Measurenents

ANATYSIS AND PRO1ECTION OF POWER SYSTE}T,S COIIRSE

BUSBAR PROTECTION

BY

G.A. I{AIII,ET

1.

BUSBAR PROTECTION

1.0 INTRODUCTION

rn the early days of the electricity suppry ind.ustry, protective equ.ipmentfor plants connected. to a busbar instalration was reried upon to crearbusbar faults' This resulted. in tine-delayed. fault clearance by time-grad.edprotections such as distance relays or overcurrent-tine relays.with present-day widely meshed power systen networks with rine sectionsvarying in length and nurnerous intermed.iate infeedsl fault elearance byzone 2 or zone J of d'istance relay can be difficult plus the inpossibilityof selective tripping of different br:s sections. rn order to maintainsysten stability and ninimise da^mag:e due to high fault levels tine-delayedtripping for busbar faults is no longer acceptable. rt is thereforenecessarJr to detect busbar faults selectivery with a rrnit form of protectionsystem.

2r0

(i)

(ii)

rt must be conpletely reliable, since the protectlon may only becalled to operate once or twi.ce in the rife of the switchgearinstallation and failure to operate under fault eonditions wourdbe unacceptable.

rt raust be absolute].y stable r:nder all through fault conditi.onssince failure to stabilise wourd cause unnecessarJr wid.espread

interruption of supply.

rt must be capable of comprete d.iscrimination between sections ofthe busbars to ensr:re that the rninimum nr:nber of circuit breakersare tripped. to isolate the fau1t. .

(ii:.)

2.

(i") It nust possess high speed of operation to minlmise da.rnage and

maintaj.n system stability.

There are two main types of busbar installation :-(i) fnd.oor or metalclad type.

(ii) outdoor type.

Indoor or metalclad switchgear is rnainly used on medium voltage systems,

but with the introduction of sF5 gas as an insulation med.ir:m, it is now

possible to have rnetalclad. busbar installations up to the highest system

voltages. rf the rnetarcrad type is fulry phase segregated interphase

faults cannot occur and only earth fault protection is required. A1I other

types of busbars should be protected against both phase and earth faultsby varior:s types of protection schemes.

1.o

1.1

TYPES OF 3US3AR TA.OTECTION SCIfiME

SASIC CIRCULATING CURRE}IT SCHBIE

This is a sinple form of r.rnit protectlon which compares the current entering

and ]eaving the busbar as shown in Fig. 1. If the curent transf orrners were

perfect there would. be no cument through the relay circuj.t. In practice

there will be spill current through the relay circuit, which must not exceed

the relay cunent setting up to the rnarinr:n through fault current.

1.2 BTASFJD DI$ENEMIAT. CIRCULATTNG CURRnIT SCFTN,IE

The basic schene using practical current transforrners cannot provide high

speed operation and guarantee through fault stability at the same tine.

A bias feature can be incorporated. The principle of operation is shown

in Fig. 2. The reray used has both bias and operating circuits. ?he

former is energi-sed by the arithrnetric sr:m of all the circuit curents

whilst the latter energised by the vector sun. A set of rectifiers and

ar:xi1iary sunnation cr:rrent transforners is required.

1.

,.1 DIRECTTONAT COMPAXISON SCIII}E

The principle of operation is shown in Fig. Ja. An internal fault resultsin eurrent in all feeders connected to the bus flowing towards the bus.

During an external fault, however, the cr:nent in the faulted feed.er willflow outwards. Contacts from all the d.irectional relays are connected inseries to energise a muLti-contact trip reIay.

An alternative amangement is to use an additional blocking relay. The

d-irectional relay is to have a changeover output contact. Al-1 the break

contacts are paralleled and connected to the blocking relay. All the make

contacts are paralle1 and connected to the trip relay through a no::nally

crosed contact from the blocking reray. This is sholn in Fig. 1lb.

The d.irectional relays are arranged to look into the bus. luring norrnal

load eonditions at least one feed.er will ca^rry outgoing current so thatthe blocking relay will nornally be energised. and there will not be contact

race to prevent tripping on an external fauIt. A tine deIay in the triprelay can be provided for this purpose.

'.4 PHASE COMPARISON SCHEME

Fig. 4 shows a simplified single phase arrangernent using high speed. relays

A and 3. Under external fault conditions the prinary fault currents are

in phaser but the cr:nent transforrner secondarJr cuxrents are out of phase.

Soth relays A and B operate, no tripping occrgs. Ilnd.er internal faultconditionsr the primary currents are out of phase but the secondary cunents

are in phase. Relay A operates in the positive half cycle of the cunent

wavefbr"m whilst relay 3 operates in the negative half cycle"

4.

1.5 FRII'IE LEAKAGE PROTECTTON

lhis is a sinple and econonical fom of busba^r protection which is ideal forthe protection of phase seg?egated. ind.oor metalclad. switchgear where earthfault protection only is reqr:-ired.. The main basic requirenent is that thefrarne of the switchgear mrrst be insulated from the tn:e earth and betweensections of the switchboard. This provision of insuration between switchboardsections is the main disadvantage of this forr of protection plus the factthat it is not possible to d.iscriminate between faults on tr*o sets of busbarsrwrning through cornmon switchgear fra.meso

).5.1 Principle of Operation.

