Measurenents

DIRECTIONAI, OVERCURRENT

AND EARTH FAULT RELAYS

BY

G. A. HADLEY

Dra: .trl

1.0

1.1

1.2

2.0

DI.?JCIIONAI C'/":R.CUPP.!I.N R.ELAY S

If fauLt cur:ent can flov in both direction through the relay

location it is necessa.ry to add directional properties to the

overcurrent relays j.n order to obtain correct (isslirnilatioa.

tro t erros which are always used in the application of

directional relays are :-

REI,AY COI'N'IECTIONS

lhis is the angle by vhich the cr.r:r:eent appli.ed to the relay is

displaced. fron the voltage applied to the relay at unitypover

factor.

REI,AY I,UXffUM TORQUS ANGIfi

This is defined. as the angle by which the cureat applled. to

the relay mrst be displaced fron the voltage applted to the

relay to produce merirmrm torque.

DIR,TTCTIONAL RILAY OPIBATING CH.{.RACTB.ISIC

In arry directional relayr ttre qrr^antity that produces one of

the fh:xes is teimed the polarising quantity. fhe polarising

qrrantity nay be voltage or cllnrent but in either case it nust be

a particular quantity that is suitable for the prrpose. If re

consider a voltage polarised rela1r as this is the most cormon

case in protective relay applications, the vector relationship

for oaximum torque would be as shorrn in figure 1.

The voltage coil fh.a i.s substantially in phase with the volta€e

coil cr:rrent vhi.ch lags the voltage applied to the relay ty an

angle o( while the cr:rrent applied. to the reLay a.nd associated

flrrx 1ead. the voltage tf an angle S. For ttre na:isr:n torque fi

i"crLd f,v nugt be 9Oo apart ald hence the cr:rent strpplied to the

relay nust be at the position shovn by the dotted'line. The

eqr:ation for the electrical torque vill be 3-

r = Kvr SrrJ (fi +{)But this is more colnnon\r expressed as 3

r = K\n Cos(e -A)

).0

).1

1.2

?AGE 2.

\,rhere I is calIed the relay oa;risum torque angle, vhich is defined

as the a:rgIe by vhich the cr.:nent strpprS.ed to the relay urus., lead.

the applied voliage in cldet to obtaill 6:visplq torque.

DIRECTICNAI, RELAY COII}IECT;C$S

If thlee phase short circuits 'rere ihe only ilpe of fault to be

considered any of the possible connections available to produce

naxi.nun torque at a given angle vould be acceptable. Eowever

to enstue co:=ect di=ectional discrinination r:nder all fault

conditions it is necessalrJr to gi.ve thought to tlre co:=ect

conneclions for any g'iven applicatioa. fhere are three standard

connections vhich i.n general ui.11 cover all applications. ?hese

a.re the J0o, the 600 and the loo connection. the names of these

relay connections give the relay relationship betveen the crrrreat

and polarisir:g voltage r:nder three phase unity pover factor

cond.itions. Of a1l these connections the nost popular is the 90o

connection, vhlch is the one nanufactrrred by GEC Meagurements.

9oo nsr.lr coNuEcrrox

This connection is applied only vhere the fault crrr=eat lags by

a ver:f 1a.rge angIe. the voltage coil is given a voltage displaced

fron the relay cr::=ent by 9Oo at urrity por.et factor. Ulth the 9Oo

connection the directiona.l eleoent vould. have no torque at unity

power factor since the relay cr:sent and. voltage are then 9Oo out

of phase as ghown in figure 2.

9oo - 45o nEI^aY coNNEcrIoN

ltre tat phase relay is srrp'plied rrittr I" cr.rrent aad, V6q volts

displaced. 45o in an antj. cloclcrise direction. fn this case, tbe

f1r:x produced by the voltage coil lags the applled voltage Yt" by

45o ana the relay uaxi.orn torque is produced vhen tbe cnlrent lagr

the qlstem phase to neutral voltage by 45o. TbiE connectioa gives

a conect d.irectional tripping zone ovet the range of cr::rrent 45o

lead,ing to 1550 lagging. The relay torque at urrlty pover factor

1.j

,.4

a \,^? Z-dsr )a

is 0.707 of the nax:.num torque and the sa.tre at zero pover factor

(9oo rae). See figure J.

