ground differential protection

7/29/2019 Ground Differential Protection

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roun~l-diffcrcnti;iI cotcction lias been

iised fb r m m y years for groun d faults

with in wye-ciinnected gcner;itors an d

transformcts, which arc e i ther sd id ly

or low-impudance groui~ded. s a result of thc in -

hcrrnr design of the diffcrentid scheme, this type

of protection does not rrqu ire timu-deliiycd oprla-

rion to crxmlinatc wit h otlicr relays. Therefore, this

m e t h o d p c n vi d es s e n s i t i v e , s e l e c t iv e , and

high-speed clcaring ofgtound faults . The merits of

8'"u"d-dilf~l-enriiil protection havu been th e r q i i c

ofsevcral p p e r s ovcr the ycars 131,141.A variety of pmrrcrive rclays have bucii used fix

gt(i t ini1-i l i ffcrential schen i c s . These i n c l i i d c

r i m e - o v e r c u r r e n t r e l ays , t o r q u e - c o n t r o l l e d

time-ovcrcurrent relays, percentage diiicrcntial, and

prnducr-typc I I V C ~ C I I I ' ~ ~ ~ ~clays. Sonic of rliese rclay

rypes have c er ta in h i t a r i o n s an d requ ire special at-

tention he givcn to their appliccirion til ensure dc-

sired op eration.

Fig. I indiciltcs rlie thcory of opcratiuti o l the

gn,ond-dilfcrrnt i ; i l schcmr for IL Inw-impcdiuice

gronndcd power transformer or r o ta ti n g m d i i n c .

In this cipplication, thc relay is connected in ti ryp i -

ca l di f fe rcn t id meth id wi th a cimeiir t ransformer

(Cl)n th e grounded nc u t rd , p rov id ing onc inpu t

to the relay, ;ind rh c residual connectinn of tlic

phase CTs providing ii second inpu t to the relay.

T h e polariry of the CT s niiist be LISsliowii to ensure

scciite a n d rcliable opcrntion. Fo r application on

power rrmsformers, a n auxiliary CT is required to

m at ch rhe CT s e c r i n d qcurrents. As shiiwn i n Fig.

1, for aground 6ult within th e gn,und-dilfcrrnt ial

zone, t h e c i r r c n t s i n th e ( T s econdary comb inc

i n the pamllrl conncctioii of th c ground-differen-

tial relay (dcvice 87N) to ciiiise o p c m i n n . Fu r

ground fa& outsidr th e differcnrial zonc, rhe sec-

ondary cur ren t simply circulates w ithin th e Cl"s

secondary circuit with 110 operation. A dctailril ex -

planarian of its operation hit rhe v a r i o u s q >p l i m

tioiis litis been addressed sufficienrly i n [11-[4].

The produc t - type rclay lias berri tlic frirrmost

relay <ifchoice for gro un d-d iff~ rm rial pplicat ions

I077.~61xiiioi~11.00020110 in i i c

in rhr IXW. l'hesr relays arc uf electromrchaniciil

induction disk design with a n upper and Iriwtr

coil, which BC C p~ilarity-sensitive.Due to th e de-

.sign and iiattire of producr-type relays, there ar e

critical areas of c onc c tn wirh rheir applicntion.

Directionnl ground -owm u-rent relays may dno

bu applied in a gn , i lnd-d i r f~r ro t ia l c h u m r . Thcsc

rrlays provide similar benefits of security and

higli-speed oper;ition inhctrnt in ground-differen-

r i d protection. Howcvcr, newer staric ;iimloji a nd

digita l relays offer addi tiona l benefits ovet th e clcc-

tmmechi in icd produc t - type relay.

