Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS 3842-9 (1977): Application guide for electrical relaysfor ac systems, Part 9: Relays for busbar protection [ETD35: Power Systems Relays]

:.

IS :3842 (part IX) -1977

Indian StandardAPPLICATION GUIDE FOR

ELECTRICAL RELAYS FOR AC SYSTEM-SPART LX RELAYS FOR BUS BAR PROTECTION

. Relays Sectional Committee, ETDC 35

Chairman Representing

fi T. S. M. RAIJ University of Roorkee, Roorkee

‘ Members%rrtx N. S. S, AROKIASWAMT Tamil Naiiu Electricity ‘Board, Madras

SHRI T. V. SUBRAMANIAM(Alterna&)S~I M. M. BANDRE Maharashtra State Electricity Board, BombaySHRIG.R.BHATIA Directorate General of Supplies and Disposals

(Inspection Wing), New DelhiDEPUTY DIRECTORSTANDARDS(TI), Railway Board, New Delhi

RDSO, LUCKNOWASSISTANTDIRECTOR STANDAREN

REUYS (A1.%rnate)DIRECTOR(CIP)SHRI A. K. GHOSE

SHRI C. GHOSE (Altirwk)SEiRID.P.GHOSH.%R1 S. GOVINDAPPASHRI I. C. JOSEPHSHRIB.R.@pTA

SHRIJ.C.Jutoij.% (Alterna&)SHRI A. D. LIUYE

SHRI A. M. KF,LKAR (Al&rn.ate)SHRI MATA PW~SHRI N. NATHSHRIT.S.NEiJI

,~HRIV. B. DESAI (Alternate)SHRI S. T. PATEL

Smu V. RAGHUPATI (Alternate)SHRI V. RADHAKRXSHNAM

SHRI M. R. NANDESHWAR(AlteIc---- ,,- 7.. m

Central Electricity Authority, New DelhiUniversal Electrics Limited, 24 Parganas

National Test House, CalcuttaKarnataka Electricity Board, BangaloreLarsen & Toubro Limited, BombayHaryana State Electricity Board, Chandigarh

Bombay Electric Supply & Transport Undertaking,Bombay

U.P. State Electricity Board, LucknowEnglish Electric Compmy of India Ltd, MadrasJyoti Limited, Vadodara

ASEA Eleetric India Pvt Ltd, Bombay

Bharat Heavy Electricals Ltd, Bhopalmtt?)

JHK1 A. lN. KAMASWA?,ly‘ire~~tfie~ of Technical Development,

SHRKR. K. GUPTA (Alternate)

(Continued on page 2)—

O Copwkht 1977 IBUREAU OF INQIAN STANDARDS

This publ@ttion is prote+ed undertheIndian Copyright Act ( XIV of 1957 ) andreproductionm whole or m partby anymeansexceptwith written permission of thepublishw shall be deemed to be an infringement of copyri~t under the said Act.

.4

.’ .

,,

i, ,, ,

,,,

Members Rcpf~titlg

StiRI P. RANoAswAMy Hindustan Steel Ltd, l&&iSoar T. C. RAJAOOF~LAN (Ahemzk)

smuu.v.R4Q Hindustan Brown Boveri Ltd, BombaySRR~ V. BALASUB RAMANIAN (Altewe)

DRB.RAMESHRAO Tata Merlin & Gerin Ltd, BombaySmu C. P. R~bu RAO (Alkmafe)

Smu A. M. SAHNI Tata Hydro-Electric Power Supply Co Ltd,Bombay

SHRI V. S. DORAI (Altcmatc)D R Y. V. SO~~AYAJULU National Physical Laboratory (CSIR), New Delhi

Soar P. SVRAYANARAYANA (Alfrmaie)SWU G. N. THADANI Engineers India Limited, New Delhi

SHRI H. K. KAUL (Altcmafc)Sxw S. P. SACHDEV, Director General, IS1 (Ex-&cio Member)

Dkector (Elec tech)

SecretarySHRI VIJAI

Deputy Director (Elec tech), IS1

Panel for Application Guide for Electrical Relays,ETDC 35 : P2

coavmrSHU G. K. THARUR

Members

f)=_” ;mGy-& (TED)

&I & GIIOSE (Alknratc)SHRI B. R. ‘GIJPTA

SHRI J. C. JUNEJA (Altwwb)!hu MATA PRAsADSHRI N. NATHSHRI V. RADHAICRISHNANSHRI V. I(ACHUPATHIDR T. S. M. %o&RI PRATAP Swoxi SATwMSHRI P. K. SENSHRI M. M. SHETHNASHRI T. V. SWRAMMYAM

Tata H dro-Electric-6

Power Supply CO Ltd,Born ay

Central Electricity Authority, New DelhiUniversal Elect&s Limited, 24 Parganas

Haryana State Electricity Board, Chandigarh

U.P. State Electricity Board, LucknowEnelish Electric Comoanv of India Ltd. MadrasBh&at Heavy Elect&a& Ltd, BhopaiASEA Ekctric India Pvt Ltd, BombayUniversity of Roorkee, RoorkeeBhakra Management Board, chandigarhPunjab State EIectricity Board, PatialaSiemens India Ltd, BombayTamil Nadu Electricity Board, Madras

Indian StandardAPPLICATION GUIDE FOR

ELECTRICAL RELAYS FOR AC SYSTEMSPART hi RELAYS FOR BUS BAR PROTECTION . . .

0. F O R E W O R D

0.1 This Indian Standard (Part IX) was adopted by the Indian StandardsInstitution on 28 January 1977, after the draft finalized by the RelaysSectional Committee had been approved by the Electrotechnical DivisionCouncil.

0.2 Modern power systems are designed to provide uninterrupted electricalsupply, yet the possibility of failure cannot be ruled out. The protectiverelays stand watch and in the event of failures, short-circuits or abnormaloperating conditions, help de-energize the unhealthy section of the powersystem and restrain interference with the remainder of it and thus limitdamage to equipment and ensure safety of personnel. They are also usedto.indicate the type and location of failure so as to assess the effectiveness ofthe protective schemes.

0.3 The features which the protective relays should possess are :reliability, that is, to ensure correct action even’after long periods of4inactivity and also to offer repeated operations under severe con-ditions ;

b)

4

4

4

selectivity, that is, to ensure that only the unhealthy part of thesystem is disconnected;sensitivity, that is, detection of the short-circuit or abnormal opera-ting conditions;speed to prevent or minimize damage and risk of instability ofrotating plant; andstability, that is, the ability to operate only under those conditionsthat call for its operation and to remain either passive or biasedagainst operation under all other conditions.

0.4 This guide has been prepared mainly to assist protection engineers inapplication of relays installed as a separate unit for protection of bus bar.Few practical examples have also been included to illustrate the applicationof these relays. However, it is emphasized that this guide has been preparedto assist rather than to specify the relay to be used. This guide deals onlywith the principles of application of bus bar protection and does not dealwith the selection of any particular protective scheme.

3

IS : 3842 (Part IX) - 1977

0.5 In the preparation of this guide considerable assistance has been dexived&om several published books and from manufacturers’ trade literature.Assistance has also been rendered by State Electricity Boards in collectingactual examples.

0.6 This guide is one of the series of Indian Standard application.&des forelectrical relays for ac systems.

1. SCOPE1.1 This guidetection covered1.2 This guideprotection.

(Part IX) deals with application of relays for bus bar pro-by IS : 3231-1965*.does not cover the principles of system design and system

2. TERMINOLOGY2.1 For the purpose of this guide, the definitions given in IS : 1885 (Part IX)-1966t and IS : 1885 (Part X)-1968: shall apply.3. GENERAL3.1 Introduction-The provision of a separate scheme of protectioncovering the bus bar zone (bus zone), as distinct from circuit protectionrelays providing a measure of protection against bus zone faults is widespreadat the present time, due to :

a) possibility of widespread disturbance to system as a result of highfault levels on present day systems, and

b) possibility of better utilization of bus arrangements.3.2 Causes of Bus Zone Faults-The more usual causes of bus barfaults may be:

a) deterioration of insulating materials;b) flashover of insulators due to lightning or system overvoltages;c) wrong application of or failure to remove temporary earth connec-

tions;d) dropping of jumper connections across bus bars in outdoor instal-

la tions ;e) short circuits caused by birds, monkeys, vermin and the like; andf) short circuits caused by construction machinery.

4. BUS ZONE PROTECTION BY CIRCUIT PROTECTION RELAYS4.0 Before proceeding to the typical bus zone protection schemes, the pro-tection afforded by circuit protection relays against bus zone faults is brieflyexplained in 4.1 and 4.2.

*Specification for electrieal relays for power system protectiou.tElectrotechnica1 vocabulary: Part IX ElcetrieaI relay%$Electrotechnieal vocabulary: Part X Eleetried power system IWOtectkHL

4

ls t 384!z(PartIx)-1977

4.1 owxamad Back-up- Consider 8 typical case shown in Fig. 1where a bus bar has connected to it two outgoing circuits and the lowvoltage side of two transformer circuits. Assuming power infeed beingpossible to the bus only through the transformers, the overcurrent relays51B on the transformer low voltage side provide a measure ofbus bar pro-tection. .fhe disadvantage of suw prot~,t.ion is its relatively slow speed.

42 DistaaceBaclc-up - Consider a typical case shown in Fig. 2, where thebus bar section has a generator-t circuit and a feeder connectedto it. Bus wne faults are covered by the following distrnce protection:

a) The second zone of the distance relays 21LB at the, faF end of thefeeder, operating in zone 2 time, and

b) The generator back-up distance relay 21 G, operating after presettime delay.

The limitations of the scheme are long operating time and poor discri-mination or selectivity.

5. BUS ZONE PROTECTION SCHEMES5.0 Categories of Bus Zone Protection Schemes - Bus zone protectionschemes can be class&xl into two main categories, namely:

a) those for use with indoor metalclad switchgear installations, andb) those for use with outdoor or open type installations.

5.1 Schemes for Xndoor Metalclad Installations5.1.1 These schemes make use of frame leakage principle, which takes

advantage of the fact that such installations are completely metal-enclo&d,the enclosure being at earth potential. Metal-enclosed or metalcladswitchgear are so constructed that phase-to-phase faults are less likely tooccur than phase-to-earth faults and these schemes therefore afford aneconomical way of protection against phase-to-earth faults.

Where it is considered inadequate to cater to earth faults cmly and wherebetter discrimination is desired, recourse shall be taken to the schemes des-cribed later for outdoor installations under 6.

5X2 Eat& L&age Principle - The switchgear metal enclosure or frame-work is lightly insulated from earth and from the sheaths and armouring ofpower and control cables as well as conduit connections. The enclosuresof the units are housed together and connected to earth at one point throughthe primary of a current transformer. The secondary of the current trans-former feeds an instantaneous overcurrent relay which, when operated,disconnects all the circuits connected to the bus bar. Principle of earthleakage protection is shown in Fig. 3.

5.1.3 CIuck Feature -A check feature should be incorporated to preventmal-operation of the protection. This check feature measures the zero-sequence current flowing from the bus bars or the zero-sequence voltage ofthe bus bars and can take any one of the following forms:

5

Is : 38a(PartI2q-1977

1BUS ZONET---_ - - _-__ “‘~

! I- I

III

TRANSFORMER 2

,:

iiJ I If3 ii1 TRANSFORMER 3 1 ---’ ;

64R - Restricted Earth Fault Protection for Transformer KV51 I- Transformer H.V Over-Current Back-up Relay (IDMTL)5ZB - Bus Bar Back-up Over-Current Relay (IDMTL)51L - Line Protection Relay (IDMTL)

SIBS-Bus Section Protective Relay (IDMTL)52 - Circuit Breaker

FIO. 1 INSTANCES OF MAL-OPERATION OF Bus BAR BACK-UP. IDMTLOVER-CURRENT RELAY (TYPICAL)

6

IS t 3&!4!2(Partx)-lm

---------1

4

a>

b)

c)

2ILA, 21LB - Line Distance Protectibn21G - Generator Back-up Distance Protection

t, - Zone 1 Time Setting of 21LA, 21LBt--Time Setting of 21G

---Zone 2 Time Setting of 21LA 21LB2 - Zone 3 Time Setting of ZILA: 21LB

52 - Circuit Breaker

FIG. 2 DISTANCE BACK-UP FOR Bus BARS (TYPICAL)

Core-balance current transformer slipped over each feeder circuitconnected to the bus bars and capable of feeding zero-sequencecurrents, the secondaries of all such core-balance current transformersbeing paralleled, to another instantaneous overcurrent relay (seeFig. 4A).

