector,

• Temperature sensor.

Floor and walls (1)

• The floor must slope to anevacuation trough which leads to a holding tank,

• Protection coating against acidon the floor and walls, up to aheight of at least 0.5 meters.

For example, asphalt for lead-acidbatteries, PVC or chlorine-based paintfor alkaline batteries.

Ventilation (2)

• Calculation of throughput

The volume of air to beevacuated depends on themaximum load current and thetype of battery. In installationscomprising a number of batteries,the quantities of air that must beevacuated are cumulative.

- vented batteries

d = 0.05 x N x Im, where

d - throughput in cubic metersper hour,

N - number of battery cells,

Im - maximum load current inamperes.

- sealed battery

The ventilation conditions in ageneral-purpose room are sufficient.

• Safety

An automatic device must stopbattery charging if the ventilationsystem fails.

• Location

Air must be drawn out from thetop of the battery room.



Figure 29: Layout of battery room..

Figure 30: Basic diagrams..

UPS configurationsSingle UPS

This is the standard double-conversionUPS (see figure 31). A single UPS canbe used to form redundant configur -ations as shown in diagrams 4 and 11.

Single UPS, see page 5 and page126 “UPS components andoperation”.

Standard diagrams (see table 16):

No. 1

No. 4

No. 11

Figure 31: Double-conversion single UPS..

Parallel UPS

Purpose of parallel connection

Parallel connection of a number ofidentical UPS units is the means to:

• Increase the power rating,

• Establish redundancy thatincreases MTBF and availability,

• Make the installation scalable.

Two types of UPS units can beconnected in parallel:

• Integrated parallel UPS units:each UPS unit includes anautomatic bypass and a manualmaintenance bypass (figure 31).The manual bypass may becommon to the entire systemand located in an externalcubicle (e.g. figure 32);

• Parallel UPS units with acentralised static-switch cubicle(SSC) (e.g. figure 33).



Single source

The load is supplied by a single set of UPS.

Multi-source

The load is supplied by more than oneset of UPS.

Selection of possible configurations

Types of possible configurationsBasic diagrams



Figure 32: Installation with three integrated parallel UPS.units and a common maintenance bypass.

Integrated parallel UPS

This configuration is upgradeable,starting for instance with oneintegrated parallel UPS unit equippedwith an automatic bypass and amanual maintenance bypass. Whenstarting with two units or whenexpanding to two units or more, acommon maintenance bypass isinstalled in an external enclosure (see figure 32).


No. 2

No. 3



Figure 33: Three parallel UPS units with a centralised static-switch cubicle (SSC)..

Parallel UPS units with a centralised

static-switch cubicle (SSC)

The static-switch cubicle comprises anautomatic bypass and a maintenancebypass that are common for a numberof modules without a bypass (seefigure 33). It is possible to have tworedundant SSCs.

Upgrading of this configurationdepends on the rating of the staticswitch. It offers the highest level ofreliability (SSC with independent UPSunits).


No. 5

No. 6

No. 7

No. 8

No. 9

Parallel connection withredundancy

The parallel configurations presentedearlier may or may not be redundant.

Without redundancy

All the UPS units are required tosupply the load. Failure of one unitmeans the entire system shuts down.

With active redundancy (N + 1, N + 2,

etc.)

Only N UPS units are required tosupply the load, even though N + 1, N + 2 or more units are installed. Thisensures a secure supply of power tothe load even if one (for N + 1redundancy) or two (for N + 2redundancy) UPS units fail or requiremaintenance.



Power distribution units (PDUs) can beused to complete this distributionconfiguration, offering:

• Load management,

• Multi-channel supply of power to the loads (dual attach),

• Isolation of parts of the installa tionfor maintenance or upgrading.

This type of configuration ensures avery high degree of availability andoffers a number of installation-upgrade possibilities.


No. 11

No. 12

Figure 35: Redundant distribution with an STS..

Optimum redundancy of non-modular

UPS

For non-modular systems, differencesin the lengths or tightening torques ofcables connecting the different unitscan lead to problems concerning theimpedance upstream and downstreamof each UPS. For this reason, the high -est MTBF is obtained for redundantsystems with just two UPS (figure 34).For modular UPS systems, moduleinterconnections are an integral part ofthe system, thereby eliminatinginstallation problems that can lowerthe MTBF as more units are added.

Figure 34: For non-modular redundant UPS systems,.the best MTBF is obtained with two units.

Redundant distribution with an STS

All the loads are supplied by more thanone UPS source (two single UPS units).Each source can be made up of anumber of parallel-connected unitsoffering active redundancy. Use of astatic transfer switch (STS) ensurestransfer of the load between the sourcesin the event of a downstream fault(while avoiding any risk of faultpropagation) or for maintenance.

Upgradeability

It must be possible to upgrade theinstallation over time, taking intoaccount both the need to expand theinstallation gradually and operatingrequirements.

Discrimination and non-propagation of faults

It must be possible to limit faults to as small a part of the installation aspossible, while enabling servicingwithout stopping operations.

Installation operation and management

Make operations easier by providingthe means to anticipate events viainstallation supervision andmanagement systems.



Criteria for comparisonTable 16 compares the standard dia -grams of this section, mainly relatedto UPS, according to the followingcriteria.

Availability

A level of availability meeting theneeds of the application. Figures arebased on:

• An estimated level of utility-power availability of 99.9% (the European average),

• An MTTR of ten hours as perstandard MIL-HDB-217-F level 2(U.S. military) and IEEE.

Maintainability

Ensure easy maintenance of theequipment under safe conditions forpersonnel and without interruptingoperation.

Selection table and corresponding ranges



Table 16..

Single-source configurations

Standarddiagramnumber

Criteria for comparison

Availability MTBF Maintainability Upgradeability Comment

1. Single UPS 99.99790% M1 = 475 000 h *4-parallel-

connected UPSunits

Reference forcalculations

2. 2 integratedparallel UPSunits

99.99947% up to 4 x M1 **4-parallel-

connected UPSunits

3. Integratedparallel unitsand externalmaintenancebypass

99.99947% up to 4 x M1 **4-parallel-

connected UPSunits

4. Isolatedredundancy 99.99970% 6.8 x M1 ** Flexible

5. CentralisedSSC 99.99968% 6.5 x M1 **

6-parallel-connected UPS

units

6. Totalisolation,singlebusbar

99.99968% 6.5 x M1 ***6-parallel-

connected UPSunits

7. Totalisolation,doublebusbar

99.99968% 6.5 x M1 ***6-parallel-

connected UPSunits

8. Totalisolation,singlebusbar

99.99968% 6.5 x M1 ****6-parallel-

connected UPSunits

9. Totalisolation,doublebusbar

99.99968% 6.5 x M1 ****6-parallel-

connected UPSunits

Multi-source configurations

Standarddiagramnumber

Criteria for comparison

Availability MTBF Maintainability Upgradeability Comment

10. Isolatedredundancy 99.99970% 7 x M1 ** No limit

11. With STS 99.99970% 7 x M1 **** No limit to thepower rating

No propagationof faults

12. STS + PDU 99.99930%The highest

level ofavailability

**** No limit to thepower rating

+ loadmanagement

**** Excellent *** Good ** Fair * Poor



Figure 36: Double-conversion single-UPS unit..

This is the basic solution for UPS installations. The double-conversion UPS unit supplies high-quality voltage, whatever the level of disturbances in the utility power.

Diagram no. 1. Single UPS

Availability of power for the load

99.99790% and an MTBF of 475 000hours, compared to a utility MTBF of96 hours.

UPS maintenance

Made easy due to the built-in bypassfor supply of power to the load duringservicing.

Possible upgrades

On site by connecting several identicalUPS units in parallel.



Diagram no. 2. Active redundancy with two integrated

parallel UPS units

Figure 37: Active redundancy with two integrated parallel UPS units..

A simple solution where the UPSunits share the load.


99.99947% and an MTBF up to fourtimes higher than that for a singleUPS.

UPS maintenance

During maintenance on one unit, theload remains protected by the other.

Possible upgrades

Several identical UPS units can beconnected in parallel and equipped with an external maintenance bypass.

Special characteristics:

• The automatic-bypass function isensured by managing the static switches,

• Centralised monitoring of thevarious modules,

• Can be used only with twoidentical units.



Diagram no. 3. Active redundancy with integrated

parallel UPS units and external maintenance bypass

Figure 38: Active redundancy with integrated parallel.UPS units and external maintenance bypass.

An upgradeable solution where thepower rating can be increased up to4000 kVA*.

Availability

99.99947% and an MTBF up to fourtimes higher than that for a singleUPS.

UPS maintenance

During maintenance on one unit, theload remains protected by the other units.

Easy upgrades

Several identical UPS units can beconnected in parallel for a low cost solution with small dimensions.

Special characteristics

• The UPS units share the load,

• The automatic-bypass function isensured by managing the staticswitches,

• Centralised monitoring of thevarious modules,

• Identical modules must be used.



Diagram no. 4. Isolated redundancy with two UPS units

Figure 39: Isolated redundancy with two UPS units..

An extremely flexible solution that cancombine heterogeneous and distantUPS units. It also offers improvedback up time and is perfectly suited to the technology implemented byCaterpillar which provide excellentwithstand capacity for load stepchanges.

Availability

99.99970% and an MTBF 6.8 timeshigher than that of a single UPS.

UPS maintenance

During maintenance on one unit, the load remains protected.


• For a single load, the two UPSunits have the same powerrating, but if there is a secondload (possible load), the rating of the backup UPS unit must beadapted correspondingly;

• No control wires between theUPS units.



Diagram no. 5. Active redundancy with parallel units

and centralised static-switch cubicle (SSC)

Figure 40: Active redundancy with parallel units and centralised static-switch cubicle (SSC)..

The solution for centralised install -ations up to 4 MVA*. Excellentreliability due to the independencebetween the units and the static-switch cubicle (SSC).

Availability

99.99968% and an MTBF up to 6.5times higher than that for a singleUPS.

UPS maintenance

During maintenance on one unit, the load remains protected by theother units and the SSC. Duringmaintenance on the SSC, redundancyof the UPS units is maintained.

Easy upgrades

Up to eight UPS units.


The UPS units share the load.

* Power rating for N + 1 reduncancy.



Diagram no. 6. Active redundancy with parallel UPS

units and total isolation, single busbar

Figure 41: Active redundancy with parallel UPS units and total isolation, single busbar..

A solution that can evolve with needsup to 4 MVA*. Excellent reliability andimproved maintainability due to thetotal independence between the UPSunits and the static-switch cubicle(SSC).

Availability


UPS maintenance

During maintenance on one unit, the load remains protected by theother units and the SSC. Duringmaintenance on the SSC, redundancyof the UPS units is maintained.

Easy upgrades



• Total isolation of the UPS unitsor the SSC for maintenance,

• The UPS units can be testedusing a test load,

• Isolation of each UPS unit andthe SSC, thus eliminating thesingle point of failure in the SSC.

* Power rating for N + 1 redundancy.




units and total isolation, double busbar

Figure 42: Active redundancy with parallel UPS units, double SSC and total isolation, double busbar..

A solution that can evolve with needsup to 4 MVA*. Excellent reliability andimproved maintainability due to thetotal independence between the UPSunits, the static-switch cubicle (SSC)and the busbars.

Availability


UPS maintenance

During maintenance on the UPS unitsand one busbar, the load remainsprotected by the other units and theSSC, which are parallel-connected tothe second busbar. Duringmaintenance on the SSC, redundancyof the UPS units is maintained.

Easy upgrades



• Transfer from one busbar to theother without disturbing theload,

• Total isolation of the UPS unitsor the SSC for maintenance,

• Isolation of each UPS unit andthe SSC, thus eliminating thesingle point of failure in the SSC.

* Power rating for N + 1 redundancy.




units, double SSC and total isolation, single busbar

Figure 43: Active redundancy with parallel UPS units, double SSC and total isolation, single busbar..

An upgradeable solution offeringimproved maintainability due to thetotal redundancy of the UPS units andthe static-switch cubicles (SSC).

Availability

99.99968% and an MTBF up to 6.5times higher than that for a single UPS.

UPS maintenance

During maintenance on the UPS units and one SSC, the load remainsprotected by the other units and thesecond SSC. During maintenance onone SSC, redundancy of the UPS units is maintained.



Easy upgrades



• Only one SSC is active, the otheris on stand-by and transfer of theUPS units from one to the othertakes place without disturbingthe load,

• During operation on the bypass,the load is split 50/50 betweenthe two SSCs,

• Total isolation of each SSC formaintenance,

• Parallel connection of the UPSunits in the output cabineteliminates the single point offailure in an SSC,

• The possibility of installing theSSCs in two separate roomsincreases system availability inthe event of fire or otherproblems.




units, double SSC and total isolation, double busbar

Figure 44: Active redundancy with parallel UPS units, double SSC and total isolation, single busbar..

A solution for two evolving loads withdifferent needs in terms of powerratings and redundancy.

Availability


UPS maintenance

During maintenance on one UPS unit and one SSC, the load remainsprotected by the other units and thesecond SSC. During maintenance onone SSC, redundancy of the UPS unitsis maintained.

Easy upgrades





• During operation of only oneload, only one SSC is active, theother is on stand-by and transferof the UPS units from one to theother takes place withoutdisturbing the load,

• During operation of the twodifferent loads, both SSCs areactive, each with a number ofassigned UPS units,

• Parallel connection of the UPSunits in the output cabineteliminates the single point offailure in an SSC,

• The possibility of installing theSSCs in two separate roomsincreases system availability in the event of fire or otherproblems.



Diagram no. 10. Isolated redundancy N + 1

Figure 45: Isolated redundancy N + 1..

Solution combining heterogeneousand distant UPS units to protect anumber of independent loads.


Greater than 99.99970% and an MTBFup to seven times higher than that fora single UPS.

UPS maintenance

During maintenance on one UPS unit,the load remains protected. However,the UPS units are not totally isolated(servicing under energisedconditions).

Possible upgrades

No limit to the power rating.



Short-circuit propagation

Impossible between the sources.


• Short-circuit capacity is lowerthan in a configuration withparallel UPS units,

• (Isc, discrimination, crest factor,etc.),

• Sizing of the backup UPS musttake into account the number of

UPS units downstream, theirpower ratings and their criticality,as well as any future plans forthe installation (generallyspeaking, the backup UPS has a parallel configuration),

• All the advantages of isolatedredundancy (diagram no. 4).



Diagram no. 11. Redundant distribution with STS

Figure 46: Redundant distribution with STS units..

The best solution in terms ofavailability, site operation and safety.It is the only solution that deals withpower distribution through to theloads. It is particularly flexible andmakes for easy adaptation ofredundancy to the needs of the load.


Greater than 99.9999%, the highestlevel of availability!

UPS maintenance

Total distribution redundancy andservicing under no-load conditionsmake for maximum safety duringmaintenance.

Easy upgrades

Using single-UPS units and with nolimit to the total power rating, up -grading is made easy by the capacityto partially isolate distributionsubassemblies.



Fault propagation

Load segmenting and the technologyemployed in STS units (break-beforemake source transfer with nointerruption to the loads) ensuresisolation of loads from disturbancescaused by other, faulty loads.

Easy operation

Automatic or manual source transfer.

Continuous monitoring of the sources(11 parameters and internal circuits).

Secure transfer of desynchronisedsources.


• The synchronisation moduleensures perfect sourcesynchronisation under allconditions (long outages, etc.),

• Selection of the load distributionfor the UPS units,

• The UPS units can be hetero -geneous and remote from theload.



Diagram no. 12 . Active redundancy with parallel UPS

and a common battery

Figure 47: Redundant distribution with STS units and PDU..



Redundancy is built into each level,including the PDUs, the STS units, theUPS units and the synchronisationmodules.

Same advantages as diagram no. 11,

plus:

• Capacity to enhance thereliability of a particular point in the installation,

• Four different supply channels to dual-attach servers.

Elimination of harmonics in installations

HarmonicsDefinition, origin and typesof harmonicsHarmonics

Harmonics are sinusoidal currents orvoltages with a frequency that is awhole multiple (k) of the frequency ofthe distribution system, called thefundamental frequency (50 or 60 Hz).

When combined with the sinusoidalfundamental current or voltagerespectively, harmonics distort thecurrent or voltage waveform (seefigure 48).



Harmonics are generally identified asHk, where k is the harmonic order.

• IHk or UHk indicate the type ofharmonic (current or voltage).

• IH1 or UH1 designates thesinusoidal current or voltage at50 or 60 Hz that exists whenthere are no harmonics (thefundamental current or voltage).

Figure 48: Distortion of H1 (the fundamental) by H3 (third-order harmonic)..

Non-linear loads are the cause

Equipment implementing powerelectronics is the main cause ofharmonics. To supply the electronicswith DC power, the equipment has aswitch-mode power supply with arectifier at the input that drawsharmonic currents.

Examples are computers, variable-speed drives, etc.

Other loads distort the current due to their operating principle and alsocause harmonics.

Examples are fluorescent lamps,discharge lamps, welding machinesand devices with a magnetic core thatcan be saturated.

All the loads that distort the normalsinusoidal current cause harmonicsand are called non-linear loads.

Figure 49: Examples of non-linear loads that cause harmonics..

Linear and non-linear loads

Utility power supplies 50/60 Hzsinusoidal voltage to loads. Thecurrent waveform supplied by thesource in response to the needs of theload depends on the type of load.

Linear loads

The current drawn is sinusoidal withthe same frequency as the voltage.The current may be displaced (angleφ) with respect to the voltage.

• Ohm's law defines a linear relationbetween the voltage and thecurrent (U = ZI) with a constantcoefficient, the load impedance.The relation between the currentand the voltage is linear.

Examples are standard light bulbs,heating units, resistive loads, motors,transformers.

• This type of load does notcontain any active electroniccomponents, only resistors (R),inductors (L) and capacitors (C).



Non-linear loads

• The current drawn by the load isperiodic, but not sinusoidal. Thecurrent waveform is distorted bythe harmonic currents.

• Ohm's law defining the relationbetween the total voltage andcurrent (1) is no longer validbecause the impedance of theload varies over one period (seefigure 50). The relation betweenthe current and the voltage is notlinear.

• The current drawn by the load is,in fact, the combination of:- a sinusoidal current called the

fundamental, at the 50 or 60 Hzfrequency,

- harmonics, which aresinusoidal currents with anamplitude less than that of thefundamental, but a frequencythat is a multiple of thefundamental and which definesthe harmonic order (e.g. thethird order harmonic has afrequency 3 x 50 Hz [or 60 Hz]).



Figure 50: Voltage and current for non-linear loads..

(1) Ohm's law applies to eachvoltage and current of the sameharmonic order, Uk = Zk Ik, whereZk is the load impedance for thegiven order, but is no longer validfor the total voltage and current.

Linear loads, non-linear loads, seepage 6 “Power quality of UPS”.

Types of harmonics and specificaspects of zero-sequenceharmonics

Types of harmonics

Non-linear loads cause three types ofharmonic currents, all in odd orders(because the sinusoidal is an “odd”function).

• Harmonics H7 - H13 - …. :positive sequence,

• Harmonics H5 - H11 - …. :negative sequence,

• Harmonics H3 - H9 - …. : zero sequence.

Specific aspects of zero-sequence

harmonics (H3 and multiples)

Zero-sequence harmonic currents (H3and odd multiples, written 3(2k + 1)where k is an integer) in three-phasesystems add up in the neutralconductor.

