.. ,

4·12fij

4.4 Mho Relll~Adminance Rela~ "44itritn

4.12•3•00!!c!'I!t0!g004-12. , ..,43.3 0pe!!!I!!Q a-1s:Ics .....

- - . . - "4.32 TO!!!!!! E!IUIDCWI

4-541.4 Oisadvan'", 01 PIlon Impedance Relay •4.2.3 Opela;!ng ~ 00 R·X0lag!3m ••••••.4.2.2 Openi:m Charaaensllcs •• ,.. .. .•• ....... .. .. ...

4.1 Otstance Rata ,..~ 4-1

Review Questions 3-93 7 DifftQJltles to Bushar protecllon 3-93.6 High Impedance Differential Protection of Busbar ' 3-83 5 CUWlaUngCUrTentProlpction 01Busbar ,,'..... ,,3-734 Framelaa a Protection of Busbar " _ _._ 3-6

}S! ,, ± , ± ,331 BystwEm3.533 Bllsha, pmlec:tiQn

3-53.2.3 Vaftag! Balance IlIfIn<1tili Relay

, 1 t •32 Typel!OfOl!'t!!!!ua!!lay§ ",,''''' U,,'" """""u,,u31.!~~~, c..' , ,

3-13,1 Introduction "

(3-1 to 3-10)",," ""Iq" I I Tt"ReyiewOuestigos. ""tll!1!!",,' tll"l "Tt

29 Universal Relay Torque Equation 4 2·28

EJtampies wrth Solutions II.'" II'''' "' !!.Ir"" III t, t. "" It"",." t." p' p' .5:31t Unbalanced Loadin 5-30

5 11Rotor enab ell d1pmtoclion

"0,,0 _tt! II' t.ll.

.- """',,"'" 5:195 10 Stator Protac:bonAgainst Intenum Faults .,....5-185,9100% Earth Fault protection ,.,5·165 8 Balanced Earth Fau" protection5-155.7 Unreslric!ed Earth Fault proteclioo,,,,.,.. ,,' ,. .

5.3 BaSICpjfferentlal prolecUon Scheme (or Generators """"",,""""""" " ..5-65.4 Basic Percentage Differential ProtocUonScheme for Generators .,..".".5-75.5 Merz·Price Proledion of Alternator Stator Windings }§

82' Ot_ «"'''*!'

$23'0.''''''

5233l)< 2 _ LoocI!!q.

5 2 3 AIInanNII!um!lq Condi!lonl .•.5.2.310.-9

S2 2 Rcax ea,e.

, ,.., , , -5 2 1 SIa!!>! fait>5 2 Generator EauliS5 1InUoductm ... '",,' 1111" It. II' to •• "",,,It, ItIt ,ee ti"' It"I", 11""'"" IttI' .. ,,1.,Ie IIlie '"" Itl II.5:1

Review OI'estionS +22

.61 0IstInce r.".~ Reily, ••• 0.' , ,',.... • •• •• • 4..194~ 3Aec*?'- ond ~. of 00sIMa RAIla'!!.. '-21

(1 .1)

The relays are compx1 and self eonrained devices ,,,,tuch an sense the nboort\\31conditions. When<l'\."erAn abnormal condition C'<isIS, the !"day contacts get closed. Thisintum closes tbe trip orcuit o( • circuli breaker, The circuit b""'M-~ are capablc 01dlsconn~ctin8 • (.ulr), ~Iem.nt, when they M. called upon to dn so by Ihl! relo)..Thue entir~ prOC\'!llindue!.... III,' o""r"nons likf (>'-':urre_ oi (.ul~ op<lr~tl"n o( ",lay,(~pM1ln8 ()r ., cil'CU't bt~.k.et And rt·'l'lov141()f r"ulty ~k!n\t"nt. Thas entire pnx'(."S... Is.-,utOlllatic 31\(:1(ast. wtuch is pos8iblt' due to e((t.'Ctive proll'ctl\t! relOlying scheme

TI\e protective relaying scheme i_I\(Jud,,~pretective CUl'Tt"I'II transformers. voltagetransformers, protective reiaY", time delay relays, auxiliary relays, secondary orelli!:!,tnp circuits etc Each. coa\p<.mC!nt pL1yS ihS owrt rote.. which is vt'1)' i1nporl"nt in th~overall operation of the ~J\e. The protective reLlying is !hi: ;eam work of .u the>;:ronlpont:nts, The pl"OteC:lfV~~I.yil'gIIJ~o provides the indlC'""uon of 1«4th_ln and t).~0( thi' r."It.