Refer to Fig. 5.

T?re principle of operation of a frame leakage schene is based on the factthat any breakd'own of the witchgear lnsulation will raise the potential ofthe frame to earth and' cause a eunent to flow in the connection between thefra'me bonding bar and earth. A current transformer connected. between thebonding bar and earth will therefore measure thi-s earth fault current and.

operate a protective re1ay. An instantarreous cunent relay such as typeCAG12 is sufficient for this application.

The current transformer ratio used is not critical provid.ed the necessarJr

fault setting can be obtained.

3.5.2

The switehgear nust be insurated. as a whore, usually by stantling it on

concrete, ta-king care that the found.ation bolts d.o not touch any steelreinforcenent. No other earth connections of any Wpe including incid.entalconnections to structural steelwork should be present. This is to ensure

that:-

5.

(i) The effective setting of the relay is not raised. by any path shwtingthe principal earth connection and cr:*ent transformer,

(ii) No spurious tripplng will take place for an external earth faultwith current flowing into or out of the switchgear fra.&e.

The insulation achieved should be greater than 10 ohns to ensr:re stabilityunder external fault cond.itions. This is illustrated by considering Fig. 5

which shows the cuFent distribution for an external earth fault. The faultcurrent splits between the switchgear frarne to earth insulation resistanceand the resistance of the earthing electrod.e. Since the latter has a valuewhich is norrnarly less than 1 ohn the cu:rent 11 seen by the relay wirl be

approximately 1CIrt of the total fault cunent. The relay setting should begreater than 1CI/o of the maxinun earth fault current to achieve stabilityfor external- faults and should. be 1g1/o of the mininr:m earth fault cr:rrent toensu.re fast operation for busbar faults.0n resistance earthed. systems r*here the earth fault current is fairly constantthere is no problern. However on soridly earthed. high vortage systems thedifference between mininrrn and maxinun ea^rth faurt levers may be considerableand this may prevent the use of fra.ne leakage protection scheme unlessspecially high insulation resistance is provided.

A11 cable glands nust be insulated to prevent circuration of spurior:s cg:rentproduced by higb voltages induced in the cable sheaths under through faultconditions causing flashover between grand and srritchgear fr4ne.on resistance earthed systems it is reconnended to use a eonmon earthingelectrode for both the power source neutral and the switchgear frame. rfseparate electrodes are used an internal fault cunent has to flow thror:gh

both eleetrodes in series. rf either or both are of high resistance orinadequate current carrying capacity the fault cunent nay be linited tobelow the relay setting.

6.

If the electrode earthing of the switehgear fra^me is the offender the potential

of the fra.me may be raised to a d.angerous value as a1l the fault curent will

flow through the frane-to-earth insulation resistance.

Fig. 5 shows the preferred arrangenent with the earthing connection from the

switchgear frame rnade between the bottorn of the earthing resistor and the

earthing electrode.

t.r.t Types of Fra.ne Leaka.ee Schelnes.

1.5,1.1 SinEle Susbar with Insulation Saniers on BothSides of 3us Section Circult Sreaker-

The seheme is shown in Fig. 5. In this case sepaxate zones of protection

are fo::med with complete discrinination between them. Faults on the zones

on either sid.e of the bus section brealer outside the bus section zone result

in tripping of the bus section brealcer and a1I circuit breakers in the fJtea

Zon€o Faults on the bus section zone result in tripping of all circuit

brea.kers.

1.5.1.2 SinEIe Busbar with Insulation Sarrier ona

In this agangement the bus section brealcer i.s insulated. on one side only

as shor.rn in Fig. 7. There will be a blind spot between the br:.s section

breaker a3d. the insulation bagier. tr'aults on zone 1 or zone 2 will trip

the bus section breaker and all breakers connected. to the busbaxs in the

faulted Zoneo To cover the blind spot a sequential tripping circuit is

used whieh is arranged. to trip circuit breakers in the non-faulted zone

if the protection in the faulted zone including the bus section breaker

remains operated. A time setting 1n the order of 0.4 seconds is used. to

allow for the breakers in the faulted. zone to trip and the protection in

that zone to reset (for a genuine fault in that zone) before initiating

tripping of the other zonse

7.

It is essential that an earthed. source of

the busbar not containing the bus section

supply is connected. to the side of

breaker.

1.5.1.1 Double Susbar.

rt is extremery diffieult to obtain d.iserimination between the two busbars

due to the practical difficurty in insuLating between them and the fact thatthe circuit breakers thenselves must also be includ.ed in the zones ofprotection.

The schene shown in Fig. B illustrates the various zones arranged. to tripall circuit breakers connected to the faulty section of the nain busbars and

al1 breakers connected to the reserve busbars.

3.5.4 Check Feature.

The main objection to the frame leaka6e protection scherne is the faet thatthe discrininating relays in the various zones of protection will operate

whenever the cunent in the current transforrner is above their effectivesetting irrespective of whether it is due to a genuine busbar fault orfaults in the secondarSr wiring.

To overcorne this diffieulty it is conmon practice to add a cheek featr:reas a second line of d.efence. This takes the fo:m of another ind.epend.ently

operated' relay to detect earth faults. This relay is non-discrj.ni.natory

and operates for both internal and external faul-ts. Both the eheck relayand diserininating relay m'st operate before tripping can occur.The various nethod.s of obtaining a check feat're &Te !-(i) Neutral cheek provided by a relay energised. from a single cugent

transforner in the pouer systen neutral.