This is because their order 3(2k + 1) isa multiple of the number of phases(3), which means they coincide withthe displacement (one third of aperiod) of the phase currents.

Figure 51 illustrated this phenomenonover one period. The currents of thethree phases are displaced one thirdof a period (T/3), i.e. the respective IH3harmonics are in phase and theinstantaneous values add up.Consequently:

• When there are no harmonics,the current in the neutral is equalto zero:

IN = I1 + I2+ I3 = 0

• When there are harmonics, thecurrent in the neutral is equal to:

I1 + I2 + I3 = 3 IH3.

It is therefore necessary to payparticular attention to this type ofharmonics in installations with adistributed neutral (commercial and infrastructure applications).

Figure 51: The third-order harmonics and.their multiples add up in the neutral.



Figure 52: When there are H3 harmonics and their odd multiples, the current in the.neutral is no longer equal to zero, it is the sum of the zero-sequence harmonics.

Characteristic harmonicvaluesThe harmonic analysis of a non-linearcurrent consists in determining:

• The harmonic orders present inthe current,

• The relative importance of eachharmonic order.

Below are a few characteristicharmonic values and fundamentalrelations used in harmonic analysis.

Further information on harmonics,see the explanations in White Paperno. 17 “Understanding PowerFactor, Crest Factor and SurgeFactor”.

Rms value of harmonics

It is possible to measure the rms valueof each harmonic order because thevarious harmonic currents aresinusoidal, but with differentfrequencies that are multiples of the fundamental frequency.



• IH1 is the fundamental component (50 or 60 Hz),

• IHk is the harmonic component where k is the harmonic order (k times 50 or 60 Hz).

Harmonic analysis is used to determine the values.

Total rms current

Irms √IH12 + IH22 + IH32 + ... IHk2 + ...

Individual harmonics

Each harmonic is expressed as a percentage, i.e. the ratio of its rms value to the rms value of the fundamental. This ratio is the level of the individual harmonic.

Hk% = distortion of harmonic k = 100 IHkIH1

Voltage and current harmonic distortion

Non-linear loads cause both current and voltage harmonics. This is because for each load current harmonic, there is a supply voltage harmonic with the same frequency. As a result, the voltage is also distorted by harmonics.

The distortion of a sine wave is presented as a percentage:

THD* % = total distortion = 100 rms value of all harmonicsrms value of fundamental

* Total Harmonic Distortion.

The following values are defined:

• TDHU % for the voltage, based on the voltage harmonics,

• TDHI % for the current, based on the current harmonics.

The THDI (or the THDU using the UHk values) is measured using the equation:

THDI% = 100 √IH22 + IH32 + IH42 + ... + Hk2 + ...IH1

Crest factor

The crest factor (Fc), used to characterise the form of the signal (current or voltage), is the ratio between the peak value and the rms value.

Fc = peak valuerms value

Below are typical values for different loads:

• Linear load: Fc = √2 = 1.414,

• Main frame: Fc = 2 to 2.5,

• Microcomputers: Fc = 2 to 3.



Figure 53: Harmonic spectrum of the current drawn by a non-linear load..



Power factor

Power factor

The power factor is the ratio betweenthe active power (kW) and theapparent power S (kVA) across theterminals of a given non-linear load.

λ = P (kW)S (kVA)

It is not the phase displacementbetween the voltage and the current,because they are no longer sinusoidal.

Displacement between the

fundamental current and voltage

The phase displacement φ1 betweenthe fundamental current and voltage,both sinusoidal, can be defined as:

cos φ1 = P1 (kW)S1 (kVA)

where P1 and S1 are the active andapparent power, respectively, of thefundamental.

Distortion factor

The distortion factor is defined as:

ν = √THDI2 = λ (as defined by IEC 60146).cos φ1

When there are no harmonics, this factor is equal to 1 and the power factor is simply the cos φ.

Power

Linear load

Across the terminals of a balanced, three-phase linear load, supplied with aphase-to-phase voltage U and a current I,where the displacement between U and I is φ, the power values are:

• P apparent = S = UI, in kVA,

• P active = S cos φ, in kW,

• P reactive = Q = S sin φ, in kVAr,

S = √P2 + Q2

Spectrum of the harmoniccurrent

Defining the spectrum of a harmoniccurrent consists in determining thecurrent waveform and the individualharmonics, as well as certain valuessuch as the THDI and Fc.

Non-linear load

Across the terminals of a non-linearload, the equation for P is much morecomplex because U and I containharmonics. It can however beexpressed simply as:

• P = S ( = power factor)

For the fundamentals U1 and I1,displaced by φ1:

• P apparent fundamental = S1 = U1I1í3

• P active fundamental = P1 = S1 cos 1

• P reactive fundamental Q1 = S1 sin 1

S = √P12 + Q12 + D2 where D is thedistortion power, due to theharmonics.

Effects of harmonics In electrical devices, harmonics

produce neither active norreactivepower, only losses throughthe Joule effect (ri2).

Loss of apparent power

Figure 54 shows that the product of avoltage at the fundamental frequencywithout harmonics multiplied by athird-harmonic current is zero at the end of one period. This is truewhatever the phase and order of theharmonic.

This is expressed by the relation S = √P12 + Q12 + D2

A part of the apparent power isconsumed by the harmonics, to noeffect.

• In rotating machines, theresulting motor torque is equalto zero and only a parasitic

pulsating torque exists, creatingvibrations,

• The only active power presentduring a voltage drop is theheating produced by theharmonic current (Ihk) in aconductor with a resistance r (r IHk2).

Figure 54: U x I products for fundamentals (top)and for fundamentals with harmonics (bottom).

Temperature rise in cables

Temperature rise due to harmoniccurrents adds to the temperaturerise due to the fundamentalcurrent.



Temperature rise in cables isexpressed as:

∞

Losses = r + Σ IHn2

n=1

Current in the neutral

The neutral must be oversized totake into account the third-orderharmonic currents and theirmultiples.

All third-order harmonic currents andtheir odd multiples add up in theneutral (see figure 55). The current inthe neutral can reach 1.7 times that inthe phases.

Consequences

Significant losses in the neutral

r Ineutral2 = temperature rise in theneutral.

Figure 55: The third-order harmonics and.their multiples add up in the neutral.

Self-polluting loads

Voltage distortion mirrors that ofthe current and increases in stepwith the sum of the impedancesupstream of the non-linear load.

Current distortion THDI, caused by theload, results in voltage distortion THDUcaused by the harmonic currents flowingthrough the various impedances fromthe source on down. Figure 56 shows thevarious forms of distortion throughout acommon electrical installation.



Risk of capacitor breakdown

In conclusion, the higher thecontent of high-order componentsin the voltage, the worse thesituation for the capacitor. It isoften necessary to use reinforcedcapacitors.

The value of a current in a capacitor is equal to:

.I = U C ω

For a harmonic current of order k, theangular frequency is equal to ω = 2π kf, and the current is equal to:

.I = 2 π k f U C

where f = the fundamental frequencyand k = the harmonic order.

It follows that the value of the currentincreases with k.

What is more, for a harmonicfrequency, there can also beresonance (1) of the capacitor(capacitance C) with the equivalentinductance (L) of the source(transformer, essentially inductive) inparallel with that of the other suppliedloads.

This resonant circuit (see figure 57)significantly amplifies the harmoniccurrent of the corresponding order,thus worsening the situation for thecapacitor.

(1) This is the case if, for a harmonicorder k, with a frequency fk = k x 50(or 60) Hz, LCωk2 - 1, where ω = 2 π fk.



Figure 56: Effects of harmonics throughout the installation..

Consequences

• Risk of capacitor breakdown,

• Risk of resonance due to thepresence of the inductors.

Certain limitations must be respected:

• U max = 1.1 Un,

• I max = 1.3 In,

• THDU max = 8%,

• Selection of capacitor type,depending on the situation, i.e.standard, class h (reinforcedisolation), with harmonicinductors.

Derating of transformers

Generally speaking, harmonicsresult in source derating that isinversely proportional to the loadpower factor, i.e. the lower thepower factor, the more the sourcemust be derated.

A number of effects are combined:

• Due to the skin effect, theresistance of a transformerwinding increases with the orderof the harmonics,

• Losses due to hysteresis areproportional to the frequency,

• Losses due to Foucault currentsare proportional to the square ofthe frequency.

Consequences

In compliance with standard NFC52-114, transformers must bederated by applying a coefficient kto their rated power, such that:

k = 1 n= ∞

1 + 0,1Σ H2nn1,6√ n=2

This is an empirical equation.

Other national standards recommendderating using a similar k factor thatdepends on the country (e.g. BS 7821Part 4, IEE 1100-1992).

Example

A 1000 kVA transformer supplies a six-pulse rectifier bridge drawing thefollowing harmonics:

H5 = 25%, H7 = 14%, H11 = 9%, H13 = 8%.

The derating coefficient is k = 0.91.

The apparent power of thetransformer is therefore 910 kVA.



Figure 57: Effects of harmonics with capacitors, risk of resonance..

Risk of disturbing generators

Practically speaking, the THDI ofthe current in the generator mustnot exceed 20%. Above, derating isnecessary.

Similar to transformers, generatorssuffer greater losses due to hysteresisand Foucault currents.

• The subtransient reactance X"dincreases as a function of thefrequency,

• The “harmonic” rotating fieldsweeps the rotor at a frequencyother than the synchronismfrequency (50 or 60 Hz).

Consequences

• Creation of parasitic torqueresulting in lower efficiency ofthe mechanical to electricalconversion,

• Additional losses in the inductorwindings and the rotor damper,

• Presence of vibration andabnormal noise.

Losses in asynchronous motors

Harmonics produce the followingeffects in asynchronous motors:

• Increases in Joule and ironlosses (stator losses),

• Pulsating torque (rotor losseswith a drop in mechanicalefficiency).

The THDU must be less than 10%to limit these phenomena.

Effects on other equipment

Harmonics can disturb operation ofthe following equipment as well:

• Non-rms trip units, resulting in nuisance tripping of circuitbreakers,

• Automatic telephone exchanges,

• Alarms,

• Sensitive electronic equipment,

• Remote-control systems.

Effect on recent UPS systems

Modern UPS systems have highchopping frequencies (PWM) and verylow output impedance (equivalent to a transformer five times morepowerful).

When confronted with non-linearloads, these UPS offer:

• Limited losses,

• Current-limiting operation,

• Very low voltage distortion(THDU < 3%).

UPS are an excellent means to supply non-linear loads.

Conclusion

Harmonics may have damagingeffects on electrical installations andon the quality of operation.

That is why international standardsstipulate increasingly preciseharmonic-compatibility levels forequipment and set limits for theharmonic content on publicdistribution systems.

Standards on harmonics, see page145 “UPS standards”.

On the following pages are apresentation of the various strategiesconcerning harmonics and theusefulness of active harmonicconditioners.



Elimination of harmonicsStrategies againstharmonicsThere are two strategies:

• Accept and live equipment to take,

• Eliminate the harmonics,conditioners.

Living with harmonicsOversizing of equipment

Given that the negative effects ofharmonic currents increase with thecumulative impedance of cables andsources, the obvious solution is tolimit the total impedance in order toreduce both voltage distortion andtemperature rise.

Figure 58 shows the results whencable cross-sections and the powerrating of the source are doubled.

Given that the THDU depends primarilyon the inductive component and thuson the length of the cables, it is clearthat this solution is not very effectiveand results simply in limitingtemperature rise.



Figure 59 shows that for the strongestharmonic currents (H3 to H7), the L /Rratio is equal to 1 for cables with across-section of 36 mm².Consequently, above 36 mm², it isnecessary to reduce the impedance by using multicore cable to createparallel impedances.

For Data Centers, see “HarmonicCurrents in the Data Center: A CaseStudy”.

Figure 58: Increased cable cross-sections.to limit distortion and losses.

Solutions to eliminateharmonicsThere are different types of solutionsto eliminate harmonics.

Filters, see page 28 “Selection of a filter”.

Passive filters

LC passive filters are tuned to thefrequency requiring elimination orattenuate a band of frequencies.Harmonic recombination systems(double bridge, phase shifting) canalso be grouped in this category.

Passive filters have two majordisadvantages:

• Elimination of harmonics iseffective only for a specificinstallation, i.e. the addition or removal of loads can disrupt the filtering system,

• it is often difficult to implementthem in an existing installation.

Active filters / active harmonicconditioners

Active filters, also called activeharmonic conditioners, cancelharmonics by injecting exactly equalharmonic currents where they arise.This type of filter reacts in real time(i.e. actively) to the existing harmonicsin order to eliminate them. Moreeffective and flexible than passivefilters, they avoid their disadvantagesand, in comparison, constitute asolution that:

• offers greater performance (totalelimination of all harmonics ispossible, up to the 50th order),

• is flexible, adaptable (action canbe configured) and reusable.



Figure 59: Influence of cable cross-section on L /R..

Table 17.

Table summing up the possible strategies against harmonics

Strategy Advantages Disadvantages Solutions

Live with harmonics

Increase theratings of sourcesand/or the cross-sections of cables.

Reduction insupply THDU byreducing thesource impedance.Reduction in Joulelosses.

Difficult in existing solutions. Costlysolution limited to reducing theresistive component for small cross-sections (the inductance remainsconstant). Requires parallel cablesfor large cross-sections. Does notavoid disturbances upstream of theinstallation. Does not comply withstandards.

Special supply fornon-linear loads.

Limits disturbancesto neighboringloads throughdecoupling.

Same as above.

Partially eliminate harmonics

Tuned passivefilters. Simple solution.

Only for one or two harmonicorders. Wide-band filters are notvery effective. Possibility ofresonance. Costly design work isrequired.

Range of passive filtersIncluding double-bridgeand phase-shiftingsolutions.

Inductors upstreamof the non-linearloads.

Reduction inharmonic currents.Limits the effects oftransientovervoltages.

Increase in THDU across theterminals of the load.

Specialtransformers.

Elimination of only certainharmonic orders. Non-standardconstruction.

Completely eliminate harmonics

Active harmonicconditioners.

Simple and flexiblesolution.

Total elimination of all harmonics is possible (up to the 25th order),adaptable (action configured) andreusable system.

Active conditioners



Active harmonic conditioners

Active harmonic conditionersCharacteristics

Active harmonic conditioners

Active harmonic conditionersconstitute a more general approach to the problem of harmonics. Theseactive filters are not only for a UPSunit, but are designed to eliminateharmonics throughout the installation.



Active harmonic conditioners areparticularly well suited to medium-power industrial and infrastructureapplications, offering conditioningcurrents from 20 to 480 A in three-phase systems with a neutral.

These solutions are presented in the following section.

Table 18 sums up the maincharacteristics.

Table 18..

Range Power level50/60 Hz systems

Main characteristics Applications

Active harmonicconditioner

20 to 480 A 380 to 415 V3 Ph+N and 3 Ph

• Filtering up to H25• Digital active

conditioning with: - analysis and

conditioning ofindividual orders,

- response time 40 msfor load fluctuations.

Filtering of medium-powercommercial, infrastructureand industrial systems,3Ph+N and 3 Ph, single-phase loads.

Advantages of active harmonicconditioning

• Wide-band solution from H2 toH25 with individual conditioningof each phase,

• It is possible to select individualharmonic orders for conditioning,

• No risk of overloads, condition -ing limits to the maximum powerrating, even if the load powerexceeds the rating,

• Automatically adapts to all typesof loads, single-phase and three-phase,

• Compatible with all systemearthing arrangements,

• Power factor correction,

• Economic, when harmonics arecut in half, losses are reduced byfour,

• Can be reused in otherinstallations,

• Upgradeable with parallel-connected units,

• Very compact,

• Simple installation, with currenttransformers upstream ordownstream.

Operating principle

The source supplies exclusively thefundamental component (IF) of theload current.

The active conditioner measures inreal time the harmonics (IH) drawn by the load and supplies them.

Upstream of point A, where theconditioner is connected, thefundamental current IF is not altered,downstream the load draws the non-linear current IF + IH.

Figure 60: Harmonic conditioning..



Operating modes

Digital mode, conditioning of

individual orders

The basic operating mode is digital,with a current sensor, analogue/digitalconversion of the current measure -ments and real-time calculation of theharmonic spectrum. This informationis supplied to the inverter forcompensation of the individualharmonic orders.

The response time to load fluctuationsis 40 ms (two cycles).

Operating diagram

The power required for conditioning isdrawn on the three-phase distributionsystem and stored in the inductor Land the capacitors charged to +Vmand -Vm respectively (see figure 61).

Depending on the sign of theharmonic current required, the pulsewidth of one capacitor or the other ismodulated. This means the sameconnection to the supply system canbe used to draw power and inject theharmonics.



Figure 61: Operation..

The power sent to the load depends on:

• The harmonic values measured,

• User requirements, set duringsystem configuration: harmonicorders to be eliminated andpower-factor correction (yes or no).

The current transformer, combinedwith an analogue/digital converter,determines the spectrum(fundamental and harmonics) of the current supplying the load.

Depending on these values and the

selecting program, a processorprepares the commands for theinverter, for execution one phase afterthe measurements.

Power factor correction is obtained bygenerating a fundamental current +90°out of phase with the voltage

Options

On 3Ph or 3 ph+N systems, the usercan decide to condition:

• All or only certain harmonics up to H25,

• The power factor

Installation modes

Parallel mode

Up to four active harmonic conditionerscan be connected in parallel at thesame point of installation. This themeans to increase harmonicconditioning capacity and/or systemavailability.

For parallel installations, a single set of sensors is required on theconditioned circuit and a wire link is used to send the load-currentmeasurements to the variousconditioners. If one conditioner shutsdown, the remaining conditionerscontinue to condition the harmonics,within the limits of their ratedconditioning capacity.

Figure 62: Parallel operation of three.active harmonic conditioners.

Cascade or in-series mode

“Cascade” or “in-series” operation ispossible, but simply requires specialsettings to avoid any interactionbetween the different conditioners.

The downstream conditionergenerally conditions a high-powerload. The upstream device conditionsother low-power outgoing circuitsand, where applicable, any residualharmonics not conditioned by the firstconditioner.

Figure 63: Active harmonic conditioners.in cascade mode.

Multi-circuit mode

In this mode, a single conditioner can condition up to three outgoingcircuits. A set of sensors is requiredfor each circuit conditioned and all are connected to active harmonicconditioners . This configuration isvery useful when the harmonics areconcentrated on a small number ofcircuits.



Figure 64: One active conditioner for several circuits..

Position in the installation

Total (or centralised) conditioning

The active harmonic conditioner isconnected just downstream of thesources, generally at the main low-voltage switchboard (MLVS) level.

Partial conditioning

The active harmonic conditioner isconnected at the main or secondaryswitchboard level and conditions a setof loads.

Local conditioning

The active harmonic conditioner isconnected directly to the terminals ofeach load

Figure 65: Three possible installation points,depending on user requirements.



Practically speaking

• Total conditioning does not poseany calculation problems,

• Partial conditioning requires afew precautions,

• For all non-compensated RCDloads (high-power variable-speed drives without inductorsfor variable-torque applications),local conditioning can guaranteeonly a THDU not exceedingcertain limits to ensure properload operation.

Position of current transformersupstream or downstream

In most of the installation modes,previously listed, two different typesof current-transformer (CT) installationcan be used with active harmonicconditioners.