A set of three current transformers on each of such circuits asin 5.1.3(a), residually connected and secondaries paralleled andconnected to the relay (see Fig. 4B).

Current transformers in the neutrals of all the windings of the powertransformer in the station connected to the bus bars, thesecurrent transformers being paralleled to the relay (see Fii. 4C).Alternatively, if paralleling of current transformers as rquired in5.1.3 (a), 5.1.3(b) and5.1.3( c is not possible, one relay per fault)feeding circuit with their contacts in parallel can also be uscdscd ,.I

7

1!

IS : 384!2 (Part IX) -1977

~&l52L1

Q

INSULATEDCABLE ENTRY

:;

CLAD

?!

INDOORINSTALLATION

52L2

XJ==J

TSWITCHGEARFRAME WoRKBONDING BARZ

1

3A AC Circuit

r ---- 16K4-I

@’---wL ,

To

/

/ ot ,

d--‘~TRIP 52LI

~~TR1p 52L2IIt

64 CH-2

ALARM3B DC Circuit

64CH — Check Relay64FL — Frame Leakage Relay

86— High Speed Re~y52 — ~rCLrit Br~ker—..

FIG.3 PRINCIPLEOF FRAME LEAKAGE PROnCTION

8

IFEEDER

4A

,..

r-

IS : 3S42 (pm IX). 1977

I J4B

,,, , ,. . -------- _--L— —

:U 1

L---------- .i

4B (Alternative) 4C

I

4D

TIEII

~;gl~

-----------’4E

AUXILIARY VOLTAGETRANSFORMER

FIG.4 METHOD OF ENERGIZINGCI-IRCKRELAY FOR FRAME LEAKAGE,PROTECTION (’I%PICAL)

9

—

● ’,

.,,y.’V

Is : 3842(PartIx)-1977

d) A voltage operated relay connected across the broken delta windingof a voltage transformer connected to the bus bars ($68 Fig. 4D orFig. 4E). The contact of the check relay is connected in series withthat of the earth leakage relay, as in Fig. 3B. In method 5.13(c)above, the check feature is inoperative, when the transformer circuitsare disconnected. Also none of the above methods are restricted,that is, they would operate for faults anywhere in the system.Hence, the check relays should have self reset contacts. If the checkfeature, as above, cannot be provided at all, the instantaneous over-current relay of 5.1.2 should be replaced by an IDMTL relay toavoid mal-operation on short-lived spurious currents.

5.1.4 A typical example of a single bus bar installation is given in Fig. 5A.The switchboard has a switchgear bonding bar, bonding all the framework‘together and a cable bonding bar. It is insulated from the framework andis connected to the station earth. The primary of the earth leakage currenttransformer connects these two bars. The secondary of this current trans-former feeds the instantaneous overcurrent relay 64FL, which is usually ofthe attracted armature type. The check relay 64CH is shown connected toa core-balance current transformer mounted on the incoming feeder. Whenboth 64CH and 64FL are operated, a hand reset master trip relay 86FL isenergised (see Fig. 5B) which trips the various feeders and also gives an alarm.A disagreement alarm is also sounded when the relay 64FL operates.

5.1.5 Application to Sectionahed and Duplicate Bus Bars - The frame leakage“principle can be applied to sectionalised or duplicate bus bar installationsalso.

5.1.5.1 Its application to a sectionalised bus bar is given in Fig. 6,where the section to the left of the bus section breaker,. the section to theright of the bus section breaker and the section contammg the bus sectionbreaker 52E, form the three zones A, B and C respectively. Here, it is neces-sary to provide insulation between the zones in addition to insulating theswitch-board from earth, each zone having its own switchgear bonding baris connected to the cable bonding bar through the primary of frame leakagecurrent transformer, the current transformer of zone C having two secon-daries (alternatively two separate current transformers can be used). Theratios of all the current transformers should be the same. A single relay64FLA is provided for zones A and C and another relay 64FLB for zonesB and C energised as shown.three zones.

A common check relay 64CHserves all the

A fault in zone A is cleared by the tripping of all breakers in zone A ~1~s52E. Likewise a fault in zone B is cleared by the tripping of all breakers inzone B plus 52E. A fault in zone C is cleared by the tripping of all breakersin zones A, B and C.

5.1.5.2’ As a variation to the scheme described in 5.1.5.1 the bus sectionbreaker may.be included in one of the zones A or B, as shown in Fig. 7.

Faults in zone A are cleared by tripping all breakers in zone A plus 52E.

10

12

Is t 3m!2(Partlx)-l!m

Faults in zone B are cleared by tripping of all breakers in zone B plus 52E.Faults occurring between the barrier and 5233 are cleared by zone B protec-tion intertripping zone A, through a timer with a delay of O-4 to O-5 seconds.A limitation is that there should be an earthed source of supply in the zonenot containing the bus section l&&x (zope A in Fig. 7). In installationswith more than one bus section breaker the earthed sources of supply shallhe so arraqed that under all swi&hing conditions a fault between any bussection breaker and the nearest barrier is always fed. .

515.3 This protection applied to duplicate bus bars is shown in Fig. 8,where the arrangement is similar to that shown in Fig. 6.break= is included in one of the zones (zone B).

The bus coupler

Faults in zone A are cleared by &ping all breakers in zone A, plus thebus section and bus coupler breakers, plus all breakers connected to thereserve bus, leaving only the circuits connected to the zone B. Faults inzone C isolate the switchboard completely.

520, 52E, 52F, 5X - Cirad Breakex64CH-Check Relay

64~LA -Zone A: Frame Leakage Relay6@LB-ZoneB:FrameLakageRelay

FIG. 8 FRUE LEAKAGE PROTECTION APPLIED TO DUPLICATE Bus BARS(TYFWXL)

14

Is: 384!4(PartIx)-1977

5*kft7= maximum through fault current,

IF = minimum internal fault current,RSE = resistance of the station earthing electrodes,RFI = switchgear frame insulation resistance,,N = ratio of earth leakage current transformers, andIpoc = minimum primary operating current.

Then, from Fig. 9 it can be seen that to prevent n&-operation on throughfaults,

&oC>&F RSE&A-RH

For reliable operation under minimum internal fault conditions, I-should be generally of the order of 0.3 1l-r. The current transformer ratio.W, is so chosen that a setting within the relay range is retained for a current

corresponding to 9. It will be seen, that &XX can be reduced, by

providing better insulation between framework and earth.

GENERATOR lRllE EARTH

FAULT,

TT’T -

I

8L____________________-__-_______C-_______--_

i____A

ZTF- Through Fault Current4 - Leakage Current through Frame Insulation on through Earth Faults

2- Ground Return Current

pI - Switchgear Frame Insulation RcsiatanceRSE - Resistance of Station Earthing ElectrodeR,- Neutral Earthing Resistance

(For Resistance Earthed System only);#CCF - Check Relay

- Frame Leakage RelayFIG. 9 MAL-OPERATION OF FRAME LEAKAGE PROTECTION UNDER

THROUGH FAULTS (TYPICAL)

15

IS :3842’ (Part IX) -1977 .

5.1.6.1 O@rating time — An overall operating time of the order of3 cycles can be obtained, at fault current of 5 times the setting.

5.1.7 A@lication .Notes

5.1.7.1 Switchgear framework insulation

a)

b)

c)

d)

e)

If the switchgear is mounted on concrete foundation, no specialinsulation is required provided it is ensured that at least 50 mmthickness of concrete is available between each grouting bolt andsurrounding metalwork like reinforcing bars.If the gear is mounted on steel channels, special insulation should beprovided between the channels and framework.All metalwork leading away from the gear such as vent exhausts,conduits, shall be fitted with insulated joints as near to the gear aspossible.If any high tension line enters the mar through bushimzs. the bushing ,supp&t &ges should be insulat;d fkmr th~ framew&& of the gea;and earthed separately.The clearance between the switchgear framework and secondarystructural steel work should not be less than 50 mm.

f) The overall frame insulation to earth should have a minimum

d

resistance of 3 to 10 ohms, the higher figure should be aimed at, whenthe earth electrode resistance is comparatively high and the frameleakage relay is very sensitive. MaI-operation can otherwise occuron through faults since check feature is not of the restricted type.Glands of all main and atiiliary cables entering the gear should beinsulated from the switchgear framework by insulating sleeves orwashers. The glands of the power cables should ha~e an insulationto withstand 8 to 10 kV.

5.1.7.2 Insulation betweenzones-At the junction of two zones an insulatedband joint shall be used and all conduits, vent-heaters and metal connectionsshall be made with insulating inserts.

5.1.7.3 Earthing of switchgear

a)

b)

e)

d)

All the metal framework of the switchgear panels and cable boxesin a zone shall be connected to a common switchgear bonding bar.The switchgear bonding bars of zones should be insulated from oneanother.If the cable sheaths are earthed at the station, all the cable glandsand sheaths should be solidly connected to a cable bonding barwhich should be insulated from the switchgear framework andconnected directly to the station earth.-Where earthing devices are used for feeder earthing, this should beconnected to the able bonding bar.

IS t 38a(PartIx)-1977

earthed within the protected zone, for example in the instrumentpanels of drawout metalclad gear, should be earthed to the cablebonding bar. In cases where an earth connection exists betweenequipment and the switchgear framework, for example the Y- haseof the voltage transformer secondary, no further earthing o Psuchcircuits should be made to the cable bonding bar.The earthing of all instrument, meter and relay cases should be doneto the switchgear framework to avoid risk of shock to operatingpersonnel.The cable bonding bar should be connected to the station earththrough the main station earth electrode and not through a separateelectrode, to avoid inclusion of additional earth resistance in theearth path.In case of resistance earthed systems the cable bonding bar shouldbe connected to the station earthing electrode, so as not to includethe neutral resistance in the earth path.

5.2 schemeafor oufhor Ins~tion53.1 The principle of frame leakage can be applied to outdoor instal-

lations as well, where the bases of all support insulators are lightly insulatedfrom the gantries and are connected together and earthed through theprimary of an earth leakage current transformer. The scheme does notpvide protection against p,hase faults and the obvious difficulty of properlymsulating all the support insulators which may include insulations of equip-ment such as isolators, makes the application of such a scheme unsatisfactory.

592 The differential principle using the current or voltage balancearrangement is in most common use for providing bus zone protection. Theprinciple is given in Fig. ‘10 which shows four circuits connected to breakers.

87 - Differential Relay5.?LI, 52L2, 52L3, 52L4, - Feeder Circuit Breaker3

FIO. 10 I~FFERENT~AL hINCIPLE

17

IS : 3842 (Part Ix) - 1977

Current transformers in each circuit are connected in current balancearrangement to feed a differential relay. Under healthy or externa1(througbj fault conditions when the sum of the currents measured by thecurrent transformers is zero, no current passes through the relay. A faulton the bus bars, on the other hand, produces a current through the relaywhich picks up and causes disconnection of the bus bar from the system thusisolating the fault. The protected zone is the area bounded by the currenttransformers.