Appl i to t ion o f Product-Type RelaysProduct-rype relays, cmnecred as shown in Fig. 2 ,

ar e constructed with iiii Lipper and lower coil on the

same slmft. l h c s r coils work in co njunction to pro-

vide an q x t a t i n g totqoe when rhc c ur re nt s cnrer

t h e p ~ 1 ; ~ t i t yc r m i n d s (mtirkcil "+") nf both coils

simulmneously [SI.

l ' h u inagnitudc of the opuraring torque is a

funorion of thc cur ren t in th c uppe r and lower coils

an d thc phase angle between thcsr c u r r e n t s , a s in -

dicated i n ( I )

T=I , t X I , , cos0 (1)

where

, ,1 =Oper ating tr irque,

I,< = Secondary rtlsiduil cutrciit, and

11 = Secondary polarizing m i r e n t .

Maximum opcrnting torque o c c ~ ~ r sh rn rhe cur-

r e n t s arc in phasr, cos 0" = l

IFff ndu i ty lppl i io l i s Mogozinne n Mnrth/April2000



Pix, I , Ap / d i u t i o , z of g r~ rmr~ -~ - rC i J / e~~en t ~ ' ~ lwo t e c t im z

"11 ' I /,"ule' tm*?lfi)rmel.

Th e operat ing c1i;mcteristic of the prnd uct re-

lay with maximum torque q>pliccl is givcii by tin

invcrsr r ime-product curve, as shown in Pig. 3.

Thcsc curves are a fLinction of th c opcra t ing t ime

and m ult iples of tap prriduct cur ren t a t maximum

torque (cu r ren ts are inphase). Howcvcr, if th e cur-

rents arc out of phase, die operating time will be

longer atid c an bc determined from the curvcs by

using ( I ) to f ind th e product currcnt .

FOIgrwnil faults outside tlie differential z onc ,

sccondary current (Illrs)will h w nto the non-po-

larity tcrmiiial of the lower coil , producing a ncga-

t ivc torque to force thc t r ipping contacts in t h r

r ipcndirect ion. Ground fimlts within the differen-

tial zone resu l t in secondary current tram th e iic~i-

trnl CT (IN,;) tllat flows thniugh th c uppe r an d

lower coils. Secondary current from rhc residual

phase CT connection (11lr;s) combines to flow

thrriugh the lower coil, as wrl l . Cor rcn t from botli

secondary circuits flows into tlie polarity terminal

ofhu th upper and lower coils simultaneously. C ~ U S -

ing tlie relay to upc rntc .

Rccausc of th e pu lal-ity-sensitivc riiiture uf rhe

elecrron~rcl~anicalroduct-type rrlay and tlie possi-

bility of current imbalance an d satoratirin, it has

hrcn recommended to cnnncct the auxiliary trans-

former in an autotl.;uisformcr configuration [31. l'nr

the application in this articlu, thc autotcansformer

CT Ixovidcs 10% more currcnt contribution from

th e phase Cl's. l 'h c additional ciirrrnt increases the

negative torqoc and hclps keep the c o n r x t s o p e n .

This aids in preventing misopcration when rherc is

currcnt imbdance or thc auxiliary CT satiil'iites.

Th is circu it mndifiicntion is a necessity for th e prod-

uct-type relay to opcratc rcl idd y a n d srcurcly.