Comparison of installation possibilities

Table 19..

Type of conditioning Advantages Disadvantages Applications

Total(MLVS level)

• Economical.• Relieves generators

(transformers,generators).

• Harmonics remain inthe downstream partof the installation.

• Cables must beoversized.

• Compliance with utilityrequirements.

• Avoid injectingharmonics upstreamof the installation.

Partial(secondary-switchboard

level)

• Avoids oversizing thecables between themain and secondaryswitchboards.

• Recombination ofcertain harmonics maymake it possible toreduce conditionerrating.

• Harmonics remainbetween the secondaryswitchboard and thenon-linear load.

• Outgoing cable to theload must beoversized.

• Large buildings.• Conditioning regularly

spaced on each flooror set of floors.

• Several circuitssupplying non-linearloads.

Local(load level)

• Eliminates harmonicswhere they occur.

• Reduces losses in allcables, up to thesource.

Costly when a numberof conditioners arerequired.

• For installations wherenon-linear loads arefew in number andhigh-powered iwthrespect to the otherloads (example: largevariable-speed drives,high-power UPS):- Examples: server

bays, lighting, high-power UPS,fluorescent lightingsystems.

CT upstream of the load

This is the most common situation.

Figure 66: Installation with one CT upstream of the load..

Installation with one CT upstream of

the active harmonic conditioners and

one CT on the switchboard incomer

This configuration simplifies matterswhen it is difficult to install a CT onthe line just upstream of the load. Thetwo CTs must have compatible andcomplementary characteristics. Thedifference between the measuredcurrents determines the necessarycompensation current.



Figure 67: Installation with two CTs, one on the switchboard.incomer and the other upstream of the conditioner.

Advantages

Elimination of the conditioned

harmonic currents

For the selected harmonics, activeharmonic conditioners are designedto provide a path for the harmoniccurrents with virtually zero impedancewith respect to that of the source.

This eliminates their flow upstreamtowards the source.

Figure 68 shows active harmonicconditioners between two linesections ZL1 and ZL2, supplying astandard RCD load that can be eithersingle or three-phase (switch-modepower supply or variable-speed drive).

The harmonic currents IHn thatpreviously flowed through impedancesZs and ZL1 upstream of the activeharmonic conditioners point ofinstallation, are eliminated.

The source now supplies exclusivelythe fundamental current If.

It is the active harmonic conditionersthat supplies the harmonic currentsIHn to the load, by continuouslymeasuring the harmonics drawn by the load.



Figure 68: Modifies the current upstream of its point of installation..

Reduction in THDU at the point

of installation

Upstream of active harmonicconditioners, the selected harmoniccurrents IHn (all or only some of theharmonics up to the 25th) areeliminated.

Total harmonic distortion upstream ofthe point of installation is calculatedas (see page 158, figure 94):

∞

THDU% = 100 Σ UHn2√n=2

UH1

where UHn is the voltage dropcorresponding to harmonic IHn.

Elimination of the harmonic current fora given order eliminates the harmonicvoltage for the same order (1).

The result is a major reduction in theTHDU, by selecting the mostsignificant harmonics.

Given that above the 25th order,individual harmonics are negligible,the THDU is practically equal to zeroand distortion is totally eliminated if itis decided to condition all harmonicsup to the 25th.

(1) In that UHn and IHn are sinusoidalcomponents at frequency nf (where fis the frequency of the fundamental),they are related by the Ohm law,taking into account the value of theconcerned impedances (Zs and ZL1)with an angular frequency nω.

Therefore:

UHn = (Zs(nω) + ZL1(nω)) IHn.

For all the conditioned harmonics, IHn= 0 and consequently, UHn = 0.

Procedure for implementingactive conditioningConclusion on activeconditioning

Precise conditioning calculations

require:

• Precise and in-depth knowledgeon the installation (sources, linesand installation method),

• Precise knowledge on the loads(harmonic and displacementcurves depending on the sourceimpedance),

• Special calculation tools,

• Analysis and simulation.

New installations

The standard rules governingelectrical installations remain valid,but an evaluation of the voltagedistortion (THDU) is required whereharmonic currents flow.

This evaluation is very complex andrequires special calculation softwareas well as in-depth knowledge of thenon-linear loads, in particular theharmonic distribution as a function ofthe upstream impedance.

Existing installations

For existing installations, a preciseevaluation of the site is theindispensable prerequisite to anycorrective action. The mathematicalrelationship between current andvoltage distortion is complex anddepends on the various componentsof the installation.



Control over harmonic phenomenarequires know-how and experience, aswell as specialised tools and software(spectrum analyser, calculationsoftware for distortion in cables,simulation software, etc.).

However, even if each solution isspecific to a given site, properprofessional techniques and rigorousmethods ensure maximum probabilitythat the installation will operatecorrectly.

Methodology

Three-step approach:

1. Site audit,

2. Determination of the most suitable

solution,

3. System installation and checks.

1. Site audit

Installation diagram

Before initiating a series ofmeasurements, we suggest you drafta simplified diagram of theinstallation, indicating the following.

• Types of equipment:

- generators: type, power rating,voltage, Usc, X"d (enginegenerator set),

- isolation transformers: voltage,power rating, type, Usc,coupling,

- distribution: type of cables,length, cross-section,installation method,

- loads: power rating, type,

- system earthing arrangementsat the various points in theinstallation.

• Operating modes:

- on utility power,

- on engine generator sets(standby power orcogeneration),

- on UPS.

• Downgraded operating modes:

- without redundancy,

- on engine generator set power.

This diagram should enable you tolocate the different measurementpoints and identify critical operatingphases (for evaluation by simulationor calculation).

Measurements

Following the previous indispensablestep, the measurement phase canbegin, starting preferably at the sourceand working downstream toward theloads drawing the harmonics, in orderto limit the number of measurements.

The quality of measurements is moreimportant than their quantity andmakes the next step easier.

Preliminary installation study

This first step ends with a preliminarystudy of the installation:

• Point(s) of installation of theconditioner(s),

• Installation conditions for theprotection circuit breakers,

• Installation of sensors (energisedconditions or not),

• Possibility of shutting down the load,

• Available space,

• Evacuation of losses (ventilation,air-conditioning, etc.),

• Environmental constraints(noise, EMC, etc.).



2. Determination of the mostsuitable solution

The previous elements are used todetermine the optimum solutionthrough:

• Analysis of the measurementresults,

• Simulation of different solutionsfor the problem encountered,

• Determination of the mostsuitable solution,

• Drafting of a summary reportwith the proposed solutions.

3. System installation and checks

This last step includes:

• Implementation of the selectedsolutions,

• Checks on performance levelswith respect to the guaranteedresults,

• Drafting of a system start-upreport.



Theoretical review

Supplying sensitive loads

Types of electricaldisturbancesPower distribution systems, bothpublic and private, theoreticallysupply electrical equipment with asinusoidal voltage of fixed amplitudeand frequency (e.g. 400 volts rms, 50 Hz, on low-voltage systems).

In real-life conditions however, utilitiesindicate the degree of fluctuationaround the rated values. Standard EN50160 defines the normal fluctuationsin the LV supply voltage on Europeandistribution systems as follows:

• Voltage +10% to -15% (averagerms values over 10-minuteintervals), of which 95% must bein the +10% range each week,

• Frequency +4 to 6% over oneyear with ±1% for 99.5% of thetime (synchronous connectionsin an interconnected system).

Practically speaking, however, inaddition to the indicated fluctuations,the voltage sine-wave is alwaysdistorted to some degree by variousdisturbances that occur on the system.



Origins of disturbances

Utility power

Utility power can be disturbed or evencut by the following phenomena:

• Atmospheric phenomenaaffecting overhead lines orburied cables:- lightning which can produce a

sudden voltage surge in thesystem,

- frost which can accumulate onoverhead lines and cause themto break,

• Accidents:- a branch falling on a line, which

may produce a short-circuit orbreak the line,

- cutting of a cable, for exampleduring trench digging or otherconstruction work,

- a fault on the utility powersystem,

• Phase unbalance,

• Switching of protection orcontrol devices in the utilitypower system, for load sheddingor maintenance purposes.

User equipment

Some equipment can disturb theutility power system, e.g.:

• Industrial equipment:- motors, which can cause

voltage drops due to inrushcurrents when starting,

- equipment such as arc furnacesand welding machines, whichcan cause voltage drops andhigh-frequency interference,

• Power electronics equipment(switch-mode power supplies,variable speed drives, electronicballasts, etc.), which often causeharmonics,

• Building facilities such as liftswhich provoke inrush currents orfluorescent lighting which causesharmonics.

Types of disturbances

Disturbances that are due to thepreviously listed causes are summedup in the following table, according tothe definitions contained in standardsEN 50160 and ANSI 1100-1992.



Table 20..

Disturbances Characteristics Main causes Main consequences

Power outages

Micro-outages Total absence of voltage ≤ 10 ms. Atmospheric conditions, switching,faults, work on the utility.

Faulty operation and loss of data (computer systems) orinterrupted production(continuous processes).

Outages Total absence of voltage for morethan one period:• short outage: < 3 minutes (70% of

outages last less than 1 s)• long outage: > 3 minutes

Atmospheric conditions, switching,faults, incidents, line breaks, workon the utility.

Depending on the duration,shutdown of machines and risksfor people (e.g. lifts), loss of data(computer systems) orinterrupted production(continuous processes).

Voltage variations

Voltage sags Reduction in the rms value ofvoltage to less than 90% of the ratedvalue (but greater than 0%), withreturn to a value greater than 90%within 10 ms to 1 minute.

Atmospheric phenomena, loadfluctuations, short-circuit on aneighboring circuit.

Shutdown of machines,malfunctions, damage toequipment and loss of data.

Overvoltage Temporary increase to more than10% over the rated voltage, for aduration of 10 ms to a few seconds.

• Quality of utility generators andtransmission systems.

• Interaction between generatorsand load fluctuations on the utilitypower system.

• Switching on the utility powersystem.

• Stopping of high-power loads (e.g.motors, capacitor banks).

• For computer systems:corruption of data, processingerrors, system shutdown,stress on components.

• Temperature rise andpremature aging of equipment.

Undervoltage Drop in voltage lasting from a fewminutes to days.

Peak in consumption, when the utilitycannot meet demand and mustreduce its voltage to limit power.

• Shutdown of computer systems.• Corruption or loss of data.• Temperature rise.• Premature aging of equipment.

Voltage spike Sudden major jump in voltage (e.g.6 kV).

Close lightning strikes, staticdischarges.

• Processing errors, corruption of data system shutdown.

• Damage to computers,electronic boards.

Voltage unbalance (in three-phase systems)

Condition where the rms value of thephase voltages or the unbalancesbetween phases are not equal.

• Induction furnaces.• Unbalanced single-phase loads.

• Temperature rise.• Disconnection of a phase.

Frequency variations

Frequency fluctuations Instability in the frequency. Typically+5%, -6% (average for ten-secondtime intervals).

• Regulation of generators.• Irregular operation of generators.• Unstable frequency source.

--These variations exceed thetolerances of certain instrumentsand computer hardware (often±1%) and can therefore result inthe loss or corruption of data.

FlickerFlicker in lighting systems due to adrop in voltage and frequency (< 35Hz).

Welding machines, motors, arcfurnaces, X-ray machines, lasers,capacitor banks.

Physiological disturbances.

Other disturbances

HF transients Sudden major and very short jumpin voltage. Similar to a voltage spike.

Atmospheric phenomena (lightning)and switching.

Destruction of equipment,accelerated aging, breakdown of components or insulators.

Short duration < 1 μs.Amplitude < 1 to 2 kV at frequenciesof several tens of MHz.

Starting of small inductive loads,repeated opening and closing oflow-voltage relays and contactors.

Medium duration < 1 μs and ≤ 100 μs.Peak value 8 to 10 times higher thanthe rated value up to several MHz.

Faults (lightning) or high-voltageswitching transmitted to the low-voltage by electromagnetic coupling.

Long duration > 100 μs.Peak value 5 to 6 times higher thanthe rated value up to severalhundred MHz.

Stopping of inductive loads or high-voltage faults transmitted to thelow-voltage system byelectromagnetic coupling.

Harmonic distortion Distortion of the current and voltagesine-waves due to the harmoniccurrents drawn by non-linear loads.The effect of harmonics above the25th order is negligible.

Electric machines with magneticcores (motors, off-load transformers,etc.), switch-mode power supplies,arc furnaces, variable speed drives.

Oversizing of equipment,temperature rise, resonancephenomena with capacitors,destruction of equipment(transformers).

Electromagneticcompatibility (EMC)

Electromagnetic or electrostaticconducted or radiated disturbances.The goal is to ensure low emissionand high immunity levels.

Switching of electronic components(transistors, thyristors, diodes),electrostatic discharges.

Malfunctions of sensitiveelectronic devices.



UPS

UPSA UPS (uninterruptible power system)is used to supply sensitiveapplications with secure power.

A UPS is an electric device positionedbetween the utility and the sensitiveloads that supplies voltage offering:

• High quality: the output sine-wave is free of any and alldisturbances in utility power and within strict amplitude andfrequency tolerances,

• High availability: the continuoussupply of voltage, within thespecified tolerances, is ensuredby a backup supply of power. The backup supply is generally

a battery that, if necessary, stepsin without a break in the supplyto replace utility power andprovide the backup time requiredby the application.

These characteristics make UPSs theideal power supply for all sensitiveapplications because they ensurepower quality and availability,whatever the state of utility power.

Components of a UPS

A UPS generally comprises the maincomponents listed as follows.

Rectifier/charger

It draws utility power and produces aDC current to supply the inverter andcharge or recharge the battery.



Inverter

It completely regenerates a high-quality voltage output sine-wave:

• Free of all utility-power disturb -ances, notably micro-outages,

• Within tolerances compatiblewith the requirements ofsensitive electronic devices (e.g. tolerances in amplitude ± 0.5% and frequency ± 1%,compared to ± 10% and ± 5% in utility power systems, whichcorrespond to improvementfactors of 20 and 5, respectively.

Note:The term inverter is sometimesused to designate a UPS, when inreality it is only a part of the UPS.

Energy Storage

The energy storage provides sufficientoperating backup time (seconds to a number of hours) by stepping in toreplace utility power as needed.

Static bypass

The static bypass ensures no-breaktransfer of the load from the inverterto direct utility power and back. No-break transfer is carried out by adevice implementing SCRs(sometimes called a static switch).

The static bypass makes it possible tocontinue supplying the load even if aninternal fault occurs or duringmaintenance on the rectifier/chargerand inverter modules. It can also servefor transfers to call on the full poweravailable upstream in the event ofoverloads (e.g. short circuits)exceeding UPS capacity.

During operation on the static bypass,the load is supplied directly by utilitypower and is no longer protected(operation in downgraded mode).

Maintenance bypass

This bypass may be used to supplythe load directly with utility power,without calling on the inverter or the static switch. Transfer to themaintenance bypass is user initiatedwith switches. By actuating thenecessary switches, it is the means to isolate the static bypass and theinverter for maintenance, whilecontinuing to supply the load indowngraded mode.



Figure 69: The UPS solution..

UPS applicationsUPS are used for a wide range ofapplications requiring electrical powerthat is available at all times and notaffected by disturbances on the utilitypower system. Table 21 presents anumber of applications.

For each, it indicates the sensitivity ofthe application to disturbances.

The applications requiring this type of installation are:

• Computer systems,

• Telecommunications,

• Industry and instruments,

• Other applications.

The required UPS typologies arepresented on page 5, “Types of staticUPS”. They include static UPSimplementing the following typologies:








Table 21..

UPS Applications

Application Protected devices

Protection required against

Micro-outages Outages Voltage

variationsFrequencyvariations Other

Computer systems

Data centers• Large bays for rack-mounted

servers• Internet data centers

***** ***** ***** ***** *****

Company networks

• Sets of computers withterminals and peripheraldevices (tape storage units,disk drives, etc).

***** ***** ***** ***** *****

Small networks and servers

• Networks made up of PCs or workstations, servernetworks (WAN, LAN) **** **** *** *** **

Stand-alone computers• PCs, workstations• Peripheral devices: printers,

plotters, voice mail** ** * * **

Telecommunications

Telecommunications • Digital PABXs ***** ***** ***** ***** *****

Industry and instruments

Industrial processes

• Process control• PLCs• Numerical control systems• Control systems• Robot control/monitoring

systems• Automatic machines

*** ***** *** *** ****

Medical and laboratories

• Instrumentation• Scanners (60 Hz)

**** ***** **** **** ***

Industrial equipment

• Machine-tools• Welding robots• Plastic-injection presses• Precise regulation devices

(textile, paper, etc.)• Heating equipment for

manufacture of semi-conductors, glass, purematerials

*** **** *** *** ***

Lighting systems

• Public buildings (elevators,safety equipment)

• Tunnels• Runway lighting in airports

** **** *** *** **

Other applications

Special frequencies• Frequency conversion• Power supplies for aircraft

(400 Hz)**** **** **** ***** ***

* low sensitivity to disturbances***** high sensitivity to disturbances

Types of UPS

Static or rotary UPSStatic or rotary UPS solutions

There are two main types of UPSwhich basically differ in the way theUPS inverter function is implemented.

Static solution

These UPS use only electroniccomponents to perform the inverterfunction. A “static-inverter function” is obtained.

Rotary solution

These UPS use rotary machines toperform the inverter function.

These UPS in fact combine a motorand a generator with a highlysimplified static inverter.



The inverter filters out utility-powerdisturbances and regulates only the frequency of its output voltage(generally in “square-wave” form),which supplies a regulatedmotor/generator set that is sometimescombined with a flywheel.

The motor/generator set generates anoutput voltage sine-wave, taking theinverter output frequency as thereference.

Figure 70: Static and rotary UPS..

Comparison

Rotary solution

The arguments often put forward infavour of this solution are as follows:

• High generator short-circuitcurrent on the order of 10 In (tentimes the rated current) thatmakes setting of protectiondevices easier,

• 150% overload capacity (of therated current) over a longer period(two minutes instead of one),

• Downstream installationgalvanically isolated fromupstream AC source due to themotor/generator set,

• Internal impedance providinghigh tolerance to the non-linearloads frequently encounteredwith the switch-mode powersupplies used by computersystems.

Static solution

Compared to the advantages of rotary

solutions

The static UPS offers the advantageslisted below.

• Operation in current-limiting modewith discrimination ensured forcircuits rated up to In/2.

These features, which are morethan sufficient in practice,prevent the disadvantages ofrotary systems:- overheating of cables,- the effects of an excessive

short-circuit current and thecorresponding voltage drop onsensitive devices, during thetime taken by protectivedevices to clear the fault.

• 150% overload capacity (of therated current) for one minute.

The two-minute overload capacity isof no practical use because mostoverloads are very short (less thanone second, e.g. in-rush currents ofmotors, transformers and powerelectronics).

• Galvanic isolation, whenrequired, by means of anisolating transformer,

• Double-conversion operationwhich completely isolates theload from utility power andregenerates the output voltagewith precise regulation of thevoltage amplitude and thefrequency,

• Very low internal impedance forhigher performance with non-linear loads due to the use ofpower-transistor technologies.

Other advantages

Static solutions provide many otheradvantages as well, due to power-transistor technology combinedwith a PWM chopping technique.

