thesis regarding shortcircuit and protection

8/13/2019 Thesis regarding shortcircuit and Protection

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SHORT-CIRCUIT CURRENT CALCULATIONSND PROTECTIVE RELAY COORDINATION FOR

INDUSTRIAL AND COMMERCIAL POWER SYSTEMS

A Thesis Presented toThe Faculty of the College of Engineering and Technology

Ohio University

In Partial Fulfillmentof the Requirements for the Degree

Master of Science

Houshang C. MohammadiAugust 986

O IO UNIVERSITYLIBR RY


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ACKNOWLEDGEMENTS

I would like to offer my appreciation and thanksto Dr. Albert J F Keri my advisor and Dr. H.W. Hill fortheir guidance and encouragement in the preparation of thisthesis.

I owe a great deal to my employer E I Dupontcompany who gave me this opportunity to continue myeducation while I was working. I would also like to expressmy thanks to Mr. R L Doughty senior project engineer atE I Dupont company who checked my work through this study.

Finally I would like to express my thanks to thefaculty and all members of staff at Ohio University who havecontributed a great deal towards this work.


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TABLE O CONTENTS

ABSTRACT xiiCHAPTER 1 Introduction 1

1.1 Statement of the problem 1.2 Literature search.3 State of the art

CHAPTER 2 Sources of short circuit 72.1 Generator 82.2 Synchronous motors2.3 Induction motors 10.4 Electric utility systems 11.5 Rotating machine reactance 12.6 Symmetrical and asymmetrical currents 15

CHAPTER 3 Short circuit current calculation 17.1 Important assumptions 8.2 Single line diagram 18.3 Impedance diagram 19.4 Type and location of faults required 22.5 Symmetrical short circuit current 22

calculation3.6 Momentary short circuit calculation 25.7 Interrupting short circuit calculation 27


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CHAPTER 4 General equipment used for protection 314 1 Overcurrent relays 3 24 2 Fuses 334 3 Circuit breakers 384 4 Generator protection 404 5 Transformer protection 44

CH PTER 5 Coordination of protective devices 475 1 Primary considerations for coordination 485 2 Data required for a coordination 51

3 Methods of coordination 524 Testing of protective devices

CH PTER 6 Conclusion 7 2

BIBLIOGRAPHY

PPENDIX 1 Impedance data 77l l ANSI standard reactance values 771 2 Transmission line impedances 781 3 1 5K V cable impedances 791 4 5K V cable impedances 80

A 1 5 Correction factors for non magnetic ducts 81


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PPENDIX 2 Belle plant short circuit study 82A2.1 Description of the work 82A2.2 12KV air circuit breaker ratings 84A2.3 System assumptions 8A2.4 Belle plant single line diagram 85A2.5 Belle plant impedance diagrams 87A2.6 Belle plant short circuit calculation 93

PPENDIX 3 System generator protection 983.1 Belle plant generator protection 98

A3.2 Belle plant bus protections 115

PPENDIX Belle plant- transformer protection 120A4.1 ~~~e HU 1 transformer differential relay 120

PPENDIX 5 Belle plant coordination curves 126and detailed diagramsA switch house. B NK No.1 and 12KV BUS 126

CR No.6. 12KV BUS A'f 128ECR No.6 12KV BUS B 130Cogeneration generator 132B NK No.7 feeder No.18 1342.4KV 600HP motor river pump house 1362.4KV 450HP motor boiler feeder pump No.5 138


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A5.8 2.4KV 75HP motor ash pump . . . . . . . . . . . . . . 140A5.9 2.4KV 200HP motor recirculating pump . . . 142A5.10 2.4KV 200HP motor F D fan boilers . . . . . 144

No.6 9A5.11 2.4KV feeders sheet No.1 B NK No.60 . . 146A5.12 2.4KV feeders sheet No.2 B NK No.59 . . 148A5.13 2.4KV feeders ECR No.5 . . . . . . . . . . . . . . . . . 150A5.14 2.4KV 150HP motor boiler No.10 mills . . . 152A5.15 2.4KV 550HP motor 1 D fan boilers . . . . . . 154

No 14 15A5.16 2.4KV 400HP motor 1 D fan boilers . . . . . . 156

No.6 9A5.17 480V feeders ECR No.8 sheet 1 . . . . . . . . . . 158

. . . . . . . . . . .5.18 480V feeders ECR.No.8 heet 2 160

. . . . . . . . . . .5.19 480V feeders ECR No.8 sheet 3 162A5.20 2.4KV 550HP motors 1 D fan boiler No.10. 164

APPENDIX 6A6.1A6.2A6.3

A6.4A6.5A6.6A6.7

Westinghouse overcurrent relay curves . . 166Type CO 7 Over Current Relay . . . . . . . . . . . 166Type CO 9 Over Current Relay . . . . . . . . . . . 167Type CO 11 Over Current Relay . . . . . . . . . . 168Type COM 5 Over current Relay . . . . . . . . . . 169Type CV 2 Under Voltage Relay . . . . . . . . . . . 170Type CV 7 Under Voltage Relay . . . . . . . . . . 171

. . . . . . . .estinghouse type Amptector 11 A 172


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A6.8 Westinghouse type DSL 206 Limiters 173A6.9 Two pole type AA12P Overload Relay 174

APPENDIX 7 Buss fuses curves 180A7.1 Type LPS RK Low Peak Dual Element fuse 180A7.2 Type KRP C HI CAP Fuses 181

PPENDIX 8 ASA Device numbers and functions 182


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LIST OF FIGURES

2.1 Total short circuit current equals sum of sources 92.2 Symmetrical short circuit currents from 1 3

four surces5 . 1 Typical time curves of CO-9 Relay 635 . 2 Portion of Belle plant relaying detailed diagram 645 . 3 Portion of Belle plant co-ordination curves . . . . . 65A2-1 Block diagram of system setup 83A2-2 Belle plant single line diagram 86A2-3 Multiplying factors for three phase line 91

to ground faultsA2-4 Belle plant simplified impedance diagram 92A2-5 Equivalent circuit for fault A (momentary) 93A2-6 Simplified equivalent circu'it for fault A 94

Wye-Delta transformation for fault A 94Equivalent circuit for fault A (interrupting) 95Wye-Delta transformation for fault A (interrup.) 96ypical time curves for the 10 sensitivity 100type CA generator relayimits for application of the CWC time curves 102

Typical time curves of the type CWC relay 103enerator over voltage curve 108enerator reactive capability curve 109achine capability curve 110


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A3 7 SDF 1 tripping relay under declining frequency 113A3 8 Operating time variations with changes in 114

time dial settingA3 9 K B relay voltage unit setting 118A3 10 K B relay current unit setting 119


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LIST OF TABLES

achine reactance and multiplying factorsuse classificationCurrent limiting characteristics of c s onetime fuseransformer ANSI data

Rating ratios for safe co ordinationsPower air circuit breaker minimum bandNSI Standard reactance valuesransmission line impedances5KV cable impedances

5KV cable impedancesCorrection factors for non magnetic ducts 8Result of Belle plant short circuit study 97Limits for application of CWC relay 1 1


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ABSTRACT

The calculation of ac short-circuit currentsessential to the selection of adequately rated protectivedevices and equipment in industrial and commercial powersystems is becoming increasingly important to the systemdesigner. Today power systems carry larger blocks ofpower are more important to the operation of the plant andbuilding and have greater safety and reliabilityrequirements. Meeting these requirements necessitates thefulfillment of certain criteria including the use ofadequately rated equipment.

This report outlines state of-the-art industrialpower system engineering practices which should beespecially valuable to industrial plant engineers andelectricians industrial power application engineers andothers who are involved with the planning of electricalfacilities for industrial plants or commercial buildings.

The method of short-circuit-current calculationhas been selected so that adequate ratings of all the aircircuit breakers were obtained. Fault protective deviceswere selected to maintain proper relay coordinationthroughout the system. These devices has been selected andset so that only the device nearest a fault opened to clearthe fault without affecting larger devices nearer the sourceof power or causing a wider outage than the minimum.

-xii-


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CHAPTER ONE

INTRODUCTION

All power systems, whether they be utility,industrial, commercial, or residential, have the commonpurpose of providing electric energy to the utilizationequipment as safely and as reliably as is economicallyfeasible. The relative importance of economic, reliability,and safety considerations may vary somewhat with the type ofsystem, but all three elements must be taken intoconsideration in any good system design, and certain minimumsafety and reliability requirements must be satisfied.

1 1 STATEMENT OF THE PROBLEM

Electric power system in today s industrial plantsand large commercial establishments handle enormousquantities of energy. A review of the trend in electricenergy usage in such establishments indicates that suchenergy usage has been doubling every seven to eight yearsand shows little signs of leveling off [ I ] Many industrialprocesses and commercial operations demand high degree ofcontinuity of electric power supply because of the greatcosts of production downtime. One of the majorconsiderations in the design of a power system is adequate


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control of short circuits, or faults, as they are commonlycalled. Uncontrolled short circuits can cause serviceoutage with accompanying production downtime and associatedinconvenience, interruption of essential facilities,extensive equipment damage, personnel injury or fatality,and possible fire damage.