5.2.3 Successful application of this principle calls for current transformerswith characteristics such that a currenti in the relay is produced only forinternal fault conditions and not under healthy or external fault conditions.However, the causes of current through the relay under such conditions canbe classified as follows:

a>

b)

4

4

At a certain excitation voltage, the different current transformerscan draw different magnet&g currents causing a spill c-urrent toflow through the relay.In bus zone differential schemes, the currents through the primariesof all the current transformers may not be the same under throughfault conditions and this would also cause different magnetisingcurrents to be drawn causing a spill current to flow in the relay.The lead burden between the current transformer and the r&ymay be different for the different current transformers causingunequal loading of the current transformers again resulting in spillcurrents due to different magnetising currents.Depending upon the instant of fault, the fault current can contain .varying proportions of unidirectional transients. The X/R ratioof the system up to the fault point determines the time constant ofthis transient. High values of such transients can cause saturationof the currem transformers resulting in the current balance beingupset.The cores of the current transformers can have remanent magne-tism, due, for example, to previous fault currents. Depending uponits magnitude and polarity ac fault currents can cause saturation,leading to unbalance.

5.2.4 The limitations mentioned in 5.2.3 can be overcome by the follow-ing methods:

a) Use of current transformers without the ferrous cores (called linearcouplers) thereby- eliminating the cause of varying magnetisingcurrents and saturation,

b) Use of medium,/high impedance differential relays,c) Use of biased differential relays, andd) Use of IDMTL relays.

52.5 Linear Couplers - The limitations mentioned in 5.2.3 like differentmagnetising currents and core saturation‘are due to the presence of the iron

1 8

Is: 3842(PartIx)-1977

core which has non-linear characteristics. Current transformers have beendeveloped with air cores, thus eliminating the cause of these limitations.

Linear couplers have nor&magnetic cores over which the secondarywinding is toroidally wound and uniformly distributed. The absence 01magnetic core limits the output of linear couplers to about 3 VA for evtr);I 000 A primary current. Linear couplers have poor response to transientcurrents which consequently undergo considerable attenuation. Specialfilters to block dc components from entering the relay are thereforteliminated.

Scheme using linear couplers usually employ the voltage balance principlewhich is shown in Fig. 11. If Ini, IP,. . . .Ipn are the primary currents in thyvarious circuits and M is the mutual reactance in ohms (assumed the samefor all the couplers), then the voltage developed across the relay,

V = I,M+I,,M+ . . . . I,&= M (&+&a+. . . Jpn)

For external faults where lPl+IP2+. . . .Ipn=O, no voltage is producecacross the relay. During internal faults, a resultant voltage is imprcssecupon the relay. A high impedance, low watt consumption (5 mW or less:voltage operated relay is employed.sources, the current in the

Linear couplers are essentially vubltagtsecondary loop being limited by the self isnpe

dance of the coupler, the relay impedance and the lead impedance, the Iatmbeing usually negligible compared to the other two. Linear couplers artmade with mutual impedances accurate to within f 1 percent and thidetermines the stability ratio of the protective system which is de&d as:

ID1 ‘P2 b

ZP,, Zp2, I,, - Primary Currents52L1, SZLZ, 52L3 - Feeder Circuit Breakers

87 - Differential Relay

FIG. 11 USE OF L INEAR COUPLER (TYPKAL)

19

Stability ratio = The maximum external fault current for non-operationThe minimum internal fault current for Operation

me stability ratio is calculated as follows taking the worst case & that w+positive error in the ‘Fault coupler’ and negative error in the ‘Source couple$;

The voltage developed by the couplers for the minimum internal fault!current 1’1 = MI’P

Th;~xi~,um error voltage developed from an external fault currentIF= *

For nonop;tion of the relay under external fault, 0.02 MI8 <MZ’F.

Taking a safety factor of 2, this gives a stability ratio of as high as 25,5.2.6 Bdad Caveat Systems - Consider the circuit of Fig. 12A CT; and

CT, are assumed to be identical current transformers having identical para-meters like ratio, resistance and similar magnetising characteristics. Undernormal conditions, therefore, they share ,%e burden in a manner illustratedin Fig. 12B.equal.

The emfdevelopcd by t&mnt transfomers are, therefore,Points ‘XX’, ‘ 2T, etc, are eq!~tcntial points along the pilots and

no current will flow if these are short-&&ted say? through a low impedancerelay. However, in practice such equipotential points are somctim~difficult to obtain conveniently, and it may be required to connect the relaysacross non-equipotential points like ‘<-&‘. When such points are bridgedby a low-impedance device an equilising current will flow through it, re-ducing the potential dif%.rence across them to zero. The voltage droparound the loop before and after the shorting is shown in Fig. 12C and itwill be seen that one of the current transformers (Crs) has’ to develop anincreased voltage compared to the other. The Cl1 being assumed to beperfect and identical the circulating current (i) between them will ~stillbalance. Thus the output of CT, is increased to E,I and that of C’& isdecreased to E,I.

1f, under the above conditions a through fault of high magnitude were tooccur! the burden of Cl, can be excessive and drive it into saturation,multmg in a current through the FIay and causing ma&operation.

Increasing the relay setting to avoid this trouble is not desirable as itincreases the primary operating current, This is particuIa14y true for highsped relays whose settings should take into account the effect of primarycurrent transients also. To a certain extent this can be applied to lowspeed relays.

Padding resistors can also be introduced in the interconnecting wiringto equalise the burdens. But this method only serves to increase the burdenson the less-loaded current transformers.

5.2.7 High/Medium ImpGdance Relays5.2.7.1 The basic circuit is given in Fig. 13 and it will be seen that the

20

Y PILOT 1\ \

21 Xl Yl PILOT 2

12A Basic Balanced Current Circuit

12B Voltage Distribution Around the Loop

126 Effect of Relay Connection at Non-Electrical Midpoint

Cl;, ( : i - Current Transformers.52L1, 1 1 P -- Circuit Breakers

87 -- Differential RelayE! . . 1 p I22 - Generated emfs of CT3 and CT,

‘I - Voltage amiss relby under load conditions when connected am fz

Frc. I.2 BALANCED CURRENT SYSTEMS ( TYPICAL)

21

IS : 3842 (Part Ix) - 1977

scheme is similar to that of Fig. 12 except that the relay circuit has a highimpedance R which may be an external impedance or may be inherent inthe relay coil. Even if the points Jr and 3s across which the relay is con-nected are non-equipotential points, very little current flows through therelay because of its high impedance. Under healthy conditions, therefore,the current transformers work as if there is an open circuit between pointsJ1 and 3s. The circulating currents in the loop and the current transformerworking voltages are not materially disturbed.

51.73 Under external fault conditions - Refer to Fig. ,13 and considera through fault at F2. Neglecting the effect of load, if all current trans-fibnaus behave ideally the voltage betwwill be of a small value depending upo

n points J1 and 3s will be zero or

elcetrical mid-points or not. %whether these two points are the

If howev , the fault current transformer dueto ita being loaded more compared to! the source current transformer isdriven into saturation its generated voltage E.? will be reduced. Dependingupon the degree of saturation this can be partial (EZ) or total (zero). Inthe worst case of total saturation in the fault current transformer and nonein the source current transformer, the latter forces the current through the‘dead’ impedance of the fault current transformer, tihich is assumed to bemainly resistive. Consequently a voltage e3 appears across the junctionpoints J1 and 3s (and therefore across the relay). For any other assumedcondition, such as partial saturation in source current transformers and totalsaturation in fault current transformer, the magnitude of the above voltageis lesser, as can be seen from Fig. 13B.

The value of this voltage can be shown to be equal to the following :

V,, = (Rs + 22) 3 or

v&s= (4 + 59) iwhere

v-=Rs

voltage across relay,= secondary resistance of fault current transformer,

r = lead resistance between relay and fault current transformer

4(one way),

i= maximum through fhult primary current (rms value),= secondary equivalent of Ii, and

X = nominal ratio of current transformer.53.73 Under in&ma1 fault co&ions-The currents delivered by the

current transformers have the same polarity with respect to the points Jrand Js. The current transformers therefore work into the high impedanceof the relay, with high voltages appearing across the current transformersecondaries as well as the relay. The ratio of voltages across the points3l and Js during internal faults to that during external faults can be as highas 10, this factor providing discrimination between internal and externalfaults.

22

Is: 3842(Partrx)-l!m

13A Principle

SOURCECURRENT CURRENTTRANSFORMER TRANSFORMER

138 Relay Setting Voltages

52~452~2, 52~3 - circuit BxakR - High-hqxdanc~87 - Relay

CTi, CT2, CT3 - Currad Trbformus

, c3 - Voltage Across Relay with Complete Saturationjn Fault Current Trans-former and no Saturation in Source Current Transformer

cl - Voltage Across Relay with Complete Saturation in Fault Current Tran+former and Partial Saturation in Source Current Transformer

Fro. 13 Hm+EmJM hPEDANCE %H~iw%

23

Is: 3842(PartIx)-1877

This is where the main advantage of high impedance relay over the lowimpedance type exists.fault current

In the latter under through fault conditions, if thetransformer saturates, the relay shunts more than 90 percent of

the secondary current.and in-zone fault.

It cannot therefore differentiate between a through

5.2.7.4 Relay Setting - The high impedance relay is so set that it doesnot pick-up for voltages produced across it during through fault conditions.As the through fault value is likely to change with time, as the generationin the system is increased, the relay setting is to be reviewed from time totime. In the initial state when fault levels are low it is desirable to adopta lower setting.

The sensitivity or the minimum primary operating current for a given relaysetting voltage depends on the excitation currents drawn by the variouscurrent transformers in parallel with the relay as well as the current drawn

’by the relay itself at that voltage. If i, is the current drawn by the relay andIn the sum of the magnetising current of the current transformers the totalminimum secondary current to be supplied by the source current transformerfor relay operation can be written as:

is = (&+I,)The corresponding primary operating current is:

I, = .N (t + I”)The minimum primary operating current can be reduced by making therelay current as small as possible and using current transformers with lowermagnetising currents. The larger the number of circuits (and currenttransfiormers in parallel) the larger is the minimum primary operatingcurrent.

5.2.8 ‘I@es of High/Medium ImgGdants Relays-The three general typesare:

a) Relays using linear resistors in series with their operating coils,b) Slays using non-linear resistors in series with their operating coils,

;t1rtlc) Rrri~ys using both linear and non-linear resistors.

5.2.8, Y Kelps using a linear resistor in series with their ojerating coil - Atypical ~13 3, is shown in Fig. 14. The relay has a low burden current ope-rated coil I ir~nected to the secondary of a saturating current transformerthrough X1 :t:ries tuned circuit. The series tuned circuit makes the relayimmune to harmonics and dc transients in the voltage across the relay. Thesaturating ( rwent transformer limits the current in the relay coil to a safevalue uncl~~~~ heavy current in zone faults. The external linear resistor Rwhich is a( 1,) ustable is connected in series with the relay.

The adv;tntages of this relay are that with linear resistor accurate settingsmay be n~tl~ and thb being continuously adjustable, a setting equal to the

24

Is : 3842(PartIx)-1977

11 b :L ___-_____-- - ______ - -I----I------ f

XX - Saturating Current TransformerLandC - Tuned Circuit

R - Linear Resistor87 - Relay

FIG. 14 RELAY USING A LINEAR RESISTOR (TYPICAL)

87-RelayC -capacitorR - Non-Linear Resistor

F I G. 15 RELAY USING NO N- LI N E A R F~SISTOR (TY_JIGAL)

25

Is: 384!2(P8rtM)-1977

calculated voltage can be chosen. Also the variation of resistor value withtemperature is not of consequence.