(ii) Residual check proirid.ed by a relay energised fron residuallyconnected cr:rrent transforuers on the incoming circuits to the

busbarso

8.

(:.ii) Residr:ar vortage check provided. by a voltage reray energised froman open delta voltage transforner supply.

cheek relays should' be self reset to eliminate the need to manuarly resetthe relay after each external fault.Fig. 9 shows a typical tripping and alarn circuit for a frame leakageprotection schene with a eheck feature.

1.5

Tlris is a r:nit type protective scheme in which cunents entering an. reavingthe busbar installation a-re compared continuously. The object is to providefast operation at a 1ov fault setting on interraal faults and. yet retainstability up to the highest possible value of short circu:it current onthrough faults' cr:rrent transfonners on each of the busbar circuits areeonnected in paraller whlch will produee a resultant cr:nent to operate arelay for internal busbar faults onry. Theoreticarl_y such a systen isunaffected by through faults, but in practice the associated. cu*enttransfo::ners may not behave ideally when the cu:rent exceed.s a certain vaIue.Errors in transfo::station dr.rs to saturation of the current transfo::mer coresnay be sufflcient to cause rnaloperation if special preeautions are not taken.

1 .5.1

consider Fig. 10. under external fault conditions the current in relay R

should theoretically be zQToo However, if one of the cr:nent transfo:mersbecones fu11y saturated' d'ue to high flux in its core, its secondanlr e.n.f.becones zero and this can be represented by a short circuit acrosg itsmagnetising impedance. This is the worst condition for stability of therelay and the high inped.ance principle is used to ensure that the relaycircuit inped.anee is sufficientry high to prevent its operation.

9.

Assuming that CT rXf becomes ful1y saturated and ignoring the nagnetising

culrent in CT rYr r the secondarJr current Iy will split up betveen ttre relay

circuit and the saturated current transforrner. The relay circuit inpedance

is adjusted so that the current flowing through the relay is less than its

cunent setting. The necessary inpedance can be calculated with a slight

safety nargin by assuming the cu:rent Iy flows th:ough the saturated. cunent

transformer only. This will develop a voltage Vj given by :

vp = Iy (Rcr * Ru)

Ttre relay circuit irnped.ance is then adjusted so that the necessary voltage

to operate the relay is greater than the voltage Vp. This voltage Vg ca11ed

the setting voltage is given by :

Yg = rnR where Ip = relay current setting.

R = relay circuit inp,edance.

fhus

i.er

Vg

iRR > Iy (Rcr + RpE)

.t. R

-1R

In order to obtain the required. value of R it is usually necessary to use

an additional resistor called stabilising resistor Rg1 in series with the

relay coil Rp.

.'. Required value of stabilising resistor :

Rst = R-RR

For an internal fault at tr' as shown in Fig. 11 the current transfolsners

will attenpt to transfom the fu1I fault cur'rent and. pass this through

the relay circuit. The voltage output Vp fron ttre current transformers

required to pass this cunent through the relay will be given by:-Vp = fF.R

= fr'.a (na* + nr;)fp

10.

Depending on the relative values of relay setting fp and fault cunents

rp and ry the value of vp can be many KV. rt is not possibre for any

practical current transforners to develop such a high value an6 severe

saturation will oecu!. The saturated output consists of spikes of very

high voltage around the points of zero f1ux. To enable fast relay oper-

ation the curent transformer should have a knee-point voltage equal to at

least twice the relay voltage setting Vg.

1.5.2 Fau1t SettinEs.

Consider Fig. 11 again. With a given relay current setting fp the overall

fault setting of the scheme is higher than Ip due to the magnetising cugent

taken by the curent transformers. This overall fault setting or the

effective setting Ig (referred. to cu:rent transformer secondary amperes)-

is given by :

rs=rP+Zrm vhere I, is the magnetising cr:rrent

taken by one current transforroer

at the setting voltage Vg.

the effective prinarJr current setting fp is0r in terms of prinary cr:rrent

given by :

rp = l:,1- + 2rm)

where r = ;":-

transforner c'*ent

For a busbar installation with n circuits :

rp = r(rn+nr6).In addition to the relay there may be a voltage limiting d.evice, a fault

setting resistor and supervision relay connected across the relay circuit.

These will be discussed in later sections. Therefore, in generar, the

effective prima:ry cur-rent setti.ng can be expressed as :

fp = t (In + nI, + Iy + fSR + Iv)

11.

where r14 = currrent taken by voltage limiting device

at Vg volts.

rsn = current taken by faurt setting resistor atVg volts.

Iv = cunent taken by supervi.sion relay at Vg volts.The value of rp shoul-d' be in the order of tq" of minimr:m fault cu*entavailable' This is to ensure sufficient fault cunent frowing throughthe relay under internal fault eonditions for high speed. operation.