Clearly, the fault must be quickly removed fromthe power system, and this is the job of the protectivedevice, the circuit breakers and fusible switches. In orderto accomplish this, the protective device must have theability to interrupt the maximum short-circuit current whichcan flow for a fault at the device location. The maximumvalue of short-circuit current is frequently referred to asthe 'Iavailable'l short-circuit current.

The purpose of this report is to provide thereader with information and procedures necessary tocalculate short-circuit currents, and also present a newmethod of coordination for protective relaying by using thetypical time curves of the relay. Calculation ofshort-circuit current and coordination of protectiverelaying for the Belle plant, (one of the biochemical sitesof E.I.dupont de Nemours Company) is shown for the entiresystem as an example for this report.

This report defines the function of systemprotection as the detection and prompt isolation of theaffected portion of the system whenever a short circuit or


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other abnormality occurs which might cause damage to , oradversely affec t, the operation of any portion of the systemor the load which it supplies.

Thi s report will be limited to short-circuitcalculation and coordination of protective relaying forlow-voltage to medium-voltage industrial plants, commercialand institutional buildings. Westinghouse protectivedevices and Bussmann fuses have been used throughout thereport, which limits our selection and information aboutother protective devices.

Chapter One introduces an outline of this reportand reviews some of relative methods concerning short-circuit calculation and coordination of protective r elayin g.Chapter Tw o explains .the major short circuit sources andtheir wavefor ms. Chapter Three explains longhand method ofshort- circuit-current calculation, which includesimpedances of all motors less than 5 hp by lumping themtogether and treated as a single impedance. Chapter Fourexplains different protective devices used for systemprotection. Chapter Fiv e explains several method ofcoordination which will be compared with the new methodpresented in this report. Chapter Six gives the conclusionon this report. Appendix 1 provides impedance data that isrequired for the Belle plant, which is used as an examplefor this report. Appendix 2 provides the Belle plantsingle-line diagram. Appendix 3 provides syst em generator


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protection for the Belle plant. Appendix providestransformer protection of protective relaying for the Belleplant. Appendix provides the complete set of coordinationcurves and detailed diagrams for the entire system.Appendix 6 provides Westinghouse overcurrent relay curvesused throughout the study. Appendix 7 provides Bussmannfuse curves used in this report and finally Appendixprovides the American Standard Association devices numbersand functions ASA) which are used for an automaticswitching equipment.

1.2 LITERATURE SEARCH

There are several texts available forshort-circuit-current calculation and relaying coordinationof industrial and commercial power system. Most noteworthyis the book IEEE Recommended Practice For Protection andCoordination of Industrial and Commertial Power Systems byD.Dalasta [ 6 ] Other good books are The Art and Science ofProtective Relaying by Russell C. Mason [ 3 ] and IndustrialPower Systems Handbook by Donald Beeman [ 2 ] These textsespecially The Art and Science of Protective Relaying willprovide an understanding of the function of protectiverelaying and their operation for protection of industrialand commercial power systems.


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1 3 STATE OF THE ART

Protection of power system involves short-circuitcalculation and relay coordination.

At the present there are several methods of short-circuit-current calculation available throughout theindustries. Longhand calculation, Network Analyzer anddigital computer techniques are the most-used methods. Ingeneral, the presence of closed loops in the impedancenetwork, such as might be found in a large industrial planthigh-voltage system, will favor using a network analyzer ordigital computer technique from an economic and time-savingstandpoint[l]. Radial systems, such as those used in mostlow-voltage and medium-voltage systems, can be easilyresolved by longhand calculations.

There are also several methods available forcoordination of protective relaying for industrial andcommercial power systems against any abnormalities whichcould reasonably be expected to occur in the course ofsystem operation. These methods are as follows:1 Coordination by tables are used as a simple check for

selectivity assuming that identical or reduced faultcurrents flow through the circuits in descending order,that is, main-feeder-branch. this method is recommendedonly for low-voltage branch circuits [6]

2 Coordination by using the device characteristic curve can


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be used for any size system. The sheet of log-log paperon which the study is being made is placed on top of thedevice characteristic curve. Proper curve then can beselected and traced. Human error is very high in thismethod which makes t undesirable to use.Coordination by using the typical time curves is a newmethod presented in this report. This method isrecommended, since relay curve is drawn by findingseveral points from typical time curve. These points arethen ploted and traced on log-log paper. calculation ofthese points require knowing the relay type, currenttransformer c.t.) ratio, available taps, ampere range ofthe relay, circuit voltage level and availableshort-circuit current. These requirements will ensurecorrect data point transformation from typical time curveof the relay to the log-log paper, which practicallyeleminates human error.


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CHAPTER TWO

SOURCES OF SHORT CIRCUIT

If adequate protection is to be provided for aplant electric system the size of the electric power systemmust be considered to determine how much short-circuitcurrent it will deliver. This is done so that circuitbreakers or fuses may be selected with adequate interruptingcapacity. This interrupting capacity should be high enoughto open safely the maximum short-circuit current which thepower system can cause to flow through a circuit breaker ifa short circuit occurs in the feeder or equipment which itprotects.

When determining the magnitude of short-circuitcurrents it is extremely important that all sources ofshort circuit be considered and that the impedancecharacteristics of these sources be known. There are fourbasic sources of short-circuit current:1 Generators2. Synchronous motors3. Induction motors4 Electric utility systems

All these can feed short-circuit current into ashort circuit as shown in figure 2.1.


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2.1 GENERATORS

Generators are driven by turbines diesel engineswater wheels or other types of prime movers. When a shortcircuit occurs on the circuit fed by a generator thegenerator continues to produce voltage because the fieldexcitation is maintained and the prime mover drives thegenerator at normal speed. The generated voltage produces ashort circuit current of a large magnitude that flows fromthe generator to the short circuit. This flow of short-circuit current is limited only by the impedance of thegenerator and the short circuit. For a .short circuit at theterminals -of the generator . the current from the generat0.ris limited only by its own impedance.

2.2 SYNCHRONOUS MOTORS

Synchronous motors are constructed much likegenerators; that is they have a field excited by directcurrent and a stator winding in which alternating currentflows. Normally synchronous motors draw A C power fromthe line and convert electric energy to mechanical energy.

During a system short circuit the voltage on thesystem is reduced to very low value. Consequently the


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motor stops delivering energy to the mechanical load andstarts slowing down. However as the prime mover drives agenerator the inertia of the load and motor rotor drivesthe synchronous motor. The synchronous motor then becomes agenerator and delivers short-circuit current for many cyclesafter the short circuit has occurred. The amount of short-circuit current produced by the motor depends upon theimpedance of the synchronous motor and impedance of thesystem to the point of short circuit.

FROM E L ECTRICU T I L I T Y S Y S T E M

URBINE GENERATOR METAL CLADSWITCHGEAR

C U R R E N TF R O MGENERATOR

SHORT- CIRCUITC U R R E N T F R O ME L E C T R I C UTlUTTS Y S T E M

SYNCHRONOUSMOTOR TOTAL SHORTCI RCUI TC U R R E N T-?OM LL

SHORT-CIRCUIT 1

SHORTURCUlTSYN MOTOR

FGUR

INDUCTIONCURRENT FROMNWCTlCE

Figure 2.1 Total short-circuit current equals sum of sourcecontributions [ ]


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2.3 INDUCTION MOTORS

The inertia of the load and rotor of an inductionmotor have the same effect on an induction motor as on asynchronous motor; that is they drive the motor after thesystem short circuit occurs. There is one major difference.The induction motor has no DC field winding but there is aflux in the induction motor during normal operation. Thisacts like flux produced by the dc field winding in thesynchronous motor.

The field of the induction motor is produced byinduction from the stator rather than from the D winding.The rotor flux remains normal as long as voltage is appliedto the stator from an external source. However if theexternal source of voltage were suddenly removed as it iswhen a short circuit occurs on the system the flux in therotor can not change instantly. Because the rotor flux cannot decay instantly and because the inertia of the rotatingparts drives the induction motor a voltage is generated inthe stator winding. This causes a short-circuit current toflow to the short circuit until the rotor flux decays tozero. The short-circuit current vanishes almost completelyin about four cycles since there is no sustained fieldcurrent in the rotor to provide flux as in the case of asynchronous machine.


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The flux does last long enough to produce enough

short-circuit current to affect the momentary duty oncircuit breakers and the interrupting duty on devices thatopen within one or two cycles after a short circuit. Hencethe short circuit produced by induction motors must beconsidered in certain calculations. The magnitude of ashort-circuit current produced by the induction motordepends upon the impedance of the system to the point ofshort circuit. The machine impedance effective at the timeof short circuit corresponds closely to the impedance atstandstill. Consequently the inertia value of short-circuit current is approximately equal to the locked rotorstarting current to the motor.