Tap mngcs imd mulr iplicr s for product-type re-

lays vary by manufacturcr, b u t t h e r a p p r d i c t

( the tap se t t ing X the mult ipl ier) wil l general ly

ninge f rom abou t 0.5A2 to 36 A . The lower nip

set t ings provide gruatcr scnsi tivi ty. I~ OW CV CI,he

hurdcn is alsogreater a t thrsc lower values a n d may

c a ~ ~ s eh e ncutrn l C T t o s a t u r a t c u n d e r

l i igl i-ground-f~ult ondit ions. Conscqucnrly, ex-

treme care niust be used to accurately calculate tlie

burden placed on thc neutral CT . Dctnils o l these

calculatinns h a w been prrsrntcd in detiiil by other

au thors [?I.An additional point to consider is th c iippliciition

of product-type relays fur guncrator ground-diffrr-

entia1 protection. Assuming equal voltxgc distr ibu-

tion across tlie griirrator windings, rhr ckiscr a

ground fault occurs to th e neutral, the lower the

voltage and imprdmcc wil l be, resulting in lowcr

hultcurrent. Increased scnritivity afforded by thr

lower tap settings is an advantage for faults of tliis

rypc, an d ma y give you a falsc sense of security. As

2



dcscribcd almvc, adjusting the relay to o p e m c f or

h i t s near th e gencrnror nciittiil by sclecting Iowcr

tap values placcsa grcxtcr b u r d c n on the ne ut ra l C X .Tor ground faults at the gcnuratnr's remiio;ils, the

miixiiiitim f,dr ctirrcnt mill flow through t11c ricw

tral CT, possibly causing saturiition

Applying Directional Ground-Overcurrent RelaysDiruotional ground-overcuI-rmt relays ar c nor-

mally unrd to pruvidc sensitive rripping fur C U I -

r e m s flowing i n on e direction only. Dircutinnal

g r [ i t i i i i i - o v e r c u r r e n t rc l ; iys c u i i s i s t of a il

overcurrent function and a ilirectioniil function.

The dirccrional function determilies rh c direcrirm

of cucrcnr flow based on a polarizing input soi~tc e.

The polarizing suurce ciln he cur r ent , voltagc, or

both. Outpur h i m the ovcrcuiirnt lun ction is cun-

rrolled by tlic clircctionnl functirin. When rlre cur -

rent excccds irs tap sert ing a i d is f lowing i n th c



the angu1;ir displaccment of th e mcasured current

and polarizing quanti ty , rhc power f low dircc t ion

cmbrdcrcmiined. As long as th e p i h i z i n g quan-

tiLy nminwiins a s teady phase position as t h c f i d t

location changes, it is a ptopcr polurizing sourcc.

Directio nd ground-overcl~rrcl l t elays can bc

used to Iprovidr sensitive ground-fault protect ion

oftransformers and generators. T hc rclay is applied

in B mnnners imihr to the p roduc t type relay as prr-

v i o u s l v d i s c u r s c d . The a r u u n d - d i r e c t i o n a l

1 I ' F i

B A1C

g'Oun'l~uvcrcurr~iIt rclny, similar to tlir prod-

uct-typc relay, provides 6,irr opcration withou t the

ne rd to coordinate with gruun&biicknp drviccs.

Bccause this scheme is not affected by phasc b u l t s

(nor involving ground) not normnl load currents,

th e relay can he sec tovery sensi t ive pickupvalues.

Applicd in a ground-differenti i l scheme, th e

directional ground-ovorcurrcnt rrlay slioulcl br

connccted u s i n g current polarization, as shown in

F i r s . 5 ;iod6.Thesr fisiircs show chat tlie relay may

flows thrnugli tlir operat e e lement tu

cxcccd it s minimum pickup set t ing.

I . .br crinnected using c itliera typical atix-

iliary CT or iln au to- t r ; ins~ ,rmer - ty i~e

(:r, T h e relay is c o n ~ ~ ~ c t c de t w r r n the

ncutnil CT sccnndary circuit and rhr

auxiliary Cl, incc the polarizing clc-

meot is polarity scnsitive, it is c nn-

nected i n series with the n e u t r d CTsecondary winding. The operat ing elc-

ment is oonnccted in pc""le1 with thc

auxiliary CT secondary winding. S imi-

la r tn tlir product typr rclay, the direo-

t i u n a l g r o u n d - ~ i v c r c u t r e i i t r e l ay

compzircs th e ph;m relat ionship bc-

tween the pda r iz ing quan t i ty and th r

me;isurcd current in the operacc circuit .