5.2.8.2 Relays using a non-linear resistor in series with th.eir ojerating coil - Atypical relay is shown in Fig. 15. The stabilitising resistor is made up of aseries of non-linear resistors chosen by a plug, the lowest step, however,being a variable linear resistor, which limits the maximum relay current ,,to a safe value. The series capacitor tunes out harmonics and blocks directcurrent. The non-linear resistor follows the law V = A-14 (where V isthe voltage, I the current and d a characteristic+ of the resistor).. Theadvantage is that the relay current at pick up can be quite small, the currentincreasing rapidly for comparatively smaller increases in voltage resultingin fast relay operation. The minimum primary operating current is mainlydependent on current transformer magnetising current.

The disadvantages are that continuous variation in resistor values is notpossible and’it will not be possible to set the relay to the exact calculatedvalue. The non-linear resistors are rather bulky and expensive, and aresusceptible to temperature variations .

,5.2.8.3 Relays using both linear and non-linear resistors-A typical relayis shown in Fig. 16. The low set unit w‘ consists of a relay connectedthrough a bridge to a series tuning unit L and C. The high set unit HS isconnected across the low set unit and consists of a relay in series with non-linear resistor. The connection of the low set relay through the bridgeensures that variations in the value of the relay resistance for setting purposeswill not affect the tuning. Under heavy internal fault conditions the highset unit operates faster because of larger currents through the non-linearresistor.

==c .

W - Low Set UnitHS - High Set Unit

LandC- Tuned Circuit,FIO. 16 RELAY Uma BOTH LINEAR AND NON-LINEAR RESISTOR (TYPICAL)

26

IS: 3942(PartIx)-1977

53.8.4 Limitation of voltage peaks - With high/medium impedancerelays, under internal fault conditions, the current transformers have thesame polarity with respect to each other, driving their currents through thehigh impedance of the relay. This can be considered as practically open-circuit condition. The current transformers can therefore be driven tosaturation resulting in high peak voltages across the relay, associated wiring,etc. For values of Vi =formula:

vr this peak voltage Speak is given by the followingv

vPeak =-2 2/T-2/ vk (v, - vk)

wherevk = the knee point voltages of the current transformers involved, andVr = the maximum rms voltage the current transformer would have

developed if there were fault current I, flowing in the relaycircuit,

= I, X (where X is the impedance of relay branch presented to Zr).To keep these voltages limited to a safe value, a non-linear resistor is

generally connected across the relay so that when the voltage reaches thedangerous value, the non-linear resistor provides an easy shunt path for thecurrent. The voltage limiting resistors are used only if the calculated valueof rpTpe,& exceeds 3 kV,and it is normal practice to choose resistors that keepthe voltage limited to about 1 kV. In the relays described under 5.2.8.1and 5.2.8.2 separate voltage limiting resistors will be necessary. In therelays described under 5.2.3.3 the resistors in series with high set unit actas voltage limiting resistors also.

5.2.8.5 Cane& tran+-fm rq&nents, for high/medium impedance relays:

4

b>

4

4

All the current transformers should have the same ratio. Auxiliarycurrent transformers for ratio correction should be avoided as a ruleas these introduce dissimilarity and make calculation of settingsdi&ult.All the current transformers should be of the toroidal or low re-actance type as the formula for voltage across the relay takes intoaccount only the resistance of the current transfformer secondary.Current transformer windings with tapped secondaries are also to beavoided unless the tapped portions are also uniformly distributedover the core.Auxiliary current transformer secondary rating of 1 A is to be tire-ferred as it reduces the voltage across the relay under through faultconditions, and allows lower voltage settings to be adopted. Thisreduces current transformer magnetising currents at pick Up voltage,thereby increasing scheme sensitivity.A high current transformer ratio of the order of 800/l or 1000/lshould be adopted without regard to the individual circuit rating.However, due consideration should be given to the maximum bus

27

coupler transfer load as well as the effective primary operatingcurrent setting while choosing the ratio. The current transformerratio should also be as high as possible irrespective of individualcircuit ratings. This reduces the secondary current for the maxi-mum through fault levels resulting in lower relay voltage setting.The effect on the minimum primary operating current has, how-ever, got to be watched. Due consideration should also be givento maximum bus coupler transfer load in selecting current trans-former ratio.The current transformer secondary resistances should be kept aslow as possible.The knee point voltage of the current transformers (defined as thevoltage on the current transformer magnetisation characteristicsat which a 10 percent increase in voltage results in a 50 percent in-crease in magnetising current) should not be less than twice thevoltage calculated across the relay under maximum through orexternal fault conditions. This is expressed by the. followingformula:

R = 2 (Rs + 5) Ir.hf

wherevk = current transformer knee point vohage,Ii = maximum short circuit current rating of the switchgear,N = current transformer ratio,Rs = current transformer secondary resistance, and.& = maximum lead burden between the relay and the

current transformer.The value of vk is usually calculated taking the current transformer

farthest from the relay taking into account maximum Y and If istaken as the current corresponding to the actual maximum faultcurrent.The magnetising currents drawn by the current transformer at therelay setting voltage should be low enough to ensure that the primaryoperating current is not more than 30 to 50 percent of the minimumfault current:

thus,

le <1

(0.3 to 0.5) 4 (min) _ irx 1

x n

wherrIe - magnetising current drawn by the current transformer;

Ii (mini - minimum .t&ult current;II = number of current transformers that are paralleled;

28

Is t 384!2(Partix)-1977

X = nominal current transformer ratio; andrr* = relay branch current at setting, including the current

drawn by voltage limiters, supervisory relays, etc.5.2.9 Typical High/Medium Imfedance &hemes - The basic types of schemes

are:a) schemes for earth fault protection only using a single pole relay,b) schknes for phase’ and earth fault protection using a single pole

relay, andc) schemes for phase and earth fault protection using a 3-pole relay.

5.2.9.1 Schemes for earth felt protection only u&g a single pole relay - Atypical scheme is illustrated in Fig. 17. The three current transformers ofeach circuit are parallelled with those of all the other circuits and the relayxonnected across these current transformers. It will be seen that the relayresponds only to zero sequence currents, that is, to earth faults only.

As most faults involve earth, there appears to be some justification inadopting such schemes, particularly for the corninstallations. However, compared to a phase andp

aratively less importantearth fault scheme, eco-

riomy is achieved only in respect of relays, as the current transformer require-ment is the same. The saving, therefore, is not of much si&%ume.Further, in systems where the earth fault currents are limited (for exampleresistance earthed systems) and with the large number of current transformas

5ZBS, 52L1, 52L2,952L3 - Circuit Breakers87-RelayN- Current Transformer Ratio

FIO. 17 EARTH-FAULT PROTECTION USING A SINGLEPOLE RELAY (TYPICAL)

2 9

IS t 3842 (Part IX) - 1977

in parallel, the minimum primary operating current may not he sticientlylow.

5.2.9.2 Schemes for phase and earth fault protection using a single pole relay -There are two typical varients, namely :

a) One typical arrangement for resistance earthed. systems is shown inFig. 18 and it will be seen that the basic connection is similar to thatof Fig. 17. However, the ratio of the current transformers in the Rphase of all the circuits is JVI, in the Yphase .AC? and in the B phaseN3 with the result that there is a current in the relay for all types ofinternal faults. The magnitudes of the current in the relay for thedifferent types of the faults depends upon the actual current trans-former ratios .Nl, .hL? and N3. With a proper choice of these ratios,the earth fault sensitivity can be made high compared to phase fault,the scheme being then suitable for systems where earth fault currentsare limited.

Phase fault protection is also provided by this scheme. Advantagesover a scheme using three relays are that only two bus wires betweencurrent transformers and the relay are required. Also the numberof auxiliary switches on the isolators in a double bus substation isconsiderably reduced. The current in the relay branch for assumedcurrent transformer ratios of 300/l, 490/l and 500/l in the R, rand B phases respectively for the various types of fault work out asgiven in Table 1.

RLa81

52BS, 52L1, 52L2, 52L3 - Circuit Breaker87 - Relay

NI, X2, N3 - Current Transformer Ratios in R, Y, and B Phases respectivelyFIG. 18 PHASE AND EARTH FAULT PROTECTION USING A SINGLE POPE RELAY

(FOR RESISTANCE EARTHED SYSTEMS) (TYPICAL)

30

IS : 3842 (Part IX) - 1977

TABLE 1 MMbSPAIUSON OF RELAY WRRENTS FOR DIFFERENT FAULTS[CZaue 5.2.9.2(a)]

TYPE OF FAULT RELAY CURRENT AS APERCEXTAGE OF THAT

ON A R-E FAULT

R-E 100T-EB-E 2R-Y 25T-BB-R iiiR-Z--B 35

b) In the other current transformers of two phases having the same ratioare cross-connected and paralleled to the current transformer of thethird phase which has half the ratio of the other current transformersas shown in Fig. 19. The Table 2 gives the comparison of relaycurrents for different type of faults.

5G?BS, 52L1, 52L2, 52L3 - Circuit Breakers87 - RelayN - Current Transformer Ratio

F I G. 19 PHASE AND EARTH FAULT PROTECTION Usma A SINOLE POLERELAY (FOR SOLIDLY EARED SYSTEMS) (TYPICAL)

31

Is: 3842(Putlx)-1977

TAE&E 2 COMPARISON OF RELAY CURRENTS FOR mYYPES OF FAuL.ls

[Cluuse 5.2.9.2(b)]

TYPX OF FAULT RELAY-AlPERcEIqTAaarmrONA R-E FAULT

It will bi seen Corn Table 2 that the sensitivity for phase fiults is gezwallybetter than for e+%h f&u18 and hence it is more suitable for solidly earthed$yst!xIn.

The advantages mentioned for the scheme under 5.2.9.2 (a) hold goodfor th+ scheme also.

@2J-87 87 87

i I &

52BS, 52L1, 52L2,52L3 - Circuit Breakers87 - RelayN- Current Tradbmm Ratio

FIG. 20 PHASE AND EARTH FAULT PR~TIXTION USING ATHREE-POLE RELAY (TYPICAL)

32

51.93 $&me for &se and earth f&t jrotection using a 3-$ola reluy (f&r ba@&y eurtbd and resistance earthed systems) -A typical scheme is shown inFig. 20. The current transformers of each phase are paralleled separatelyusing four bus wires. Three relays, one per phase? are required, and con-neeted between phase and neutral. The sensitivny for phase and earthfiults is the same. The number of current transformers in parallel is lessthan that in the other schemes, thereby reducing the minimum primaryoperating current. This scheme is very widely used on the more important ,sub-stations especially at the higher voltages. !

SS.10 Biased DaJmential Relays - T&z technique of biasing is extensivelyused to increase the stability ratio of di%rential schemes such as trans-former protection, pilot wire protection, etc, and is advantageously used forbus protection also. Biasing makes it possible to use reduced stabilizingreaiator values in circulating current high impedance schemes which in turnrelax the requirements of the current transformers but the schemes tend tobe xnore complicated.

Tho.biased schemes as applied to bus bar protection fall in two majorcptegories:

a) with resistance stabilising, and

5) without resistance stabilising.

53.10.1 Biawd scAemes using res&ance stabilising - The principle ofbiased scheme is shown in Fig. 21A. The relay has a single bias winding87 B which is fed with the algebraic sum of rectified currents correspondingD the individual circuit currents.

The operating coil is shunt connected, with a series connected stabilizingrcsbtor. The operating coil itself can be ac or dc operated and will haveto be fed through a rectifier bridge if it is the latter as shown in Fig. 21 B.Under normal conditions of load only the bias coil remains energised. Whena through fault occurs causing saturation in the current transformers, thespill curre&s so set up flow through the operating coil. However, thesecondary equivalent of the through current flowing through the bias wind-ing, restrains the relay against operation.

The stabilizing resistors ensure that the overall relay branch impedanceis not low enough to shunt a major portion of the fault current. Its valueis chosen such that only that much of current flows through the operatingcoil which can be safely handled by the bias coil.