2.4 ELECTRIC UTILITY SYSTEMS

The electric utility system or the supplytransformer from the electric utility system are oftenconsidered sources of short-circuit current. Strictlyspeaking this is not correct because the utility system orsupply transformer merely delivers the short-circuit currentfrom the utility system generators. Transformers merelychange the system voltage and magnitude of current butgenerate neither. The short-circuit current delivered by atransformer is determined by its secondary voltage rating


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and impedance the impedance of the generators and system tothe terminals of the transformer and the impedance of thecircuit from the transformer to the short circuit.

As illustrated in figure 2.2 the totalsymmetrical short circuit current usually has severalsources. The first includes generators either in the plantor in the utility system or both. The second comprisessynchronous motors. The third source is induction motorswhich are located in every plant and building. Becausethese currents decay with time due to reduction of flux inthe machine after short circuit the total short-circuitcurrent decays with time. So even though only thesymmetrical part of the short-circuit current is consideredthe magnitude of current-is highest at the first half cycleafter short circuit and is of lower value a few cycle later.Note that the induction motor component disappears entirelyafter one or two cycles [ 2 ]

2.5 ROTATING MACHINE REACTANCE

The impedance of a rotating machine consistsprimarily of reactance and is not one simple value as it isfor a transformer or a piece of cable but is complex andvariable with time. For example if a short circuit isapplied to the terminals of a generator the short-circuit


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T O T A L ASYMMETRICAL CURRENTO C O M P O N E N T

AC COMPONEHT

a) Symmetrical b) AsymmetricalFigure 2.2 Symmetrical short-circuit currents from four

sources [ 2 ]


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4current behaves as shown in figure 2.2a (generator). Thecurrent starts out at a high value and decays tosteady-state value after some time has elapsed from theinception of the short circuit. Since the field excitationvoltage and speed have remained relatively constant withinthe short interval of time considered, the reactance of themachine may be assumed-to explain the change in the currentvalue-to have changed with time after the short circuit wasinitiated.

Expression of such a variable reactance at anyinstant requires a comlicated formula involving time as oneof the variables. Therefore, for the sake ofsimplification, three values of reactance are assigned togenerators and motors for the purpose of calculatingshort-circuit current at specified times. These values arecalled the subtransient reactance, transient reactance, andsynchronous reactance and are described as follows:1 Subtransient reactance (XI is the apparent reactance

of the stator winding at the instant short circuitoccurs, and it determines the current flow during thefirst few cycles after short circuit.

2) Transient reactance (X d) determines the currentfollowing the period when subtransient reactance is thecontrolling value. Transient reactance is effective upto onehalf second or longer, depending upon the design


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of the machine.3) Synchronous reactance (Xd) is the reactance that

determines the current flow when steady state conditionis reached. It is not effective until several secondsafter the short circuit occurs; consequently, it is notgenerally used in short-circuit calculations [ 2 ]

A synchronous motor has the same kind of reactanceas a generator, but it is of a different value. Inductionmotors have no field coils, but the rotor bars act like theamortisseur winding in generator; therefore, inductionmotors are said to have subtransient reactance only. Referto Appendix (impedance data) for the multiple value ofreactances used for the momentary and interruptingshort-circuit calculations for this study.

2.6 Symmetrical and Asymmetrical Currents

The words symmetrical and asymmetricaldescribe the shape of the ac waves about the zero axis. Ifthe envelopes of the peaks of the current waves aresymmetrical around the zero axis, they are calledsymmetrical current . Figure 2.2a shows the symmetrical

short-circuit currents from four sources combined intototal. If the envelopes are not symmetrical around the zero


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16axis, they are called asymmetrical current . Figure 2.2bshows the asymmetrical short-circuit currents plus the dccomponent from all sources.

Most short-circuit currents are nearly alwaysasymmetrical during the first few cycles after the shortcircuit occurs. The asymmetrical current is at a maximumduring the first cycle after the short circuit occurs and ina few cycles gradully becomes symmetrical as shown on figure2.2b.

Asymmetrical currents are analyzed in terms of twocomponents, a symmetrical current and a dc component asshown on figure 2.2b. As previously discussed thesymmetrical component is at a maximum at the inception ofthe short circuit and decays to a steady state value due tothe apparent change in machine reactance. In all practicalcircuits, that is, those containing resistance, the dccomponent will also decay to zero as the energy representedby the dc component is dissipated as I loss in theresistance of the circuit. The rate of decay of dccomponent is a function of the resistance and reactance ofthe circuit. In practical circuits, the dc component decaysto zero in from one to six cycles [Z]


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CHAPTER THREE

SHORT-CIRCUIT-CURRENT CALCULATIONS

The maximum magnitude of short-circuit currentmust be known in order to coordinate protective devices aswell as to select adequate interrupting ratings. Asmentioned earlier, there are three basic methods ofcalculating short-circuit current: network analyzer, digitalcomputer, and longhand calculation. For a radial medium-voltage system the longhand method is feasible and fairlysimple to use. Determination of short-circuit current forthe Belle plant, which is used as an example in this report,is done by this longhand calculation method.. Since thesystem contains a three-winding transformer and other loops,delta-wye network transformations is used to combineimpedances. Method of combining impedances are included inAppendix 2. The following steps identify the basicconsiderations in making short circuit calculations.

Make certain assumptions in a way that simplifies thecalculation and also maximum short-circuit current canbe calculated.

2 Prepare system single-line diagram which it shouldinclude all significant equipment and components.

3 Prepare system impedance diagram which should display


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18the interconnected circuit impedances that control themagnitude of short-circuit currents.

4 Identify Type and location of faults required for thesystem.Calculate symmetrical short-circuit current for the

system.Calculate interrupting and momentary short-circuit

currents for all identified locations, so that properprotective devices can be selected for the system.

3.1 IMPORTANT ASSUMPTIONS

Certain simplifying assumptions are made for thiscalculation. n important assumption is that the fault isiibolted . That is, it has zero impedance. This assumptionnot only simplifies caculation, but also applies a safetyfactor since the calculated values are a maximum, andequipment selected on this basis is rarely stressed beyondits full rating. Furthermore a three-phase fault should beassumed, because maximum short-circuit current is requiredfor device selection.

3.2 SINGLE LINE DI GR M

The system one-line diagram is fundamental to


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19short circuit analysis. It should include all significantequipment and components and show their interconnections.Refer to Appendix 2 section 112.4 for the complete Belleplant single-line diagram.

3.3 IMPEDANCE DIAGRAM

The impedance diagram displays the interconnectedcircuit impedances that control the magnitude ofshort-circuit currents. Impedance diagrams should bepatterend for momentary and interrupting short-circuitcurrent calculations. These impedance diagrams are derivedfrom system single-line diagram, by replacing all elementsof the single-line diagram with their calculated per-unitreactances. Specified reactances of a motor, generator, ortransformer should be used. The resistance of allgenerators, transformers, reactors, motors, andhigh-capacity buses above 1 A rating) is so low,compared with their reactance, that their resistance is notconsidered in impedance diagram.

Reactance of the cables 6 volts and highershould be considered in impedance diagram. Appendix showsthe tables used for selecting a reactance of these cablesfor the Belle plant impedance diagram.

After it has been decided what elements of the


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single-line diagram are to be considered in the impedancediagram, the mechanics of making the impedance diagram andof determining the short-circuit current magnitude used inthis report are as follows.1) The first step is to decide whether to use ohms, per

cent ohms, or per-unit ohms to represent the variouscircuit impedances in the impedance diagram. Ohms arenot recommended because of the difficalty of convertingohms from one voltage base to another without error.This report is listing the impedance or reactance datain per-unit.The second step in making an impedance diagram is to

represent every generator and motor and utility supplyby a reactance connected to a zero impedance bus orso-called inf nit bus This bus represents theinternal voltage of the generators and motors. Thesereactances can be found as follows:Utility X=[MVAbase [MVAUtilityI P l lGenerator X=(XIfd%) MVAbas MVAgenerator P.U.Motor X=(XI1d%) MVAbase/KvAmo t r P.U.

Since most industrial plants contain many motorsunder 50 hp, Unlike other methods available for the shortcircuit study, this report recommends that induction motorssmaller than 50 hp should be lumped together and treated asa single impedance on the secondary side of the supply


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21transformer. This will slightly reduces the equivalentreactance of the system, which means, the calculated valuefor the momentary and interrupting short circuit will beslightly higher. This will create a safety factor for thesystem protection when selecting protective devices.

