3 key components of online double conversion ups

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PSA double conversion UPS converts the incoming

alternating current (AC) to a direct current (DC), so

it can power the system’s battery, and then inverts

the DC back to AC for powering equipment –

hence the name “double conversion.” Take a look

at how a UPS’s components work together so you

can better understand your system and ensure

your mission critical load remains online.

The UPS system acts as a source to the loads

connected to it and as a load to the electrical

mains.

3-1

Efficiency 98-99% 95-96% 95-96% 96-97% ~ 98%

Impact on Low Load

No Leading PF No No No

3-2

Architecture

Input current

waveform

THDi <33% <10% <5% <10% <5% PF ~ 0.8 0.92 0.99 ~ 0.8 0.99

Compatibility with DG

Requires Oversizing

Requires Oversizing

Requires Oversizing No Oversizing No Oversizing

Cost Low Medium Medium High High Operating Efficiency (THDi&PF)

No impact Good at full load

Good at all Load levels Medium Good

at all load Levels

6 pulseSCR Rectifier

6 pulse with harmonic filterPassive … ……. Active

12Pulse Rectifier IGBT Rectifier

As shown in the above figure, an online double Conversion UPS

has 4 major components

RECTIFIER

• INVERTER

• BATTERY

• STATIC SWITCH

The rectifier acts as a load to the electrical mains. The primary

objective of the rectifier is to

a) Convert the incoming power supply (AC) to DC

b) Charge the battery

It also has a hidden objective which is to draw a sinusoidal

current from the mains and also to ensure the current drawn is

in phase with the voltage waveform so that the current

harmonic distortion injected on the mains is less and the power

factor is better.

RECTIFIER

Normal mode

Battery

Ac Input

Ac InputRectifier

BatteryCharger(optional)

Inverter

BypassSwitch

Batteryto Dc

Converter

Ac Output

Ac Input

Stored energy mode

Bypass mode

DC Link

Bypass (prime or standby)

IEC 487/9962040-3D IEC:1999

Figure 1 Double Conversion UPS

The rectifier in a three phase UPS is

designed to operate under nominal input

voltage of 415V and frequency of 50Hz.

Taking into consideration voltage

fluctuations, the rectifier is typically

designed to operate with a input specific

voltage range of ±15% and frequency

range of ±6%.

In general the best rectifier topology should

have high efficiency, high power factor (PF)

and low current distortion(THDi).This will

ensure good compatibility with Genset and

also reduce the need to oversize the DG

set, incoming transformer and cable sizing

for supporting the UPS.

The technology of the UPS has evolved and

different technologies are being used in the

rectifier of the UPS. A short comparison of

different rectifier technologies is given in

the table below.

© Copyrights Reserved

INVERTER

INVERTER

The primary objective of the inverter is to convert DC power to

AC power and to support the loads. The DC power can be

either from the rectifier or from the battery connected to the DC

bus of the UPS System.

The inverter is a critical component as this acts as a source to

the critical loads connected to it. As a source, the inverter has

to support the loads with sinusoidal voltage waveform under

below conditions:

a) Zero break power from mains to battery mode,

b) Static and dynamic loading conditions,

c) Overload conditions

d) Linear and Non-Linear loading conditions

e) Faster fault clearing

f) Overload handling capability

There are two main Inverter topologies namely with transformer

in the inverter output and transformer less inverter topology.

Transformer based and Transformer Less Inverter

In transformer based inverter the primary

objective of using a transformer is to setup

the inverter output voltage as the DC bus

voltage will be generally around 600V DC

and the inductance required as a part of

output LC filter will be incorporated in the

same.

In a transformerless UPS, The DC bus

voltage is increased to 800V DC and a

DC-DC booster circuit will be introduced

between the battery and the DC bus.

Refer section 5 for the details and selection

of right topology based on the applications.

Also the actual inverter bridge can have two

level swithcing or three level switching

which is explained is detailed in the next

section.