If B=bias ratio, andIs=the secondary equivalent of through fat& current,

Then permissible spill current=B. Is.

33

IS : 384!2 (Part Ix) - 1977

21A Principle

21 B Alternative Connection for Relay Operating coil

R7LI - Relay Bias Winding870~ - Relay Operating Winding

R - Stabilizing Resistor5X.1, %%? -- Circuit BreakersCTI, CT2 - Current Transformers

FIG. 21 BIASED SCHEMES USING RESISTANCE STABILIZING

34

Is : 5842(Partlx)-1977

If only one of the current transformers saturates completely and othercurrent transformers have no saturation, the impedance of the loop acmssthe relay is (Rs+2r)

where

R, = the current transformer secondary resistance, andr = lead impedance between the saturated current &an4kmx

and the relay (one way).

The relay current under this condition = IS x (Rs +2r)5+ (R~f2r)

where <= relay branch impedance

For non-operation of de protection, therefore,

‘1~ (ReSBr)5+ (Rt1+22) < R’ ”

or

a) <>v, if (R8+2r) is very small compared with z.

b) <>q- (RB+2r), if 5 and (%+2r) are of the Paae

order of magnitude.Usually the value of 4 obtained f&m (a) also satisfy equation (b) abow.

53.10.2 Biad sdmzes without resistance stabilizing - The COWare identical to that for the scheme described in 5.2.10.1, the differencebeing in the relay itself. ‘A typical relay consists of a sensitive telephonetype movement fed by a transductor tiu a bridge rectifier. The transductorhas a high permeability three-limb core. The bias winding is uniformlydistributed over the middle limb. It is tapped in the centre and connectedto the operating winding which is distributed equally over the two outerlimbs. The short circuited winding on the middle limb provides a smooth-ing effect for the dc bias flux.as in Fig. 22A.

The output winding is on the two outer limbsUnder through fault conditions, the bias current in both

the half-cycles nearly saturates the core and even with spill currents in theoperating winding, no output current is produced. Under internal fkultconditions, the bras effect is very much reduced, and the current in theoperating winding is very much higher, thereby resulting in an outputcurrent.

This can be readily understood by referring to the Fig. 223. The dc biasflux in the centre limb determines the point of operation of the transductoron the BH curve. On through faults the current is available during bothhalf-cycles m either half of the bias winding (that is II and b). Also the

35

magnetic core is nearly saturated with the &as flux and so no output isavailable to operate the telephone relay. Even if any spill current exists inthe operating winding it is not capable of producing enough output to causethe relay operation.

During external faults the dc bias effect is reduced considerably since thebias currents in the windings a and b flow during alternative half cycles. Onthe other hand the operating winding carries currents dtig both the halfcycles. Consequently the output winding develops enough output #operate the relay.

S.!L103 In the practical application of the schemes, summation cur-rent transformers (one per set of main current transformers) are normallyused. These summation current transformers have a tapped primary towhich the three phases of the main current transtormers are connected, thesewndary of the summation current transformers prowding a single phase

I POsrrlvE BUS

NEGATIVE BUS 1 .TI

* COMMON BUS

22A Transductor Relay

FIG. 22 BIASED SCHEME WITHOUT RESISTANCE STAEILIZED(TYPICAL) - Co&

36

THROUGH FAULT IN-ZONE FAULT

FAULT CURRENT INAMPERES

228 Relay Currents During Faults

A - Current available for internal faultB - Current for operation (through faults)C - Current for operation (internal fault)

FIG. 22 B IASED SCHEME WITHOUT R ESISTANCE STABILIZING (TYPICAL)

output. The connection for one set of main current transformers is shownin Fig. 23. The advantage of using summation current transformers are:

4b)4

44

a single relay is used for all three phases;a de&rite bias current is available for all types of external faults;lead burden on main current transformers is less, provided thesecurrent transformers are located judiciously;secondary cabling is reduced; andauxiliary switch requirement in double bus bar arrangement isreduced.

The main drawbacks are:a) the setting for various tapes of faults is different, needing careful

analysis; andb) bias effect is less for phase faults than for earth faults.

37

I8 t 3842 (Part IX) - 1977

POSITIVE Busk

07Br87OP

52L I, 52L2 - Circuit BreakersSCI - Summation Current Transformer

878 - Relay Bier Coil870P - Relay Operating Coil

FIG. 23 USE OF SUMMATION CURRENT TRANSORMERS IN BIASED SCHFMES(TYPICAL)

6. ~SI?LI?LI~~NOF BUS BAR PROTECTION TO OUTDOOR

6.1 Cnmmon Bus Bar Arrangements and Cmrrent TransformerLocationa

6.1.1 Five common layouts have been illustrated in Fig. 24A to 24E. Thesimplest of all, the single bus bar arrangement Fig. 24A does not pose any

38

IS: 3842(PartIx)-1977

difficult problem at all from the protection application point of view as nocomplicated current transformer switching is involved. The other forms oflayout, Fig. 24B to 24D do sometimes involve a certain amount of currenttransformer switching which however, depends to a great extent, upon thelocation of the current transformers themselves. The different possiblelocations of the current transformers for the various circuits have beenmarked (X for bus couplers, Y and YT foT feeders and 5 for bus sectioncircuits). The degree of coverage afforded by the protection under anygiven mode of primary circuit switching, is also largely decided by thecurrent transformer location. The ideal layout from the application view-points, therefore, would be that which needs minimum or no current trans-former switching, but which affords maximum coverage by the protectionunder all contemplate modes of switching. The mesh layout in Fig. 24Ealso, presents no difficulty as far as application is concerned. In fact, it doesnot at all require a separate form of bus protection since the circuit pro-tection itself adequately covers the bus bars against faults.

6.2 The Practical Scheme of Bus Protection for Outdoor Installa-tion - The practical scheme for an outdoor installation essentially has thefollowing features usually incorporated:

a) Zonal discrimination,b) Check feature,c) Secondary wiring supervision,d) dc circuit for selective tripping, ande) Alarm indication (annunciation).

65.1 <onal Discrirninatitin6.2.1.1 With a view to limiting the extent of outage of circuits to the

barest minimum in the event of a bus fault it is the normal practice &I dupli-cate and/or sectionalise the bus bars, with the circuits distributed over thevarious sections. However, the full advantage of such precautions can berealised only if the bus protection can discriminate the faulty section hrnothers and isolate the same selectively.

63.1.2 A practical scheme of protection is therefore to be designed tocover the entire bus bars in two or more zones as required, each zone beingcomplete by itself, with the necessary protective and tripping relays.

How the zones are created for a typical duplicate bus bar execution isgiven in Fig. 25A. The secondaries of the current transformers of all thecircuits connected to a particular bus bar/section are paralleled selectivelyand connected to the associated differential relay. Since the bus couplerand bus section switches are common to the zone on either side, currenttransformers are also required on either side for these circuits in an idealCasC.

The duplicate bus bars as given in Fig. 25A are sectionlized by isolators.

39

24A Single Bus Bar

248 Transfer Bus

- MAIN BUS

1 I/-u-w+/

Z’dZ

T

J; //

T / /- OUPLtCAlE

24C Duplicate BusFIG. 24 Bus BAR LAYOUTS SHOWING PROTECTION CURRENT

TRANSFORMER I&CATIONS (TYPI~AL)-CO~

BUS

40

Is r3wz(PartIx)-1977

r 1-y MAIN BUS

1 11 /_r

, OUPLCATE SUS

JRANSFER BUST I I

24D Duplicate Transfer Bus

24E Mesh Bus

FI G. 24 Bus BARLAYOUTS SHOWINGPROTECTION% RENTTRANSFORMERLOCATIONS(TYPICAL)

The isolators being non-automatic devices which cannot tri, 1 to isolate thefaults as breakers do, current transformers are not required to 1 ‘: providedon either side of them. Thus, when the isolators are kept closed the entire

41

IS g 38W (Part lx) - 1377

7”“/r-/+/

52BS, 52BCA, 52BCB, 52A, 52B - Cixmit Breakers

25A AC Circuits87 - Relay

cos - Cut Off Switch 86 - Trip Relays 87 - Relays256 Type I Trip Circuits

Fig. 25 ZONES IN BUS BAR I’R OTECTION (TYPICAL)-cOntd42

r - - - - - - - i

COS A 66A 86CHe 52A

cos-c--__

660 86CH~_-!L.??~!?-o---, TRIP 528

cos-D 66D0-l- o-o--------o-

COS-A 66Cti-TRIP 52 BCA

[Similarly for 52BCBand 528s)

COS - Cut-Off Switch86 - Trip Relays87 - Relay52 - Circuit Breaker

25C Type II Trip Circuits

FIG. 25 ZONES IN Bus BAR J?ROTECWION (TYPICAL)

43

I8S3WpiUtM)-l977

duplieafe bus bar can be considered as a single zone. On the other handwhen the isolator/isolators is/are opened, the two sections of the duplicatebus bars which are electrically separate, now, should be considered as twoseparate zones. Consequently, the current transformers groups of twozones (C and D, in this instance) are connected together or disconnectedthrough the normally open auxiliary switches of the isolator. It will how-ever be noted that when the differential relay is of medium or low impe-dance type such paralleling of relays of two zones can increase the primaryoperating current of the protection. It is therefore, eential in those casesto disconnect one of the two relays and tie together the corresponding dctrip circuits. This is also achieved th&ugh the use of auxiliary contacts ofisolators (see Fig. 25B and 25C).

Further, the feeders, in a duplicate bus layout, are capable of being con-nected to either of the bus bar and fault to earth existing between the localisolator 89LA and the current transformer, the feeder is energised from theremote end inadvertently (ses Fig. 26A).

When an earth fault occurs in a solidly earthed s tern, some of the faultcurrents can ffow through the conductors of the feedy”er, if it is isolated, fromboth ends A and B earthed at these ends through earth switches. Thesecurrents also can damage the current transformers if they are left open-circuited.

Of course, the above is true only if the current transformers are locatedtowards the line side of the feeder isolator/earth switches. Thus the chancesof the above happening are less in the case of oil circuit-breakers with bush-ing current transformers than for air blast breakers with separate outdoorcurrent transformers.

Automatic shorting of current transformers through auxiliary switches asshown in Fig. 26B and 26C can sometimes lead to further complicationswith layouts shown in Fig. 24C, for example, the mere fact that both isola-tors al and a2 are open does not necessarily mean that the circuit is diswn-netted from the bus bars. In such cases, therefore, manual unshorting andreconnection of the current transformer secondaries as required, will bemost ideal.

6.2.1.3 Rejeater relays for curwnt transf~ secondary switching - In cases,where a scarcity of isolator contacts exists, it may become necessary to usecontact multiplying relays. Depending upon the type of isolator, that is,manually-operated or motor/pneumatic-operated, these relays can be oftwo types.

65.1.4 Relays for manually-opsratcd isolator - A typical method of usingrelays for contact multiplication in the case of manually-operated isolators isindicated in Fig. 27. It will be noticed that normally closed contacts of theisolator are used to energize the relays. This is considered a better arrange-ment compared to using normally open contacts of the isolator as it resultsin a ‘&l-safe scheme. This will he evident from Fig. 27 because it can be

89 - IKdator26A Earth

268 Change-Over TypeAuxiliary Switches

26C Break-Make AuxiliarySwitches

Fault

FIG. 26 USE OF ISOLATOR AUX&IARYSWITCHESFOR&RRR~TRANSFORMERS

l----------,II

II

III

tIII

III

L--

I IZONE A

ZONE B

Is :3642(PartJx)-1977

seen that, in the event of a failure of dc supply to the relays, no current trans-former gets open-circuited as would be the case otherwise.