Table Al-1 in Appendix 1 shows the positive-sequence reactance of the short-circuit sources with theirmultiple factors. These multiple factors are used whendetermining momentary and interrupting short-circuit currentfor selecting circuit breakers. For example, for a 6 hpmotor in branch circuit, the positive-sequence reactancefactor is 1.2 for momentary calculation and 3.0 forinterrupting case. The reciprocal of the locked rotorcurrent factor, which is normally six times full loadcurrent, has to be multiplied by these factors for thecorrect Thevinin reactance of the load as shown below.Xth momentary) =l. 1/6) base KVA KVA load) p u.Xth interrupting)=3.0 1/6) base KVA KVA load) p uKVA 1oad) power factor) motor hp)

3) The third step is to add the reactance of cables, buses,transformers, current transformers and circuit breakers,in their proper location to complete the impedancedigram. Appendix 2 section A2.5) shows the calculationof these reactances for the Belle plant.


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3 4 TYPE ND LOCATION OF FAULTS REQUIRED

All buses should be identified. The locationwhere short circuit studies are required should be selected.In many studies, all buses are faulted. The type of short-circuit currents we required is based on the short-circuitrating of the equipment located at the faulted bus. FigureA2-1 in Appendix 2 shows the block diagram of system setupwhich includes the fault locations calculated for the Belleplant. Locations were mainly picked on main buses andfeeders

3.5 SYMMETRICAL SHORT-CIRCUIT CURRENT CALCULATION

After completing the impedance diagram andinserting the values of reactance or impedance for each partot the diagram, it is necessary to reduce this network toone equivalent value. Longhand method of combiningreactances is used in this report. If there are threereactances in the system, the following shows how to combinethem.a) Combining series reactances:

X1+ +X3=X =equivalent reactanceeX1, ,X3=reactances of circuit components

b Combining parallel reactances:


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X =equivalent reactancee

Some sytems are such that they cannot be reducedby merely combining series and parallel reactances. Forexample, the equivalent impedance diagram of the Belle plantis shown in Appendix 2 figure A2-5). In order to reducethis circuit to a single reactance, wye-delta transformationis used. By these transformation any commonly encounteredsystem impedance diagram can be reduced to one equivalentreactance.

The calculation to derive the symmetricalshort-circuit current is I=E/X where E is the system drivingvoltage and X is the equivalent positive impedance.

When calculations are made in per-unit there arefour base quantities base KVA base voltage, baseimpedance, and base current. When any two of the four areassigned values, the other two values can be derived. It iscommon practice to assign study base values to KVA andvoltage. Base current and base impedance are then derivedfor each of the voltage levels in the system. For example,the KVA base assigned for the Belle plant is MVA Thenominal line-to-line system voltages are normally used asthe base voltages. Following formulas apply for shortcircuit calculations:


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Basic per-unit relationship:Per-unit voltage=actual voltage/base voltagePer-unit current=actual current/base currentPer-unit impedance=actual impedance/base impedanceChange from percent on an old base to per unit on a newbaseXpUNEW= XpUOLD) NEW KVABASE OLD KVABASE)

OLD KVBASE NEW KVBASESymmetrical phase short-circuit current in per unit p.u.)I .u. E .u.jZp.u. , symmetrical 3 phase short-circuitcurrent in amperes, I=I base) [ZP.U.Symmetrical phase short circuit KVA, KVA=KVA base) /Z p uwere, I base)=KVAbase/\/S KVLL, ase

When calculations are made in ohms, symmetricalthree-phase short-circuit in amperes will be I=E /Z whereL-nE =line to neutral voltage and is the equivalent networkL-nimpedance in ohms per phase.

Calculation for several points at differentvoltage level for the Belle plant is done in Appendix 2,section A2.6. Calculation of fault duties is done for bothmomentary and interrupting current for different faultlocations.


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3.6 MOMENT RY SHORT CIRCUIT CALCULATION

The fact that the current changes in magnitudewith time has led to the establishment of two bases ofshort-circuit-current ratings on power circuit breakers: 1)the momentary rating or its ability to withstand mechanicalstresses due to high short-circuit current and 2) theinterrupting rating or its ability to interrupt the flow ofshort-circuit current within its interrupting element.

Calculation of the precise rms value of anasymmetrical current at any time after the inception of ashort circuit may be very involved. Accurate decrumentfactors to account for the dc component at any time arerequired, as well as accurate factors for the rate of changeof the apparent reactance of the generators. This precisemethod may be used if desired, but simplified methods havebeen evolved whereby the dc component is accounted for bysimple multiplying factors. The multiplying factor convertsthe rms value of the symmetrical interrupting) ac wave intorms amperes of the asymmetrical momentary) wave including adc component.

The magnitude of the dc component depends upon thepoint on the voltage wave at which the short circuit occurs.For protective-device application, only the maximum dccomponent is considered, since the circuit breaker must be


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26applied to handle the maximum short-circuit current that canoccur in a system.

In the general case for circuits rated above 6volts, the multiplying factor to account for dc component is1.6 times the rms value of the ac symmetrical component atthe first cycle. For circuits 6 volts and less, themultiplying factor to calculate the total current at thefirst cycle is 1.25 when the circuit breaker is applied onthe average current in three phases. These factors arelisted in table 3.1 [2]

Since the short-circuit current is maximum at thefirst-cycle, the short-circuit current must be determined atthe first-cycle to determine the maximum momentary duty oncircuit breaker. To determine the short-circuit current atthe first cycle, it is necessary to consider all sources ofshort-circuit current, that is, the generators, synchronousmotors, induction motors, and utility connections. Thesubtransient reactances Xud) of generators, synchronousmotors, and induction motors are used in the impedancediagram.

Procedure for determining momentary currentconsists of calculating E Z the line to neutral voltage atMthe breaker divided by the equivalent momentary systemimpedance at that point. Since the dc component is presentat this time, it is necessary to account for it by the use


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of a multiplying factor as mentioned/ earlier, to find thefirst-cycle total short-circuit-current duty per-unitcurrent. Breaker application 600 volts or above) will beproper if E ZM times a factor of 1.6 does not exceed themomentary current rating of the breaker.

Where IIC is the first-cycle short-circuit asymmetricalcurrent. Refer to Appendix 2 section A2.6) for the Belleplant momentary short-circuit current calculation.

3.7 INTERRUPTING SHORT CIRCUIT CALCULATION

To check the interrupting duty on a power circuitbreaker, the short-circuit current should be determined atthe time that the circuit-breaker contacts part. The timerequired for the circuit-breaker contacts t part will varyover a considerable rang, because of variation in relay timeand in circuit-breaker operating speed. The fewer cyclesrequired for the circuit-breaker contacts to part, thegreater will be the current to interrupt. Therefore, themaximum interrupting duty is imposed upon the circuitbreaker when tripping relays operate instantaneously. Toaccount for variation in the circuit-breaker operatingspeed, power circuit breakers have been grouped into severalclasses, such as eight-cycle, five-cycle, and three-cycle


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circuit breakers.In industrial plants eight-cycle circuit breakers

are generally used. Normally the induction-motorcontribution has disappeared and that of the synchronousmotors has changed from the subtransient to the transientcondition before the contacts of these circuit breakerspart. Therefore in calculating the interrupting duty oncommonly used power circuit breakers generator subtransientreactance and synchronous-motor transient reactance are usedand unlike other methods which induction motors areneglected induction motor transient reactance are used.This will cause the calculated interrupting short-circuitcurrent to be higher than the actual value which creates asafety factor for selection of circuit breakers for theentire system.

Instead of specifying a time at which theshort-circuit current is to be calculated it is determinedby specifying the generator and motors reactances and usinga multiplying factor. These factors are listed in Table 3.1

The procedure for determining interrupting currentconsists of calculating E / Z I the line to neutral voltage atthe breaker divided by the equivalent interrupting systemimpedance at that point and then applying multiplyingfactors to determine total current at the time of breaker


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29contact parting. This total current is compared with thebreaker total current interrupting rating.

Reactance values used with NSI standard C37.51969 for motors transformers generators and utilitysources are listed in Appendix 1 table Al-1.


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Table 3.1 Machine Reactance and Multiplying FactorsUsed in Simplified calculation of Short circuitDuty [I

Machine Reactances to Use

Synchronous Synchronous Inductione a t o s Motors Motors

EquipmwttMultiplying Focmr

t be Appliedto CalculatedSyrnmehicoiValue*

None

Type of Short-circuit

GeneralCaset

Rating

1 I

Subtmnsient 15

Subtransient 1 27

Sp&lCaret

Subtronsient(x )

N o g l u t1x1

Subtransient(xl)

Subtransientx?

with Rated InterruptingTimes of cydes(Refer to the TotalCurrent Rating k r i s -ASA C37.6-1964)

Fuses and fusedCutouts(o ba e 1500 volts

Subtransientx?

Transient

L-V Power Circuit BreakersL-V Molded-core CircuitQr#kersL-V Motor Controllers (In-corporaiing Fusesor M o id d -rose Circuit Breakers)

L-V Fuses1-V Buswoy

k s &racing imL-V Switchgear1-V SwitchboardsL-V Motor -contro i CantersL-V Panelboards

Power Circuif Ereakors(above 6 volts) Available

Mocncntary-AsymmetricalAmperes Available

Interrupting-AsymmetricalAmpares Available

xl)

Subtmnsient(x )

Subtransientx?