3-3

Transformer Based

Figure 2 Transformer Based & Transformerless Inverter

Transformer less

Figure 2 shows the configuration of Inverter with transformer

and in a transformerless or transformer free configuration.


Two-Level and Three-Level Inverter

Two-Level Inverter

The two-level inverter has been widely used for a range of power levels. The schematic of the topology is shown inFigure 3 and 4.The two-level inverter switches between two voltage levels of +Vdc and –Vdc. The switching voltage will be the full DC bus voltage which is generally 600 to 800Vdc which demands the usage of IGBT with higher voltageing of 1200V to reduce the impact of voltage stress.

As a result of PWM switching at higher frequency, the output voltage waveform generated contains higher distortion and which increases the size of the choke / inductor must be increased to smoothen it into a sinusoidal waveform.

The two-level inverter is a very simple design without anycomplex circuits. The two-level inverter will have a lowerconduction loss but higher switching loss making the two-level inverter less efficient at higher switching frequencies.

Three-Level InverterThe schematic of three-level inverter is shown in 5 and 6. A three level inverter will have three switching states,+Vdc/2,0,-Vdc/2. The effective switching voltage of IGBT will be 400V and the IGBT voltage ratings will be 600V. However in a three level inverter we use 4 no’s of 600V IGBT in series for each leg.

As a result of three level switching, the resultant output wave-form is more sinusoidal with lesser distortion, which will reduce the size

TWO AND THREE LEVEL INVERTER

Figure 5 Three Level Inverter Bridge

Figure 6 1Symbolic Representation ofThree Level Inverter Bridge

of choke/inductance required to smoothen the voltage waveform eventually reducing the losses across the chokes.

The switching losses of a three-level inverter is lesser but the conduction losses are higher compared to a two level inverter.

In a three-level inverter,while the number of switching devices are more, the overallefficiency of the inverter can be better than a two level inverter. But actual efficiency of either topology depends on the IGBT used, the switching frequency as well as the losses in the output choke (Inductor).

Figure 3 Two Level Inverter Bridge Figure 4 Symbolic Representationof Two level Inverter Bridge

3-4


3-5

COMPARISON OFTWO-LEVEL INVERTERAND THREE-LEVELINVERTER

Comparison of Two-level Inverter vs Three-Level Inverter

Description Two-Level Inverter Three-Level Inverter

Output Waveform

Higher distortion, required bigger Choke to smoothen the waveform

Lower distortion, Lower size of Choke to smoothen the waveform

Higher switching loss due to high

switching speed and lower conduction losses

Lower Switching loss due to lower

switching frequency and higher conduction loss due to more

devices.

Control Circuit Simple & Easy Complicated control algorithm

No of Devices 6 devices of 1200V each 12 devices of 400V each

0Two-level Three-level

20

40

60

80

100

120

Switching Losses

63%

Conduction Losses

63%

Conduction Losses

37%

Switching Losses

21%

Loss

es P

ropo

rtion

(%)

Losses of theDevices

Switching Voltage

800V DC,Higher Voltage Stress on devices

400V DC,Lower Voltage Stress on devices


Energy Storage

When electrical service is disrupted (i.e., mains failure), the UPS continues to support the load connected to it through its energy storage system. The UPS may provide power fordurations ranging from 10 to 20 seconds to several hours. Shorter duration UPSs are designed to carry the load during the start-up of back-up electrical generators, typically diesel engine driven generators, and to enable a smooth transition to the generator as the power source.

In many cases, the UPS is designed to provide power for 5 to 30 minutes. The purpose is to enable an orderly shutdown of operations thereby avoiding an abrupt shutdown, which would otherwise cause equipment damage, product/work losses or a security/safety hazard. The under-desk UPS for PCs is an example.

UPS with enough energy to provide power for several hours are somewhat rare. A key reason is that, in most situations, it is less expensive to store energy in the form of diesel fuel (for generators) if backup power is needed for several hours.