9oo - Joo a:ux col{irEcrroN

The tar phase relay is supplied. r,rith f" cu:=ent and Y6. volts

displaced. bV JOo in an anti clocknise direction. In this case

the flux produced by the voltage coil lags the applled voltage- ('\

VU" by 60- arr<i the relay naxinr:m torque is produced nhen the

cr:nent lag:s ihe q;sten phase to neutral voltage ty 5Oo. thie

con:ection g'ives a corect directional trippir€ zone over the

range of cunent JOo lead.ir:g to 1!0o lagg'ing. The relay torque

at wrlty pover factor is 0.5 of the relay 6avinnm torque and at

zero polrer factor (9Oo r"g) is 0.865 for marioru torque. See

figltre 4.

DIRECTTONAI, OVSRCURRE{T RE,AY PERrcR}TANCE

A relay designed for quadrature connectj-on and having a ndrimln

torque angle of JOo is recomended. when the relay is beJng used

for the protectioa of plain feed.ers with zero sequence soulcc

behind. the relaying point.

In the case of tralsfo::ner feeders or feeders whlch have a zero

sequerpe source in front of the reIay1 a qr.Iadrature connected'

relay is ls6errnend.ed but it is preferable vhen protecting tltis

type of feeder that the directional relay is designed to have a

naxi.nr:o torque angle of 45o.

ltre 9Oo - 45o connection i s necessa.LT i.n transfo:mers and

transformer feeders, to en$re co:rect reLay operatlon for faults

beyond. the star/d.elta transfo:tet. Ihis connection should. be used

nherever single phase directional relays are applled to a ci:scult

in which the cr:nent d.istribution is 2:1:1.

4.0

P:GE 4.

'The vector diagra:as shown on figules 5ar 5b and5c indicate the

onerating and non-:perating regions of a qr:ad,ratr:re connected.

reray a-nd ihe aa.:ci.arrm ar:gular d.isplacenent of the cr:r=ent appliedto the 1e141 froro the appried voltagel for arr types of faultconditions. ?hey also shor* the ad,valtaEn of using a relay haring

a roo lflA for the proteetion of pla-in f eed.ers and a relay havfu:g

a 45o l{fA for the -orotection of transfortrer feed,ers.

ftrr=e fauLt conditions roay theoretically cause oal-operation ofthe directional:eray. They are phase to phase to gror:nd. on a

plain feeder i phase io gror:nd fault on a tralesfomer feed,er rrith

the zelo seguence source in front of the reray, and, phase to

phase fauli on a transforuer with the relay looking into tbe deltewinding of ihe :ran s-t'o::!er.

rt strould, be reoerobered., however, ihat these d.iagrams are dranre

to illustrate theoretical, naxj.mro positive and, negative a.r:gular

displacenent of the applied cunent froo the appli.ed. voltage aad.

that in actual fact it can be shorrn that the posaibiuty of oql-operation in practice is very reoote.

DIRECTIONAI COMTROL

With inverse ti-oe relays it is essential that the d.lrectional r:altd.oes not a11ov the overcrrrrent r:rrit to start rntil the farrltcurrent florrs in tbe operation direction. Connecting the onercumeat

and directional relay contacts in serj.es Ls not satisf,actory as,

although this vould prevent tripping, the qvercrrnent relay could,

start to operate before the cr:rrent flor was in the correct d,irection

resulting in possible contact race and loEs of d.iscrinination. Tbe

copper shading ring used on the no:maI inductioa disc overcurrent

relay is replaced try a vor:nd shading coi1, the circrrit of which isconpleted ty the closing of the directional relay contacts.

5.0

6.0

D.n:f :a---VP Ja

PARAIIEL TEEDMS

rf non-directional overcllcrent relays are appri.ed to pararler

fee<iels any faults occr:rring on ary one line will inevitabLy,

ir:espective of the relay setting chosen, isolate both lines

anci conpletely disrlpt the supply. To en$ue discriminative

operation of the relays dr:ring line faults, it is usual uiththis type of qystero io d.esign and connect relays n{ anO nlsuch ihat they will only operate faults occulring: on the

protected line in the direction indicated. try the arroys. see

figr.rre 6. With parallel feeders to ensure correct di.scri-ulnatlon

during line faults, it is important that the co:rect directiona1tl

relay R1 or R2 operates before ttre nondj.rectional relays R1 and

R2.. For ttris reason relays R'r ana ni "r" given lor*er tirne

settings than relays R1 and R2 and also lorer cuneat settlngs. -The usual practice is to set relays n.l ana n[ to 50?6 of the

nonoal fu1l load of the circuit tnrt care sust be talen to en$rre

that the relays are capable of carrying without d.amage, twice

their setting cu:=ent continuously.