Since this arrangement requires the relays to be kept continuously ener-gised, the relays should be continuously rated for this application. Alsothe normally closed auxiliary switch should open earlier than the closing ofmain contacts and close later than the opening of the main contacts. Dupli-cation of the auxiliary contacts of the isolator also is recommended thoughthe same for the relay contacts may not be really necessary since usually therelay contacts are more reliable than the auxiliary switches of isolators.

F I G. 27 RELAY FOR CURRENT TRANSFORMER SWITCHING (MANUALISOLATORS) (TYPICAL)

69.1.5 Rdays for motor-o&rat.d isohm -A typical method -of usingrelays for motor-operated isolators is given in Fig. 28. The relay should bean electrically reset type having separate ‘operating’ coil OP and ‘resetting’coils RES. When the ‘operating’ coil is energised the contacts close andlatch in, when the ‘reset’ coil is enesgised the latching is removed and thecontacts open out. An additional pole of the control switch CS of theisolator energises the relay at the same time as the closing command is givento the isolator. The relay picks up and its contacts make the secondarycurrent circuits, therefore, much earlier than the isolator main contactsmake. Similarly when the opening command is given, the reset coil ofthe relay does not get energised till the isolator has acutally opened.

Further, the contacts of the relay being ‘latch-in’ type, there is no fear of

46

IS:3842(PartrX)-1977

the contacts open-circuiting the CT secondary in the event of a dc faihut.Also the relay coils are not kept continuously energised.

Normally closed contacts of the isolators are required for the relay opera-tion and should be duplicated for added reliability.

CLOSING OPENING

CS - Control SwitchOP - Operating Coil

RES - Kese Ming Coil

F I G. 28 RELAY FOR C URRENT TRANSFORMER SWITCHING ( RE M O T ECONTROLLED ISOLATORS) (TYPICAL)

6.2.9.6 There is one disadvantage in using relays for switching currenttransformer secondaries. It would involve in drawing the individual cur-rent transformer leads to the control room where the relays are locatedbefore paralleling and connecting to the differential relay. Thus, thesecondary lead resistance and consequently the knee-point voltage require-ments of the current transformers will be increased. One way to avoid thiswould be to locate the switching relays in the switchyard itself inside localcontrol rooms. This would also reduce the amount of cabling betweencontrol room and switchyard.

6.2.1.7 Location of current transformers - Various methods of locatingcurrent transformers for bus bar protection on the feeder circuits, bus sectioncircuits and bus coupler circuits had been shown in Fig. 24. For feeder

47

circuits two alternative locations namely Yand ffhave been shown. Whatis to’bc observed from Fig. 24 is that dl the bus bar protection currenttransformers have been so positioned with reference to the associated circuit-breaker that the latter is fully covered by the xone of protection each currenttransformer deflncs. This is very essential to isolate effectively fults any-where on the bus bar sections covered by these zones, leaving no blind spots.

However, it will not always be possible to locate the current transformersas shown in Fig. 24 due to economical reasons.outdoor current transformers.

This is especially true withIn such cases alternative locations as shown

&Fig. 29 have to be accepted. These alternative locations, however, wouldfirll‘ short of the ideal in the matter of full xonal discrimination, as cannaturally be expected. It will thus be clear from Fig. 30A that faults inlocations such as F continue to be fed even after the associated protectionoperates to trip the circuit-breaker. In the case of feeders equipped with3-step distance protection, such f&t&t arc not left for long but would even-tually be cleared by the remote end protection operating in zone B time.However, such delayed clearance (say 0.25 second) is not acceptable inmany cases and effective measures against fults in such blind spots have tobe taken. They usually take the form of an intertripping relay.

6 9 . 1 . 8 Iat&#ing b~tw,ren zosw -,Typical cases of intertripping be-tween adjacent zones of protection-have been shown in Fig. 30. Refefiing60 the arrangement shown in Fii. 30A it will be seen that a fault at F r;auscs&me A protection to operate and trip all circuits connected to zone A, includ-

ing &e bus section breaker 5ZBS. However, the fault is continued to be fedfmm xone B without being detected by zone B protection because it is out-side aone B. However, the tbct that wne A has operated and still the faultco&mes to be fed are indications that the fault is in the ‘blind spot’. Thisis detected by the interlocked ovcrcurmnt relay 511 which is energised by atbi& set of current trans&mers and is triggered for operation by wne Aprotection as indicated. Relay 511, when operated will bypass the wneB di&rential relay contacts to trip all circuits connected to wne B.

In Fig. 30B the operation of an interlocked overcurrent relay for feedercircuit is shown. The particular location of current transformers shown hasbeen chosen only to illustrate the trip circuit selection for intcrtripping.

Alternatively a timer can be used in place of the interlocked overcurrentrelay as shown in Fig. 30C which is self explanatory. In order to be f&m-&rally the same as an interlocked overcurrent relay, the timer should besuch that it requires a continuous signal till the expiry of the set to ‘ve atip impulse. To the same end, the contacts that encrgise the timer srouldbe self-reset, preferably that of the diSrentia1 relay itself. The settingadopted on the timer, should give allowance for the time of operation ofzone A protection plus the opening time of the breakers.

This method of intertripping would be found very advantageous on gene-rator circuits where the bus protection is required to inter-trip the generator

48

PRO

‘BUSPROTECTION

BUS OR F E E D E RPROTECTION

BUS OR FEEDERPROTECTION

BUS OR FEEDER BUS OR FEEDER BUS

29A Ideal 298 Preferred Arrangement* 29C Preferred

*Outdoor Current Transformers when infeed from feeder side is larger.IOutdoor Current Transformers when infeed from bus side is larger.

FIG. 29 C URRENT T RANSF~RMER LOCATION (TYPICAL)

OR FEEDER -

Arrangement?! i

8

\ I

PR O T E C T I O N

PROTECTION

I i U

:II

:________-______J

30A Bus Coupler or Bus Section

30B Feeder Circuits

87CH

clJ--@R

2- Timer87_4 - Zone A Differential Relay878 - Zone B Differential Relay

87CH - Check Differential Relay

5ZZ- Interlocked Overcurrent Relay52BS - Bus Section Breaker

R - Resistor

NOTE - Dotted Iine indicate circuitry when a timer is employed instead of 51I.

FIG. 30 INTERTRIPPING BETWEEN ZONES (TYPICAL) - Contd

IS: 384!2 (Part IX) - 1977

2 -.Timer 87CH - Check Differentiai Relay87A - Zone A Differential Relay 51Z- Interlocked Overcurrent Relay878 - Zone B Differential Relay R - Resistor

30 C Use of Timer

Ncrrp. -Dotted line indicate circuitry when a timer is empIoyed instead of 5fJ.

F I G. 30 INTERTRIPPING BETWEEN ZONES ( TYPICAL)

circuits. In such cases due to the generator decrement the conventionalinterlocked overcurrent relay may not operate for certain faults.

When the bus protection is required to intertrip the feeder protection,the method of such inter-tripping would depend to a large extent upon thelength of the feeders and the type of protection provided for than.

a) for short feeders, direct wire dc intertripping can be attemptai.b) for longer feeder-s having distance protection with carria inter-

tripping or blocking arrangement the interlocked ovacurrent relaycan be used for sending the intertrip signal or de-blocking theprotection.

c) with unit type pilot wire protection, the interlocked overcurrentrelay can be made to open or short the pilot wires depending uponwhether the protection is current or voltage balanced to cause inter-tripping of the remote circuit breaker, provided the fault infecd issufficient to operate the remote end protection.

6.2.1.9 Interlocked overcurrent relays - These are conventional relays,specially designed for this application, available with the various s-claymanufacturers. Alternatively an ordinary overcurrent IDMT relay withslight modification, can also be used.

6.2.1.10 Conventional type interlocked overcurrent relqys - A relay of thistype is diagramatically represented in Fig. 31. This is a single pole IDMTdisc type unit having a built in summation current transformer to fead theoperating coil of the relay. The relay has a wound shading coil instad ofthe short-circuited slug ofthe normal IDMT relays. This shading wi&ng

.51

IS s3S42 (Part IX) -1977

M!4~N CURRENT r.—.— .—.— .—.T%iNSFORMER -1

II II 1~

J ,NTER}R,P

511 — Overcurrent RelaySCT — Summation Current Tran~ormer

86 — Trip Relay (of bus protection) of the CircuitOP — Operating CoilSH — Shading Coil

(FIG. 31 CONVENTIONAL INTERLOCKED OVERCURRENT RELAY

is not kept short-circuited, but brought out t: separate terminals on the relay,and will be shorted only when the intertrlppmg protection (for example,zone A in Fig. 30) operates.

The summation current transformer supplies the relay with a single phaseoperating current for all types of faults (for a t ical summation current

Ptransformer design see Fig. 23). The value o the summation currenttransformer secondary currents for diflerent types of faults of the samemagnitude, however, are different for the reason that the effestive stationarycurrent transformer ratio changes with the type of fauhs. Thus with theconnections shown in Fig. 31 the relayhas a better sensitivity towards cer-tain types of faults than to others. Typical settings for a particular make ofrelays are shown in Table 3.

TABLE 3 EFFECTIVE SETTINGS OF INTERLOCKED OVERCURRENT RELAYS(Clause6.2.1. 10)

TYPEOFFAULT EFFECTIVElSETTIN~(PEROE;EmATEO

R-E 3.546

;E 69R-Y ‘ 138

138;:: 69R- T-B 80

.

52

Is:3842(PartIx)-1977

At 5 times the setting current, the relay has an operating time of 84second with time multiplier set at unity.

The variation of fault settings makes it very essential to check for positiverelay operation for all types of faults. Especially, for generator circuitswhen relay operation depends upon generator infeed: The applica-tion needs careful study, because of the generator decrement mentionedunder 6.2.1.8. The application of these relays for bus-coupler and bus sectioncircuits does not usually pose any problem, though here too it is advisableto check for relay operation with the expected primary switching modes.

6.2.1 .I1 Three pole IDMT relay as interlocked overcurrent relay-A standardthree pole IDMT relay with settings of say 20 to 30 percent of 1A or 5A canalso be used as an interlocked overcurrent relay, provided these relays havewound shading coil separately brought out. The method of connection ofthe relays is given in Fig. 32. The relay connection, it will be noted, isexactly similar to that of any other 3 pole overcurrent relays. But setting,lower than full load can be adopted on these, the shading winding beingkept open-circuited under normal conditions. However, the thermalwithstand consideration sometimes preclude the use of the lowest setting inthese relays.

6.2.1.12 Use of single pole overcurrent relay as interlocked overcurrent relay -This method is based on a connection of current transformers given in5.2.9.2 (b) and is given in Fig. 33. It is better suited to the generatorcircuits where the effect of detiement is very much felt and where the ,current transformer ratios are more or less kept unaltered.

The peculiar connection of current transformers effectively replaces thesummation current transformer. A conventional IDMT relay with 10 to40 percent setting and 0 to 1.3 seconds definite minimum time-lag can beused. There is no necessity for a special relay with wound shading coil.The relay is normally kept short-circuited through normally closed contactof the bus bar protection trip relay so that it does not pick up on load. ThisJVC contact should have a high breaking capacity of about 50A.

It is, however, recommended that the relay mentioned under 6.2.1.11 beused wherever possible as it has the least amount of complexity fromapplication point of view.