The rulny will opcmte w h m the cur-

runts from the polarizing clemcnt and

th e oper;itc c lement ilrc in-phase with

currcnt flow into tlie polarity side of

ench c lement , a n d sufficient current



ncut ion toward th e autotransfnrmrr. tlim 0. 5 A

must f low though t h e operate element from

non-polarity toward polarity. However, s incc the

current flow through the opctate clrnicnt is 1x0"

out ofphase with the polarizing quanti ty , th e relay

will not opera tc . As previnusly mentioned, rlicsetwo qunnti t ics must be in pliasc (indicating a n in -

ternal fault) for tlic relay to operatr.

F i g . 8 s how s t l ie r e s u l t s fo r an i n r c r n d

A-pliasc~to-ground kiult . Th e primary ground-

fault ciir rent will flow into the pol;irity side of th e

ne ut ra l CC, similar to c h i . e x t e m d grriiind fiiult,

and will prodnc c il nrcondary ciirrent. IN[:, of 5.0

A. The phase A primary O I I I P C L I ~f 200 A f lows i n t o

thr polarity sidr cif the pliasc CT, ixoducing a sco-

ondnry residual ciirtcnt, TRVp, o f 0 .5 A tlmt iliiws

in to thc polar i ty sidc of the 1o:i autotranslormer .

Th e auto-transformcr pr i~ luces senna la ry cur-

rent , lRps, of 5 .5 A that c i r c uhtc s in tbc secomlciry

circuit andinto th e polarity side of thc relay's oper-

ate clement. Tlic neutral CT sccondary current,iNv, also flows into th e polarity side of th e d a y ' s

operate element, pKOdUCing ii tota l operate c ur re nt

of 10.5 A. Since the polarizing current atid r h u 01-

c ra t rc u r r rn t i i r c i n p l m c , a n d t h r o p c r x r c u r r e n t is

above t he pickup srtting, th e rclay will opcratr for

thc given fiult c i ind i t iws .

Comparison Anulys is

Comparing th e opcrmion duscrihcd previoosly for

tlie product-type relny an d th e operation of th e di-rectioncil ground-overc urrent rcl;iy, it q2peilrs tliat

bnrl i relays perfnrni satisfactorily l i ir internal and

external ground faults rising the ziutotransformer

CT connection. Brith methods will aki i pcrfrirm

satisfactorily w h u n using a st:indarcl C'l connec-tion, as shown i n Fig. 5, as long as th e fault current

is limited toil viilue such tbnt tlic ;oixiliary CT s do

nut go into saturation a d tlic circuit is cur-

rcnt-balanced. IS t h e circuir is inot billiinccil, any

current imbalance rhnt might occur dur ing ii

ground fault would find a path r11rougIi li c upercitc

clrment of th e rclay. If this irnhalnnce is above rb c

pickup set t ing, t h c relay will operate. Such would

bc thr CBSC for ti n internnl fault wherc the iiuxiliary

CT s arc dr ivcn in to sa tu ra t im

Auxiliary CT satunition is typically not a problem

with single-source impedance-gti,undi.d power sys-

tems. However, for inrcrnal ground laults with mul-

tiple ground soiirccs, tlic fault current ctmribution

from th e phasr CTs is incrcasrd. As il result, it is pos-

sible to drive tlic residunlly connectcd auxiliary C'X

winding into snturatir,n. The effect this wriuld lm-

duce for a n internal fault is a reductim olthe magni-

tude ofcurtent thmugh th e relay's opernte element hy

rcducing the current contrilmtim fcnm th e l h s c

CTs. l'his should liavc little e lkc t on the priiperopcr-

ation of thr relay. Sofficicnt ripcratr cwcciit s l~ould

exist from th e ncutral CT contribution to cause th e

relay to operate.