Symmetrical AmperesAvailable

Interrupting--Symmetrical+aperesor MVA

Subtransient( x 7

Subtransient


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CHAPTER FOUR

GENERAL QUIPM NT USED FOR PROTECTION

We usually think of an electric power system interms of its more impressive parts, the big generatingstations, transformers, high voltage lines, etc. Whilethese are some of the basic elements, there are many othernecessary and fascinating components. Protective relayingis one of these [ 3 ]

The function of protective relaying for systemprotection and coordination is to minimize damage to thesystem and its components and to limit the extent andduration of service interruption whenever equipment failure,human error, or acts of GOD occur on any portion of thesystem. The relaying equipment is aided in this task bycircuit breakers that are capable of disconnecting thefaulty element when they are called upon to do so by therelaying equipment.

Overcurrent protection device selection andcoordination is an engineering decision that should be madeto protect the system from short-circuit or fault currents.

Protection for electric systems is an art as wellas science and should be designed with the followingobjectives in mind:


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1 Prevent or minimize damage to equipment.2 Minimize the effect of the disturbance on the

un-interrupted portion of the system, both in its extentand duration.

3 Minimize interruption of power.4 minimize the effect on the utility system [4].5 prevent injury to personnel.

The isolation of short circuits requires theapplication of protective equipment which will sense anabnormal current flow and remove the affected portion fromthe system. The sensing device and interrupting device maybe completely separate, interconnected only through externalcontrol wiring, or they may be the same device or separatedevices mechanically coupled to function as a single device.Equipments used for protection are overvurrent relays,fuses, and circuit breakers. The following sections explaineach of these devices in detail.

4.1 OVERCURRENT RELAYS

Overcurrent relays are sensing devices only andmust e used in conjunction with some type of interruptingdevice to interrupt a short circuit and isolate the affectedportion of the system. These relays may be eitherdirectional or non directional in their action. They may be


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33instantaneous or time delay in response. Various timecurrent characteristics, such as inverse time, very inversetime, extremely inverse time, and definite minimum time areavailable over a wide range of current setting. Theovercurrent relays are generally available in the followingcurrent ratings:

Range Taps0 .5 -2 .5 0 .5 0 .6 0 .8 1 .2 1 .5 2 .0 2 .0 2 .51.5-6 .0 1 .5 2 .0 2 .5 3 .0 3 .5 4 .0 6 .04.0-16.0 4.0 5.0 6.0 7.0 8.0 10 12 16The relays can be specified to have either single

or double circuit closing contacts for tripping either oneor two circuit breakers.

Directional relay consists of two units, anovercurrent element and a directional element. The contactcircuits are arranged insuch a way that tripping occurs onlywhen current has proper relationship to the voltage withpower flow in the tripping direction. The actual trippingof the circuit is done by. a contact on the overcurrentelement, the overcurrent element does not operate until thecurrent is flowing in the proper direction and is above thepickup setting. The overcurrent element cannot operate on afault in the nontripping direction. For typical applicationrefer to Appendix 5 section A5.3 device No. 7 CWC type)

The instantaneous element is set for a current


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4

higher than that which should operate the time-delayelement. The contacts ot this element are either connectedin parallel with the contacts of the time-delay element orthey aconnected to seperate terminals. This element ismainly used when time interval required between twoovercurrent relays can not be made in the short time region.Refer to Appendix 5 section A5.12) for instantaneoussetting used in portion of the Belle plant.

The time-current characteristics for a variety ofrelays used in this report are shown in Appendix 6. Thesecharacteristics give the contact closing times for thevarious time dial settings when the indicated multiples oftap current are applied to the relay. In Chapter severalmethod .of coordination for these relays will be discussed.

4 2 FUS S

Fuses are the oldest and simplest of allprotective devices. The fuse is both the sensing andinterrupting device. They are installed in series with thecircuit and operate by the melting of a fusible link inresponse to the current flow through them on an inverse timecurrent basis. They are one-shot devices since theirfusible elements are destroyed in the process ofinterrupting the current flow. Fuses may have only the


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ability to interrupt short circuit current up to theirmaximum rating or the ability to limit the magnitude ofshort circuit current by interrupting the current flowbefore it reaches its maximum value.

As shown in table 4.1, several types of fuses arecompared. Each fuse has a different characteristic. Forexample, code-type fuses are not recommended because of lowinterrupting capability and high melting current 70 timesfuse rated current) at 0.01 second. Current limiting Amptrapfuses have high interrupting capability and very low meltingcurrent 3.3 times fuse rated current) at 0.01 second.These fuses are mainly recommended for branch circuits andshould not e used for protection of transformers. Sincetransformers have an inrush current sometimes 12 times fullload current). This type of fuses will melt before thetransformer is energized. Chase Shawmut C-S) one-time fuseis the best choice for most applications because of theirlow cost and ratings.


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Table 4 . 1 Fuse Classification

Code fuse NEC)Semi lag fuseRowan AIR-SEAL

i one timedual element fuseC-S new trionicn

Current limitingAmptrap 6 11Limitron

G.E. CLF-JSolid statecurrent limitin

Melting currentAverage multiplesof rating amperesmelt in melt in0 . 0 1 s 10 s

70 3

30 3.525 3

30 520 645 5.5

15 2.513 2 .512 3

3 .3 2

Interruptingcapability

amperes

3,000

50,000 ii

50)000I

1 O O O O O

10

100, 000100 000100 000

100,000


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37COST OF FUSES [51

Approximate costs of fuse types, for given,amperage, relative to NEC type are.

NEC fuseC-S one timeRowan air sealC-S trionicFusetronLo-peakCurrent Limiting

Semi lag and dual element fuses of smaller size than NECsize may be applicable, with proportionaly reduced cost. Onthis basis, the Chase-Sha~ut C-S) one-time fuse appears theoptimum for all uses within its interrupting capability of50 000 amperes.

CURRENT LIMITING CHARACTERISTICS OF C-S ONE TIME FUSESChase-Shawmut C-S) one-time fuses are current

limiting, but not so much as Amptraps. 30-ampere one-time fuse is about as current limiting as 60-ampereAmptrap. Table 4.4 hows the comparison of Chase-Shawmutone-time fuses and Amptrap fuses.


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Table 4 2 C-S one time fuses vs Amptrap fuses

50,000 7 8 11 16 24 35--- ---- - -- ---- -C-S) ONE-TIME fuse rating:

Available currentRMS amperes )

Fuses, and their characteristics which have been

Let- through currentthousands of amperes peak AMPTRAP

Amptrap fuse rating)

used in this study is shown in Appendix 6

4 3 CIRCUIT BREAKERS

Circuit breakers are interrupting devices only andmust be used in conjunction with sensing devices to fulfillthe detection function. In the case of medium-voltage


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391-72.5 KV circuit breakers, the sensing devices are

separate protective relays or combinations of relays. Inthe case of low voltage under 1000 volts) circuit breakers,sensing devices may be external protective relays orcombination of relays. In most low voltage application,either molded-case circuit breakers or other low voltagecircuit breakers having series sensing devices built intothe equipment are used.

The ratings which apply to circuit breakers andthe actual assigned numerical values reflect the mechanical,electrical, and thermal capabilities of their majorcomponents. Basic ratings are1) Rared voltage2) Rated frequency3) Rated continuous current4 Rated interrupting current5) Rated short-time current

The basic overcurrent trip device characteristicused on molded-case circuit breakers and low voltage circuitbreakers are long-time delay and instantaneous. Thecombination of these characteristics provides time delaytripping for those low-level short circuits or overloadsthat persist, and instantaneous tripping for higher levelshort circuits.

New molded-case circuit breakers are equipped with


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40

a short-time-delay characteristic in place of theinstantaneous characteristic. The resulting combination oflong-time-delay and short-time-delay characteristicsprovides delayed tripping for all levels of current up tomaximum allowable available short-circuit-current limit ofthe circuit breaker without instantaneous trip element.

These new breakers are used for the Belle plant,so that better protection can be obtained throughout thesystem without using instantaneous trip element. Refer toAppendix 6 section A6.7) for the device characteristiccurve, which shows available settings for long-time-delayand short-time-delay pickup values.

4 4 GENERATOR PROTECTION

The protection of generators involves theconsideration of more possible abnormal operating conditionsthan the protection of any other system element. nunnecessary generator outage is undesirable, but one shouldnot try to avoid it by the omission of otherwise desirableautomatic protection.

The practice of using centralized control isincreasing, which requires more automatic equipment and lessmanual supervision. Such practice requires more automaticprotective relaying equipment to provide the protection that


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41was formerly the responsibility of attendants [ 3 ]

The following protective relays are used for thegenerator protection. Refer to Appendix 3 for the type andsettings of protective relays used for the Belle plantgenerator protection.