3.5.1 Throueh Fau1t Stabilitv.The stabilj'ty lini't of a busbar protection scheme is based on the naximunthrough fault curent. rn general this takes the value of the associated.switchgear ratj-ng irrespective of the existing or anticipated fauLt 1eve1s.As shol'n previowly the stability limit is governed. by the relay clrcuitsetting voltage. This must not be less than the stability voltage of thesysten, which is caleulated by assuming that the maxinr:rn through faultcurrent flows in through one cilrrent transforrner and out through a second.

onet the latter being assumed to be the most remote (in terrns of secondarxr

Lead resistance) from the relay associated with the zone concerned. rt isfurther asswaed that the d.c. conponent of the offset prina:ry fault c'rrentcompletely saturates the second. cr:nent transforraer whirst the first one

continues to transforn perfeetly.

1.5.4 Fault SettinE Resistors.

These are used to increase the effective primary fault setting by creatinga shunt resistance across the relay circuit as shown in Fig. ,rz, They areuseful where a standard relay with a given setting i_s used. for arl thebusbar installations to achieve a given primary faurt setting throughout.

12.

1.5.5 Cheek Feature

A second line of defence is considered good practice in most schemes ofbusbar protection, not to give security against maroperation of theprirnarSr protection d'ue to inherent defects but to prevent incorrect trippingas a resurt of damage to wiring and equipment from extra'eous sourees.A check feature is provid.ed by duph_cation of the primary protection usinga second set of curzent transforners on all circuits other than bus sectionand coupler units. The check systern is ananged in a sinilar manner tothe prinary protection but fozns one zone only covering the whole of thebusbars and does not discriminate between faults in the various sectionsof the busbars.

under i'n-zone faurt eonditi.ons, the high impedance relay circuit constitutesaJl excessive burden to the eu:rent transformers, leading to the developnentof a high voltage the waveform of which will be highly distorted with a peakvalue many tines the nominal saturation voltage. AD approximate forrnulabased on experimental results conmonly used for checking the magnitude ofthe peal< voltages vp deveroped by a current transformer und.er internalfault eonditions is gi.ven by :

Yp = 2rmvFEf where v6 = eurrent transfor:mer

knee-point r.m.s.

voltage.

Vp = maximurn ?orDrso voltage

that would be produced

if the cunent transforser

did not saturate.

13.

This fornula does not hold for the open eircuit condition and is inaccuratefor very high burd'en resi'stance values that approximate to an open circuit,ft only appl_ies for values of V6 less than v.F. ft should therefore be used

2

as a guide to the possible peak voltage.

rf there are a number of current transformess in parallel the peak voltagevp as caleulated above for a single transforuer wi]] be reduced by the shuntconductance of the other current transformers.

The insulation of the cu:=ent transformel second.ary winding and reray willnot be abre to withstand the very high voltages that can be produced..where neeessary the voltage is rinited to less than ]kv peak by the use ofnon-linear resistors caLled metrosirs eonnected in paraller with the relaycircuit as shown in Fig. 1J.

The voltage/current characteristic of a netrosir is given by :v = crp where the voltage v and current r are

peak values.

I(rms) = o.52I

C = a constant depending on the

metrosi.l construction i.e. the

size and number of individ.ual

dises used in the metrosil and.

whether connected. in series orpara11e1.

F = a constant in the range O.2 toO.25.

14.

The values of c and F are chosen so that the voltage across the metrosil islinited to less than lkv peak at the maximr:n fault current. The value of c

nust al'so be sufficiently high to restriet the current tal<en by the netrosilat the relay setting voltage Vg so as not to ad.versely affect the primary

fault setti.ng. Acceptable metrosil cuments are approxinately JgmA for use

with 1A current transforrners and. 1OOrnA for use with 5A cu:rent transfomers.

An approximate value of r(rms) at relay setting voltage vg(rms) can be

caLculated from :

,fr vs

Suitable metrosils are chosen based. on :

(i) relay setting voltage.

(ii) rated. cunent transformer second.arxr current i.e. required prirnary-

cunent setting.

(:-ii) rnaxirm:m eurrent transfo:rner second.arxr current under fault eonditions.

J.6.7Open Circuited Current Transformers and Wiring

when a cur=ent transformet secondary wind.ing or corueections between currenttransfor:ners and the relay cireuit become open circuited, the resultaltout-of-balanee current will flow through the paralleI combination of relay,metrosilr fault setting resistor and cu:rent transfoxmer magnetising

inpedance. This nay cause the proteetion to operate for load. or through

fault conditions dependi.ng on the effective primary setting.The condition of an open circuit can be d.etected. by neasr.rring the voltage

across the relay circuit by a sensitive voltage-operated. relay as shown inFig. 14. This eelay is set to operate when the out-of-balance cument

equals about 1U/o of the least loaded feeder conneeted. to the busbars or2! amperes whichever is the greater.

= c (r(*");F( 0.52 )

'15.

rf accr:rate details of current transformer nagnetising characteristics areavailable, the required setting ean be calculated.. Checks should be doneon site to ensuee that the relay will not operate due to normar unbalancewith the system and protection healthy.

operation of the supervision relay is arranged to give an ala* that thebusbar protection is faulty and to short circuit the buswires if this isneeessarJr to prevent danage to the protective relay and stabirising resistors.when the busbar protection has a faurt setting below fulr road of theconnected feeders it is very 1ike1y to operate .ue to an open circuitcurrent transforrner' rn this case a check feature is required to preventtripping. At the sarae tine it is inportant that the buswires are shortcircuited via the supervisi-on relay to prevent therrnar damage to theprotecti've relay and' stabilising resistors which woul-d othe::wise renaincontinuously picked up und.er road conditions.The supervision relay must have a tine delay to prevent its operation dueto genui.ne busbar faults. A time delay of about J seeonds is used.