• Simplified overall design, with areduction in the number of partsand connections, and in thenumber of possible causes offailure;

• Capacity to react instantaneouslyto utility-power amplitude andfrequency fluctuations by meansof microprocessor-controlledswitching regulation based ondigital sampling techniques. Thevoltage amplitude returns toregulated conditions (± 0.5% or ± 1% depending on the model) inless than 10 milliseconds for load



step changes up to 100%. Withinthe indicated time interval, sucha load step change produces aload voltage variation of lessthan for example ± 2%;

• High, constant efficiency what -ever the percent load, which is amajor advantage for redundantUPS units with low percent loads.A static UPS unit with a 50% loadmaintains high efficiency (94%),whereas the efficiency of a rotaryUPS drops to the 88-90% range(typical value), which directlyimpacts on operating costs;

• Redundant configurationsproviding high availability in theframework of ultrareliable supplysystems (e.g. for data centres);

• Possible integration in redundantarchitectures with separatefunctions that facilitatemaintenance by isolating parts of the installation.

Rotary systems integrate the UPS, thebackup power and the generator as asingle component, thus making itimpossible to separate the functions.

Consider also the following non-negligible advantages:

• No wear on rotating parts, henceeasier and faster maintenance.For example, rotary systemsrequire checks on the alignmentof the rotating parts and thereplacement of the bearings after2 to 6 years is a major operation(lifting equipment, heating andcooling of the bearings duringthe replacement).

Conclusion

Given the advantages just presented,static UPS are used in the vastmajority of cases, and for high-powerapplications in particular.

In the following pages, the termuninterruptible power supply(UPS) is taken to mean the staticsolution.

Types of static UPSStandards

UPS

Due to the vast increase in the numberof sensitive loads, the term “UPS”now includes devices ranging from afew hundred VA for desktopcomputers up to several MVA for datacentres and telecommunications sites.

At the same time, different typologieshave been developed and the namesused for the products on the marketare not always clear (or evenmisleading) for end users.

That is why the IEC (InternationalElectrotechnical Commission)established standards governing thetypes of UPS and the techniques usedto measure their performance levels,and those criteria were adopted byCenelec (European standardisationcommission).

Standard IEC 62040-3 and itsEuropean equivalent EN 62040-3

define three standard types(topologies) of UPS and theirperformance levels.



UPS technologies include:




AC input power

These definitions concern UPSoperation with respect to the powersource including the distributionsystem upstream of the UPS.

The standards define the followingterms:

• Primary power: power normallycontinuously available which isusually supplied by an electricalutility company, but sometimesby the user's own generation,

• Standby power: power intendedto replace the primary power inthe event of primary-powerfailure.

Practically speaking, a UPS has one ortwo inputs:

• Normal AC input (or Mains 1),supplied by primary power,

• Bypass AC input (or Mains 2),supplied by standby power(generally speaking via aseparate cable from the samemain low-voltage switchboard(MLVS).

UPS operating in passive-standby mode

The UPS is installed in parallel to the utility and backs it up. Thebattery is charged by a charger thatis separate from the inverter.

Operating principle

• Normal mode:- The inverter operates in

passive standby mode,- The load is supplied by utility

power via a filter whicheliminates certain disturbancesand provides some degree ofvoltage regulation,

- The standards do not mentionthis filter and speak simply of a“UPS switch”. They also indicatethat “additional devices may beincorporated to provide powerconditioning, e.g. ferroresonanttransformer or automatic tap-changing transformer”.

• Battery backup mode:- When the AC input voltage is

outside specified tolerances forthe UPS or the utility powerfails, the inverter and thebattery step in to ensure acontinuous supply of power tothe load following a very shorttransfer time (generally lessthan 10 ms). The standards donot stipulate a time, but doindicate that “the load [is]transferred to the inverterdirectly or via the UPS switch(which may be electronic orelectromechanical)“,

- The UPS continues to operateon battery power until the endof battery backup time or utilitypower returns to normal, whichprovokes transfer of the loadback to the AC input (normalmode).



Figure 71: UPS operating in..passive-standby mode.

Advantages

• Simple diagram,

• Reduced cost.

Disadvantages

• No real isolation of the load with respect to the upstreamdistribution system,

• Transfer time. It operates withouta real static switch, so a certaintime is required to transfer theload to the inverter. This time isacceptable for certain individualapplications, but incompatiblewith the performance requiredby more sophisticated, sensitivesystems (large computer centres,telephone exchanges, etc.),

• No regulation of the outputfrequency, which is simply thatof the utility power.

Usage

This configuration is in fact acompromise between an acceptablelevel of protection againstdisturbances and cost.

The mentioned disadvantages meanthat, practically speaking, this type ofUPS can be used only for low power

ratings (< 2 kVA) and cannot be usedas a frequency converter.

UPS operating in line-interactivemode

The inverter is connected inparallel with the AC input in a

standby configuration, and alsocharges the energy storage. It thusinteracts (reversible operation)with the AC-input source.

Operating principle

• Normal mode

The load is supplied withconditioned power via a parallelconnection of the AC input andthe inverter. As long as the inputpower is within tolerances, theinverter regulates fluctuations inthe input voltage. Otherwise(reversible operation), it chargesthe battery. The output frequencydepends on the AC-inputfrequency.

• Backup mode

- When the AC input voltage isoutside specified tolerances forthe UPS or the input powerfails, the inverter and theenergy storage step in toensure a continuous supply ofpower to the load. The powerswitch (e.g. static switch) alsodisconnects the AC input toprevent power from theinverter from flowingupstream.



- The UPS continues to operateon backup power until the endof energy storage backup timeor input power returns towithin tolerance, whichprovokes transfer of the loadback to the AC input (normalmode).

• Bypass mode

This type of UPS may be equippedwith a bypass. If one of the UPSfunctions fails, the load can betransferred to the bypass AC inputvia the maintenance bypass.

Figure 72: UPS operating in.line-interactive mode.

Advantages

• The product has improvedefficiencies due the fact that notall power is being broken downand rebuilt as with a doubleconversion UPS.

Disadvantages

• No real isolation of the load withrespect to the upstreamdistribution system, thus:- sensitivity to variations in the

utility voltage and frequentdemands placed on theinverter;

- influence of downstream non-linear loads on the upstreaminput voltage;

Usage

This product is primarily used whereefficiency is a driving factor in theproduct purchase.

Double-conversion UPS

The inverter is connected in seriesbetween the AC input and theapplication. The power supplied to the load continuously flowsthrough the inverter.

Operating principle

• Normal mode

During normal operation, all the power supplied to the loadpasses through therectifier/charger and inverterwhich together perform a doubleconversion (AC-DCAC), hence thename. The voltage is continuouslyregenerated and regulated.

• Backup mode

- When the AC-input voltage isoutside specified tolerances forthe UPS or the input powerfails, the inverter and theenergy storage step in toensure a continuous supply ofpower to the load.



Figure 73: Double-conversion UPS..

- The UPS continues to operateon backup power until the endof energy storage backup timeor input power returns tonormal, which provokestransfer of the load back to theAC input (normal mode).

• Bypass mode

This type of UPS comprises astatic bypass (sometimes called a static switch) that ensures no-break transfer of the load fromthe inverter to direct utility powerand back.

The load is transferred to thestatic bypass in the event of thefollowing:

- UPS failure,

- load-current transients (inrushor fault currents),

- overloads,

- end of energy storage backuptime.

The presence of a static bypassassumes that the input and outputfrequencies are identical, whichmeans it cannot be used as afrequency converter. If the voltagelevels are not the same, a bypasstransformer is required.

The UPS is synchronised with thebypass AC input to ensure no-breaktransfers from the inverter to thebypass line.

Note. Another bypass line, often calledthe maintenance bypass, is availablefor maintenance purposes. It is closedby a manual switch.



Advantages

• Complete regeneration of theoutput power, whether it comesfrom the utility or the battery,

• Total isolation of the load fromthe distribution system and itsdisturbances,

• Very wide input-voltage range,yet precise regulation of theoutput voltage,

• Independence of the input andoutput frequencies, thus ensuringan output frequency within stricttolerances. Capacity to operate asa frequency converter (if plannedas such), by disabling the staticswitch,

• Much higher performance levelsunder steady-state and transientconditions,

• Instantaneous shift to backupmode if input power fails,

• No-break transfer to a bypassline (bypass mode),

• Manual bypass (generallystandard) to facilitate maintenance.



Disadvantages

• Reduced efficiency driving higherowning and operating cost andincreased heat rejection.

Usage

This configuration is the mostcomplete in terms of load protection,regulation possibilities andperformance levels. It notably ensuresindependence of the output voltageand frequency with respect to theinput voltage and frequency.

Conclusion

Double-conversion UPSs representthe vast majority of the medium

to high-power systems sold (90% ofthe overall UPS market). This is due totheir numerous strong points inmeeting the needs of sensitive loadsat these power ratings and is largelythe result of the inverter positioned inseries with the AC input.

UPS components and operation

Components of a UPSThe information that follows concernsthe double-conversion UPS, thetechnology most commonly used forpower ratings greater than 10 kVA.

General diagram of a UPS

The various items in figure 74 havebeen assigned numbers thatcorrespond to the sections on thefollowing pages.



Figure 74: Components of a UPS..



Power sources and UPS inputs

Practically speaking, a UPS has one ortwo inputs:

• Normal AC input (or Mains 1),supplied by primary power,

• Bypass AC input (or Mains 2),supplied by standby power(generally speaking via a separatecable from the same main low-voltage switchboard (MLVS).

AC Input Power, see page 121

UPS connection to both the primaryand standby-power sources (UPSinputs supplied by two separatecircuits from the MLVS) isrecommended because overall systemreliability is increased. However, if twoseparate circuits from the MLVS are notavailable, it is possible to have both ACinputs (normal and bypass) suppliedby primary power (second cable).

Management of transfers between thetwo input lines is organised as follows.

• The UPS synchronises theinverter output voltage with thatof the bypass line as long as thelatter is within tolerances. It isthus possible, if necessary, forthe static switch to transfer theload to the bypass AC input,without a break (because the twovoltages are synchronised and inphase) or disturbances (becausethe standby power is withintolerances) for the load;

• When standby power is notwithin tolerances, the inverterdesynchronises and transfer isdisabled. It can, however, bycarried out manually.

Components of a UPS

Rectifier/charger (1)

Transforms the AC power from theprimary-power source into DC voltageand current used to:

• Supply the inverter,

• Charge and float charge thebattery.

Inverter (2)

Using the DC power supplied by the:

• Rectifier during normaloperation,

• Battery during autonomousoperation, the invertercompletely regenerates asinusoidal output signal, withinstrict amplitude and frequencytolerances.

Battery (3)

Makes the UPS autonomous withrespect to the utility in the event of:

• A utility outage,

• Utility-power characteristicsoutside specified tolerances forthe UPS.

Battery backup times range from 6 to30 minutes as standard and can beextended on request. Depending onthe duration of the backup time, thebattery is housed in the UPS cabinetor in a separate cabinet.

Static bypass (4)

A static switch is used to transfer theload from the inverter to the bypasswithout any interruption* in thesupply of power to the load (no breakbecause the transfer is performed byelectronic rather than mechanicalcomponents). The switch is possiblewhen the frequencies upstream anddownstream of the UPS are identical.

Transfer takes place automatically forany of the following reasons:

• Voluntary shutdown of the UPS,

• An overload exceeding thelimiting capacity of the inverter(this transfer can be disabled),

• An internal fault.

It can also be carried out manually.

* No-break transfer is possible whenthe voltages at the inverter output andon the bypass AC input aresynchronised. The UPS maintainssynchronisation as long as thestandby power is within tolerances.

Manual bypass (5)

A manual switch is used to transfer theload to the bypass for maintenancepurposes. The switch is possible whenthe frequencies upstream anddownstream of the UPS are identical.

The shift to manual-bypass mode iscarried out using manual switches.

Manual switches (6, 7, 8)

These devices isolate therectifier/charger and inverter modulesand/or the bypass line for servicing ormaintenance.

Battery circuit breaker (9)

The battery circuit breaker protects thebattery against excessive discharge,and the rectifier/charger and inverteragainst a battery short-circuit.

Upstream isolating transformer (10)

(optional equipment)

Provides UPS input/output isolationwhen the downstream installation issupplied via the bypass.

It is particularly useful when theupstream and downstream systemearthing arrangements are different.

Voltage-matching transformer (11)


Adapts the voltage to the desiredvalue.

Filters (12)


• Upstream of the rectifier/charger,when it is of the thyristor-basedGraetz bridge type, a harmonicfilter (see page 25) reduces thecurrent harmonics resulting fromthe switching of the rectifierthyristors. This reduces thevoltage distortion on theupstream busbars resulting fromthe flow of harmonic currents(the level required is generally<5%). What is more, these UPSfrom Caterpillar are equippedwith an oversized neutralconductor installed as standardto overcome the consequencesof third-order harmonics andtheir multiples which flow in theneutral conductor;

• Downstream, UPS implementingnew PWM-chopping techniquesmay be directly connected to non-linear loads. This techniquemakes it possible for UPS fromCaterpillar to maintain the THDUbelow 3%.





Built-in communication (13) (14)

In addition to the need for a user-friendly human/machine interface foreffective monitoring of UPS operation,it is today increasingly important forUPS to communicate with theirelectrical and computing environment(supervision systems, buildingmanagement systems (BMS),computer management systems, etc.).

UPS from Caterpillar are designedwith built-in capacity for totalcommunication and include:

• A user-friendly human/machineinterface (HMI) with an advancedgraphic display and mimic panel.The interface is built up aroundself-monitoring and self-diagnostic systems thatcontinuously indicate the statusof the various UPS components,in particular the batteries.

For example:

- the Digibat systemcontinuously monitors thestatus of the battery with fullbattery management features,

• A large selection ofcommunication cards compatiblewith market standards:- Network Management Card

(Ethernet);- Modbus – Jbus card (RS232

and RS485);- Relay card (dry contacts) for

indications;These cards can be used to implementsupervision, notification, controlledshutdown and Teleservice functions.

Human-machine interface andCommunication: see page 56.

Upstream and/or downstream

distribution and protection devices

(15) (16)


The UPS can be supplied with thefollowing equipment:

• Upstream LV circuit-breakers forthe AC inputs (normal andbypass),

• Upstream LV switchboard withcircuit-breaker protection for theAC inputs (normal and bypass),

• Downstream LV switchboardwith circuit-breaker protectionfor the different outgoingcircuits.

Main characteristics of UPScomponentsThese characteristics are based on the main technical specificationspresented in the IEC 62040-3 /

EN 62040-3 standards on UPSperformance requirements.

Certain terms used here differ from thecommon jargon and a number of newfeatures have not yet been assimilatedby manufacturers. New terms orcharacteristics used by the standardare indicated between parenthesesand preceded by an asterisk.

For example, the title of a section“input current during battery floatcharging”, a commonly used term, isfollowed by (*rated input current), theterm used in the standard.

Note that a number of numericalvalues are indicated as examples.

They are, for the most part, drawnfrom the technical characteristics ofthe corresponding UPS or indicatedsimply for the purposes of theexample.

AC input power

Number of phases and system

earthing arrangement

The AC-input supply (primary power)is three-phase + neutral. Single-phaseinputs are not used for the powerlevels dealt with here.

The system earthing arrangement isgenerally imposed by standards (IT, TT, TNS or TNC).

Normal AC input

The normal AC input is supplied withutility power for the rectifier/charger,within the specified tolerances.

• Example: 400 V rms ± 15% at afrequency of 50 or 60 Hz ± 5%,three-phase.

Bypass AC input

The bypass AC input is supplied withstandby power. Practically speaking,this a cable connected to a utilityfeeder in the MLVS other than the one supplying the normal AC input.

In general, it supplies voltage with thesame characteristics as that of theprimary power.

• Example: 400 V rms ± 15% at afrequency of 50 or 60 Hz ± 5%,and a short-circuit current Isc2 =12.5 kA. The short-circuit currentis important information for thedownstream protection devicesin the event of operation via thestatic or maintenance bypass.

Supply of separate primary andstandby power is recommendedbecause it increases overall systemreliability, but is not mandatory.However, if two separate circuits from the MLVS are not available, it is possible to have both AC inputs(normal and bypass) supplied byprimary power (second cable).

Rectifier/charger

Floating voltage

This is the voltage supplied by therectifier/charger which keeps thebattery fully charged.

It depends on the batteries used andthe manufacturer's recommendations.

Input current during battery float

charging (* rated input current)

This is the current, under normaloperating conditions, required tosupply the inverter at its rated powerwhile float charging the battery.





Example: for a 100 kVA UPS with abattery backup time of 10 minutes,this current is I input float = 166 A while floatcharging the battery.

Input current during battery charging

This corresponds to the currentrequired to supply the inverter at itsrated power while charging thebattery. It is consequently higher thanthe previous current and is used tosize the charger input cables.

Example: for the same UPS asabove, the input current is I inputfloat = 182 A, i.e. higher thanabove because it is necessary tocharge the battery.

Maximum input current

This is the input current with the UPSoperating under worst-case conditionsof permitted overload, with the batterydischarged. It is higher than the aboveinput current during battery charging(due to the overload current) but islimited in time (as is the overload).

Example: for the same UPS asabove, can accept a 25% overloadfor ten minutes and a 50%overload for one minute. In theworst-case situation with thebattery charging, the input currentcan reach:

I input max. = 182 A x 1.25 = 227.5 Afor ten minutes,

I input max. = 182 A x 1.5 = 273 A forone minute.

Beyond the above limits, the UPSinitiates no-break transfer of the loadto the bypass line and automaticallytransfers back when the overload hasended or been cleared by thecorresponding protection devices.

Battery (* energy storage means)

Type

A battery is characterised by its type(vented or sealed lead acid, ornickel/cadmium) and how it isinstalled. Caterpillar proposes sealedlead-acid batteries mounted incabinets.

Service life

This is defined as the operatingperiod, under normal usageconditions, for which the batterysupplies at least 50% of the initialbackup time.

For example, if a UPS is suppliedas standard with sealed lead-acidbatteries with a service life of tenyears or more. This type of battery,rated for 30 minutes of backuptime, will contractually supplyonly 15 minutes at the end of thespecified service life.

It may supply more if it has been used under optimum conditions(notably concerning the temperature).However, it is contractuallyguaranteed not to supply less, unlessused improperly.

Operating modes

The battery may be:

• Charging. It draws a chargecurrent (I1 charge) supplied bythe rectifier/charger,

• Float charging.The battery drawsa low, so-called floating current(I1 floating), supplied by therectifier/charger, which maintainsits charge by compensating foropen-circuit losses,

• Discharging.The battery suppliesthe inverter until its shutdownvoltage is reached.

When this voltage, set by the batterymanufacturer, is reached, the batteryis automatically disconnected to avoiddamage by deep discharge.

Rated voltage

This is the DC output voltage that thebattery supplies to the inverter.

Example: 450 V DC.

Capacity

Battery capacity is expressed inampere/hours.

Example: for a 100 kVA UPSequipped with a battery offeringten minutes of backup time and aservice life of five years, thecapacity is 85 A/h.

Number of cells

Number of single battery cells makingup the entire battery string.

Example: the battery of a 100 kVAUPS comprises, for a given type ofbattery, 33 cells providing 13.6 Veach, for a backup time of tenminutes.