There are different technologies of energy storage solution available in the market like

a) Batteryb) Flywheelsc) Ultra capacitors

The selection of right energy storage system depends on

• Required runtime/backup time• Power density/footprint • Weight • Lifespan / cycle count • Reliability • Cost of Ownership (Initial cost /Maintenance cost)• Operating temperature

Energy Storage system - battery

Battery is the most critical component in the

UPS and is also considered as heart of the

UPS System. Without, battery the UPS is

just a power conditioner.

The purpose of the battery is to provide the

energy necessary to supply the load when

the mains supply in not available.

Cost of battery is a major component on the

final price of the UPS solution proposed to

the customer.

A battery is an electrochemical device that

stores energy at one time for use at another.

The battery uses electrical energy to store

energy in chemical form which is converted

to electrical energy during the discharge of

the battery.

The UPS battery may furnish power to the

inverter for a few seconds, many minutes,

or hours. The battery capacity is

determined by the amount and duration of

power the inverter has to deliver to the load

from the battery.

ENERGY STORAGE

ElectricalEnergy

ElectricalEnergy

ChemicalEnergy

Batteries

AC/DC DC/AC

3-6


Types of Battery

Three common varieties of battery chemistries popularly used in UPS applications are: a) Lead Acidb) Nickel Cadmiumc) Lithium Ion

Lead Acid Battery

The storage battery or secondary battery is such battery where electrical energy can be stored as chemical energy and this chemical energy is then converted to electrical energy as and when required. The conversion of electrical energy intochemical energy by applying external electrical source is known as charging of battery. Whereas conversion ofchemical energy into electrical energy for supplying theexternal load is known as discharging of secondary battery. During charging of battery, current is passed through it which causes some chemical changes inside the battery. Thischemical changes absorb energy during their formation.

When the battery is connected to the load, the chemicalchanges take place in reverse direction, during which the absorbed energy is released as electrical energy andsupplied to the load. Now we will try to understand theprinciple working of lead acid battery and for that we will first discuss about lead acid battery which is very commonly used as storage battery or secondary battery.

TYPES OF BATTERY

The main active materials required toconstruct a lead acid battery are

• Lead peroxide (PbO2).• Sponge lead (Pb)• Dilute sulfuric acid (H2SO4).

The positive plate is made of lead peroxide. This is dark brown, hard and brittlesubstance.The negative plate is made of pure lead in soft sponge conditions. Dilute sulfuric acid used for lead acid battery has ratio of water to acid = 3:1.

During discharging• Both of the plates are covered with PbSO4

• Specific gravity of sulfuric acid solution falls due to formation of water during reaction at PbO2 plate.• As a result, the rate of reaction falls which implies the potential difference between the plates decreases during discharging process.

During charging

• Lead sulfate anode gets converted into lead peroxide.• Lead sulfate of cathode is converted to pure lead.• Terminal potential of the cell increases.• Specific gravity of sulfuric acid increases.

The lead acid battery is further classified as • Sealed Maintenance Free (SMF) VRLA Battery• Tubular/Flooded Battery• Tubular Gel VRLA

Battery

Lead Acid Nickel Cadmium Lithium Ion

Flooded

Valve regulatedLead Acid battery

VRLA / SMF

Iron proshpate

Nickel, Cobalt,Magnesium

3-7


LEED ACID & Ni-CdBATTERY

SMF (Sealed Maintenance Free) battery is a battery which doesn't require topping up due to negligible water loss. It is designed in such a way that it cannot be opened or refilled. These batteries are safe, maintenance free and are suitable for most UPSapplications. The SMF battery will have an additional safety valve which release the excessive formation of hydrogen, as a result of overcharging, in to the atmosphere.

SMF battery works on a recombination technology where the hydrogen gas evolved during the charging process, isconverted to water with the help of oxygen present inside the battery container.

The typical cyclic performance of the battery is less and islimited by the operating temperature and the charging profile.The SMF battery delivers higher power at highertemperatures but the life of battery comes down significantly

The SMF battery needs to be installed in a controlledenvironment to maintain the temperature at 25-27 deg C and an additional hydrogen sensor in the battery room isrecommended for installation.