1.5.8

3.5.9.1

An inportant advantage of wing high impedance relay in a circuratingcurrent systen is the ability to predict the protective scheme performancein terus of prirnary fault setting and through fault stability by calculationwithout heary-cuEent eonjunctive tests. The valid.ity of the eal-culationis based on the assumpti-on that all the current transforuers €Lre of low-reactance type' A 1ow-reactance cunent transformer is d.efined as one ofwhich a lcror+Iedge of the seconda:qr exciting eu*ent, secondarxr windingresi'stance and turns ratio is sufficient for an assessnent of its performance.This covers current transformers with uniformly distributed windings orwhose core leakage flux is negligiblq.

16.

1.6.8.2

l{1th hi-gh inpedance circulating current schemes, it is of the utmost

importance that the l-ead. burdens between the various sets of cu*enttransformer be kept as low as possibre in order to obtain the requiredstability and sensi-ti-vity. rt is therefore ad.visable to run the buswiresin the forrn of a closed' ring between all the cireuit breaker control cabinets.This avoids the need for numerous radial loops between the current transfo'mersand the bus zone panel which would be required if the buswires were formed inthe bus zone panel.

A closed ring consisting of cores in multicore cables affords increasedsecurity against maloperation which may result from unbalancing of theprotection due to inadvertent d.isconnection of bus wires. rt also provid_es

easy extensicn of the protection when new circuits are to be connected intothe protection zone.

An example of rwrning a nulti-core cable ring in the case of a double busbararrangeinent is as fol1ows :

(i) current transforrners to marshalling kiosk.(ii) marshal-1ing kiosk to ar.rxiliary switches in the busbar serector

isolators.

(iii.) loop between marshalling kiosks.

The size of conductor no::ma11y used for the interconnecting pilots is 2.5rr2.Howevet, it is occasionally necessanxr to use para11e1 cores to reduce thebu.rd.en.

j.6.9.1

rn a lot of cases such as a d.ouble bus amangement where on-load transfer ofa circuit is posslble, current transfonner outputs are switched. to thecorrect buswires by means of auxili-ary switches on the selecting isolators.These auxiliary srvitches should close before the mai-n isolator closes and

should' open after the main isolator opens to ensure stability during sr+itchingoperati.on. This is shorvn in Fig. 15.

18.

The reserve busbar is then included within the feed.er protection. The

discrininating zone current transforrners on the bus coupler for the reserve

bars are no longer required and are used to replace the check zone current

transfor^mers on the line circuit breaker. Sometimes an additional cunent

transforner may be provided on the bus coupler specifically for this pgrpose,

in which case the reserve bar discrirninating eurrent transformers can be

short circuited. during bypass cond"itions.

1.5.8,) Current Transformer Locatj,on.

The three alternative arrangements as shown in Fig. 18 are :(i) curent transfo:mers for feeder and busbar protection overlapping

(u)( ii.i )

(i)

the circuit breaker.

all current transformers on rine side of circuit breaker.

alr cument transforiners on the busbar sid.e of circuit breaker.

rn this arrangerrent faults at F1 and F2 are cleared correctly by

the busbar and feeder protection respectively. Faults at F3

between the circuit breaker and feeder protection current trans-

forrners will be creared by the busbar protection and possibly also

by the remote end. of the feeder protection. No 'nneeessary

disruption to loads will result from this.

Faults at F4 will be seen by the feeder protection but also by the

busbar protection resurting in unnecessarJr tripping of the br:sbars

for what is essentially a feeder fault. This is the nain disad.vantage

of this anrangement.

This is the uost cortrnon amangement where all the current trassformers

are on the feeder side of the circuit breaker. However, there isa blind spot at point F, where faults are seen by busbar protection

but not seen by the feed.er proteetion. With this arrangernent it istherefore required to intertrip the renote circuit breaker when busbar

protection operates.

(ii)

19.

fntertripping can be achieved, by unstabilising the feeder protection

and ean be instantaneous or tirne delayed to a1low clearance of

faults on the busbar sid.e of the circuit breaker before intertripping.

(iii)

Alternatively an interlocked. overcurrent relay can be used. to intertrip

the renote circuit breaker. fhis relay in the form of a polyphase

induction disc is interlocked. wittr the br:sbar protection by means

of a shading wind.ing which is closed when the busbar protection

operates o

\,r,hen all the current transformexs are located on the busbar sid.e

of the circuit brealcer a fault al T1 betveen the current trarrsformers

and circuit breaker will continue to be fed. fron the busbars after

the circuit breaker has been tripped by the feed.er protection. 4n

interlocked overculrent relay whieh is interlocked with the feeder

protection is required to ensure that the busbars are only tripped

for this condition and not for faults on the feeder.