Floating voltage

This is the DC voltage used tomaintain the battery charge, suppliedby the rectifier/charger.

Example: for a 100 kVA UPS, thefloating voltage is between 423and 463 V DC.

Backup time (* stored energy time)

This is the time, specified at thebeginning of the battery service life,that the battery can supply the inverteroperating at full rated load, in theabsence of the AC-input supply.

This time depends on the UPS percentload.

• For a UPS operating at full ratedload (100% of rated power), theend of the battery backup time isreached when the battery voltagedrops to the shutdown voltagespecified by the manufacturer. Thisprovokes automatic shutdown ofthe UPS.

• For a UPS operating at a lowerpercent load (e.g. 75%), theactual backup time may belonger. However, it always endswhen the battery shutdownvoltage is reached.

Recharge time (* rated restored

energy time)

This is the time required by the batteryto recover 80% of its backup time (90%of its capacity), starting from thebattery shutdown voltage. Therectifier/charger supplies the power.

Example: for a 100 kVA UPS, therecharge time is eight to tenhours, depending on the batteryand the backup time. Note that theprobability of the battery beingcalled on to supply power twicewithin such a short period is low. This means the indicatedrecharge time is representative of actual performance.

Maximum battery current (Ib)

When discharging, the batterysupplies the inverter with a current Ibwhich reaches its maximum value atthe end of discharging. This valuedetermines battery protection andcable dimensions.

Example: for a 100 kVA UPS, thiscurrent is Ib max = 257 A.





Inverter

Rated power (Sn)

(* rated output apparent power)

This is the maximum apparent powerSn (kVA) that the inverter can deliverto a linear load at a power factor of0.8, during normal operation understeady-state conditions.

The standards also define thisparameter for operation on batterypower.

Theoretically speaking, it is the sameif the battery is correctly sized.

Active output power (Pa)

(* rated output active power for

linear or reference non-linear load)

This is the active power Pa (kW)corresponding to the apparent output power Sn (kVA), under themeasurement conditions mentionedabove. This value may also beindicated for a standardised referencenon-linear load.

Example: the previous UPS, with arated power of 100 kVA supplies anactive power of Pa = Sn x 0.8 = 80kW.

Rated current (In)

This is the current corresponding tothe rated power.

Example: again for a 100 kVA UPSand an output voltage of 400 V,this current is:

In = Sn = 100000 = 144.3 AUn √3 400 x 1,732

Apparent load power (Su) and percent

load

This is the apparent power Sn (kVA)actually supplied by the inverter to theload, under the selected operatingconditions.

This value is a fraction of the ratedpower, depending on the percent load.

Su ≤ Sn. and Tc = Percent load (%) = Su / Sn.

Example: for the UPS mentionedabove, if the inverter supplies 3/4of its rated power (75% load), itdelivers an apparent power of 75 kVA, which under standardoperating conditions (PF = 0.8)corresponds to an active loadpower of

Pa = Su x PF = 75 x 0.8 = 60 kW.

Load current (Iu)

This is the current corresponding tothe load power, that is, to the percentload in question. It is calculated fromPu as for the rated current, where thevoltage is the rated voltage Un (valueregulated by the inverter).

Example: for the UPS mentionedabove (75% load)

lu = Su = 75000 = 108.2 AUn √3 400 x 1,732

which is the same as:

Iu = In x Tc = 144.3 x 0.75 = 108.2 A

Efficiency (η)

This is the ratio of active power Pu(kW) supplied by the UPS to the loadto the power Pin (kW) that it draws atits input, either by the rectifier or fromthe battery.

η= Pu / Pin.

For most UPS, efficiency is optimumat full rated load and drops sharplywith lower percent loads. Due to theirlow output impedance and no-loadlosses.

Output voltage Un

Number of phases

The output can be three-phase (3ph-3ph UPS) or single-phase (3ph-1phUPS), depending on the situation.Note that the upstream anddownstream system earthingarrangements may be different.

Rated output voltage

In general, it is the same as that of the AC input. However, a voltage-matching transformer may beinstalled.

Static characteristics

These are the tolerances (maximumpermissible variations) for theamplitude and frequency of the outputvoltage under steady-state conditions.Stricter than those applying to utilitypower, they are measured for normaloperation on AC-input power and foroperation in battery backup mode.

• Output voltage variation

The amplitude tolerance is expressedas a percentage of the nominal rmsvalue and may be adjustable.

Example: for a 100 kVA UPS, thevoltage 400 V rms ± 1% may beadjusted to ± 3%.

The standards also stipulate a ratedpeak output voltage and the tolerancewith respect to the rated value.

• Output frequency variation

The tolerance is expressed as apercentage of the rated frequency.

Example: for a 100 kVA UPS, 50 or60 Hz ± 0.1% during normaloperation on primary power and ± 0.5% in battery backup mode.

Frequency synchronisation with

primary power

The inverter supplies an output voltagewithin the previously mentionedtolerances, regardless of the disturb -ances affecting the upstream power.

To that end, the UPS:

• Monitors the voltage parameters(amplitude, frequency, phase) forthe primarypower source todetermine whether they arewithin specified tolerances,

• Reacts to any drift in parametersso as to:- readjust the inverter (phase and

frequency) to the standby power,as long as the drift remains with -in tolerances, in view of loadtransfer, if necessary,

- transfer the load to batterypower as soon as the drift goesoutside tolerances.

The new IGBT and PWM choppingtechnologies used in UPS allow anexcellent adaptation to thesevariations.

Example: for UPSs, the maximumvariation in frequencycorresponding to the tolerance is50 Hz x 0.5% = 0.25 Hz.

Frequency synchronisation withbypass AC power is possible from 0.25to 2 Hz, in 0.25 Hz steps. Practicallyspeaking, this signifies that frequencyvariations may be monitored at dF/dt= 0.25 Hz/s and readjustment carriedout within 0.25 to 1 second.





Dynamic characteristics

These are the tolerances undertransient load conditions.

Some UPSs are capable ofwithstanding the following conditions.

• Load unbalance

For unbalance in the load voltage(phase-to-neutral or phase-to-phase) of:

- 30%, the output voltagevariation is less than 0.1%,

- 100% (one phase at Pn and theothers at 0), the output voltagedoes not vary more than 0.2%.

• Load step changes (voltage

transients)

For load steps from 0 to 100% orfrom 100 to 0% of the rated load,the voltage does not vary morethan:

± 2% on utility power;

+ 2% to -4 % on battery power.

Overload and short-circuit capacity

• Overloads

- 1.1 In for 2 hours,

- 1.5 In for 1 minute,

with no change in the outputtolerances.

• Short-circuits

Beyond 1.65 In, inverters mayoperate in current-limiting modeup to 2.33 In for 1 second,corresponding to:

I peak max. = √2 x 1.65 In = 2.33 In.

Beyond this value, the invertertransfers the load to standby power orperforms a static shutdown (self-protection feature).

Total output-voltage distortion

UPSs must guarantee performancelevels for all types of loads, includingnon-linear loads.

Example: some UPSs limit thevoltage total harmonic distortion(THDU) in output power to thefollowing levels for:

• 100% linear loads:- THDU ph/ph < 1.5 %,- THDU ph/N < 2%,

• 100% non-linear loads:- THDU ph/ph < 2 %,- THDU ph/N < 3%.

General note.The standard specifiescertain of the previously mentionedperformance levels for output powerduring normal operation andoperation on backup power. Ingeneral, they are identical.

Summary diagram for maincharacteristics

Figure 75: Diagram showing the main characteristics (see the following list)..



Normal AC input:

• Voltage Un + 10% to - 15%,

• Frequency f + 4% to - 6%.

Bypass AC input:

• Voltage Un + 10% to - 15%,

• Frequency f + 4% to - 6%,

• Short-circuit current Isc2(withstand capacity of the staticbypass).

Rectifier/charger:

• Floating voltage,

• Input currents:- rated (battery float charging),- maximum (battery charging).

Battery:

• Backup time: standard 5, 6, 8, 10,15, 20, 30, 60 minutes, longertimes on request),

• Service life: 10 years or longer,

• Maximum current Ib max.

Inverter:

• Apparent output power:- rated: Sn (kVA),- load power: Su (kVA) = Sn x Tc%.

• UPS percent load Tc% = Su / Sn,

• Active output power:- rated: Pn (kW) = Sn (kVA) x 0.8,- load power: Pu (kW) = Su (kVA)

x PF = Sn x Tc% x PF = Un Iu PF,

• Efficiency: η Pu / Pn = 93%,

• Static characteristics (output-voltage tolerances understeady-state conditions):- amplitude: Un ± 1% adjustable

to ± 3%,- frequency: f ± 1% during normal

operation, f ± 0.5% in batterybackup mode,

- inverter output voltagesynchronised (frequency andphase) with that of the standbypower as long as the latter iswithin tolerances,



• Dynamic characteristics(tolerances under transientconditions):- maximum voltage and

frequency variations for loadstep changes from 0% to 100% or 100% to 0%: Un ± 2%, f ± 0.5%,

• Output voltage distortion:- 100% non-linear loads

THDU < 2%,

• Overload and short circuitcapacity:- overloads: 1.5 In for 1 minute,- short-circuits: current limiting

to 2.33 In for 1 second.

Load:

• Load current (Iu),

• Power factor PF.

UPS operating modesNormal mode (on utility power,see figure 76)

The UPS draws the AC utility powerrequired to operate via the rectifier/charger which provides DC current.

Part of the utility power drawn is usedto charge or float charge the battery:

• I1 floating, if the battery isalready fully charged,

• I1 charge if the battery is not fullycharged (i.e. charging following arecent discharge).

The remaining current is supplied tothe inverter with generates an output-voltage sine-wave within the specifiedamplitude and frequency tolerances.

Backup mode (on battery power,see figure 76)

The energy storage steps in to replaceprimary power and supplies thepower required by the inverter for theload, with the same tolerances as innormal mode.

This takes place through immediatetransfer (the energy storage is parallelconnected) in the event of:

• Normal AC-input failure (utility-power outage),

• Normal AC input outsidetolerances (degradation of utility-power voltage).

Figure 76: Normal mode and battery backup mode..

Bypass mode (on static-bypassline, see figure 77)

A static switch (SS) ensures no-breaktransfer of the load to the bypass ACinput for direct supply of the load bystandby power.

Transfer is automatic in the event of:

• An overload downstream of theUPS exceeding its overloadcapacity,

• An internal fault in the rectifier/charger and inverter modules.

Transfer always takes place forinternal faults, but otherwise ispossible only if the voltage of thestandby power is within tolerancesand in phase with the inverter.

To that end:

• The UPS synchronises theinverter output voltage with thatof the bypass line as long as thelatter is within tolerances.Transfer is then possible:



- without a break in the supply ofpower. Because the voltagesare in phase, the SCRs on thetwo channels of the staticswitch have zero voltage at thesame time,

- without disturbing the load. Theload is transferred to a bypassline that is within tolerances.

• When standby power is notwithin tolerances, the inverterdesynchronises and operatesautonomously with its ownfrequency. Transfer is disabled.

It can, however, by carried outmanually.

Note 1. This function greatlyincreases reliability due to the verysmall probability of a downstreamoverload and a standby-power failureoccurring at the same time.



Note 2. To ensure correct operation of the bypass line, discriminationmust be ensured between theprotection device upstream of thebypass AC input (on the MLVSoutgoer) and those on the UPSoutgoing circuits (see information on discrimination that follows).

Maintenance mode (on maintenance bypass, see figure 77)

Maintenance is possible withoutinterrupting load operation. The loadis supplied with standby power via themaintenance bypass. Transfer to themaintenance bypass is carried outusing manual switches.

The rectifier/charger, inverter andstatic switch are shut down andisolated from power sources. Thebattery is isolated by its protectioncircuit breaker.

Figure 77: Bypass mode and maintenance mode..

UPS configurationsParallel UPS with redundancy

“Types of possible configurations” isentirely devoted to a presentation ofthe various configurations. Below issome additional information onparallel connection for redundancy.

Configurations, see “Types ofpossible configurations”.

Types of parallel configurations

There are two types of parallelconfigurations.

• Integrated parallel UPS units

This upgradeable configurationcan be started using a single UPSunit with an integrated staticbypass and manual maintenancebypass. For configurations withmore than two UPS units, acommon maintenance bypass ishoused in an external cubicle(see figure 78).

• Parallel UPS units with a

centralised static-switch cubicle

(SSC)

The static-switch cubiclecomprises an automatic bypassand a maintenance bypass thatare common for a number ofUPS units without a bypass (see figure 79).

This configuration, lessupgradeable than the previousdue to the rating of the bypass,offers greater reliability (SSC andUPS units are independent).

• Modular UPSs

UPSs of the modular range aremade up of dedicated andredundant modules (power,intelligence, battery and bypass).

Modular design with plug-inpower modules improvesdependability, in particularmaintainability and availability,as well the upgradeability of theinstallation.

Redundancy

Redundancy in parallel configurationscan be N + 1, N + 2, etc.

This means that N UPS units arerequired to supply the load, but N + 1or N + 2 are installed and they allshare the load.

See the following example.

Example

• Consider a critical load with a 100 kVA rating,

• 2+1 redundancy:- 2 UPS units must be capable

of fully supplying the load ifredundancy is lost,

- each UPS unit must thereforehave a 50 kVA rating,

- 3 UPS units normally share the100 kVA load, i.e. each supplies33.3 kVA,

- the 3 UPS units normallyoperate at a percent load of33.3 / 50 = 66.6%,

- integrated parallel UPS unitsare each equipped with a staticbypass. Transfer is managedsuch that the three UPS unitstransfer to the bypasssimultaneously, if necessary.





Figure 78: Integrated parallel UPS units with common maintenance bypass.and 2+1 redundancy. Operation with all units OK (redundancy available).

• Loss of redundancy:- one UPS unit shuts down, the two

remaining units operate at 100%,- the faulty UPS unit can be serviced

due to the maintenance bypass.

Figure 79: Integrated parallel UPS units with common maintenance bypass.and 2 + 1 redundancy. Operation following loss of redundancy.

Electromagnetic compatibility (EMC)

ElectromagneticdisturbancesElectromagnetic disturbances

All electromagnetic disturbancesinvolve three elements.

A source

A natural source (atmosphere, earth,sun, etc.) or, more often, an industrialsource (electrical and electronicdevices).

The source generates disturbancesthrough sudden (pulse) variations inelectrical values (voltage or current),defined by:

• A wave form,

• A wave amplitude (peak value),

• A spectrum of frequencies,

• A level of energy.

A coupling mode

Coupling enables transmission ofdisturbances and may be:

• Capacitive (or galvanic), forexample via transformerwindings,

• Inductive, by a radiatingmagnetic field,

• Conducted, by a commonimpedance, via an earthingconnection.

A victim

This is any device likely to bedisturbed, and which malfunctionsdue to the presence of thedisturbances.



Examples

Sources

In low-voltage installations, sourcesinclude suddenly varying currentsresulting from:

• Faults or short-circuits,

• Electronic switching,

• High-order harmonics,

• Lightning or transformerbreakdown.

Frequencies may be low (< 1 MHz) for power frequencies and theirharmonics or high (> 1 MHz) forlightning.

Coupling

• Capacitive: transmission of alightning wave via a transformer.

• Inductive: radiation of amagnetic field created by one of the previously mentionedcurrents. Radiation creates aninduced electromotive force, thatis an induced disturbing current,in the loops of conductors madeup of the cables supplyingdevices and the earthingconductors of the devices.

As in indication, a radiation of 0.7 A/mcan disturb a video monitor.

That corresponds to the field created2.2 m around a conductor carrying acurrent of 10 A.

• Conducted (commonimpedance): increase in thepotential of an earthingconnection.



EMC standards andrecommendationsDisturbances

Emission, immunity, susceptibility

An electric device is installed in anenvironment that may be more or lessdisturbed electromagnetically. It mustbe seen as both a source and possiblevictim of electromagneticdisturbances.

Depending on the point of view, onmay speak of:

• The emission level for a source,

• The compatibility level for anenvironment,

• The immunity and susceptibilitylevels for a victim.

These notions are discussed on the next page in the section ondisturbance levels defined by thestandards.

Disturbance levels

Standard IEC 6100-2-4 defines anumber of disturbance levels for EMC:

• Level 0: no disturbance,

• Emission level: maximum levelauthorised for a user on a publicutility or for a device,

• Compatibility level: maximumdisturbance level expected in agiven environment,

• Immunity level: level ofdisturbance that a device canwithstand,

• Susceptibility level: level startingat which a device or systemmalfunctions. Consequently, fordevices and equipment that areconsidered:

- Sources, limits (emission

levels) must be set fordisturbances emitted bydevices to avoid reachingcompatibility levels,

- Victims, they must alsowithstand disturbance levelshigher than the compatibilitylevels, if they are exceeded,which is permissible on atransient basis. These higherlevels are the immunity levels.

EMC standards set these levels.

List of EMC standards, see thesection on page 146 on EMCstandards.

Figure 80: EMC disturbance levels for.disturbing/disturbed devices.

Measured values

Devices are subjected to tests.

Five major values are measured:• CE - conducted emissions,• RE - radiated emissions,• ESD - electrostatic discharges,• CS - conducted susceptibility,• RS - radiated susceptibility.

Figure 81: Five major measurement values..



The tests require major resources,namely a Faraday cage for conductedemissions and susceptibility and ananechoic chamber for radiatedemissions.



UPS standards

Scope and observance of standardsScope of standards

Standards cover the followingaspects:

• UPS design,

• Safety of persons,

• Performance levels,

• Electrical environment (notablyharmonic disturbances andEMC),

• Ecological environment.

Standards on UPS have become muchmore precise, notably with thecreation of the European EN standardsand their harmonisation with a part ofthe previously existing IEC standards.

Observance of standards andcertification

Observance of standards guaranteesthe reliability and the quality of a UPS, its compatibility with the loadssupplied as well as with the technical,human and natural environment.

Statement by a manufacturer ofconformity with standards is not, initself, a sufficient indication of quality.Only certification by recognisedorganisations is a true guarantee ofconformity.

To that end, performance levels ofUPSs from Caterpillar with respect to standards are certified byorganisations such as TÜV and Veritas.

CE marking

CE marking was created by Europeanlegislation.

It is mandatory for free circulation of goods in the EU.

Its purpose is to guarantee, throughrespect of the correspondingEuropean directives:

• That the product is notdangerous (Low-voltageDirective),

• That it does not pollute(Environment Directive) and itselectromagnetic compatibility(EMC Directive).

Before placing the CE marking on aproduct, the manufacturer must run or have run checks and tests whichensure conformity of the product withthe requirements in the applicabledirective(s).

It is NOT a certification standard ormark of conformity.

It does not signify that the productcomplies with national or internationalstandards.

It is not a certification as defined byFrench law (law dated 3 June 1994).

What is more, the CE marking isplaced on a product under theexclusive responsibility of themanufacturer or the importer. It doesnot imply inspection by a certifiedexternal organisation.

Not all labels carry the sameimplications for manufacturers.

Conformity with standards andspecified levels of performance mustbe certifiable by an organisation. Thisis not the case for CE marking whichauthorises self certification.

Main standards governingUPSUPS from Caterpillar comply (certifiedby TÜV and Veritas) with the mainapplicable international standards.