PERCENTAGE DIFFERENTI L RELAYSThese relays are used for generators rated 1000

KV or higher [ 3 ] Percentage differential relaying is thebest for the short-circuit protection of stator windings,because of its high-speed instantaneous capability.

Generally, the practice is to have thepercentage-differential relays trip a hand-resetmulticontact ,auxiliary relay. This auxiliary relaysimultaneously initiates the following: 1) trip mainbreaker, 2) trip field breaker, 3) shut down the primemover and 4) operate an alarm. These will result inminimizing damages to the generator.

SOLID STATE UNDER-FREQUENCY RELAYSThese relays are used for automatic load-shedding.

When a system overload occurs, under-frequency relays mustdisconnect load to arrest frequency decline. The output ofgenerating plants may be impaired below 5 7 5 7 . 5 Hz, soshedding must be completed before this level is reached.


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OVER-VOLTAGE RELAYSThis type of relays are used for hydroelectric and

gas-turbine generators. It operates by first causingadditional resistance into the generator or exciter field.Then if over-voltage persists the main generator breakertrips.

OVER TEMPERATURE RELAYSUnbalanced three-phase stator currents cause

double-system-frequency currents to be induced in the rotoriron. These current quickly increase rotor temperature andcause serious damage to the generator if it continues tooperate with such conditions. These relays will preventgenerators from overheating.

LOSS-OF-EXCITATION RELAYSWhen a synchronous generator loses excitation it

operates as an induction generator running abovesynchronous speed. Round-rotor generators are not suited tosuch operation because they do not have amortisseur windingsthat can carry the induced rotor currents.

Most systems cannot tolerate the continuedoperation of a generator without excitation. In fact ifthe generator is not disconnected immediately when it loses


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4excitation, widespread instability may very quickly develop,and a major system shutdown may occur. When a generatorloses excitation, it draws reactive power from the system,amounting to as much as 4 times the generator s rated load.This will cause extensive instability in the system. Themost selective type of loss of excitation relay is adirectional-distance type operating from the a-c current andvoltage at the main generator terminals [ 3 ]

REVERSE POWER RELAYSThese relays are used to detect motoring.

Motoring protection is for the benefit of the prime mover orthe system, and not for the generator. However, it isconsidered here, because it is closely associated with thegenerator.

FIELD GROUND-FAULT PROTECTION REL YSBecause field circuits are operated ungrounded, a

single ground fault will not cause any damage or affect theoperation of a generator in any way. However, the existenceof a single ground fault increases the stress to ground atother points in the field winding when voltages are inducedin the field by stator transients. Thus, the probability ofa second ground occuring is increased.


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44Note: refer to Appendix for the types and settings used

for protection of the Belle plant generator.

4.5 TRANSFORMER PROTECTION

The function of transformer protection is toprotect the transformer. Transformer protection is requiredto automatically disconnect a transformer from the powersystem for any of the following reasons.1 To prevent higher-than-rated temperatures from

developing in the transformer from excessive loadcurrent and, thereby, causing rapid deterioration ofinsulation or conductors. Protection provided to.achieve this is known as overload protection.

2) To prevent mechanical and thermal effects of largethrough currents from causing permanent deformation or

other damage to the transformer. Such protection isknown as short-circuit protection.

3) To minimize the spread of damage inside a faultedtransformer, and minimize power-system disturbanceresulting from transformer faults.

TRANSFORMER SHORT CIRCUIT CAPABILITIESThe ability of a transformer to withstand through

short-circuit current is defined as its ANSI point . This


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45point specifies the magnitude and duration of maximumpermissable through current that a transformer can withstandwithout sustaining damage.

Values of the ANSI point vary from 25 times therated full-load current for 2 seconds, for a transformerwith four percent or less impedance, to 1 4 . 3 times the ratedfull load current for 5 seconds, for transformers with sevenpercent impedance as shown in table 4 . 5 . To properlyprotect the transformer, the primary fuse must actuallyclear in the stated time at 87 percent of the current valueof a delta-delta bank, and 58 percent of the value of adelta-wye bank.

Table 4 3 Transformer ANSI dataI sym)rms 87 SYM. 58 SYM. TIME s)rms rms

Fault PH-PH FAULT L N AllowableDelta-Delta Delta-Wye

4 orless5

6

7 ormore

1

2

3

4

25 RatedCurrent20 X RatedCurrent

1 6 . 6 X RatedCurrent

1 4 . 3 X RatedCurrent

22 X RatedCurrent17 RatedCurrent14 X RatedCurrent12 X RatedCurrent

14 RatedCurrent11 RatedCurrentX RatedCurrent

8 X RatedCurrent


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TRANSFORMER INRUSH CURRENTpower transformer primary power fuse as well as

any other protective device used in a transformer protectionscheme must allow the transformer to be energized withoutactuating the protective device on the magnetizing inrushcurrent.

The precise magnitude and duration of inrushcurrent vary from one transformer to another and for anyspecific transformer can only be determined by test.Commonly used estimates of magnetizing inrush currents forprimary and secondary substation transformers range from anequivalent of 8 to 12 times full load rms current with aduration of 0.1 second. The power or current-limiting fuseshould be selected so to be capable of carrying at least 12times the full-load rated primary current of the transformerfor 0.1 second without damaging the fuse in order to passsafely the inrush currents which occur during switchingoperations.

Refer to Appendix for tranformer protection usedthroughout the Belle plant.


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CHAPTER FIVE

COORDINATION OF PROTECTIVE DEVICES

The coordination study of an electric power systemconsists of an organized time-current study of all devicesin series from the utilization device to the source. Thisstudy is a comparison of the time it takes the individualdevices to operate when certain levels of normal or abnormalcurrent pass through the protective devices.

coordination study or revision of a proviousstudy should be made for an existing plant when new loadsare added to the system or when existing equipment isreplaced with higher rated equipment. coordination studyshould also be made when the available short-circuit currentof the source to a plant is increased. This studydetermines settings or ratings necessary to assurecoordination after system changes have been made.

The objective of a coordination study is todetermine the characteristics ratings and settings ofovercurrent protective devices which will ensure that theminimum unfaulted load is interrupted when the protectivedevices isolate a fault or overload anywhere in the system.At the same time the devices and settings selected mustprovide satisfactory protection against overloads on the


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48equipment, and interrupt short circuits as rapidly aspossible.

5.1 PRIM RY CONSIDERATIONS FOR COORDINATION

In order to obtain complete coordination of theprotective equipment applied, it is necessary to obtain allof the following requirements.

SHORT-CIRCUIT CURRENTSThe following information on short-circuit

currents should be provided for every bus on the system.1) Maximum and minimum to 3 cycle momentary) total

rms short-circuit current2) Maximum and minimum cycle to 1 s interrupting duty)

total rms short-circuit currentThese short-circuit current values are obtained as

described in Chapter Three.The maximum and minimum to 3 cycle momentary)

currents are used to determine the maximum and minimumcurrents to which instantaneous and direct-acting tripdevices respond, and to verify the capability of circuitbreakers, fuses, switches, and bus bracings.

The maximum 3 cycle to 1 s interrupting) currentat maximum generation will verify the ratings of circuit


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49breakers, fuses, and cables. This is also the value ofcurrent at which the circuit protection coordinationinterval is established. The maximum 3 cycle to sinterruptig) current at minimum generation is needed to

determine whether the circuit-protection sensitivity of thecircuits is adequate.

TIME INTERVALS FOR COORDINATIONWhen plotting coordination curves, certain time

intervals must be maintained between the curves of variousprotective devices in order to ensure correct sequentialoperation of the devices. These intervals are requiredbecause relays have overtravel, fuses have damagecharacteristics, and circuit breakers have certain speeds ofoperation. These intervals are often called margins.

When coordihating inverse-time overcurrent relays,the time interval is usually 0.3 0.4 seconds [ 6 ] Thisinterval is considered between relay curves, either at theinstantaneous setting of the load side feeder circuitbreaker relay, or at the maximum short circuit current whichcan flow through both devices simultaneously, whichever isthe lower value of current. The interval consists of thefollowing components:Circuit breaker opening time 5 cycles)Overtravel

08 seconds.10 seconds


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5Safety factor .12 .22 seconds

This margin may be decreased if field tests ofrelays and circuit breakers indicate that the system stillcoordinates with the decreased margins. The overtravel ofvery inverse and extremely inverse time overcurrent relaysis somewhat less than for inverse relays allowing adecrease in time interval for carefully tested systems to0.3 seconds. 0.3 Seconds time interval is used forcoordination of the Belle plant overcurrent relays.

When circuit breakers equipped with direct-actingtrip units are coordinated with relayed circuit breakersthe coordination time interval should be 0.3 .4 s.