1.6.9 Typi.cal Susbar Protection Using HighI_grpedance Circulating Cr:gent Scheme

Figure 1! shows the current transfoxmer cireuits for a typical busbar

station layout comprising one bus section and two bus coupler cireuit

breakers. The busbar arrang€ment enables three zones of protection to

be obtained, and the cr:rrent transfoxme?s a"re connected to provide over-

lapping at the bus section and bus coupler circuit breakers.

Discrininating Featureo

Three cunent transformers axe fitted on all incoming and outgoing circuits

and also on both sides of the br:s section and bus coupler circuit breakers.

The star points of all the current transformers are connected. to a'buswire

which is earthed. via a removable 1ink. The other terminals of the current

transformers in the same zone are connected to three more buswires, all

current transfo::mers in the same phase bei.ng connected to the sarne buswires,

thus giving a set of three busvires per zone and a eommon neutral buswire

for all the three zoneso

20.

One triple pole re1ayl type CAGJ4 (device 8l) and stabilising resistor isconnected across the busr,rires to give phase and earth fault protection.

rf a fault occrus outsid'e the protected. zone, the currents entering an6

leaving the zone are equal and the current transfor:mers affected. willcirculate cu:=ent through the buswires. The schene is so designed. thatthe voltage necessary to operate the relay is greater than the voltageacross the buswires under maxj.mum through fault cond"itlons, so that therel-ay will not operate

'nder such circumsta'ces.

rf a fault oceurs insid"e the protected. zone, the balance of seeond.arxr cu:rentwill be disturbed and the relay will operate.

fiscrininative tripping is obtained by the introd.uction of isoLator auxiliaryswitches into the current transforrner circuits. The auxiliaqy switch contaetsare silver plated, with two switches in paraller per phase, in order tomj-n:inise the possibility of high arr:riIiary switeh contact resistance.

Check Feature.

This is simil-ar in operating principle to the d.iscrfuninating feature, butno current transfolsners are fitted. on the bus section and bus couplercircuits, and the cornplete busbar installation is considered as one overaLlzorl€r The crrFent transfonners are fitted on all i.nconing and outgoingcircuits and' again all the current transfo::ners in each phase are paralleledonto the buswires.

A triple pole relayr type cAci4 (device BJ) and stabilising resistor isconnected across the buswires.

Continuous Buswire Supervision.

To guard against the possibirity of faults in the cerentsecondar;r wiring and interconnecting pi.lots a static relay(device 95) is connected. across the busbar protection zone

transfo:rners

type VTX

busr.rires.

C1Lta

The relay has a voltage setting ad.justable between 2 and 1d vo1ts, and an

inherent time delay of J seconds. Operation of the llr'IX relay sound.s an

alarn and. takes the affected zone out of service by shorting the appropriate

buswires.

The following faults are covered :

* Open eircu-ited current transformers.

l+ Broken eurrent transfor.ner pilots.* Crossed current transfo:rner pilots.

Tripping Circuj-ts.

The tripping circuits are arranged. so that the accidental operation of any

one of the circulating current relays (d.evice 8l) does not cause inad.vertent

tripping of a group of cireuit breaters. Both the rDiscrirainatingr and

rCheckf high imped^ance voltage differential relays must be energised before

the respective nain tripping relays (deviee )6) arc energised. one ma-in

tripping relay is required for each feed.er circuit breakerr altd two such

relays are required. for each bus section and each bus coupler circuit breaker.

This method presents several advantages 3

* Spare contacts are always available on the arr:riliary tripping relays, and.

can be used when required for intertripping duties.

* The zone of protection rnay be easily extended to cover additional circuit

breakets.

* The front appearance of the busbar protection panel is not dependent on

the nwrber of circuit breakers in the zone.

Alarm Circuits.

Audible and visual alarrns are given under the following cond.itions :

* Busbar fauIt.* Busbar protecti.on faulf,y.

tt Battery voltage low.

22.

The r3usbar Proteetion Faultyr alarm is time

being given und.er busbar fault condltions.

Figure 20 shows a WpicaL arrangement of the

indicating circr:-its.

delayed to prevent such an alarrn

d.c. tri.ppingr alarrn and

1.6.10 Busbar protection Using Separate DifferentialRelavs &rg Each Circlrit Brea]<er.

J.6.10.1 laslg_Sgbgg.

Fig. 21 shows the basic arrangement. This scheme is particularly suitable

for use where individ.ual relay roons are used on the 400kV busbar install-

ation j.n U.K. Each room acconmodates both the feeder and busbar protection

on a per-circuit basis. Each circuit breaker has its own individual

discrirninating and check high impedance relays, coincident operati.on of

which trips the breaker. Each breaker also has its onn individ.ual trippihg

battery. 0n this basi.s buswire selection is limited to the a.c. circuits

with no isolator switches in the d.c. trip circuits. This will provide

greater secr:rity against mal-tripping and greater reliability of tripping.

Another axrangement shown in Fig. 22 uses two tripping routes for each

circuit breaker. Direct tripping from the high inpedance relays associated

with each circuit breaker ls via a discrete busbax protection trip relay.