Safety• IEC 60950-1 / EN 60950-1

Information technologyequipment - Safety - Part:General requirements

• IEC 62040-1/ EN 62040-1

Uninterruptible power systems(UPS) - General and safetyrequirements for UPS.

• IEC 62040-3 / EN 1000-3

Uninterruptible power systems(UPS) - Method of specifying thetest and performancerequirements.

• IEC 60439

Low-voltage switchgear andcontrolgear assemblies.

• LV directive: 2006/95/EC

Electrical environment,harmonics and electromagneticcompatibility (EMC)

Harmonics

• IEC 61000-2-2 / EN 61000-2-2

Compatibility levels for low-frequency conducted disturbancesand signalling in public low-voltage power supply systems.(see table 22)

• IEC 61000-3-2 / EN 61000-3-2

Limits for harmonic currentemissions (equipment inputcurrent ≤ 16 A/ph).

• IEC 61000-3-4 / EN 61000-3-4

Limits for harmonic currentemissions (equipment inputcurrent > 16 A/ph).

• IEC 61000-3-5 / EN 61000-3-5

Limitation of voltage fluctuationsand flicker.

• EN 50160

Voltage characteristics of publicnetworks (see table 23).

• IEEE 519

Recommended practices andrequirements for harmoniccontrol in electrical powersystems.

EMC

• EN 50091-2

UPS - EMC.

• IEC 62040-2/ EN 62040-2

Uninterruptible power systems(UPS) - Electromagneticcompatibility (EMC)requirements.

• EMC Directive 2004/108/EC

For equipment liable to cause orbe affected by electromagneticdisturbances.

Quality

• Design , production and servicingin compliance with standard ISO9001 - quality organisation.

Ecological environment

• Manufacturing in compliancewith standard ISO 14001.

Acoustic noise

• ISO 3746

Sound power levels.

• ISO 7779 / EN 27779

Measurement of airborne noiseemitted by computer andbusiness equipment.





Tables on harmonic-compatibility levels

Table 22..

Table 23..

Compatibility levels for harmonic voltages according to the type of equipment

as indicated in standard EN 50160.

Order of the voltageharmonic generated

Class 1(sensitive systems and

equipment) % offundamental

Class 2 (1)

(industrial and publicnetworks) % offundamental

Class 1(for connection of

major polluters) % offundamental

2 2 2 3

3 3 5 6

4 1 1 1.5

5 3 6 8

6 0.5 0.5 1

7 3 5 7

8 0.5 0.5 1

9 1.5 1.5 2.5

10 0.5 0.5 1

11 3 3.5 5

12 0.2 0.2 1

13 3 3 4.5

TDHU 5% 8% 10%

(1) Class 2 corresponds to the limits of Table A of standard IEC 61000-2-2 / EN 61000-2-2

Compatibility levels for individual harmonic voltages in low voltage networks

as indicated in standard IEC 61000-2-2 / EN 61000-2-2.

Odd harmonics non-multiple of 3

Odd harmonics multiple of 3

Even harmonics

Harmonic ordern

Harmonicvoltage as a %of fundamental

Harmonic ordern


Harmonic ordern


5 6 3 5 2 2

7 5 9 1.5 4 1

11 3.5 15 0.3 6 0.5

13 3 21 0.2 8 0.5

17192325

>25

21.51.51.5

0.2 + 0.5 x 25/n

>21 0.2 1012

>12

0.50.50.20.2

Resulting THDU < 8% (for all harmonics encountered among those indicated)

Energy storage

Possible technologiesEnergy storage in UPS

The energy-storage systems used byUPSs to backup the primary sourcemust have the followingcharacteristics:

• Immediate availability ofelectrical power,

• Sufficient power rating to supplythe load,

• Sufficient backup time and/orcompatibility with systemsproviding long backup times (e.g. an engine generator set or fuel cells).



BatteriesThe battery solution

Batteries are by far the most commonlyemployed solution today for energystorage in UPSs. They are the dominantsolution due to low cost, proveneffectiveness and storage capacity, but nonetheless have a number ofdisadvantages in terms of size,maintenance and the environment. Atthe power ratings under consideration,they offer backup times in the ten-minute range, enough to ride throughlong outages and wait for start-up of anengine generator set for extendedruntime.

Electrochemical energy storageusing batteries, where applicablebacked up by a thermal enginegenerator set, is the commonlyused solution to protect criticalloads using a UPS.

Figure 82: Energy storage using a battery and an engine generator set for long backup times..



Types of industrial batteries

Battery families

A battery is a set of interconnected cells.

Depending on the type of cell, thereare two main families of batteries:

• Lead-acid batteries,

• Nickel cadmium batteries.

Cells may also be of the:

• Recombination type sealedbatteries.

The gas recombination rate is at least95% and they therefore do not requirewater to be added during service life(hence the term “sealed”),

• Vented type vented batteries

They are equipped with ports to:- release to the atmosphere the

oxygen and hydrogen producedduring the different chemicalreactions,

- top off the electrolyte by addingdistilled or demineralised water.

Batteries used in a UPS

The main types of batteries used inconjunction with UPSs are:

• Sealed lead-acid batteries, used95% of the time because they areeasy to maintain and do notrequire a special room, thesebatteries can be installed in officeenvironments and in any position,

• Vented lead-acid batteries,

• Vented nickel-cadmium batteries.Vented batteries impose greaterconstraints in terms ofmaintenance (checks on theelectrolyte level) and theirposition (only in the verticalposition).

Lithium-polymer batteries are currentlybeing studied for use in UPSs.

Battery selection, see page 54.

Installation modes

Depending on the UPS range, thebattery capacity and backup time, thebattery is:

• Sealed type and housed in theUPS cabinet,

• Sealed type and housed in one to three cabinets,

• Vented or sealed type and rack-mounted.

Cabinet mounting

This installation method (see figure 83)is suitable for sealed batteries. It is easyto implement and offers maximumsafety.

Batteries installed on racks:

• On shelves (figure 84). This install -ation method is possible forsealed batteries or maintenance-free vented batteries which do notrequire topping up of theirelectrolyte,

• Tier mounting (figure 85). Thisinstallation method is suitablefor all types of batteries and forvented batteries in particular, aslevel checking and filling aremade easy.

Figure 83: Cabinet mounting..

Figure 84: Mounting on shelves..

Figure 85: Tier mounting..

Constraints on batteriesAtmospheric constraints

The batteries supplied with UPSs aretypically designed to operate underthe following conditions:

• Optimum temperature range:15°C to 25°C,

• Optimum relative humidityrange: 5% to 95%,

• Atmospheric pressure: 700 to1060 hPa (0.7 to 1.06 bars).

For other operating conditions, pleaseconsult us.

Access

Access must be provided for testingoperations.

• Battery installed in UPS cabinetor other cabinet: comply with theclearances indicated in the“Dimensions and weights”.

• Battery installed on racks: selectan installation method suited tothe type of battery.

• Preliminary work: this aspect isimportant as it involves safety. It is discussed in page 58.

Main battery parameters

Backup time

For a given battery, the backup timedepends on:

• The power that must besupplied, a low value increasesthe available autonomy,

• The discharge conditions, a highdischarge rate makes possible alower shutdown voltage and thusincreases the backup time,

• Temperature, within therecommended operating limits,the backup time increases withincreasing temperature. Note,however, that a high temperatureadversely affects battery servicelife,

• Aging, battery backup timedecreases with the age of thebattery.

Service life

A battery is considered to reach theend of its service life when its realbackup time has fallen to 50% of the specified backup time.





Battery management

DigibatTM

To manage the previously mentionedparameters, all UPS from Caterpillarcome as standard with themicroprocessor-based DigibatTM

battery-monitoring system (dedicatedDSP for real-time processing).DigibatTM, an easy-to-use system, offersadvanced and flexible functions as wellas physical and computer-aidedprotection for the battery. It provides ahigh level of safety, true measurementof the backup time and optimisesbattery service life. Some functionsincluded are:

• Automatic entry of batteryparameters,

• Measurement of the real backuptime remaining, taking intoaccount the age of the battery, thetemperature and the load level,

• Estimate of remaining battery life,

• Battery test to preventivelydetect battery-function faults,

• Regulation of battery voltagewith respect to the temperatureto optimise battery life,

• Automatic battery-discharge testat adjustable time intervals.

The service life of a battery is basicallyenhanced by:

• Providing protection againstdeep discharge,

• Correct charger settings, inparticular the ripple factor of the charge or float current,

• An optimum operatingtemperature, maintainedbetween 15°C and 25°C.

Recharge mode

The charge cycle takes place in twosteps:

• Step 1, a constant current limitedto 0.1 C10 (one tenth of thebattery capacity for a ten-hourdischarge),

• Step 2, a constant voltage, at themaximum permissible value. The charge current regularlydecreases and reaches thefloating value.

Figure 86: Battery charge cycle..



Figure 87: Digibat TM.

Protection includes:

• Protection against deep discharge(depending on the discharge rate)and battery isolation using acircuit breaker which automati -cally opens when the backuptime, multiplied by two plus twohours, has elapsed,

• Limiting of the recharge current inthe battery (0.05 C10 to 0.1 C10),

• Progressive audio alarmsignalling the end of the backuptime,

• Numerous automatic tests.

Temperature monitoring

UPS can also be equipped with theTemperature Monitoring module usedto:

• Optimise the charger voltagedepending on the temperature in the battery room,

• Warn the user if presetpermissible temperature limitsare exceeded,

• Refine the estimate on batterybackup time carried out by thestandard system $.

Natural ventilation of battery cabinetsavoids battery temperature rise.

Environment Sensor is also a simplemeans to monitor temperature andhumidity. It can be used to launchshutdown when combined withsoftware running the module.



UPS / generator-set combinationUse of a generatorLong backup times

An engine generator set is made up of an internal-combustion enginedriving a generator that supplies thedistribution system. The backup timeof an enginegenerator set depends onthe quantity of fuel available.

In some installations, the requiredbackup time in the event of a utilityoutage is such that it is preferable touse an engine generator set to back up utility power (figure 88).

This solution avoids using largebatteries with very long backup times.

Though there is no general rule in thematter, a generator is often used forbackup times exceeding 30 minutes.Critical installations requiring veryhigh availability levels and with highdown-time costs (e.g. data centres)systematically combine UPS andengine generator sets.

The battery backup time of the UPSmust be sufficient for generator start-up and connection to the electricalinstallation. Connection is generallycarried out on the main LVswitchboard using an automaticsource-changeover system. The timerequired for changeover depends onthe specific characteristics of eachinstallation, notably the start-upsequence, load shedding, etc.

Figure 88: UPS / generator-set combination..



UPS / generator-setcompatibility

A number of factors must be takeninto account when using an enginegenerator set to provide long backup-time power to a UPS.

Load step changes

In the event of emergency conditionsrequiring connection of theinstallation to the generator set, heavy loads can result in high inrushcurrents which can cause seriousgenerator-set operating problems.

To avoid such phenomena, UPSs fromCaterpillar are equipped with a systemensuring gradual start-up of the charger.The walk-in lasts approximately tenseconds. What is more, when utilitypower returns, the charger may bestopped gradually via an auxiliaryswitch in order to avoid disturbing the other loads.

Figure 89: Gradual start of the UPS rectifier during operation on generator power..

Capacitive currents

The generator can supply onlyrelatively low capacitive currents (10 to 30 % of In). When an LC filter isinstalled, the main difficulty lies in thegradual start-up of the rectifier ongenerator power, when active power is equal to zero and the generatorsupplies only the capacitive current forthe filter. Consequently, the use of LCfilters must be correctly analysed toensure that operation complies withmanufacturer specifications.

Use of compensated LC filters with acontactor solves this problem. ForUPSs with a PFC rectifier, compatibilityis total.

LC filters and generators, see page 26.

Respective UPS and generator power

ratings

A UPS equipped with a PFC rectifierhas a high input power factor (greaterthan 0.9).

The engine generator set can thereforebe used to maximum effectiveness.

For LC filters, compensated filters with a contactor solve the problemconcerning capacitive currents.

Compatibility of power ratingsbetween modern UPSs andengine generator sets avoids all problems of derating.



Stability of generator frequency

During operation on engine generatorset power, fluctuation in the generatorfrequency may occur due to variationsin the speed of the thermal motor forwhich the regulation functions are notinstantaneous. These variations aredue to changes in the load. Examplesare start-up of the engine generatorset itself (until it reaches its ratedspeed), start-up of other loadssupplied by the engine generator set(elevators, air-conditioning systems),or shedding of loads.

This may create problems with a line-interactive UPS whose outputfrequency is identical to that of theinput. Generator frequency variationsmay lead to multiple transfers to theenergy storage (frequency outsidetolerances) and returns to input power(when the inverter has stabilised thefrequency, but the generator itself hasnot yet stabilised), resulting in“hunting” phenomena (instabilityaround the frequency setpoint).

With a double-conversion UPS, theregulation of the output power by theinverter avoids this problem.

Double-conversion UPS are totallycompatible with the frequencyfluctuations of engine generatorsets. This is lesser so with line-interactive UPS.

Harmonics

The subtransient reactance X"d of agenerator is generally higher than theshort-circuit voltage Uscx of atransformer (two to four times higher).Any harmonic currents drawn by theUPS rectifier may have greater impacton the voltage harmonic distortion on

the upstream busbars. With PFCrectifier technology, the absence ofupstream harmonics avoids thisproblem.

Review of inrush currents

On start-up, a number of loads causemajor inrush currents (switchingsurges, startup peaks), which last a certain time.

For the UPS, these currents representan apparent load Sa (kVA) that isgreater than Sn (kVA), which can be supplied under steady-stateconditions.

The value of Sa to be taken intoaccount in sizing UPS power iscalculated on the basis of these inrush currents.

Below are indications on thesecurrents caused by common loaddevices.

Motors

Motors are generally of the three-phase asynchronous type (95% of allmotors). The additional powerrequirement corresponds to the start-up current defined by (figure 90):

• Id (5 to 8 In, rated rms value) fora time td (1 to 10 seconds),

• Imax = 8 to 12 In, for 20 to 30milliseconds.

The power drawn that must be takeninto account (neglecting the peakeffect of Imax) is:

Sa (kVA) = Un Id √3 during td.

LV/LV transformers

Transformer switching produces currentpeaks with amplitudes that are damped



according to an exponential decaywith a time constant (see figure 91).

• i = I1st peak exp -t/τ where τ is afew cycles (30 to 300 ms),

• I1st peak = k In (where k is given,generally 10 to 20).

Indications generally include thenumber of cycles the phenomenonlasts and the value of the variouspeaks as a percentage of I1st peak.

The corresponding inrush current isgenerally calculated on the basis of(see example):

• Sa (kVA) = Un I1st peak √3 , i.e.Sa (kVA) = k Un In √3 during thenumber of cycles,

• Example of an inrush currentdamped in four cycles with:- 1st peak (100%): k In (k from

10 to 20),

- 2nd peak 30 %: 0.3 k In,

- 3rd peak 15 %: 0.15 k In.

The total of the rms values of thecurrents corresponding to the variouspeaks (Ipeak / √2) (1) is:

k ln (1 + 0,3 + 0,15) = K ln 1,45 ≈ k ln√2 √2

This is roughly equivalent to the valueof the first peak alone.

(1) Considering the current peaks as sine waves; note that somemanufacturers indicate an rms valueof Ipeak / 2.

Computer loadsSwitch-mode power supplies are non-linear loads. The current for asingle-phase load has a wave formsimilar to that shown in figure 95.There can be a peak during the first

half wave of approximately 2 In.However, it is generally much lowerthan this and can be neglected.

Figure 90: Curve for direct online starting.of a three-phase asynchronous motor.

Figure 91: LV/LV transformer..switching current.

Figure 92: Computer load starting current...



HarmonicsHarmonicsOrigin of harmonics

The increasing use of computing,telecommunications and power-electronics devices have multipliedthe number of non-linear loadsconnected to power systems.

These applications require switch-mode power supplies which transformthe voltage sine wave into periodicsignals of different wave forms. Allthese periodic signals of frequency fare the product of superimposedsinusoidal signals with frequenciesthat are multiples of f, known asharmonics (see the section“Characteristic harmonic values”dealing with the Fourier theorem onpage 159). Figure 93 illustrates thisshowing the initial current (thefundamental) and the third-orderharmonic.

Figure 93: Example of harmonics...

The increased presence of harmonicsis a phenomenon that concerns allelectrical installations, commercialand industrial, as well as residential.No modern electrical environment isexempt from these disturbancescaused by devices such as PCs,servers, fluorescent tubes, air-conditioners, variable-speed drives,discharge lamps, rectifiers, staticpower supplies, microwave ovens,televisions, halogen lamps, etc. Allthese loads are termed “non-linear”.

Consequences of harmonics

Harmonics disturb, increasinglyseverely, all sorts of activities, rangingfrom factories producing electroniccomponents and data-processingsystems to pumping stations,telecommunications systems,television studios, etc., because theyrepresent a significant part of thecurrent drawn.



There are three types of negativeconsequences for users:

Impact on the electrical installation

Harmonics increase the value of therms current with respect to that of the rated sinusoidal current. The result is temperature rise (sometimessignificant) in lines, transformers,generators, capacitors, cables, etc. The hidden costs of accelerated agingin such devices can be very high.

Impact on applications

Harmonic currents circulate in thesource and line impedances, thusgenerating voltage harmonics whichlead to voltage distortion on thebusbars upstream of the non-linearloads (figure 94).

The distortion of the supply voltage(upstream THDU - Total harmonicdistortion in voltage) may disturb theoperation of certain sensitive devicesconnected to the these busbars.

What is more, for TNC systems whereN and PE conductors are combined to form a PEN conductor, the zero-sequence third-order harmonicscumulate in the neutral conductor.This unbalance current in the neutralcan disturb circuits interconnectinglow-current devices and may requireoversizing of the neutral.

Figure 94: Voltage distortion due to reinjection of harmonic currents by non-linear loads.

Impact on the available electrical power

Harmonics represent an outright lossof current (up to 30% more currentconsumed). The user must pay morefor less available power.

Precautions

General

There are a number of traditionalsolutions to limit harmonics:

• Installation of tuned passive filters,

• Installation in parallel of severalcables with medium-sized crosssections,

• Separation of non-linear loadsand sensitive loads behindisolating transformers.

However, these solutions have twomajor disadvantages:

• Limitation of harmonics is effectiveonly in the existing installation (theaddition or removal of loads canrender it ineffective),

• Implementation is difficult inexisting installations.

Active harmonic conditioners (seepage 98) avoid these disadvantages.Much more effective than othersolutions, they may be used with alltypes of loads and can selectivelyeliminate harmonics ranging from the2nd to the 25th order.

UPS

• Due to the rectifier/charger, a UPSis a non-linear load for its powersource. UPS from Caterpillar offerperfect control over upstreamharmonics by using “clean” PFCrectifiers or filters.



Upstream of the UPS, the total voltagedistortion remains within limits thatare acceptable for the other devicesconnected to the same busbars.

Characteristic harmonicvaluesCurrent values

Harmonic expansion of a periodic

current

The Fourier theorem indicates thatany periodic function with a frequencyf may be represented as the sum ofterms (series) composed of:

• A sinusoidal term with frequencyf, called the fundamentalfrequency,

• Sinusoidal terms with frequenciesthat are whole multiples of thefundamental frequency, i.e. theharmonics,

• A DC component, whereapplicable.