6.2.2 Check Feature - Mal-operation of bus bar protection can result inwide spread system failure. It is therefore considered judicious to monitorits ‘operation’ by some form of check relay. It should not, however, beconcluded that such a step is expedient due to some inherent flaws in thescheme itself.’ Specially when high impedance circulating current schemeis used the fact that the inherent factor of safety in the setting calculationsis quite high and that any departure from the assumed conditions for c&u-lation only results in an increase of the safety factor, rules out the possibilityof mal-operation from the design point of view. The provision of a checkfeature is, therefore, purely a measure against mal-operation caused by

53

IS : SW (Part IX) - 1977I’

SH&k51JY

S

&ISllR

. OP

1

TO INTER TRIP

I-

I 66-2

86 - Trip Relay51- Overcurrent Relay

OP - Operating CoilSH - Shading Coil

FIG. 32 USE OF ORDINARY OVERCURRENT RELAY FOR INTERLOCKING

USING 3 POLE EARTH FAULT RELAY (TYPICAL)

Fault Relay Current as Percentage ofthat on a R-E Fault

__-

R-E 100

B’:;100200

R-T 200

;;100300

R-Z--B 260

.N-- Current Transformer Ratio51I- Interlocked Overcurrent Relay%A - Trip Relay

FIG. 33 S INGLE POLE IDMT RELAY U SED ASOVERCURRENT RELAY ( TYPCIAL)

54

INTERLOCKED

IS : 3842 (Part IX) - I977

external w&s. Incidently it also allows low settings to be adopted insome cases as discussed later.

6.2.2.1 The ideal check feature should possess the following charac-teristics :

a) The check feature should be provided by a relay which is physicallydifferent from the main relay.

b) It should pick-up for all types of faults that the main protection iscapable of detecting.

c) The check feature should be at least as fast if not faster than themain protection for a given type of fault.

d) The source which feeds the check relay should be physically differentfrom that which feeds the main protection.

e) The check feature should operate only for faults within the mainzone/zones of protection and not for external faults.

6.2.2.2 Check feature using separate sets of current transformers and relays -A.se.parate set of cores on the circuit protection current transformers is addedwith the ratios the same as for the main discriminating cores. The secon-daries of all these current transformers are paralleled in groups, ‘one pre-phase, and connected to a differential relay as in the case of the main pro-tection. I-Iowever, no attempt is made to switch these current transformersecondaries. The contact of the check relay is then wired in series with themain relay(s) contact for selective tripping (as shown in Fig. 30). Thus, solong as the fault lies within the overall check zone, this feature is operative.Also, being identical to the main protection in design and execution thecheck feature is as fast as the main feature. Finally both features arephysically independent which makes this the ideal form of check features.Costwise also, this is not an unattractive proposition, and is therefore re-commended whenever possible.

The check contact is invariably recommended in the negative lead of thetripping relays, shunted by a resistor of appropriate value as shown inFig. 30. This would prevent the trip relays getting energised when andif the positive gets applied to the relays inadvertently as could very wellhappen in the cable trenches during testing. The resistor across thiscontact takes care of the corrosion problems.

6.2.2.3 Checkfeature using two differential relays in series (relay du$ication) -This is comparatively a less than ideal solution since the current transformersfeeding both the relays are the same. The duplicated relays can be con-nected to the bus wires in two different manners, depending upon whetherthe relay is a high or medium impedance type.

For high impedance type relays (as defined in 5.2.8.2 and 5.2.8.3), thetwo relays can be connected in parallel across each zone bus wires. Theircontacts will then be connected in series as shown in Fig. 34A. The relays

55

being high impedance type, connecting two in parallel does not increase theprimary operating current of the protective system appreciably (see Fig.34B).

The relay setting voltage can be worked out ignoring the presence of thesecond relay and both the relays can be given the same current settings.

For relays of the medium impedance type (see 55.8.1) connecting thetwo relays in parallel is not advisable for the fear of the primary operatingcurrent (POC) being adversely affected. The solution is to connect thetwo relay elements in series, with a single common stab&sing resistor asshown in Fig. 34C. Again both the relays are given the same settings and

34A Relay Contacts in Series 34B With High Impedance Relays

ZONE A RY

8

R R Rz!z01 87 07

1 I 1

87 87 87

2 2 2

34C With Medium Impedance Relays

87 -Relay

zG - -:x-

FIG. 34 CHECK FEATWRY BY &SLAY &PLIoATION (TYPICAL)

58

Is:384!2(Partlx)-1977

the stabibsing resistor value can be worked out from the following formulafor a given voltage setting:

V = ir (R,+2R,)where

V = voltage setting,.2r = setting current (same for both the relays),R, = stabilising Resistor value, and.Rr = relay impedance at setting current &.

6.2.2.4 Check feature with inhmnt phase f&t and external earth fault check -This is indicated in Fig. 35. When there are three differential relays, one

for each phase, two of them will and should operate for a phase fault. Henceby merely connecting the relay contacts two by two in series as shown inFig. 35, an inherent phase fault check results.

A separate earth fault check relay, described in 6.2.2.5 and 6.2.2.6 canbe added, and its contacts can be connected.

6.2.2.5 Earth fault check relay, zero sequence m&age oprated - This is asensitive instantaneous overvoltage relay with low settings en~gigi;~nb~~open-delta voltage of a bus-connected voltage transformer.fault occurs on bus bars there appears a zero sequence voltage across theopen delta terminals which causes the relay to pick-up. When a solidphase-to-earth fault occurs on the bus bar the voltage that appears acrossthe relay would be rated line voltage of the tertiary winding, assuming noarcing and a solidly-earthed system. However, arcing cannot be precludedaltogether and a low setting of about 20 percent should be adopted for thisrelay, to ensure fast operation under such conditions.

TO TRIPRELAYS

64CH- Earth Fault Check Relay87-DiGremtial Relay

Fro. 35 BUXLT-IN I?HASB FAULT CHECK

57

rs:3842(PartIX)-1977

When the main voltage transformers do not have a tertiary winding, a1 : 1 ratio auxiliary voltage transformer connected star/open delta can beemployed. Further when there are two or more main voltage transformers,one per zone, as many check relays will be required. Also fuses should beavoided on the secondary side of the auxiliary voltage transformer. Theprimary fuses can, however, be retained. Suitable switches to short-circuit the check relay contact when the check relay supply fails also have tobe provided as required.

6.2.2.6 Earth .fault check relay, zero sequence cuwent operated - Thoughmost of the methods of connection of current transformers described under5.13 can be adopted for outdoor installations also, the most common oneis that using current transformers mounted in the transformer neutrals.However, it has’the disadvantage that it is not available when the trans-formers with earthed neutrals are kept disconnected from the bus bars.

When there are more than one transformers, a neutral current transformeris required for each, the secondaries of all such neutral current transformersbeing paralleled and connected to the relay. The relay can be a currentoperated type with a setting range of 10 to 40 of rated current transformersecondary current.

6.23 SGGondary Wiring Sumn

6.2.3.1 Any protective system which requires the balancing of secon-dary currents from two or more sets of current transformers over pilotsdepends heavily on the integrity of these pilots for correct operation. In ahigh impedance circulating current scheme, where the relay operation isclosely associated with the magnitude of the voltage across the pilots at therelay terminals, an open-circuiting or short-circuiting of the leads of any oneof the current transformers would seriously interfere with the performanceof the protection. The effect of an open-circuited current transformer is thesame as that of an internal fault. In such cases a voltage proportional tothe primary current in the open-circuited current transformer. can appearacross the relay. If this current is less than the primary operating currentsetting of the protection there may not be any danger, but a subsequentthrough fault can cause an unwanted trip. Similarly with a short-circuitedpilot an internal fault can go undetected, depending upon the location ofshort circuit. It is therefore customary to employ some kind of secondarycircuit supervision scheme to detect and alert the operational staff aboutsuch secondary wiring faults.

6.2.3.2 Continuous supervision by means of an externally injectedalternating current through the test windings of the current transformers isone of the methods available though it is not popular because of the compli-cations created for double bus bar installations. Further, with bushingmounted current transformers the provision of test windings imposes addi-tional design limitations. Another method is to incorporate a millivolt-meter with a push button in the secondary winding.

58

Is :3842(PutM)-1977

The more popular scheme is the self powered one using the load currentitself for the supervision. A very sensitive voltage operated relay is con-nected across the bus wires, in the same manner as the main differentialrelay. The relay is given a voltage setting such that for a given set of mainrelays and current transformers the primary operating current is 10 percentof the load current of the least loaded current transformer or 25A, whicheveris higher. In practice, the load details are not exactly known and hence afigure of 25A is taken for calculation purpose.

The supervision relay is usually followed by timer relay with a range of2 to 10 seconds, set to operate at about 3 seconds. The time delay helps toavoid unwanted alarms in the case of through faults as well as internalfaults.

The supervision relay is also normally arranged to short-circuit the buswires thereby affording thermal protection to itself (as such relays are notnormally continuous rated) and also preventing unwanted tripping by themain protection, should a through fault occur further to its operation.

Where the supervision relays are not provided with built-in timer, asingle external timer can be used with a protection scheme having more thanone zone (and hence as many supervision relays). The method of connec-tion is shown in Fig. 36.

The effectiveness of the supervision feature can be fully real&d only if thebus fault-levels are high enough to adopt a comparatively high setting onthe differential relays. If such is not the case, narrower margin betweenthe settings of the main and supervision relays can result in themal-operationof the main relays, before the supervision relays get a chance ‘to short-circuitthe bus wires. Where such a narrow margin exists, the check feature is amust.

6.2.4 DC Circuits for Selective Tri@hg - The design of the dc trippingcircuit should take into account all the checks built into the bus bar protec-tion. Further, it should also ensure that all the circuits contributing to theflow of fault current should be tripped out with minimum delay. Indouble bus layouts the circuits are capable of being connected to more thanone zone and the trip circuit should be arranged to have only the correctcircuits tripped in the event of any one zone differential relay operation.

There are mainly two types of trip circuits employed which fulfil all theseconditions .

6.2.4.1 Tyfie I tr$ circuits (using individual feeder trip relays) - Hereindividual hand-reset trip relays are provided for each circuit and two suchrelays for bus couplers and bus section. The dc trip circuit is an image ofthe actual primary connection, as shown in Fig. 25B. The feeder triprelays are selected through the auxiliary switches of the isolator to dc buswires of either zone A or zone B. The trip relays of bus coupler (and bussection, if any) are permanently connected to the dc bus bars, one each,

TO SHORT ZONE ABUSWIRES

2- Timer95- Supervision Relay of Zone A

FIG. 36 ARRANGEMENT OF SUPERVBION RELAYS AUXILIARY ELEMENTSWITH COMMON TIMBER FOR ALL ZONES (TYPICAL)

6.2.4.2 ?jlpd 11 ~F$J circuits (using multi-contact master trij relays) -Thisarrangement is illustrated in Fig. 25C with only a single master trip relayfor one zone. The trip selection is achieved in the circuit-breaker trip coilcircuits again, with the help of isolator auxiliary switches (in the case offeeders). For the bus coupler (and bus section, if any) contacts of zone Aand zone B master trip relays are connected in parallel without any selectionarrangement.

6.2.4.3 Comparison of the two tQ,bes of trip circuits - For large, importantand likely-to-expand installations the type I circuit is to be preferred eventhough it may call for a large number of trip relays, for the followingreasons: *

4

b)

It facilitates expansion. The usual practice is to mount the circuittrip. relay on the individual circuit panels, with bus bar protectionpanels accommodating only the clifherential relay, supervision relaysand others which are common for the bus protection. Hence anaddition of a circuit to the existing installation would not need any’modification of the bus bar protection panel either by way of addi-tional drilling or wiring.Without the provision of a check feature, the type II trip circuitinvolves greater risk compared to type I arrangement in as much asa mal-operation of the trip relay can shut down the entire station.

60

rs:3842(Partlx)-1!m_-

c) With type I trip $reuS&e selection is not done in the trip coil cir-cuit unli~irrt~e II trip circuit. The latter therefore increases thecablingm the breaker trip circuit which is undesirable.

d) Type I trip circuit has the further advantage that regular provingtest of the circuit tripping can readily include proving the trippingfrom the bus bar protection trip relay. But with multi-contactrelay, such proving tests need careful consideration to avoid wrongtripping of circuits other than the one being tested.