Advantages and Limitation

Tubular Batteries have openings at top to add distilled water for maintenance and safe running. These batteries are very rugged and used in Cyclic application. These batteries last longer due to robust design and are suitable for harsh environmentapplications.

The tubular battery can be installed in any environment(other than closed air conditioner room) with proper ventilation and air exchanges as hydrogen evolution from the battery is higher when compared with SMF buttery.

Tubular Gel batteries require no topping of water and is a sealed, valve regulated lead-acid deep cycle battery that uses

a gel electrolyte. These type of batteries are rugged and suitable for cyclic applications but are maintenance freecompared to flooded tubular batteries.

Nickel cadmium cell (Ni-Cd)

The active components of a rechargeable Ni-Cd battery in the charged state consist of nickel hydroxide (NiOOH) in the positive electrode and cadmium (Cd) in thenegative electrode. For the electrolyte, usually caustic potash solution (potassium hydroxide) is used. Due to their low internal resistance and the very good currentconducting properties, Ni-Cd cells can supply extremely high currents and can be recharged rapidly.

These cells can operate over a largetemperature range, from +60°C down to -20°C. The selection of the separator (nylon or polypropylene) and the electrolyte (KOH, LiOH, NaOH) is also of great importance. These constituents influence the voltage conditions in the case of a high current discharge, the service life and theovercharging capability of the cell. In the case of misuse, a very high-pressure may arise quickly.

For this reason, these cells are equipped with a reversible safety valve, which can act several times. NiCad cells offer a long service life (depending on the type ofapplication and charging unit up to 2000 cycles).

3-8


ADVANTAGES ANDLIMITATIONS OFLEAD ACID BATTERIES& Ni-Cd

Advantages • Inexpensive and simple to manufacture — in terms of cost per watt hours, the VRLA Battery is the least expensive. • Mature, reliable and well-understood technology — when used correctly, the VRLA Battery is durable and provides dependable service. • Low self-discharge —the self-discharge rate is among the lowest in rechargeable battery systems. • Low maintenance requirements — no memory; no electrolyte to fill. • Capable of high discharge rates.

Limitations • Cannot be stored in a discharged condition. • Low energy density — poor weight-to-energy density limits use to stationary and wheeled applications. • Allows only a limited number of full discharge cycles — well suited for standby applications that require only occasional deep discharges. • Environmentally unfriendly — the electrolyte and the lead content can cause environmental damage. • Transportation restrictions on flooded lead acid — there are environmental concerns regarding spillage in case of an accident. • Thermal runaway can occur with improper charging.

3-9


Advantages and Limitations of Lead Acid Batteries

• Simple storage and transportation — most air freight companies accept the Ni-Cd without special conditions. • Good low temperature performance.

Limitations • Relatively low energy density — compared with newer systems. • Memory effect — Ni-Cd must periodically be exercised to prevent memory affect. • Environmentally unfriendly — Ni-Cd contains toxic metals. Some countries are limiting the use of Ni-Cd battery. • Has relatively high self-discharge — needs recharging after storage.

• Fast and simple charge — even after prolonged storage. High number of charge/discharge cycles — if properly maintained, the Ni-Cd provides 2000 charge/discharge cycles.• Good load performance — Ni-Cd allows recharging at low temperatures.

• Long shelf life – in any state-of-charge.

• Forgiving if abused — the Ni-Cd is one of the most rugged rechargeable batteries.

• Economically priced — the Ni-Cd is the lowest cost battery in terms of cost per cycle.• Available in a wide range of sizes and performance options — most NiCd cells are cylindrical.

Advantages

Advantages and Limitations of Ni-Cad Batteries

COMPARINGDIFFERENT TYPESOF BATTERY

3-10

Gassing / fuming No gassing / fuming, can be installed anywhere

No gassing/fuming, can be installed anywhere.