HOW TO RE D URVESbasic understanding of time-current

characteristic is essential to any study. Time 0 isconsidered as the time at which the fault occurs and alltimes shown on the curve are the elapsed time from thatpoint. The curves that are drawn are response times sincefor a radial system all the devices between the fault andthe source experience the same current until one of theminterrupts the circuit.

coordination curve is arranged so that theregion below and to the left of the curve represents an areaof no operation. The curves represent a locus of a family


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UJ

of paired coordinates which indicate how long a period oftime is required for device operation at a selected value ofcurrent.

Reading current along the abscissa, the time orrange of times in which any device is expected to operatecorresponds to the ordinate or ordinates of the curveplotted. Relay curves begin at a point called pickup (theminimum value of current which will cause the relay to closeits contacts) and end at the maximum short-circuit currentto which the device under consideration can be subjected.single curve should be drawn for any device underconsideration.

5 2 DATA REQUIRED FOR A COORDINATION

The first requisite for a coordination study is asingle-line diagram of the system. This diagram should showthe following data.1 Apparent power and voltage ratings as well as theimpedance and connections of all transformers2) Nameplate ratings and subtransient reactance of all

major motors and generators3) Conductor sizes, types, and configurations4 Current transformer ratios5) Relay, direct-acting trip, and fuse ratings,


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5characteristics, and ranges of adjustment

Refer to Appendix 2 section A2.4) for the Belleplant single-line diagram, which includes the aboveinformation.

The second requirement is a complete short-circuit-current study as described earlier in this Chapterand also in Chapter Three.

The third requirement is the time-currentcharacteristics and typical time curve this curve is usedfor the new method of coordination in this report) of allthe devices under consideration.

The forth requirement is starting currents andaccelerating time of large motors.

Once this information is .assembled, it is thennecessary to select the protective devices so that theyperform their assigned function of protecting individualpieces of equipment and operate so that only the minimumamount of circuitry associated with the fault is isolated.

5.3 METHODS O COORDINATION

There are several methods of overcurrent relaycoordination available to minimize the effects of suchsystem abnormalities on the system itself or on theutilization equipment which it supplies. These methods are


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as follows.1 Coordination by table2 Use of device characteristic curve3 Use of typical time curve of device new method

presented in this report)The following section will explain these method in

detail with some examples.

METHOD ONE COORDINATION BY TABLEThis method mainly uses fuse as the protective

device for the coordination of the system.I n fuse applications, coordination is achieved

through the use of selectivity ra tio table s. Table 5.1shows a typical selectivity schedule for variouscombinations of fuses. This schedule is limited to severalclass and type of fuses. This table is used as a simplecheck for selectivity assuming that identical or reducedfault currents flow through the circuits in desending or der,that is , main-feeder-branch. A coordination study may bedesired when the simple check as outlined is no t sufficient,and can be accomplished by plotting fuse time-currentcharacteristic curves on log-log graph paper.

Fo r example, when the largest branch device is a100 A current limiting fuse, the main fuse may be a 200 AArnptrap r 100 A dual-element or one-time fuse.


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54Table 5.2 shows the coordination between the

branch or load protective device and main or supplyprotective device when a power air circuit breaker is usedin main or supply circuit. For example, for a currentlimiting fuse used in branch or load circuit, multiple of2 . 5 times the ratio of 80 percent long time pickup to 50 0percent short time pickup of power air circuit breakershould be used for proper coordination. This multiple is1.25 when the ratio of 160 percent long time pickup over100 0 percent short time pickup is used.

Some of the basic rules of coordination for thismethod are [6]1) Branch fuse-clearing time must be less than the melt time

of the main fuse. That means under the total maximumclearing I t of the largest fuse on the load side shouldnot exceed 90 percent of the melting I t of the supplyside fuse. I t values must be used t o check coordinationin the current-limiting range . Below the current-limitingrange the time/current characteristic curve s may becompared as follows: 1) add 10 percent to the curvecurrent values for the downstream fus e, 2) subtract 10percent from the curve current values for the upstreamfuse. The resulting curves should allow at least a 10percent current margin between the two fuses based uponthe downstream fuse. The result is a minimum fuse size


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s p r e a d w hi ch s h o u l d c o o r d i n a t e w i t h o u t damage t o u p s t re a mf u s e . More s p r e a d w l l o f c o u r s e i n c r e a s e a s s u r a n c e o fc o o r d i n a t i o n .

2 B ra nc h d e v i c e c l e a r i n g t im e m u s t be u n d e r 80 o f t h es e n s i n g t i m e o f a b r e a k e r .


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Table 5.1 Rating Ratios for Safe Coordination

Branch or Main or supply deviceload device minimum multiple of branch device)

current limiting, dual element, semi lag,c-sAmptrap,limitron, trionic and one-time and


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Table 5.2 Power air circuit breakerMinimum band

Branch or load Main or supply devicedevice (minimum multiple of branch device) -

80 long time pickup 160 long time pickup500 short time pickup 1000 short time pickuq

-current lim.dual elementC S one timeNEC

Molded caseair circuitbreaker50 A100 A100 A(heavy duty)225 A600 A

ratio ratio-- -- -- -- -- -- ---- .2.55.0 1b25.0 2.510.0

4.0

5.07.0

4.0

3.0

5.0

2.02.53.5

2.01.5


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8METHO TWO USING DEVICE CHARACTERISTIC CURVES

In order to start drawing curves a voltage leveland multiple scale current in ampere should be selected andmarked on the bottom of the log-log paper. Then propermultipliers for the various voltage levels considered in thestudy are calculated. protective device characteristiccurves are then placed on a smooth bright surface such as awindow pane or a glassed-topped box with a lamp in it. Thesheet of log-log paper on which the study is being made isplaced on top of the device characteristic curve thecurrent scale of the study lined up with that of the devicecharacteristic. The curves for all the various setting andratings of devices being studied may then be traced orexamined [4]

METHO THREE USING DEVICE TYPICAL TIME CURVESThis method present a new way of drawing

co-ordination curves which has been practiced throughout theBelle plant coordination study. This method is recommendedfor more accurate protective relaying coordination. Insteadof device characteristic curve Typical time curves ofovercurrent relays used in this method. These curves showtime versus multiples of tap value current. Eachovercurrent relay has several tap value and time dialsettings. Knowing the amount of maximum short circuit


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59current and current transformer ratio is essential. Bydividing several values of short circuit currents to c.t.ratio and the tap value, several multiples of tap values canbe found. Refering to the curve time for these multiples oftap value can be found depending upon what time dial settinghave been selected. At this time the short circuit currentand the time for several points are known. These points arethen ploted and traced on log-log paper. Now let s assumethat an overcurrent relay at a different voltage level mustbe coordinated with this relay. The minimum interval timeof 0 3 seconds must be allowed between the two relays. Inorder to illustrate this method of coordination and decisionmaking, a portion of Belle plant detailed relaying diagramis shown as an example in figure 4.2 and the related curvesare shown in figure 4 3 Figure 4.1 shows the typical timecurves of CO 9 overcurrent relay. The following exampleillustrates this new method.

EX MPLEFigure 4.2 shows a portion of the Belle plant

overcurrent detailed drawing. There are two overcurrentrelays involved in this portion of the system, which willneed coordination with the incoming power company feederovercurrent relay. Let s assume that fault A happened onthe syst em. in order to isolate the fault from spreading to


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other portions of the system unit 2A breaker must operatefirst so that only feeder 2A shuts down. If the shortcircuit is not cleared yet then bank No.1 primary mustoperate. Finally if the short circuit is still not clearedthen power company No.2 primary feeder must operate.

Figure 4.3 shows the CO-9 type overcurrent relaycurves that was selected and ploted for maximum protectionof this portion of the system. Following data are requiredfor the selection of these curves.No.2 primary feeder overcurrent relay dataCircuit: Bank No.1 primaryRelay type: Westinghouse CO-9C.T. ratio: 200/5=40/1Available Taps=1.0 1.2 1.5 2.0 2.5 3.0 3.5~4.0 5.0 6

7.0 8.0 10.0 12.0Ampere range of relay=l-12Circuit voltage: 6 kV levelMaximum available short circuit at 6 k level: 14 300 AUnit A overcurrent relay dataCircuit: Unit ARelay type: Westinghouse CO-9C.T. ratio: 800/5=160/1Available taps= same as relay No.1Ampere range of relay=l-12Circuit voltage: 12 kV level


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61Maximum available short circuit at 12 kV level: 19,400 A

Since Bank No.1 is upstream in the system relativeto Unit 2A, its overcurrent relay should be drawn first andthen unit 2A overcurrent curve should be drawn to coordinatewith that.Relay No.1 settingsC.T.=200/5=40/1Let s select the following setting to see if propercoordination can be obtained.Selected tap=12Selected time lever=lOSelected multiple current=lOSelected voltage level=46kVMultiple of tap value=(short-circuit

current) (tap) (C. atio)Pickup* value= (tap) (C. ratio)*Pickup is the minimum value of the current that can startthe relay to close its contact.Point No. : 14,300 A /(12) (40)=29 off scale on figure 4.1Point No. 9,000 (12) (40)=18.75 multiple of tap value

