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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
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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
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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
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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
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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
$111c a w Although significantly higher, the bur-
den attr ibuted fiom electromechanical directional
ove rc ur rc nt rcliiy is within mason. However, au ex -
amination of th e burilcn placcd on th e 200015 CT
indicates consiilcrably high er values, especially for
tlic clrctmmec hanical ilcvices. l'here arc ;dso majur
variatioiis betwcen the saturated and non-siitii-
rzited conditions. A majnr cont r ibu t ing factor is
the rcflectim of th c burilcn through tlic ziuxiliary
CT, where the base burden must he multiplied bythe square of th e turns-ratio of thc CT. 'l'his will
pmduce considerably highcr values fo r the e l e c m -
mechanical relays,which havc an inlirrently higher
value ofbase impvrlancc th;m thc mit ic - type rclnys.
On t h e a t l i e r hz i nd , s t a t i c d i r e c t i o n a l
gri'iiiid-ovcrcurrei,t relays will perform adequately
for both i n t c t m l a i d external faults using either
t l ic sr;indard auxiliary CT connection or t h e
a u t o t r m s h r m e r CT connection.
SummuryThe us c of groond diffcrentinl for thr protcction ot
iml,e'Iancc-groun~led transformcrs and generators
hiis been increasing over the last several pears. Bc-
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I
Statii Ano log Dirert ionnl Overcurrent 1 0.70
I
0.63 41.37 36.27
I
I
I
I
I
I
1 WithZOOO/SCT 1 W i t h 2 0 0 0 / 5 C 1 I Wi th 200/5C1 1 Wl~nZO;$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).',,,,.