Relay l y p e Polarking Element (RI OperateElement ( (2)

Elettromerhonita Product(AI 0.42 3.52

Eleilromethonirol Predud (E) 2.87 0.38

Ele~tromeihonitalDiredionol Ovenurrent 0.33 8.10

Stotit Anolog Diredionol Ouerrurronl c 0.1 c 0.1

Stotk Oigi tol Oiredionol Oveaurrent 1 0 . 2 <0.2

________

However , for cxternal groun d S i u l t s where th e

6iult ciiirent is sufficient codrive t1ieCfs into satu-

ration, misupc rnt ion could OCCLIC.This rundition is

cumpnundeil hy tlic fact that electrumechanical

product-rypc relays p l ~ en extrcmcly high bur-

clcn on rlir CTs. Tahlc I inc l ica t rs typical burdun

values fix the types of relays described i n this

a r r i c l e ~ e l e c t r o m e c l i a i i i c a l product and direc-

tional uvercurren t , sciitic iulal~g,ndstiitic digital

ilirccriorial ovemitrent.

T;iblc I1 shows the rclarive burden on tlie phase

tincl neutral CTs, considering the rcflocted imped-

ance through th e 1 :1 il-Aauxiliary CT forsaturmed

aiid nilti-saturiiteil conditions (i.e., the Values given

fu r the 20015 CT evaluiltion include the 200015 CTfo r th e satunited conclit ion). Thesc rrsrdts arrhascd

on typicnl CT winding impednnce an d lead rcsis-

tnncu Y B I U C S , with rclay burden from Tablc I. Thc

rctlected burden on th c neu t rd CT is minimal in

$111c a w Although significantly higher, the bur-

den attr ibuted fiom electromechanical directional

ove rc ur rc nt rcliiy is within mason. However, au ex -

amination of th e burilcn placcd on th e 200015 CT

indicates consiilcrably high er values, especially for

tlic clrctmmec hanical ilcvices. l'here arc ;dso majur

variatioiis betwcen the saturated and non-siitii-

rzited conditions. A majnr cont r ibu t ing factor is

the rcflectim of th c burilcn through tlic ziuxiliary

CT, where the base burden must he multiplied bythe square of th e turns-ratio of thc CT. 'l'his will

pmduce considerably highcr values fo r the e l e c m -

mechanical relays,which havc an inlirrently higher

value ofbase impvrlancc th;m thc mit ic - type rclnys.

On t h e a t l i e r hz i nd , s t a t i c d i r e c t i o n a l

gri'iiiid-ovcrcurrei,t relays will perform adequately

for both i n t c t m l a i d external faults using either

t l ic sr;indard auxiliary CT connection or t h e

a u t o t r m s h r m e r CT connection.

SummuryThe us c of groond diffcrentinl for thr protcction ot

iml,e'Iancc-groun~led transformcrs and generators

hiis been increasing over the last several pears. Bc-



I

Statii Ano log Dirert ionnl Overcurrent 1 0.70

I

0.63 41.37 36.27

I

I

I

I

I

I

1 WithZOOO/SCT 1 W i t h 2 0 0 0 / 5 C 1 I Wi th 200/5C1 1 Wl~nZO;$Cl INot Snturn ted Soturo ted Not Soturn ted

eloy lype

Eleilromeihonirol Product (AI 4.44 0.97 383.37 100.9Eleitromeihoniial Produit (8) 3.75 3.41 69.37 61.85

Eleitromethonirol Direitionol 8.93 0.88 841.37 107.07

ciliiscof its iiihcrcnt selrctivity and spccd ofopcra-

r i o i i , g r o u n d d i f f e r e n t i a l p r o v i d e s c x c e l l e n r

protect ion ;rgziinst g r o u n d faults w i t h i n t l ic

grwndeil winding of the cqu ipment . HOW~VCI,

several factors s h u u l d be consirlcrcd wlieii npplying

groun~l~diCf~eren t ia1rotection ro a power sys tem.

'L'hcsc t i c t u r r include rhr rypc a n d racings of thc

c q u i p " r beingprorecl-cd, wlierhcr it is ii nrw i i i -

sr;illarion or a rrrmfit project, an d the typc of rekiy

being used.