High gassing / fuming, separate battery room with exhaust system is essential.

High gassing/fuming, separate battery room. No gassing /fuming, in maintenance free Ni-Cd battery can be installed anywhere

Topping up of electrolyte No topping uprequired normally No topping-up required normally Topping up required frequently No topping-uprequired normally. Maintenance Free Ni-Cd doesn’t need topping up of electrolyte

Charging current level High Lower Lowest High

Space requirement Small cell size, Low space requirement.

Small cell size, Low space requirement.

Large cell size, Large space required.

Moderate space required.

Stacking Horizontal or vertical Horizontal or vertical (in tiers) Vertical stacking only. Vertically in Tiers

Transportation in charged condition

Easy Easy Not possible. Transportation in uncharged (unfilled) condition recommended.

Easy

Self-discharge during storage, at an average temperature of 25°C.

50% self-discharge in 6 months. Recovery easy.

50% self-discharge in one year. Recovery easy.

Self-discharge is very high. Long duration storage not recommended. Recoverydifficult.

Self-discharge is low and can be stored upto 1 year

Cyclic Life (to 80% DoD). 1400 cycles at an average temperature of 35°C in normal environmental condition

Better than 2100 cycles at an average temperature of 35°C in normal environmental condition

2000 cycles 2000-2500 cycles

Float life at 25°C Good Good Good Good

High temperature performance

Average, but temperature compensation provision required

Good Good Good, but temperaturecompensation provision required

Low temperture performance

Good Good Poor Good,can operate upto -20 deg C

Stratification Negligible, no boost charging required.

Negligible, no boost charging required.

Prominent, requires frequent boost charging for prevention.

Not possible

End cell voltage 1.75V/cell 1.75V/cell 1.85V/cell 1.1V/CellCapacity at very low rate of discharge

Good Good Average

Deep discharge recovery Average, after 4 to 5 charge/discharge cycles

Average, after 4 to 5charge/discharge cycles

Poor, hard sulphation prevents recovery.

Quick and Fast

Charge efficiency Excellent, 6 to 8 hours for 90% recovery. cycles

Slightly poor, 8 to 10 hours for 90% recovery cycles

Poor, 12 to 14 hours for 90% recovery.

Excellent, 6 to 8 hours for 90% recovery. Quick & Fast

Under-chargedperformance

Average Good Poor

Overcharging Poor, damages the battery Good Good Good

Performance under partial state of charge

Good Good Poor Good

Charging Requirement Constant voltage & Current charging

Constant voltage & Currentcharging

Periodical boost charging at 2.7V/cell essential

Constant voltage & Currentcharging

Thermal runaway Probable, yet rare Not possible Not found Not found

Risk of internalshort-circuiting

Remote Remote High, due to active material shedding

Remote

High temperature performance

Average, but temperature compensation provision required

Good Good Good, but temperaturecompensationprovision required


Feature VRLA (AGM) Tubular GEL VRLA Tubular Flooded Nickel Cadmium

Advantages • High energy density — potential for higher capacities. • Relatively low self-discharge — self-discharge is less than half that of Ni-Cd and NiMH. • Low Maintenance — no periodic discharge is needed; no memory.

Limitations • Requires protection circuit — protection circuit limits voltage and current. Battery is safe if not provoked. • Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect. • Moderate discharge current. • Subject to transportation regulations — shipment of larger quantities of Li-ion batteries may be subject to regulatory control. This restriction does not apply to personal carry-on batteries. • Expensive to manufacture — about 40 percent higher in cost than Ni-Cd. Better manufacturing techniques and replacement of rare metals with lower cost alternatives will likely reduce the price. • Not fully mature — changes in metal and chemical combinations affect battery test results, especially with some quick test methods.

3-11

LITHIUM IONBATTERY

Lithium Ion batteryLithium-ion batteries offer several advantages over traditional valve-regulated, lead acid batteries commonly used in UPSs today. A much longer life span, smaller size and weight, faster recharge times, and declining prices have made lithium-ion batteries an appealing energy storage technology option for energy storage.