6.2.4.4 Other requirements of bus bar protection trip circuit &sign:a) The bus bar protection trip circuit should be separately fused and

should not be connected with other tripping circuits. The wiringshould be preferably separated from others and for this reason aseparate panel as indicated in 6.2.4.2 is usually provided.

b) The trip and alarm supplies should be separately supervised andalarm for their failures should be provided.

c) Protection cut-off switches should be provided for each zone.6.2.5 Alarms and Indicatiorz

6.2.5.1 The following alarms and indications should be generallyincluded:

a) Common Alarms1) Bus bar protection defective (audible $-visual)2) Bus bar fault do3) Trip supply failure do4) Alarm supply failure do

b) For Individual <ones1) Zone operated (audible+visual)2) Zone in commission (visual)3) Zone out of commission (visual)

6.2.6 Scheme for Extra High Voltage (EHV,’ Installations -. This scheme isparticularly suited to EHV substations (‘400 kV and above) where thedistance between two circuit-breakers is great and it may be required torestrict the discharge capacitance of dc cabling on the battery. It wouldhowever require high impedance differential relays (as against mediumimpedance relays) whose operating current is only a few milliamperes. Atypical scheme of connections is illustrated in Pig. 37.

It will be seen that each circuit has its own ac (high impedance difler-ential) relay, the bus coupler and bus sections being provided with two each(in this respect the arrangement is similar to the ordinary scheme describedearlier with type I trip circuits). Also each circuit has its own check relay.There are no dc trip relays and hence no dc trip relay selection using auxi-liary switches is necessary. This results in considerable amount of saving ofcabling between main control room and switchyard, which, in such largeinstallations, more than ofBets the cost of added high impedance relays.

61

w

t tTRIP 52 A TRIP 52BC

tTRIP 528

CT - Current Transformer87 - Differential Relay

87CH - Check Relay5.2 - Circuit Breaker

FIG. 37 SCHEME FOR EXTRA H IGH VOLTAGE INSTALLATION (TYPICAL )

1s :3942(Partn!q-1977

‘I& scheme can be used in case the number of circuits are not largeotherwise very high primary fault setting will result which may not beacceptable on systems when fault current is limited.

6.3 Application Problems

6.3.1 Bypassing of Circuits

6.3.1.1 In duplicate bus bar installations it is sometimes the practice toarrange for bypassing the circuit-breakers for maintenance purposes. Fur-ther, in the transfer bus layouts, such facility is always built-in. When acircuit is bypassed, the bus coupler will have to take over the control of thebypassed circuit including protection and tripping. From the point of viewof the bus bar protection, the bypassing arrangement needs careful study.

6.3.r.2 The following aspects of the bypassing arrangement have con-siderable bearing on the design and execution of the bus protection scheme :

a) Th type of bypassing arrangement - 11; under bypass condition, thesecondaries of the current transformers on some of the circuits needreconnection/short-circuiting the same has to be done manually,using test links provided the bypassing is carried out using jumperconnection. However, with isolators doing this job, their auxiliary Iswitches can be advantageously used to dc the switching of the cur-rent transformer secondaries.

b) The akgree ofjexibility providedfor bypass - In double bus bar layoutsif the bypassing can be done on to either of the bus bars, the switch-ing required for the bus coupler current transformers would becomeslightly complex. This is illustrated in Fig. 38 where the manualswitching using test link blocks is shown. Use of auxiliary contactsof the bypass isolators in this case will further complicate the matters.

c) Location of current transformers --The ideal location of the bus pro-tection current transformers is outside of the zone of bypass and thislocation should always be chosen, unless the breaker is bulk oil type,with bushing mounted current transformers.

From consideration of minimum amount of current transformersecondary reconnection, a bypass arrangement where the currenttransformers are located outside the zone of bypass and where thebypassing is always done only to one of the bus bars (say, the dupli;cate bus bar) is the ideal one. These reconnections can be at-tempted manually, using the test link blocks or automatically(wherever possible) using auxiliary switch contacts. In either case,wherever a current transformer is to be disconnected altogether, itshould first be short-circuited, to avoid any inadvertent damage.Thus, special test links which can make before break or auxiliaryswitches which can break before make, should be employed for thispurpose.

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_ ZONE A

I7

ZONE 0

J/ /

M1/ /BP

CJ

TLK

/

NEUTRAL BUS&IRE (CHECK1

Narp, 1 -Link position shown is for feeder bypassed on to Zone A @l-&&disolator closed).

Nm 2 -Dotted position for feeder bypassed on to Zone B (dotted-circled &+&torClosed).

BP - Bypass IsolatorILK - Test Links

FIG. 38 METHOD OF SWITCHING THE CHECK ZONE CURRENT TRANSFORYBRSON Bus COUPLER , wrrr-x DUAL BYPASS FACILITY (TYPICAL)

6.33 Cabling - In high impedance scheme, it is very essential to planthe layout of the cabling for current transformer interconnection in theswitchyard carefully in order to get the best performance from the scheme.The cabling should aim at reducing the lead burden of the current tram+formefs to the minimum, the lead burden, in this case, being defined as thatcontributed by the loop impedance between the current transformer secon-dary terminals and the point at which the relay is to be connected. Thiscan be reduced considerably by the following precautions:

4

b)

Instead of bringing aii current transformer leads to the relay panelbefore paralleling and connecting to the relay, a set of such parallel-ing bus-wires should be formed in the switchyard, between marshal-ling kiosks of each circuit. From a suitable point, which is the stub,connections for the relay should be tapped, this point being suchtha.t it results in the smallest lead length between it and thefarthest current transformer.The leads from the current transformer terminal boxes should ter-minate in the test link block mounted in the marshalling kiosk ofthe associated circuit. From there, they should be routed throughthe isolator auxiliary switches (if selection is required) back to themarshalhng kiosk before forming the paralleling bus wires men-tioned above.

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IS : 3842 (Part IX) - 1977

c) When relays are used to switch current transformer secondaries dueto, say lack of enough number of contacts on auxiliary switches, it ispreferable to have such relays located right at the switchyard.

If for some reasons, this practice of cabling is not followed, it has to beensured that the cable size is suitably increased to reduce the burden betweencurrent transformers and relays.

7. OTHER TYPES OF BUS BAR PROTECTION7.1 Apart from the types of protection discussed so far which are most com-mon, the following types are also sometimes used:

a) Directional comparison relaying,b) Bus transformer differential relaying,c) Phase comparison relaying, andd) Distance protection.

However, IS their use is very limited, these are not covered in this guide.

8. SETTING CALCULATIONS FOR PRACMCAL INSTALLATION8.1 For the practical installation (typical) shown in Fig. 39 the calculationsfor arriving at the various settings are as given below:

2iOkV MAIN BUS1 1 1 1' 1 1 1 T/ / / / / / / /

PIG. 39 TYPICAL INSTALLATION

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IS : 3842 (Part IX) - 1977

a>b)4d)4f >

dh)3k)44

P)

System - 220 kV, solidly earthed,Switchgear breaking hapacity - 7 500 mVA at 220 kV,.hformal load on each feeder - 250 A,Maximum load on any circuit - l-5 x Normal rating,Current transformers ratio-800/l A (for magnetising curve, seeFig. 40))Lead length (between marshalling kiosk and fault current tralrrformers) -100 m (one way),Leads - 71.029 (cu), resistance at 20°C being 5-9 ohms/km,Relay burden - 1 VA at setting,Relay setting range - 20 to 80 percent of 1 A,Supervision relay burden - 2-3 mVA at setting,Supervision setting range - 2 to 14 V,Maximum number of circuits/zones under normal working:

i) Main zones - 5 (inclusive of bus coupler)ii) Check ZO~ULS - 8

Metrosil characteristics - V&s& = 900 (I&~)“‘z6.8.2 Calculations

8.2.1 Through Fault Stability Limit - Maximum throug: 5t;: 1pOrjmary

current Is (based on switchgear breaking capacity) =2/3x 220 x 10s

= 197OOA

The secondary equivalent If =~=24$jA

8.2.2 Relay Setting Voltage:RCT = 2.73 ohmsRL = 0.59 ohms (for 100 metres)

Therefore, relay Setting VOhge = Itx(&r+t?h,)= 24.6’(2-73+0-59 x2) = 96 V .

The relay can therefore be set for 100 V. Also the current transformershave a knee-point voltage of 302 V (hrn Fig. 40) which is more than twicethe setting voltage above. Hence they are adequate.

8.2.3 Relay Setting Current - This should be chosen, based upon the per-missive maximum primary operating current of the protection. The Pr;-mary operating current should not be more than 30 percent of the minimumfault current. Also it should he higher than maximum load encounteredon an): feeder (say 125 percent of maximum load).

Taking the latter figure, the primary operating current should be notless than 1 a25 x 250 x 1.5 = 470 A. .

The exciting current drawn by the current transform- at relay setting of100 V = 11.3 mA (from Fig. 49).

The current drawn by metrosils (if any is required) is given by%Ex 100 = 900 * I$*~

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2

1 I I I 1 I I I II

.ll o-1 0.2 o-3 04 O-5 O-6 O-7 O-6 0.9 1.0 1-l

SECONDARY EXCITING AMPERES~EE TABLE FOR HULTIPLVIN~ FACTOR)

FIG. 40 MAGNETIZING CURVE FOR CURRENT TRANSFORMER (TYPICAL)

Therefore, or Ip = O-43 mA.8.2.3.1 Tk main .zoncs

Primary operating currentCurrent transformer ratio = IR-kn - ‘m%d’ metrosil (neglecting the

current drawn by super-vision Telays) .

470Therefore, 800 =

5 x 11-3 +0*43IR+ 1 0 0 0 >

where IR is in A

Therefore, IR = O-53 AThe nearest higher setting is O-6 A (60 percent)The correspkding primary operating current

>3 800 = 527 A.

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IS : 3842 (Part IX) - 1977

8.2.3.2 For check feature with the same current setting on the relay,

Primary operating current = j x ‘ll gzoa43 >I 800 = 552 A.

8.2.4 Stabilising Resistor Setting (For both Main and Check zones)Setting voltage = 100 V

Total shunt impedance required = g = 167 ohms

1Relay impedance at setting =-(o.6)2 = 2.78 ohms

Hence stabilizing resistor value = 167-2.78 (say, 164 ohms).

8.2.5 Metro& (For both Main and Check <ones)Peak voltage during internal fault is given by

v, = 2IE d/vk (Vi--&)where

vk = knee point voltageV, = 1, x relay branch impedance

=24.6x167=4lOOVvk = 302 V (from magnetising curve, see Fig. ,40)

’. . ‘VP& = 2t/3 4302 (4 16()_302) = 3 030 V (that is 363 kV)Hence metrosils are required.

8.2.6 Supervision Relay Settings - Taking the primary, operating currentas not less than 25 A. We have the secondary equivalent current,

25= 800 = 31.25 mA

This would be shunted byi-a) the current transformers;.b) the differential relay branch;c) the metros&; andd) supervision relay itself, depending upon the voltage setting adopted

on the supervision relay. Choosing the latter as 5 V for the first setof calculations we have the following:1) The magnetizing currents drawn by the current transformers

(from Fig. 40) = 1.35 mA,

2) The di&rential relay branch current = 5 x 1 0 0 0167

=3omA,

3) The metrosil current at this voltage will be negligible,4) The current drawn by supervision relay itself

+nA=o.46mA.

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85.6.1 For main feature

Effective primary operating current = 800 (1~35~5+30+046) A1000

8.2.6.2 For check feature

= 30 A (approiimately).

Effective primary operating current = 800 (1.35 x 8+30+046) = 41 A1000

Hence 5 V setting on supervision relay is adequate.

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