The other route uses separate contacts on the individr:a1 circult breaker

check and dj.scrininating high-imped.ance relays to energise a second trip

relay associated with each circuit-breaker. fn this case the separate

contacts on all the high-impedance check relays ate connected in parallelt

the combination of which is in series with a single contact on the individual

high-impedance discriminating relay associated. with each circuit breaker.

a7-.)o

1.6.10.2 Basie Scheme with Ad.d.itional DiscriminatinE anda

Fig. 2) shows the anangement in a simplified forn. The additional relays

increase the tripping reliability" A sectionalised back tripping systern

with check and discrininating back tripping bus wj-res (each double pole

switched.) is incorporated which operates onto the ind.ivid.uaL busbar

protection trip relays. Operatj.on of the zone check and discrirninating

relays energise the back trip receive relay through the back trip buswires

and busbar selector isolator aiuiliary switches. TLre receive relays in

turn pick up the busbar protection trip relay to trip the breaker.

I'urther reliability of the tripping circuits is obtained by using separate

back trip receive trip relays as shor,m in Fig. 24o This provid.es virtual

dupli.cation of the fault detection and tripping functions apart fron curent

transformers and associated bus wiring"

1.5.10.1 Cheek Zone Sectionalisation

Provision of a single check zone covering all the br:sbars has the advantage

of relative simplicity and economy. At large busban stations having a

large mrmber of circuits it nay be necessary to depart from this approach

due to the difficulty i.n obtaining a suitable primary fault setting. fn

these cases sectionalised check zones are used. This provision leads to

greater security and flexibility during construction, naJ.ntenance and

cornnissionlng. It requi.res additional cunent transforrners at the sectioning

points and separate high impedance relays per check zon€r

I+

bCTERNAL trAU UT

rNTER-NAL EAUr-l

FIC, 1 BASTC CIRCULATING CURR.ENT STHLME

L5

OPERATING COIL

BIAS COILS

INTERNAL FAULT

+EXTERNAL FAULT

BIAS CURRENT CO|L 'o'

OPERATING CURRENT

BIAS CURRENT COIL.b,

FIGURE 7LOW IMPEDANCE BIASED DIFFERENTIAL SCHEME

POSITIVE HETT CYCLE

+

+

LXTLRNAL trhULTNEGATIVE HALF CYCLE

CTJRRENT TRANSEEONDAR\ C\'RR.ENT

KELNY OPEBATI3I.I

INTE.RNAL FA.ULT

cuRRetr TRANSFOF.t,tERSEcON!ARy CTTR.F.ENT

A R.ELAY oPERATI0N

t-

FE-E.DE.R X FTeDER -(

POqITIVE HALF CYCLE

l <--| <--

A A

FlG.4 PHA:E coMPARi tON SCHEMEtrEEXER Y

f-9

1r =I,+12

r F('nmfI

I

I

I

I

L

I

I

_l-

I

I

I

swffc.HGEAg

---TI

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ar+Iz

-tI

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-JBoIJDTNG 8AR.

REr-AY

GErJ6(A'ro(

gY9{6r,1 €AGrH|rJGRgsrsrAxcE

I,+12EACTH 8AR

A

EAP:nrrrJG tttict|aD€(€sr3rAPeE

FrG.5. OPERATTNG PRINCIPLE OF FRAME LEAKAGE PROTTCIIQN

?*:t _ Z.ONE Z

lr.lsutAfioN BePP€R .

Zot'tE 3-t

t.

l'i----r-l iTRIP h'

t?rp'B'

FIG.6 FRAME LEAKAGE 5CHEMI \\IITH DOUBLE INSULATION BARRIER

rI

I

I

I

t_

7jl-uL/1

-s/t-

-lI

I

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2ct

ZONE 1

r--- - -1

t,-rTRtP C

FIGURE 7FRAME - LEAKAGE SCHEME WITH SINGLE

INSULATION BARR IER

INSULATION BARRIER

0.2-1SEC.

fo

/,vSazaVaat B.sfPeR

Z.va JZa*s./ ZavaZIt

Zz-d

)/L+(Lc(L\l_:

rI

t'I

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--- ./'T . Y'xF

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,11

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3t

INouT

lrEtodI I css'zt

r* olrrrl 6+22-l

E:4t t css-zz

TPtp SUPPLY

SuP€lvtSro^/

AIR€n64Cl{-Z 64zl_2 ?alv

61a2-Z

74- |LAHP

tdOIJT tu"z€l

Arierr suPft.Y

Sv?6 tvl 6 rotJ

FlG.g. TYPICAL TRIP AN ALARM CIRCUI@WITH DOUBLE INSULATION BARRIER ANb CI{ECK FEATURE

LTAl(AG

)4.

Q.et Rr-v

i

Rsr

)*.

tI

leI

i

VR = rv (R.r*Prr)

Vs>Ve

Ia R > .Ty (€.x* R"r)

R > rv (Rr.x+(cr)Ig

Psr = R- Re

F'IG TO PRINCIPLE OF HIGH IMPEDANCE PROTECTION

3f

FIG.1I INTE-RI{AL FAULT Ot.\ !\IGH MCI\EME

Vr=It'R

= r, - !L (R.. * R'*)' Is

E{(e.ct.\v<. Setti^qIs=I(r2lm

-- Ietn I^?r\'^o*., eftecL\,.re sett\^1

ip : T'(r*+nI*)

tJ'rL\

'^rit\

Cr r c,.l'i tS

c-irtr"\tt

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+ i.* + lu)