Application of the Fourier theorem tothe currents of non-linear loadsindicates that a periodic current I(t), ofwhatever form at frequency f (50 or 60Hz), is the sum of harmonic sinusoidalcurrents defined by:

where

• IH1 is the rms value of thefundamental current at frequencyf (50 or 60 Hz),

• ω = 2 π f is the angular frequencyof the fundamental,

• φ1 is the phase displacementbetween the fundamental currentand the voltage,

∞

I(t) = IH1√2 sin(ωt + φ1) + Σ IHn√2 sin(nωt+φn)n=2

• IHn is the rms value of the nth

harmonic, at frequency nf,

• φn is the phase displacementbetween the nth harmonic currentand the voltage.

It is important to evaluate theharmonics (n ≥ 2) with regards to thefundamental (n = 1) to determine towhat degree the function differs fromthe fundamental.

To that end, the values shown next aretaken into account.

Current individual harmonic content

This value expresses the ratio inpercent between of the rms value ofthe given harmonic and that of thefundamental.

Ihn% = 100 IHnIH1

All the harmonics present in a givencurrent with the indication of theirrelative importance (Ihn values)constitute the harmonic spectrum ofthe current. Generally speaking, theinfluence of the orders above the 25this negligible.



Current total harmonic distortion

This distortion is called THDI (TotalHarmonic Distortion where I is for thecurrent). It expresses the ratio betweenthe rms value of all harmonics (n ≥ 2)and that of the fundamental. The THDIis also expressed in terms of theindividual harmonics.

THDI%=100 Σ IHn

2 n=2

∞

√

IH1 =100 = Σ

n=2

∞

√ IHn 2

IH1 Σ

n=2

∞

√ (Ihn%)2

Note. Harmonic contents aresometimes expressed with respect tothe complete signal Irms, and not thefundamental (IEC documents). Here,we use the definition of the CIGREE,which uses the fundamental.

For the low harmonic contentsanalysed in the following pages, the two definitions produce virtuallyidentical results.

Rms value of a current with harmonics

The rms value of an alternatingcurrent with a period T is:

After calculation and using harmonicrepresentation, this can be expressedas:

∞

Irms = Σ IHn2√n=1

where IHn = rms value of the nth

harmonic.

Irms = √ 1 T

T O ∫ l(t)2dt

The rms value is also expressed as:

• Ihn = Ihn% / 100 (individual levelexpressed as a value and not asa percentage),

• THDI = THDI% / 100 (distortionexpressed as a value and not asa percentage).

The rms value of the current is that ofthe fundamental, multiplied by acoefficient which is due to theharmonics and is a function of thedistortion.

One effect of harmonics istherefore to increase the rmsvalue of the current, which canlead to temperature rise andtherefore require oversizing ofconductors.

The lower the distortion, the less needfor oversizing.

Irms = IH12 + √ Σ IHn

2 n=2

∞ or:

Ieff = IH1 1+ √ hence: Σ n=2

∞

IHn 2

IH1

Irms = IH1 1+ √ Σ n=2

∞ Ihn2 = IH1 √ 1+THDI2



Example

Input current of a three-phase rectifier.

Figure 95: Example of the spectrum of a harmonic current..

The value under the square root signis:

332 + 2.72 + 7.32 + 1.62 + 2.62 + 1.12 +1.52 + 1.32 = 1164

consequently THDI% ≈ 34% and THDI =0.34.

Ieff = IH1√1 + THDI2 = IH1 √1 + 0.342 =1.056 x I1

The rms value of this current istherefore 5.6% higher than the rmsvalue of the fundamental, i.e. than the rated current containing noharmonics, with a correspondingtemperature rise.

THDI%= Σ n=2

∞

√ (Ihn%)2 Voltage values

At the terminals of a non-linear load,through which a distorted periodic ACcurrent flows, the voltage is alsoperiodic with a frequency f and it isalso distorted with respect to thetheoretical sinusoidal wave. Therelation between voltage and currentis no longer governed by Ohm's linearlaw, because it is applicable only forsinusoidal voltage and current. It ispossible, however, to use a Fourierexpansion for the voltage and todefine, similar to the current and withthe same results, the following values:

Voltage individual harmonic content

UHn% = 100UHnUh1

The harmonic spectrum can also becalculated for the voltage.



THDU%=100 Σ UHn

2 n=2

∞

√

IH1

=100 = Σ n=2

∞

√ UHn 2

UH1 Σ

n=2

∞

√ (Uhn)2

Voltage total harmonic distortion

THDU for Total Harmonic Distortion,where U is for the voltage.

Rms value of a voltage with harmonics

Which, similar to the current, can alsobe expressed as:

The rms value of the voltage is that of the fundamental,multiplied by a coefficient which is due to the harmonics.

Power values

Power factor in the presence of

harmonics

On the basis of the active power at theterminals of a non-linear load P (kW)and the apparent power supplied S(kVA), the power factor is defined by:

λ = P(kW)S(kVA)

This power factor does not expressthe phase displacement between thevoltage and the current because theyare not sinusoidal. However, it ispossible to define the displacementbetween the voltage fundamental and the current fundamental (bothsinusoidal), by:

cos φ1 = P1(kW)S1(kVA)

Urms =UH1 1+ √ ΣUhn2= IH1√ 1+THDU2

n=2

∞

Irms = √ ΣIHn2

n=1

∞

where P1 and S1 are the active and reactive power, respectively,corresponding to the fundamentals.

Standard IEC 146-1 defines thedistortion factor:

ν = λcos φ1

When there are no harmonics, thisfactor is equal to 1 and the powerfactor is simply the cos φ.

Power in the presence of harmonics

• Across the terminals of abalanced, three-phase linear

load, supplied with a phase-to-phase voltage u(t) and a currentI(t), where the displacementbetween u and i is φ, the apparentpower in kVA, depending on therms values U and I, is:

S = UI√3

The active power in kW is: P = S cos φ

The reactive power in kvar is: Q = Ssin φ

Where:

S = √P2 + Q2

• At the terminals of a non-linear

load, the mathematical definitionof P is much more complexbecause U and I containharmonics. It can however be expressed simply as:

P = S λ (λ = power factor)



If U1 and I1 are the fundamentalsdisplaced by φ1, it is possible tocalculate the corresponding apparent,active and reactive power by:

S1 = U1I1√3 P1 = S1 cos φ1 and Q1 =S1 sin φ1. The total apparent power is:

S = √P12 + Q12 + D2

where D is the distortion power, dueto the harmonics.



Non-linear loads and PWM technology

Non-linear loadperformance of UPS usingPWM technologyImportance of the UPS outputimpedance

Equivalent diagram of an inverter

output

With respect to the load, an inverter isa perfect source of sinusoidal voltageV0 in series with an output impedanceZs. Figure 96 shows the equivalentdiagram of the inverter output when a load is present.

Figure 96: Equivalent diagram of an inverter output..

Effects of different load types

• For a linear load, the impedancesZs, ZL, Zc are considered at theangular frequency ω = 2 π fcorresponding to the distributionfrequency (f = 50 or 60 Hz),giving V0 = (Zs + ZL + Zc) I,

• For a non-linear load, theharmonic currents drawn by theload flow through theimpedances. For the fundamentaland each individual harmonic, therms values of the current and thevoltage are related similarly andcan be expressed as:

- for the fundamental: U1 = (Zs + ZL + Zc) I1

- for each harmonic order k: UK = [Zs(kf) + ZL(kf) + Zc(kf)] IK



The impedance values are consideredat the frequency kf of the given order.

Voltage distortion decreases with theindividual levels of the voltageharmonics UK / U1.

These levels are related to those of theharmonic currents IK/ I1 by the equation:

[Zs(kf) + ZL(kf) + Zc(kf)] / (Zs + ZL + Zc).

Consequently, for a given loadcurrent spectrum, the individualvoltage harmonic levels and thetotal distortion (THDU) decreasewith the impedance of the sourceand the cables at the givenfrequencies.

Consequences of non-linear loads

To reduce the effects of the harmoniccurrents (THDU at B and C), it isnecessary, to the greatest extentpossible, to:

• reduce the line impedance,

• ensure a low source impedance atthe various harmonic frequencies.

Good behaviour on the part of aUPS supplying non-linear loadsrequires a low output impedance atthe various harmonic frequencies.

Below is a presentation of theadvantages of the PWM (pulse widthmodulation) chopping technique in thisrespect.

UPS operating principle

Chopping of the DC voltage by the

inverter with filtering

An inverter is made up of a converterthat transforms the DC power suppliedby the rectifier/charger or the batteryinto AC power. For example, on asingle-phase UPS, there are two waysto convert the DC power, using either a half bridge (see figure 97) or a fullbridge (see figure 98).

The square-wave voltage obtainedbetween A and B is then filtered toproduce a sinusoidal voltage with alow level of distortion at the output.



Figure 97: Half-bridge DC/AC converter. Figure 98: Full-bridge DC/AC converter..

Practically speaking, the switchesshown in figures 97 and 98 are IGBTsfor which it is possible to control therelative on and off times.

By controlling the on and off times, itis possible to “distribute” the voltageover the reference sinusoidal wave.This principle is known as PWM (pulsewidth modulation). It is shown in asimplified manner, with five square-wave pulses, in figure 99. The area ofthe voltage sinusoidal wave is equalto that of the square-wave pulses usedto generate it. These areas representthe power supplied by the inverter tothe load

T

over a given time, i.e. ∫ VIdt0

The higher the chopping frequency(the higher the number of square-wave pulses), the better the regulationwith respect to the reference wave.Chopping also reduces the size of theinternal filter required on the LCoutput (see figure 100).

Figure 99: DC/AC converter output voltage with.five square-wave pulses per half-wave.



Figure 100: Inverter output filter..

PWM inverters

PWM chopping

The PWM (pulse width modulation)chopping technique combines high-

frequency chopping (a few kHz) of the DC voltage by the inverter andregulation of the pulse width for theinverter output, to comply with areference sinusoidal wave.

This technique uses IGBTs (insulatedgate bipolar transistors) offering theadvantages of voltage control andvery short commutation times. Due to the high frequency, the regulationsystem can react quickly (e.g. 333nanoseconds for a frequency of 3 kHz)to modify the pulse widths within agiven period.

Comparison with the referencevoltage wave makes it possible to maintain the inverter outputvoltage within strict distortiontolerances, even for highlydistorted currents.

Functional diagram of a PWM inverter

Figure 101 shows the functionaldiagram of a PWM inverter.

The output voltage is continuouslycompared to the reference voltageVref which is a sinusoidal wave with a very low level of distortion (< 1%).

The difference in the voltage ε isprocessed by a corrector, according to a transfer function C(p), intended toensure the performance and stabilityof control.

The voltage from the corrector is thenamplified by the DC/AC converter andits control system with a gain A. TheVm voltage supplied by the converteris filtered by the LC filter to supply theoutput voltage Vs.

Practically speaking, it is necessary to take into account the impedance ofthe output transformer when it exists,to obtain the total inductance L. Often,the inductance is built into the trans -former, which is why it is not includedin diagrams.

Figure 101: Functional diagram of a PWM inverter..



Output impedance of a PWM inverter

It is possible to represent the aboveDC/AC converter and filter as a seriesimpedance Z1 and a parallelimpedance Z2 (see the left-hand side of figure 102).

The diagram can be modified todisplay the output impedance Zs.

Figure 102: Equivalent diagram of an inverter as seen from the output..

The ratio Z2 is the transfer

Z1 + Z2

function of the filter, noted H(p).

To simplify, C(p) x A is replaced byμ(p) which represents the transferfunction of the correction andamplification.

The equivalent diagram (right-handside of figure 110) shows:

• V'm = voltage measured underno-load conditions, i.e.:

V'm = Vm Z2

Z1 + Z2

• Zs = impedance measured at theoutput with V'm short-circuited, i.e.:

Zs = Z1Z2

Z1 + Z2

Figure 103: Transformed functional diagram of a PWM-chopping inverter equipped.with an output-voltage regulation system with modulated chopping frequency.



It is possible to show that the inverteroutput impedance Zs in this case isequal to:

Z' s ≈Z1

μ (p)

This means that in the regulation passband, the inverter output impedance isequal to the filter series impedancedivided by the correction andamplification gain.

Given the high gain in the regulationpass band, the output impedance issignificantly reduced compared toimpedance Z1 of an inverter withoutthis type of regulation.

Outside the regulation pass band, theinverter output impedance is equal tothat of the filter, but remains lowbecause it corresponds to theimpedance of a highfrequencycapacitor.

Consequently, the output impedanceis a function of the frequency (seefigure 104).

The free-frequency PWM (pulsewidth modulation) techniqueconsiderably limits the outputimpedance.

Comparison of differentsourcesOutput impedance of varioussources

The curves in figure 104 show theoutput impedances for varioussources with equal output ratings as a function of the AC frequency. Theimpedances are plotted as a percentof the load impedance Zc.

• Transformers and generators -the curve is a straight linecorresponding to the effect of the inductance L (the term whichrapidly becomes dominant in thereactance with respect to theresistance and which increaseslinearly as a function of thefrequency).

• Modern inverters implementingthe PWM chopping techniquewith modulated choppingfrequency - at all harmonicfrequencies, the Zs/Zc ratio is:- less than that noted for other

sources,- low and virtually constant.

Conclusion

The PWM inverter is the sourceoffering by far the lowest outputimpedance in the presence ofharmonics. It is clearly the best sourceon the market in terms of its aptitudeto minimise the voltage distortioncaused by non-linear loads. It is five to six times better than a transformerwith an identical power rating.

The new generation of UPSimplementing IGBTs and thePWM chopping technique withfrequency modulation are the bestsources of sinusoidal voltage,whatever the type of currentdrawn by the load.



Figure 104: Output impedance of different sources depending on the frequency..

Free-frequency chopping

The chopping fronts do not necessarilyoccur at fixed intervals. Choppingadapts to the requirements of theregulation, i.e. the rate of change ofthe reference. The width of thecommutation fronts decreases (thechopping frequency increases) as therate of change of the reference sinewave increases. Conversely, the widthof the commutation fronts increases(the chopping frequency decreases) as the rate of change of the referencedecreases. On the whole, the averagechopping frequency is the same asthat for the fixed-frequency technique(approximately 3 kHz). But regulationis better because the commutationaccelerates in the zones where the rateof change is high (see figure 106).

Free-frequency choppingFree-frequency chopping

Free frequency is an improvement tothe PWM technique.

PWM chopping can use either of twotechniques (figure 105).

Fixed-frequency chopping

The chopping fronts occur at fixed,regular intervals corresponding to thechoppingfrequency over one period.

The width of the pulses (square-wavepulses) can be modulated to conformto the reference within the fixed timeinterval.

The two sine waves shown in thediagram correspond to the tolerance (< 1%) around the reference sine wave.



Figure 105: PWM chopping with fixed-frequency and free-frequency regulation..

Figure 106: Regulation employing free-frequency commutation..

It can reach eight commutations permillisecond, i.e. a regulation time aslow as 125 nanoseconds (compared to 300 ns for the fixed-frequencytechnique).

The free-frequency techniqueincreases the precision of thevoltage regulation in PWMinverters compared to the fixed-frequency technique.



PFC Rectifiers

Standard and PFC rectifiersUPS units draw power from the AC distribution system via arectifier/charger. With respect to theupstream system, the rectifier is anon-linear load drawing harmonics. In terms of harmonics, there are twotypes of rectifiers.

Standard rectifiers

These are three-phase rectifiersincorporating SCRs and using a six-phase bridge with standard choppingof the current.

This type of bridge draws harmoniccurrents with orders of n = 6 k ± 1(where k is a whole number), mainlyH5 and H7, and to a lesser degree H11and H13.

Harmonics are controlled by using a filter.

“Clean” PFC (Power FactorCorrection) rectifier

This type of rectifier comprises built-inIGBTs and a regulation system thatadjusts the input voltage and currentto a reference sine wave. Thistechnique ensures an input voltageand current that are:

• Perfectly sinusoidal, i.e. free ofharmonics,

• In phase, i.e. an input powerfactor close to 1.

With this type of rectifier, no filters are required.

PFC rectifiers

Operating principle

The principle behind PFC rectifiersconsists in forcing the current drawnto remain sinusoidal. To that end, theyuse the PWM technique presentedearlier.

The principle is that of a “voltagesource” converter (see figure 107),whereas the active harmonicconditioner uses a “current source”converter.

The converter acts as a back-electromotive force (a “sinusoidalvoltage generator”) on the distributionsystem and the sinusoidal current isobtained by inserting an inductorbetween the utility power and thevoltage source.

Even if other non-linear loads increase the voltage distortion on thedistribution system, the regulation canadapt to draw a sinusoidal current.

The frequency of low residualharmonic currents is the frequency ofthe modulation and of its multiples.Frequency depends on thepossibilities of the semiconductorsused.



Figure 107: Operating principle of a clean “voltage generator” converter..

Implementation

Single-phase rectifier

Figure 108 shows the operation of a single-phase rectifier.

Voltage modulation is obtained by acontroller that forces the current tofollow a sinusoidal current reference.

Transistor T and diode D make up thevoltage modulator. The voltage u thuschanges between 0 and Vs accordingto whether transistor T is in the on oroff state. When transistor T conducts,the current in inductor L can onlyincrease as the voltage is positive andu = 0.

Therefore:

di=e>0dt L

When transistor T is off, the current in L decreases, provided that Vs isgreater than V, so that:

di=e-VS>0dt L

For this condition to be fulfilled,voltage Vs must be greater than thepeak voltage of V, i.e. the rms value ofthe AC voltage multiplied by √2.

If this condition is fulfilled, the currentin L can be increased or decreased atany time. The variation of the currentin L with time can be forced bymonitoring the respective on and offtimes of transistor T. Figure 109 showsthe evolution of current IL with respectto a reference value.



Figure 108: Diagram of a clean, single-phase rectifier drawing a sinusoidal signal..

Figure 109: Evolution of current IL with respect to the reference..



Figure 110: Diagram of a clean, three-phase rectifier drawing a sinusoidal signal..

Three-phase rectifier/charger

The basic circuit arrangement isshown in figure 110. It is similar to thatin figure 108, with the inductor placedupstream of the rectifiers; theoperating principle is also the same.The monitoring system controls eachpower leg and forces the currentdrawn on each phase to follow thesinusoidal reference.



Glossary and bibliography

GlossaryActive harmonic conditioner

Active harmonic conditioners (AHC)are used to eliminate the harmoniccurrents flowing in an electricalinstallation and consequently limitvoltage and current distortion (THDUand THDI respectively) to a givenpercentage. The conditionercontinuously analyses the harmoniccurrent drawn by the load and injects,on a realtime basis, an identicalcurrent with the appropriate phase.The current supplied by the sourceremains virtually sinusoidal, whateverthe operating conditions. Theconditioner automatically adapts tochanges in the installation and coversthe entire low-frequency harmonicspectrum (H2 through to H25). Activeharmonic conditioners are also calledactive filters.

ANSI (American National Standards

Institute)

U.S. organisation in charge ofstandardisation. Traditionally, it isassisted in this task by scientificorganisations such as the IEEE (Instituteof Electronics and Electrical Engineers).

Availability of an electrical installation

Availability is the probability that the installation will be capable ofsupplying energy with the level ofquality required by the supplied loads.