RING MATNS

fhe more usua.l application of directional relays is to ring nains.

In the case of a ring systero, fed at one point only the relays at

the generation end and at the nid-point substation, where the

setting of both overetrrent relays are identi.cal, the relaSrs ca:r

be nade nondirectional, prorrided. that in the latter case the relays

are located on the sa.rne feeder, one at each nrbstation.

In this respect it is interesting to note that whea the nrmbers

of feeders in the ring is an even number, the tso relays with the

salre operating tine are at the sane substation and nill have to be

directional vhereas when the nurober of feeders is odd., the tro

relalrs with the sane operating time are at d.ifferent strbstations and

therefore, do not need to be directional. Also at inte:ned.late

subsrarions it will be noted that whenever the times of the

nro relays at a zubstaiion a:e d.ifferent, rhe d,ifference iaoperating iime is never less tha.n the gradir€l inte:rra1 of 0.4

seconcis and. ccnsequently it is pe:misslble for -uire relay ,rith

the larger operating ii.ne to be non-iirectional.fhe us:a1 practice for grading relays in a' iutercornected.

rysteo is to open the ring at the supply poiat a.nd. to grad,e

the relays firsi cloclvise and then alti. clochrise. Thusl the

relays looking i.n a croclcwise direct:.on aror:ad, the rir:g are

arranged to irip in the sequence 1 - Z - t - 4 -, _ 5 and the

rerays looldng i.n the anti croclarise d.i.rectioa are atrRanged, to

trip in the sequence 1'-2t-t - i- >'-eJ rhe a.-=oya iadlcate

the directron i.n vhich the power oust flon in ord.er that the

d.irectional r:ni.tE rrill close thelr contacts and, prepare ttre

overcucrent elements for operation. Ttre double headed, atrro\rs on

each of the tvo feeders at the gene:ating statioa ind,j.cate non-

directional relays, directional features beiug r:naecessar5r at

these points, because power can flov in one d.irectioa only, that

is out of the genelating station. At all other poiats single head.ed,

arlors are shorn. These indicate directional relays connected so as

to operate with porrer flor in the direction of tbe a.rrow rhich is iaeverT case flom the strbstation bus bars and into the protected, line.See fig:rrre 7.

lhis rule iE invariable and applies to alr forms of d,irectional

relays. selection of the faulty section is W tfuqe a:ld, fault pover

direction. Fault pouer has tuo paths I and y. rt dlvid.es betreea

the tvo paths in t}te inverse ratio of their inped.a.nces alrd. passes

through all the substations ln the ri.ng. fhusl at everT suistatioa

one set of relays will be inoperative because the power flor isa€aiust the amov ond. the other set operative because ilre frov Ls

rith the Eutrrolr. In every case it riII be for:nd, tlrat tlre tlne

Drr-? (a Ag! JO

D^at ?r.1v! I a

settints of the :e1ays :hai are inoperative are shorter thaa those

of the operative relays, except in the case of strbstation C vhere

the settings happen to coincide. In this way all relays wlth

short tine on sections betveen the faulty one and the generating

station ale prevented from operation. The others, vhich a.re

operative a:e graded dovnward.s tovard.s the fault and the last to

be trave:esed. oy the fault current, namely that on the faulty

feeder section, has the strortest tioe and operates first., lhis

applies to both paths to the fau1t. Consequently tJee faulty

section is the only one to be isolated and srpply is naintained.

to all zubstations.

l,/hen grading ri,ng qystens vith nore than one infeed. (say tvo

sou.rces of srpply) tne best nethod of approech is to either z

(i) open ttre ring at one of the strpply pointa by neans of a -suitable high set instantarreous overcurent relay and then

p=oceed. to grade the ring as in the case of a sing3.e infeed.

(ii) treat the i5rter-connector betveen the tvo sources of supply

as a continuous bus, separate fron the rin6: and protect it

by means of a urrit systen of protectioa such as pi.Iot vire

relays. Then proceed to grade the ring as in the case of a

single infeed.