For this value and time lever of 10, time willbe equal to 1.2 seconds from figure 4.1.Point No. 7,000 (12) 40) 14.6 then time=l 3 secondsPoint No. : 5,000 A (12) (40)=10.4 then time=l .5 secondsPickup value= (12) 40) 480 A


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62These points should be plotted and traced for the

relay curve as shown on figure 4.3 (curve number 1).Relay No.2 settingsC.T.=800/5=160/1Let s select the following setting, so that propercoordination can be obtained with the upstream relay.Selected tap=7Selected time lever=7Selected multiple current=lOSelected voltage level=12 kVMaximum available short circuit at 12 kV=19,400(19,400) (12 kV) 46 kV) =5,060 at 46 kV levelPoint No. 5060/(7) 160)=4.5 at 46 kV level

For this value and time lever of 7, time=2.1 sPoint No. 4000/(7) 160) =3.57 then time=3.0 sPoint No. : 3500/ 7) 160) =3.12 then time=3.9 sPickup value=(7) (160) (12 kV (46 kV)=292 at 46 kV level

These points also should be plotted on figure 4.3.As shown unit 2A overcurrent relay (curve number 7) willoperate first in case of fault and will clear the faultbefore Bank No.1. primary relay operates, since there isabout 0.6 second time interval allowed between the curves.Both relay were tested and field checked for properoperation based on the method recommended on section 4.7.


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OV R C U R R E Y T R E L Y5 0 6 0 t i E R T Z

VaSY,

MULTIPLES O T P V L U E C U R R E H T

F i g u r e 5 1 T y p i c a l t i m e c u r v e s of t y p e CO 9 R e l a y [ ]


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I K V US ' 0

-a3;7

Figure 5 2 Portion of Belle plant Detailed elaying Diagram


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Figure 5 3 Portion of elle plant Coordination curves


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5.4 TESTING OF PROTECTIVE DEVICESThis requires that the system or protective device

be subjected to abnormal electrical conditions and theoperation of the system or devices compared tomanufacturers specifications for these conditions.

MOLDED CASE CIRCUIT BREAKERSIn testing a molded-case circuit breaker, several

points must be remembered.1 Nameplate rated voltage must be available at the input

terminals throughout the test.2) The values of current are high and voltage is low,

therefore it is.advisable to use connections having theshortest possible length and largest,.cross sectional areabetween test unit and circuit breaker.. In some cases,pieces of bus bar may be used for these connectors.

3) The connection to the circuit breaker must be tight.4 The circuit breaker tested one pole at a time.5) Trip devices must be allowed to fully reset before

performing a check test.Molded-case circuit breaker should be tested for

(1) timing and (2) instantaneous pickup. The recommendedvalues of test current is three times the circuit breakertrip unit rating. The tripping time must be measured andcompared to the manufacturer s specified values or curves.


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Molded-case circuit breakers may be relativelyprecise; however, the published time-delay characteristicindicates a wide band of operation. The electrical testwill reveal circuit breakers that will not trip, those thattake abnormally long to trip, and those that have no timedelay. If the test reveals that the circuit breaker istripping within plus or minus 15 percent of the outsidelimits of its published curves and this tolerance does notaffect the electric system coordination or stability, thecircuit breaker should be considered satisfactory otherwiseshould be rejected.

n electrical test for pickup of the instantaneousunit should be run to verify that the circuit breaker istripping magnetically. Testing at one of the lowercalibration marks is satisfactory. The adjustment may beset to the lowest calibration point to verify that the unitwill pick up. If the instantaneous unit picks up at theminimum calibration point, then pickup will be assumed to bewithin manufacturer s tolerances.

LOW-VOLTAGE POWER CIRCUIT BREAKl3R [ ]Most of these circuit breakers are equipped with

one series overcurrent trip device per phase. Theelectrical test must be run on each individual trip device.The operation of any one of these devices will trip all


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8poles of the circuit breaker.

The recommended test for a low-voltage powercircuit breakers are (1) timing (long-and short-time delayunits if the circuit breaker has both type trip units), and(2) instantaneous pickup.

The recommended values of test current for longtime delay is three times the trip unit setting, and forshort-time delay it is one and one-half times the circuitbreaker short-time delay setting; determine theinstantaneous pickup. If the circuit breaker does notoperate within the tolerances shown by the manufacturerstime current curves, then suitable adjustment should be madeas recommended by the manufacturer.

PROTECTIVE REL YSThe protective relay is the brain of the electric

protective circuit. It is the relay that senses an abnormalcondition and then sends the message to other devices on thesystem. Therefore it is imperative that any relay work bedone in a very through manner.

timing check should be made to see that therelay closes its contacts within a specified time for agiven abnormal value of current. Normally this test is runwith the relay tap in its designated position. It issuggested that a test current of four times pickup be used.


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9

Based on above methods all the protectiveequipment used in the Belle plant were field-checked andtested for proper operation and coordination for the entiresystem. The 12 kV molded-case circuit breakers were testedas follow:

Minimum pickup long time delay LTD)Minimum pickup was checked by applying 1 percent

less than pickup current for 10 minutes. The breaker didnot trip.Time delay LTD)

1 Percent pickup value was selected and appliedto the relay. The time was measured until the breakertripped. The time measured was within the shaded area ofthe characteristic curve for the test current used.

Minimum pickup short time delay STD)The LTD was blocked to be sure it does not trip

the breaker. The breaker was closed and set to zero. Theoperate button was pushed and test current gradually wasincreased. The current at which the STD armature starts toclose is the minimum pickup.

All the overcurrent protective relays were testedfor proper operation as follow:


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Zero adjust testThis test is to determine that relay contacts

close when the dial is set at zero. With the continuitylight connected across the terminals, time dial was manuallyturned until the indicator light on the test set glowed.The reading was at zero.

Pickup testThis test is to determine the minimum operating

current needed to close the relay contacts for anyparticular tap setting. The pickup value selected for thistest, was equal tap value plus or minus 5 percent. Byalternately increase and decrease current, the point wherecontinuity indicating light flickers was found and recorded.This flickering indicates that the contacts are justmaking and breaking. The recorded values in some relayswere higher than tap value, the spiral-spring tension wastoo great and was adjusted for the correct tap value.

Time current characteristicsA timing check should be made to see that the

relay closes its contacts within a specified time for givenvalues of current. Three times tap value was used as theminimum amount of test current for the timing check. Timer


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selector switch was put in N.O.MOMENTARYtt osition, theinitiate push button was jogged by using the main andvernier controls and adjust the test unit to pass the testcurrent through the relay coil. The timer selector switchwas changed to N.O.MAINTAIN1' nd the main ammeter presetpointer was set to a value just under the test current.Then the timer was reset to zero and the initiate pushbutton was pressed. This puts test current on the relaycoil and starts the timer. When relay contacts close, thetimer will stop and current will be removed from relay. Thetest current and time were compared with 3 times tap value.Some relay operating were too fast and the time dial settingwas increased up to 1/2 division.


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CH PTER S I X

CONCLUSIONS

This report has illustrated methods of longhandshort-circuit-current calculations and new method ofcoordination by using typical time curves of the protectiverelays for quick isolation of the affected portion oflow-voltage and medium-voltage industrial and commercialpower systems. The method of longhand calculation is notrecommended for large power systems since these plantscontain many closed loops. Network analyzer or digitalcomputer technique is favorable from an economic and timesaving standpoint for these systems.

s a result of the study done for the Belle plantelectrical short-circuit-current calculation andcoordination of protective devices which was used as anexample for this report; show that 12-kV 2.4-kV and480-volt feeder breakers have adequate momentary andinterrupting ratings. Short-circuit duties of the aircircuit breakers installed at the purchased-power substationshould be reviewed again for proper operation and adequateratings when significant load is to be added that timethe 12-kV bus tie breakers could be opened. Then only onetransformer would supply each 12-kV and worst-case


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7symmetrical interrupting duty would be reduced toapproximately 250 MVA. This will minimize potential damageto the syste m its components and the utilizationequipment it supplies.

Based o n the values of the short-circuit-currentscalculated for the Belle plant proper protective deviceswere selected and coordinated for the detection and promptisolation of the affected portion of the system whenever ashort-circuit occurs in the system. The recommended relaysettings associated with these devices were field-tested forproper operation based o n the method of testing mentioned insection 4 7

This report will be a good reference manual forindustrial plant engineers electricians industrial powerapplication engineers and others who are involved with theplanning of electrical facilities for low-voltage andmedium-voltage industrial plants or commercial buildings.


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BIBLIOGRAPHY

System engineering apparatus distribution salesdivision. "Short circuit current calculations forindustrial and commercial power system^, ^ GeneralElectric Co., September 1978.Donald Beeman. "Industrial Power Systems Handbo

thesis regarding shortcircuit and protection

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