Similar to the lead- and nickel-based architecture, lithium-ion uses a cathode (positive electrode), an anode(negative electrode) and electrolyte as conductor. The cathode is a metal oxide and the anode consists of porous carbon. During discharge, the ions flow from the anode to the cathode through the electrolyte and separator; charging reverses the direction and the ions flow from the cathode to the anode.

When the cell charges and discharges, ions shuttle between cathode (positive electrode) and anode(negative electrode). On discharge, the anode undergoes oxidation, or loss of electrons, and the cathode sees a reduction, or a gain of electrons. Charge reverses the movement.

All materials in a battery possess a theoretical specific energy, and the key to high capacity and superior power delivery lies primarily in the cathode. For the last 10 years or so, the cathode has characterized the Li-ion battery.

Common cathode material include:• Lithium Cobalt Oxide (or Lithium Cobaltate • Lithium Manganese Oxide (also known as spinel or Lithium Manganate)• Lithium Iron Phosphate• Lithium Nickel Manganese Cobalt (or NMC) and • Lithium Nickel Cobalt Aluminum Oxide (orNCA)


Advantages and Limitations of Li-ion Batteries

3-12

DIFFERENT TECHNOLOGIES OF LITHIUM IONBATTERY

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ENERGY STORAGESYSTEM – FLYWHEEL

Advantages

3-13

Composite Rim

Hub

Motor

MagneticBearing

Shaft

VacuumChamber

Energy Storage System - Flywheel

Flywheel stores electrical energy in the form of kinetic energy during charging process and during the discharging thekinetic energy is converted into electrical energy.

A typical system consists of • A rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination of electric motor/electric generator.• First generation flywheel energy storage systems use a large steel flywheel rotating on mechanical bearings. Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel and are an order of magnitude lighter.


Advantages and Limitations of Flywheel

Magnetic bearings are necessary; in conventional mechanicalbearings, friction is directly proportional to speed, and at suchspeeds, too much energy would be lost to friction.

The flywheel has a vacuum chamber on which a motor is held in a magnetic bearing. During charging process, the motor rotates at 1000rpm in clock wise direction to store the electrical energy in the form of kinetic energy. During discharge the motor acts as a generator and will convert the kinetic energy back to electrical energy

• High energy density — potential for higher capacities.• Low Maintenance — no periodic discharge is needed; no memory.• Flywheels are not affected by temperature changes unlike chemical rechargeable batteries• Shorter time to recharge• Long Life >20 years

• Can be used only for a short backup time, in few seconds

• High power applications with shorter backup time

Limitations

ENERGY STORAGESYSTEM – SUPERCAPACITORS

Advantages • Short duration runtime critical applications • Compact foot print and power density • High working ambient temperatures • ECO friendly low environmental impact • High energy efficiency and low running costs • Lower Total Cost of Ownership (TCO)

Limitations • High self-discharge • Ride through for shorter power outages in seconds.

Energy Storage system – Super CapacitorsSuperCaps (also known as ultracapacitors or electric double-layer capacitors) provide an alternative source of DC power to traditional rechargeable batteries. Super capacitors are high density energy storage devices with a capacitance (energy density) of up to 10,000 times that of conventional electrolytic capacitors.

Super capacitors or double layer capacitor store energy much in the same way as a conventional capacitor, hence the amount of stored energy can be described by: A double layer capacitor consists of two electrodes, a separator, electrolyte, two current collectors andhousing.

A very high capacitance is obtained in this way. Super capacitors are suitable for high power applications and offer very quick response times and high efficiency. Disadvantages are comparatively low energy density, high self-discharge and high cost. Small units exists, larger sizes are under development. Typical power ratings are 1kW-250 kW and efficiencies in the ranges of 85-98%

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Advantages and Limitations of Super Capacitors

3 key components of online double conversion ups

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