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1

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or tp = T.(I**oI^+Im

3 c(-

lop = r** lae

FTG-12 USE Dtr FAULT SETTTNG RESISToR

35

tI

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iai

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Ftc -13

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Met"osiI c\'ovoctertstic !: C l'

V= fiV*=

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^FVs = C [s]F

16

ri)l r)llJr-cTr

7*,

- -T r-_rl

-j t-6Ta-

1r

I

{ ,Crtr4t'

I

_l

I,:L.+1,+I.I.IEALTHY CD}.T}\T TA}J

tvl

i

CTr OPEN CIRCU lT , Lr FLo$rs THRDUGH MAGNET|STUG

IuFgUnUCE AN} g"EL$Y CIRIUIT IN PARALLLL

trIC.I4 SUFER.VISION AGAINST OPEN C\RCUITE} C.T..ga --

Voltac,€. m"o's.rrel by

y : l, (R

lC supervision ra\a.1

supe.rvisicn te,la1

ll zma ll zH3 il4.,o)

seLl',*.. - V.o

Crt-o{-bo.lo.,.ce. cr,rrrc-nt- to opc-vate t\'L: )4'" l:s * 9 vsP

{ - 4, ?-r'q7

I

supetvls't6n Ta\ai

97

NORMAL CIPER ATIhIG .(-oNAr-r,r qN

i\.-- + \ f4ain s< reserve zon€-gultinate/y pr"lleled byf€sefvg b.rsbar se.\ectota*rihory switches

ON-LoN} TRANSFER

FIC.15 BUSBAR SELECTOR A\-n.(ILIARY SWITCHES RECUIREMETIT

.<4

38

FIG.15. CIRCUIT BREAKTR BYPA5S.

{J* f

2

C/t/

A.c -

6ursx-=€S

/7

A.C.EttswrR€S

2tl/<//4

FIG.IT.. CIRCUIT BREAI€R, AND C.T. BYPASS.

CreorrT??o-t€giost

&lsgA€Pf.reer,o^t.

Cr€errrfe?org"",ot

Sr.rs8t{?{Xor6ctrroU

rrvr€nLoc,1gFb

ov€e!uCl€N?Qgtlv.

$W€Eir.€Dorrtler€Arrres.A\,

ClectrtTEo'recrro{

gurBAe

P€"lSeTorr,

Co) oV€&APPTNG c.-rs.

(b) ArL c.ts ov t-r^/€ sr0e

oF c'Perrff 6CsAEEl? .

(C). Al,t c.Tj oC G,lrsgAe srbE

oF crlevrT &f;eAglQ,

FIG.IB IFFECT OF C.T. LOCATION ON BUSBAR PRO]ECTION PIRFORI4ANCE

+o

t1

IEu36qN;

aarJasr.r=<a(,e:Y1gr-a<(Jff (r!:<sgzzo<NVt

2U.E

uE=ctiol3=Ua.E<>5(J;3ioEdt(JE

o!o

aaC

coc

a

L

(

l

I

3s6Et>ar>TJfsvrc

!- :<d(JJpgAra fJ

5?sUz2o<N('

-lu-lE-le-l -Jz

oN

v,(,g

I(,u,

U

=o

-!

a=€! i?5:-atcEa?-oEEA-o3i;3EIe

th

:Ez=3g

o

U2tnRExB(,sV,-)C)o

< ar.,an

-GC-3tgN;

Qr

95R.X-1

95CnX-1

Zonc indrcating rcl.y typ€ VAA l3Alarm cancrllarion rclay rypc VAKD.c. vohs rupcrvision rclay rypc VAX l2High impcdencc circularing currcnt relay rype CAG 348ur wircl supcrvision rclay rypc VTX 3l

87CH-1

74-l

EUZZER

95X Zooc bus wirec ahorting rctay rypc VAJHI3CSS Conrrol schcror :wirchtl lndicering lemp protcction in scrvic:L2 lndicating larnp protecrion out ol servicc

93M2-1

95CH-l

30[v''|-1

95Mlx-193M2X-t

css-r'f2llll a7e-1

Ial

css-R

87CH-2

ssMf -t

95R- 1

30Mr -'r

30R-l

MlM2N

o.c. BuswtREs

87M1 -2

3074808795

FlG.20. DOUBLE BU5BAR scnEne.orve RerRv pEe.zorue . o.c.crrcurrs.

+1_

h_-.e*js-r1iEilTtl @;r @6,

2z_--a o-G)Ts---olo-{F)

IYlrn I?

@*r

@;rI2o

zN.lgr@,1

trt

.J,

FIG.ZI BUSBAR. PROTECTION WITH SEPARATE RELAYS PEF.

CIRCUIT BITEAKtR -:.\::i :IHE!I3

zz--rre\---o io-rO..F I

I

Iry-ro-*is .?a?

f+-f?I

5s__ _ _ol

Flc.2?

TRIPPI.NG ROUTE

v3

@OIbl

,J

FlG . 23

crRcU_!_a_F

O--€ r-:r-l:

@-*.].H2t

ryi'o

CIRCUIT BREAKER .BAsIc sc}IEME WIT.H ADBITID\IALREunvs pgn znxE ANn.BhqK TRtpplNG rActL!.TlELJdtrHSEPARATE TRIP R.ELAYS

fto- .1-v-olo-

_,I

FlrA,

FtG .24