Availability (%) = (1-MTTR

) x 100MTBF

Practically speaking, the lower theMTTR (fast repair) and the higher theMTBF (time without failure), thehigher the availability.

Backup time

Time during which the UPS cansupply the rated load with power fromits energy storage under nominalconditions when the normal ACsource fails. This time depends on thebattery. Typical backup times are 6, 8,10, 15 or 30 minutes.

Battery circuit breaker

DC circuit breaker that protects thebattery circuit of a UPS.

Battery, recombination

Battery with a gas recombination rateat least equal to 95%. No water needbe added over battery life, which iswhy such batteries are commonlyreferred to as “maintenance free”batteries.

BMS (Building Management System)

System used to control and monitorall building utilities and systems froma central location. It is generallycomposed of sensors, actuators andprogrammable controllers connectedto a central computer (or severalcomputers) equipped with specificsoftware.

Charger

Device associated with the rectifierand used to supply the battery withthe electrical power (DC current)required to recharge and/or floatcharge the battery, thus ensuring theavailability of backup power.

Cos φA measure of the phase displacementbetween the current wave and thevoltage wave observed at theterminals of a linear load.



Cos φ1

A measure of the phase displacementbetween the fundamental currentwave and the fundamental voltagewave observed at the terminals of anon-linear load.

Crest factor (Fc)

The ratio between the peak value of acurrent and its rms value.

Fc = Ipeak

Irms

Discrimination

System whereby a fault trips theprotection device of the faulty loadcircuit only. Protection devices onneighbouring circuits and upstreamare not tripped.

Distortion factor (ν)

Factor measuring the effect ofharmonics on the power factor at theterminals of a load supplied with ACpower.

ν = λ

cosφ1

λ : power factorcos φ1 : cos φ of the fundamental

EMC (Electromagnetic compatibility)

Possibility of a device to operatenormally when installed near otherdevices, given the disturbancesemitted by each device and theirmutual sensitivities.

EN (European Normalisation)

Label used for European standards.These standards are issued byCENELEC. Following acceptance bythe member countries, thesestandards enter into force and replacethe national standards.

Fault tolerance

A fault-tolerant system can continueto operate following a fault, but in adown-graded mode.

Down-graded operation is generallyaccompanied by an alarm to signalthe fault(s). It is generally possible torepair the system rapidly and return tonormal operation, without shuttingdown the system. UPS operation onthe static bypass is a type of fault-tolerant operation.

Float current

DC current that maintains the batteryat nominal charge, corresponding tothe float voltage. This currentcompensates for open-circuit losses.

Float voltage

DC voltage applied to the battery tomaintain its charge level. This voltagedepends on the type of battery, thenumber of cells and themanufacturer’s recommendations.

Fourier theorem

Theorem stating that any non-sinusoidal periodic function (offrequency f) may be represented as asum of terms (series) made up of:

• A sinusoidal term with frequencyf, called the fundamentalfrequency,

• Sinusoidal terms withfrequencies that are whole-number multiples of thefundamental frequency, i.e. the harmonics,

• A DC component, whereapplicable.

The series may be expressed, where nis a whole number, as:

∞

Y(t)=Y0 + ΣYn √2 sin (nωt + φn)n=1

n = 1 corresponds to the fundamental,n > 1 corresponds to the nth harmonic.

Free-frequency chopping

Chopping technique where thefrequency increases or decreasesdepending on the variation of areference value. Contrary to fixed-frequency chopping, this techniqueincreases regulation during majorvariations and reduces it whenvariation is low. This improvesregulation with respect to thereference value.

Harmonic

Sinusoidal term of the Fourier seriesexpansion of a periodic function.

The harmonic (or harmoniccomponent) of the nth order ischaracterised by:

Hn(t) = Hn√2 sin(nωt + φn)

Hn is the rms value of the givenharmonic component,

• ω is the angular frequency of thefundamental, related to thefundamental frequency by ω = 2π f,

• φn is the phase displacement ofthe given harmonic componentat t = 0.

Harmonic distortion, individual

Ratio between the rms value of an nth

order harmonic and the rms value ofthe fundamental.

Hn% = 100 YnY1

Harmonic distortion, total (THD)

Ratio between the rms value of allharmonics of a non-sinusoidalalternating periodic value and that of the fundamental.

∞

This value may also be expressed as afunction of the individual distortion ofeach harmonic Hn = Yn /Y1 by:

∞

For current and voltage, these valuesare called THDI and THDU respectively.

Harmonics, current and voltage

Any periodic current (frequency f) that is not sinusoidal is made up of a set ofsinusoidal currents (see Fourier),including a fundamental (frequency f)and harmonics at various frequencies nf(where n is a whole number). A voltageharmonic corresponds to each currentharmonic. The instantaneous and rmsvalues are related by Ohm's law, wherethe terms are both sinusoidal.

If Zsn is the voltage source outputimpedance at frequency nf (angularfrequency nω), then Un = Zsn x In.Consequently, for each currentharmonic, there is a voltage harmonicthat depends on the source outputimpedance at the correspondingfrequency.

D%=100 Σ Hn2

n=2

∞

√

D%=100 Σ Yn

2

n=2

∞

√Y1





HF interference

High-frequency parasitic current thatis either conducted (electrostaticorigin) or radiated (electromagneticorigin) by a device.

(IEEE) Institute of Electrical and

Electronic Engineers

Assists ANSI (American NationalStandards Institute) in definingstandards for electric and electronicequipment.

IIK

A protection index indicating thedegree of protection againstmechanical shocks as defined byEuropean standard EN 50102. The IK code includes 11 values from IK01to IK10, corresponding to differentenergy levels expressed in Joules.This code is complementary to the IP code.

Inrush current

Transient currents observed in anelectrical installation when devices are energised. These currents aregenerally due to the magnetic circuitsof the devices. The effect is measuredby the current’s maximum peak valueand the rms current value it generatesduring the time it lasts.

Inverter

UPS subassembly that recomposes a sine-wave output (regulated andwithout breaks) using the DC currentsupplied by the rectifier/charger or thebattery. The main elements of theinverter are the DC/AC converter, aregulation system and an output filter.

IP (International Protection)

A protection index defining the abilityof electrical equipment to withstandcertain environmental conditions. It iscomposed of two digits (e.g. IP 20)defined by standard IEC 529 andincluded in standard EN60529. Eachdigit corresponds to a certain degreeof protection with respect to a givenexternal influence.

• First digit (0 to 6): degree ofprotection against penetration of solid bodies,

• Second digit (0 to 7): degree ofprotection against penetration ofliquids,

• Additional letter (A to D): safetyof persons.

The IP code may receive an additionalletter (A to D) when the protectionprovided persons against dangerousparts is better than that indicated bythe first digit. A - protection againstaccess by the back of the hand, B -protection against access using afinger, C - using a tool with a diameterof 2.5 mm, D - using a tool with adiameter of 1 mm. When theprotection of persons is the onlyrelevant factor, the two IP digits maybe replaced by “X” (e.g. IP XXB).

Example. IP 30D

3 = protection against solid bodieslarger than 2.5 mm.

0 = no protection against water.

D = protection against access using a tool with a diameter of 1 mm.

ISO 9000

Standard defining procedures and systems used to attain aninternationally recognised level of production quality. ISO 9000certification is recognition that thequality system effectively complieswith the standard. Certification iscarried out by an official organisation(e.g. AFAQ), unaffiliated with eitherclients or suppliers or the companyitself. The certificate is valid for athree-year period with yearly auditsand checks.

IT system

System earthing arrangement inwhich the neutral is isolated from theearth or connected to the earth via ahigh impedance and the variousexposed conductive parts areconnected to the earth via individualearthing circuits. An alarm (generallyan insulation-monitoring device IMD)must signal the appearance of a firstinsulation fault.

The installation must be de-energizedimmediately in the event of a secondinsulation fault.

Load, linear

Load for which the input voltage andcurrent are both sinusoidal, withpossible phase displacement(inductive and/or capacitive loads).Linear loads include only resistances,inductors or capacitors.

The Ohm law applies to both theinstantaneous and the rms values. U = Z I, where Z is the equivalentimpedance of the load (constantduring each period).

Examples of linear loads: lightingsystems, motors, transformers.

Load, non-linear

Load drawing an input current that isperiodical, but not sinusoidal, with aharmonic component. For this reason,the input voltage is also distorted byharmonics. Generally speaking, non-linear loads comprise active electroniccomponents that vary the loadimpedance over each period. The Ohmlaw applies to the instantaneousvalues, but the equivalent impedanceof the load is variable. As a result,there is no simple law for the rmsvalues, as is the case for linear loads.Examples of non-linear loads: switch-mode power supplies for computers,rectifier bridges using SCRs, variable-speed drives, fluorescent lighting.

Load power

Apparent power Su (kVA) that a UPSinverter supplies under given loadconditions. It is less than or equal tothe rated output Sn (kVA).

The ratio Su/Sn defines the percentload of the inverter.

Magnetic-susceptibility level

Level of electromagnetic emissionstarting at which a nearby device orsystem malfunctions.

Management-Pac™ (software)

Intended for network administrators,this totally SNMP-compatible softwarecan manage and supervise an entirepark of UPS.

Micro-outage

Total absence of power for a durationof less than one half cycle (< 10 ms at50 Hz).

MLVS (Main low-voltage switchboard)

The low-voltage switchgear assemblyused to distribute power immediatelydownstream of the HV/LV transformer.





MTBF (Mean Time Between Failures)

Expected value of the duration(expressed in hours) of normaloperation of a repairable devicebetween failures. The MTBF is anindication on the reliability of a device.

MTTF (Mean Time To Failure)

Expected value of the duration(expressed in hours) of normaloperation of a non-repairable device(i.e. one for which an MTBF cannot becalculated). The MTTF is an indicationon the reliability of a device.

MTTR (Mean Time To Repair)

Expected value (or statistical averageif available) of the time required torepair a device. This includes the timerequired to detect the cause of thefailure, repair it and start the systemup again.

Noise level

Acoustic decibel level (dBA) represent ing the sound power of asource measured according tostandard ISO 3746.

Off-line

A UPS where the inverter is off duringnormal mode.

On-line

A UPS where the invert is on innormal mode.

Percent load

The ratio Su (kVA) / Sn (kVA) betweenthe load power Su and the ratedpower Sn of a UPS.

PFC (Power Factor Correction) (rectifier)

PFC is an electronic regulation devicefor the UPS input rectifier thatmaintains the input current sinusoidaland in phase with the utility voltage. It avoids drawing harmonic currentsupstream of the rectifier and thus theneed for a filter.

Power factor (λ)

Ratio between the active power P (kW)supplied to a load and the apparentpower S (kVA) supplied to said load by an AC power supply.

λ = PS

Power, primary

Power normally continuouslyavailable which is usually supplied by an electrical utility company, butsometimes by the user's owngeneration. Primary power isconnected to the normal AC input of the UPS.

Power, rated

Apparent power Sn (kVA) that a UPScan deliver under given loadconditions defined for cos φ = 0.8.

Power, standby

Power intended to replace the primarypower in the event of primary-powerfailure. When standby power isavailable, it is connected to the bypassAC input of the UPS.

PWM (Pulse Width Modulation)

A high-frequency chopping techniquefor UPS inverters using a means ofregulation enabling rapid modificationof pulse widths over a single period. It is thus possible to maintain theinverter output voltage withintolerances, even for non-linear loads.

Rectifier/charger

UPS component that draws utilitypower to supply the inverter and tofloat charge or recharge the battery.The alternating input current isrectified and then distributed to theinverter and the battery.

Redundancy, active redundancy

N + 1, N + 2, etc.

Parallel UPS configuration in whichseveral UPS units (N + 1, N + 2, etc.)with equal outputs are parallelconnected and share the load. In theevent one UPS unit (N + 1redundancy) or more fail (N + 2, N + 3,etc.), the other units pick up its sharewithout any interruption in the supplyof power to the load. The remainingunits are sufficient to continuesupplying the load as long as thereare at least N units.

Redundancy, isolated

UPS configuration in which one orseveral UPS units operate on stand-by, with no load or only a partial load,and can immediately back up a faultyUPS unit by no-break transfer of theload, carried out by a static switch.

Reliability

Probability that a device willaccomplish a required function undergiven conditions over a given periodof time.

Short-circuit voltage of a transformer

(Uscx %)

Relative measurement (%) of theinternal impedance of a transformer.This short-circuit impedance iscommonly called the short-circuitvoltage because it is measured duringa short-circuit test (shorted secondarywinding subjected to a current set toIn). For most common three-phasetransformers, the value rangesbetween 3 and 6%.

Source impedance

It is possible to consider that a load is supplied by a perfect voltagegenerator Uo, in series with aninternal impedance Zs, where:

• Uo is the voltage measuredacross the load terminals, if theload is equal to zero (loadterminals in an open circuit),

• Zs is the source impedance, i.e.the equivalent impedance asseen from the load terminals(open circuit), obtained by short-circuiting the upstream voltagegenerator(s).

Static switch

Power-electronics device that can beused to switch from one source toanother without interruption in thesupply of power. In a UPS, transfer isfrom normal AC power to bypass ACpower and back. Transfer withoutinterruption is possible due to the factthat there are no mechanical parts andthe ultra-fast switching capabilities ofthe electronic components.





Static Transfer Switch (STS)

An STS carries out transfer,automatically or manually, of one or more three-phase loads, from apreferred source to an alternate orreserve source without interruption. If the preferred source fails, transfer is automatic.

Subtransient reactance of a generator

set (Uscx %)

Relative measurement (%) of theinternal impedance of an AC generatorduring harmonic phenomena. Thisreactance, also called the longitudinalsubtransient reactance of thegenerator, is sometimes identified asX"d.

For most common generators, thevalue ranges between 15 and 20%. It can drop to 12% for optimisedsystems and to 6% for special devices.

System earthing arrangements (SEA)

Standardised system for theinterconnection and earthing ofexposed conductive parts and theneutral of a low-voltage electricalinstallation. There are threestandardised arrangements:

• TN system, with the TN-C and TN-S versions (exposed conductiveparts connected to the neutral),

• TT system (earthed neutral),

• IT system (isolated neutral).

THDI

THD for Total Harmonic Distortion andI for current. This is the ratio betweenthe rms value of current harmonicsand the rms value of the fundamental.

∞

This value may also be expressed interms of the individual harmonics, e.g.Ihn = In / I1 using the equation:

∞

THDU

THD for Total Harmonic Distortion andU for voltage. This is the ratio betweenthe rms value of the voltageharmonics and the rms value of thefundamental.

∞

This value may also be expressed interms of the individual harmonics, e.g.Uhn = Un / U1 using the equation:

∞

Tolerances (%)

Permissible limits to the variation of aquantity around its nominal or ratedvalue, expressed as a percentage.

THDI%=100 Σ In

2

n=2

∞

√I1

THDI%=100 Σ Ihn2

n=2

∞

√

THDU%=100 Σ Uhn2

n=2

∞

√

THDU%=100 Σ U2

n=2

∞

√U1

TN system

System earthing arrangement inwhich the exposed conductive partsare interconnected and connected tothe neutral, the latter being connectedto the earth. The installation must bede-energized immediately in the eventof an insulation fault. There are two TNsystems, TN-S in which the neutral (N)and the protective conductor (PE) areseparate, and TN-C in which the twoconductors are combined to form asingle conductor (PEN).

TT system

System earthing arrangement inwhich the neutral and the exposedconductive parts are directly earthedvia individual earthing circuits.

The installation must be de-energizedimmediately in the event of aninsulation fault.

Ultracapacitors

An ultracapacitor (double-layerelectrochemical capacitor) is made up of two porous, metal-carbonelectrodes placed in a non-aqueousorganic electrolyte.

This technology offers very highcapacitances (> 1 000 farads).

UPS (Uninterruptible Power System)

An electrical device providing aninterface between the normal sourceof power, usually the utility, and anelec trical installation generallyincluding sensitive loads (computers,instrumentation, etc.).

The UPS supplies sinusoidal ACpower free of disturbances and withinstrict amplitude and frequencytolerances.

It is generally made up of a rectifier/charger, an inverter, an energy storagefor backup power in the event of utilityoutages, a static bypass and amaintenance bypass. The bypassesmake it possible to supply the loaddirectly with standby power,bypassing the rectifier/charger and inverter line.

Transfer to the static bypass isautomatic and without a break inpower to the load if the inverter failsor a downstream overload exceedsUPS capacity. Transfer to themaintenance bypass is carried outusing manual switches.

UPS operating in double-conversion

mode

A UPS in which the inverter isconnected in series between thenormal AC source and the load. Allpower supplied to the load flowsthrough the inverter which completelyregenerates the voltage and isolatesthe load from disturbances on theutility. This type of UPS can alsosupply the load with utility powerdirectly via a static bypass followingno-break transfer to a separate ACinput. This function ensures thecontinuity of supply if an internal faultoccurs. What is more, this type of UPSis systematically equipped with amaintenance bypass.

UPS operating in line-interactive mode

A UPS in which the inverter isconnected in parallel to the AC inputand also charges the energy storage(interactive operation in reversiblemode).





UPS operating in passive-standby mode

The UPS is connected in parallel to thenormal AC source to provide astandby power source.

This configuration, a cost-savingcompromise, is used only for lowpower ratings (≤ 3 kVA) because itdoes not isolate the load from thesource and lets through inrushcurrents. What is more, it requires arelatively high transfer time (≈ 10 ms)to inverter power in the event of apower outage or a major disturbanceon the utility.

Standards

• IEC 60529 / EN 60529: Degrees of protection provided byenclosures (IP index).

• IEC 60417: Graphical symbols fordiagrams.

• IEC 60742: Isolating transformersand safety isolatingtransformers.

• IEC 60947: Low-voltageswitchgear and controlgear.

• IEC 60950-1 / EN 60950-1:

Information technologyequipment - Safety - Part 1:General requirements.

• IEC 62040-1/ EN 62040-1:

Uninterruptible power systems(UPS) - Part 1: General and safetyrequirements for UPS.

• IEC 62040-2/ EN 62040-2:

Uninterruptible power systems(UPS) - Part 2: Electromagneticcompatibility requirements.

• IEC 62040-3 / EN 1000-3:

Uninterruptible power systems(UPS) - Part 3: Method ofspecifying the test andperformance requirements.

• IEC 61000-2-2 / EN 610002-2:

Compatibility levels for low-frequency conducted disturbancesand signalling in public low-voltage power supply systems.

• IEC 61000-3-2 / EN 61000-3-2:

Limits for harmonic currentemissions (equipment inputcurrent ≤ 16 A per phase).

• IEC 61000-3-4 / EN 61000-3-4:

Limits for harmonic currentemissions (equipment inputcurrent > 16 A per phase).

• IEC 61000-3-5 / EN 61000-3-5:

Limitation of voltage fluctuationsand flicker.

• EN 50091-2: UPS -Electromagnetic compatibility.

• EN 50160: Voltage characteristicsof public networks.

• IEEE 519: Recommendedpractices and requirements forharmonic control in electricalpower systems.

• EMC Directive 2004/108/EC: Forequipment liable to cause or beaffected by electromagneticdisturbances.

• European LV directive:

2006/95/EC,

• ISO 3746: Determination ofsound power levels of noisesources.

• ISO 7779 / EN 27779:

Measurement of airborne noiseemitted by computer andbusiness equipment.

Bibliography



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