The r;itings of a tr;inshrmer UT gcnerntor will

de tc rmine thc n o r m a l loid current ilnd th e avail-

able 6 u l t c u r rc n r. T h e load current wil l affect th e

m i 0 of t h e CTs usrd and determine the rcquire-

Lnciits for th e auxil iary CT . l h ewailable fiiult cu r-

renr will be i i fitctor in dercmmining tlic level of

imprdance groon ding necessary.

Th e type or inst;dlation wil l a lso affect the a p

plication of gn,iind-rliffuruiitial pruccction. If th c

instdlarion is new, the selcction of impcdancr

grounding Irvcl, n u i n C1' ra t io , ;ind nuxili;iry C1'

ratio provide grexrrr Intitiide tiit th e proreorion cri-

giiiccr to KllS l l fC proper protect ion Icvcls. If th e i i i -

stallation is B retrofi t t o exist ing eqi i ipmcnt , tlie

selrctions tire typical ly l imited The impei lmce

grounding is nOKmally in plzicc alre;aly, as w e t he

phase an d iicutral CTs. It t hen buconies nrcussnry

for tbeprotcotion cng in re r to se lect thcproper ; iox-

iliary Cl' fatiomd rclay that will best fir thc sys-

rein for rlic given condit iom.

As indicared in this arriclc, there iirc dil icrent

relay types that may he iiscil CO providcground-dif-

fcrcotial prrituction. The mcthod used in tlie p a x

has prirnnrily been thc electrnmcchanical prod-

u c t - t y p e re lay . Howrvcr , sttntic dcs ign d i r rc -

tinnal~ovcrcnl.l-enr clays iirr ga in ing aourprance i I

rhisapliliuationasa resu1riiftiieir;aldcd benefits.

Thc clecrromcclianic;il rcliiy provides adeqiiiitcp m t e c t i a n [or nios r cmdi t ions . However , Cl sat-

urntion is a concern duc to the inhcrcn t ly highcr

burden v:rlocs of thrsc devi ce s. l h e tap sclcction

uscd for grncrntor applicat ions should idso bc

consirlcrcd c;irc€iilly to rnsurc adequa te wind ing

ground-faault pm tection while l i m i t i n g t h e bur-

den placed on th e C'r circuits . In addi t ion , thc

a u x i l i a r y C T m a y hc c o n n e c t e d i n a n

nototninsfimner configurarion to h e l p d i m i n a r e

possible f i t lsc t r ipp ing .

Sratic dirrcrinnnl overcurrenr rclays provide se -

lective, sensitive, higli-spced prmecrion ;@tistgrmund faults within the equipment . Onc major

;idvanrage with stxtic analog or stat ic digital relays

is tlrir lower bimdcn r a ti n g . l h i s Iuww burden rc-

duces th e chancrs nf onusing CT saturation, I c d

in g ti 1 misoperatinn.

In addit ion, rhc r educed burden ofs ta t i c relays

rimy ;dlmv the sclcction of lower-excitation-class

CTs, resillring in reduccil physical sixice reqnire-

n e n r s , as well as addiriunnl cost savings. Staric TU-

la y clesign also p o v i d e s greater accuracy, rediiceil

niaiiiteiiance rcciuirrmrnts, an d niost likely, ; ddci l

calxthility at reduced cost compared tn tlie elcc-

tromecimnical devices.

Either r c l q design, sIatic 01 clectromcchanical,

m ay be applicd for ground-diffcruntial-pl-otuction

~ c l i e m u ~n impcdnnce-gl.uui,'l~l tmnsIornms nncl

gencrators.Uachapplication should hc rvaluared rc-

gard ing C T brudrn a n d sxturation to ~ ~ I S U C Croper

operation. l 'hc protection cngirierr s lm t l d carcfdly

consiilcr the applicarion f a o t t m ourlinrcl here to ell-

si re l i i ir thcdesireil Icvcl ofprotcction isprm&cl.

References[11 l l l l l i G,,idejir i'?,,*C<,i"< iW',J ,i/>/~/i<',,i",,.,i I'n,,a 'I).',,,,.

ground differential protection

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