DIRECTTONAL EARTH FAULT RELAYS

Ttrese relays are sinilar in construction to the overcrrnent relays

but are polariseil by residual voltage or current. The polarising

voltage is obtained fron the seconda4r of a three ptraee voltage

transformer connected in broken delta. It is essential to ensure

that the co:rect voltage ls fed. to the relay that the volta€p

transforser prinarSr neutral is earthed and that it be a three

phase, five linb type or consist of three single lhase unitg.

Cunent polarisation is no:mally obtained by connecting a cu:rent

transfo:nnar ln a locaI transfo::uer neutlal.. If voltage lnlarlsatioa

7.o

P.A.GE 8.

7.1

7.2

is used. a 45o lfIA is nor:lalry used for so]-idly earthed systeos

and oo or 14o for lesi.stance earthed systeas. rf the relay iscurrent polaised, the polarising er:reat is pbase si:.ifted before

being fed into the relay as oilviousry in this case the operating

and polarising c'Jrients being fed into the lelay are in lhase.

YOTTAGE POI.ASISED EARTE IAUT,T RET,AYS.

soue care is necessa-ry when using voltage polarised. relaye on

solidly earthed systems, as ihe residual volta6e r:oader single

phase to earth fault conditions vil1 be eqr:a1 to ttre phase to

neutral voltag'e at the fault location for a solld, eartb faulton1y. Any line inpedance betveen the farrrt point and the reray,

or resistance in the fault itself wilr tend to reduce the valr:c

of thg voltage and it can be ve{ srna}l if the line i.opedaace

betveen ihe fault poi.nt and the relaying point is large conpared -with ihe source impedance behind, the reray. wittr noderrr

directional re3a;rsrhouever, vhich viI1 operate d,orn to 1!6 of

no:mal voltage no trouble ihould be experienced. see figure g.

CIJRRSIT POLARTSD EARTII FAUT,T RgtAYS

As already nentioned, cr::rzent polarised relays nay be polaried

by a cu:=ent transforuer connected. in the powet transforaer

neutral. 0nly certain iypes of pouer transfomers, hoveve!, are

suitable as sources of polarisir:S crrment, as in soEe the

d.irection of the culrent in the neutral can reverse d.epending

upon the fault position a.nd the ratio of qysteu zeto sequeBee

impedances.

tl:e convention usr:ally adopted iE based on the assrmption that the

direct:onaI relay rrill operate and close its contacts vhen both

the cu:rent in the operating coil and the polarislng coil fLov inthe same direction. ff the cr.urent in either coil reverses theu

the relay nil1 restrain.

PAGE 9.

A srar/star poret transforner is not sLitable for polarislng

relays even if both star points are earthed.. A cr::=ent ira"nsr'o:uer

ln one neutral worrld not be suitable as the ctrrrent vould reverge

ilepending upon uhich side of the transfor:ner the fault is on.

Paralleling tvo cunent transfo::roersrone in each neutral

connectionrvill not be satisfactozTr as the resultant cr:rrent vould

ZQTOC

Three winding or two winding power transforrers nith one rrindiDg

delta connected are stri'uable for relay polarisatioa. Provid.ed

the star point is earthed, then a cr:rreat tcansfomer in this

neutral can be used. to supply the relay. fn the case of three

vinding transforners, if tvo star connected vindlngs have the

star point earthed, then cunent transfo::mers in each neutaal

connected in para1le1 roust be used having ratios iuversely

proportional to the pover transforoers voltage ratio. An alteraative

to this is to use one current transforuer rdthin t}re delta vind.ing

provided rhat no load. is taken from the delta. ff loed is takeu

frorn ihe delta vinding it is necessarJr to use a cureat transfo:voer

in each leg of the delta to prevent unbalanced. load. or fault ctment

pro<iueing inco:rect polari sing curlent.

DUAI, POLARISED EARTII FAiJLT REI'ATS

As the polarising current for current polarised, eartb fault relaye

is taken fron a cr:rrent transfo::rner in a local povel transfol:ner

neulurall this nay be lost if ttre particular transfo:oer is sritched

out of serrrj.ce a.nd for this reason voltage polarisation is ln

general nore reU.ab1e. However, as pointed out, in soliiily earthed

q;steos where the zero sequence source iopedasce is saIL the value

of the residual volta€p can be very lor and iirral po)arised, relaya,

with both current and voltage a.re used. It should be notedr holevert

that nith uodern relays the possibiltty of voltage polarised' relays

failing'to operate is very remote ald that for aIL practical

7.5

o^

8.1

PAGE 10.

concj.iions riais possibiliiy can in 6eneral be lgnorea.

I:IgtIigE! A.ND PET:3.SI CCIL f;.R,T]IE! STST ,TS

The ope:aiion of earth fauli inciica'tion relays on sysieos ea.rthed.

rhrough a Petersen Coil or totaLly insrrlated systero is dependent

on ihe capaciilve curent floving in the healthy feederg and, vhen

a Petersen Coil is used on the culrent drre to the strppressj-on coil

fLoving in the faulty phase.

In ttre case of overhead lines the najorif of earth f aults are of

a transient nature a:ld it is prefened, that these farrlts shal1 not

1ead. to autooatj.c isolation of ttre faulty liae. It is d.esirablel

however, that an indication should be given of suetained systeo

faulis in order that the systerD nay be supe:rised. continuously aad

so that the farrlty section of the netvork is indlcated.

For {etection of a qystem earth fault, a supersensitlve dlrectior:a"I

relay t1rp€ NSS is used vith one coil polarised froa either a voltage

transfo:uer open de1ta, or an ar:xiliary voltage vinding oa the

Petersea Coil, and a second. coil, the operating coi1, energ:ised from

resid.r:aIly connected. cr-Irrent t:ansforners ln the line oae crr:=ent

tra.nsfo:aer in each phase.

IEERSAN COIT EARIED STSTN'T

lhe d.iagram in figure 9 strows a system of radial feedersr vith a

phase to ground, fault on the tCt phase of one of the feeders. No

cr:rrrent vilI flov in the tCr phase of the healthy feeders as th€rJr

will be at earth potentia,l. Capacitlve crrrzent vill flov ia the

healthy phases of all feeders to earth and back to the soulce via

the fault. The vector $.rn of the cr:gents ln the crrr=ent coil of

the relay on the faulty feeder fs is proportional to :

i"" + i"r 1i" + iL --2i. + it*h"re i"" + Icb E i"

9.2

PAG3 11.

The vector diagra.rn of the cur:ents in the sound phases shone that

the total wartage conponent of the cugents is in the restrainir:g

quadralt, hence the relays on the healthy feeders will not operate.

Hovever, the current in the faulty feeder, shon that the wattage

conponent of the cur:ents is in the operating quadrant, and hence

the relay in the faulty feeder vill operate.

The crrnent tra-nsfomers are of a special design, class 0.2, having

an erceptionally lor phase angle error and because of this'canrot

be balalced. accurately for currents greatly in excess of rated.

current. To overcone the slight phase angle inaccuracies of the cr:rrent

transforter a cotrpensating resistance is fitted in series rith the reI.-

voltage coile which has the effect of effectively noving the zone

of operation of the relay, so that definite operation of the relay

i.s assured on the faulty feed.er, and that the relays connected.*to the

healthy feeders will be restrained.. The relay ls provid.ed. wittr

oor,rlA.

INS{;'LAtED SfSEnt

fhe dia6ran in figrre 10 shows a systen of radial feed.ers, wit]r a

phase to gror:nd. fault on the tCt phase of one of tlre feed.ers. llhe

residual cu-r=ent flowing in the cu:rent coil of the relay on the

faulty f eeder, neglecting the effect of r"agnetlsing cr:rrent, is

proportional to the 2 I" where fC is the. vector $!! of the cr:nents Ln tht

healthy phases Is. arld fg6. Since the systen is an insulated. onel

the fault has ttre effect of raislng the neutral point of the systeu

by a voltage equivalent to the phase voltage and the voltages of the

healthy feeders W8.As in the case of the Petersen Coil earthed systen, the effect of

the oa4netising currentg ruust be counteracted. by nea.ns of a

cornpensating resistance connected. in series trlth the voltagp coil

of the relay. It is not necessarlr, holever, to take such precautiona

?AGE 1 2

a8 ln the case of the Petersen coil earthed rystem, in estirnating

the value of resisrar:,ce necessafir to shift the operatir:g qr.adrant

of the relayrin order rhat the rerays on the hearthy feed.ers

restrain. The relay is provided. wiih a 9oo reading l{rA.

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