Protective re,.;'in& IS one of the S<',,~.. I fc.tu res of th~ power system dl'$lll'"E,''"'Y p.lfl ()I the power 'Y"lcm Is proreeted. The facton. otfll'<ting th,) choke 01prolt'(tlon are Iyp<' ond fObng 01 ~ulpn"'"I, 1()C,tlon of I"" ~Ulpmcnt, Iy!"'" "I lo"hs,.b",,,mal "",diU""" Mid Ctl8t.

The protective rday!ng is used to 8"'. an alarm or 10 COI>b<'prompl "''mo, ..1 01any dement o( power S)'Btem fm'" ""r"iC<'wMn that elem<nt behaves abno,"","y.Tbe abnormal beholvJOUf0( an element might cause damagr or interference \\lithint·'(t"('ti\'eoperation of ~ 01 the system, TIM' protective rebving mlmmlses the damageto thE: \!(I\upn1cot (and ink"fT\lptiot'ls II.) th(· service WN..'1'\tlectric4\1 FotUUt'l'occues. Almgwith .som,~ other equ:l~ts the rel:.)'s 1'w.!lpto minlm.i.w d~g~ And ilnpt\)v~ the~i(1l·.

1.1 Introduction

Protective Relaying

In • p,C)k'('t, ve relaying scheme, Ihl' drcult breakers 3t\' plaeed .1t the IlvpraptUtcpa<nto ,um lhAt any .1~me"l of the ''IIt1~poWe' 6y>tem (d" be dlscnnn('Cu\J foer..-potlnng work, 'ISwl operllllnl\ and. maintenAnce t\.~l'lll'\.""men~ and also uMfr.abnt_1.rmlllconditiON> IIi.e she.)rt circuits, Th'ls 8; protectiv.: co ...~ring i:t provided anMJn.cje.xh <Ie........1of 1M 5)'StmI.

A prot.cth'~ tont 15 rbe separate zone which Is eslabli5hed around eoKh .)'St>:m,-Ieml'lll The Sign,fj("",,, o( such n proIKti •• %on"Is Ihol .ny fault "",,mOtt ",til", •~''' ... 'O!W "ill <.Ll.... Ihc tripping of rel.y. whkh c.u" .. open,nl.l of .11 tho nrcuUbrr.t.kt~ IOCAt,td ""Itrun .hat ZOI'~,Thl: v,rlou,s (C)mIJOllClll$ which .11(" pro\ wed ""Ilh~ prOC«llv..• I«>nt.~~n.'gll'nCtl'lors, tra.n~'ormc"". tl~"sn,is5i'()n lines, btlj bat). cabb.".p.l01O'" etc. No PO" of the system 1$ I<lt unproreered. n.., Fig 1.1 >1>0"" tlwV.Jnc)U$ protective zones, used In a system

1.3 Protective Zones

TIl\! (.lutts can not be completely uvoided b\~t C;St) be mlnlflliied. ThI.l5 thepro!«ti\.," ~) Lng plays an import-Ant role in seJ\Slng the bulb. mi"un17.,ngthe effects of f.ul15 and minimiting the damage due to the faults,

1.2 Functions of Protectlvo Relaying"The various (unctIOn> of proreeuve relaying are:t. The prompt remO\ral of the componcont which tS behaving abnormal I) by

Ck:>$lJlSthe Inp CJrclkitof circuit breaker or to sound an ......arm

2 ru d'f<ON1<'(l the abnormolly 01"'r,'li1\8 pUrl 8(1 •• IU .,·old lhe d.""'jt_ '"Int~rfcrroc..("\\ "h.,l c(lt"(tlvc Clp"r.tt"tn of the r~t'itof \)"'Sttm

J Tn p,"''''''''1 1M ,ub'.qu<I,1 Ilult. by dbc"""ecting ,he aboonn.,lIy """,.hng

"'"..4. To d_ tlw fDulty po" as quickly us possJ.ble $0 as 10 min,ml$e ,hedamage to Iht- f.ulty pari il,.,U. For example, if there Is • winding f.ult lit •

machine and ,f ,t peni.ls for a long ume Ihen tlu!.... " • p"",bility 01 thed.mase 01 the enuec winding A•• 11.'011'his. If " I> d''''OfII\''<lro qukl.h· then0,>1),f(ow (\.l~ ""'I gf' dalll.ged in.~'c,dof the ,'ntil .. "lndmS

;; To "",'riet the spreading of lhe effect of fault causing least Interference to therest of the "".Ithy system. Th'" b)' disconnecting the f.tully part. the faulteiiects gel localised.

6 To impro\"'e ~ s}'stcm performance, system rclidbaJlty. syseen shlbult} .nd~rvlCecontJnUltv

Protlctlvo RelayingPtolec1l<>n .nd Switcl>gear

1." Primary end Backup ProtectionTh<> prot,'Ction provided by the protective "'~yln8 equipment can be co~

u,tu two typ'-" .. :1. Prim..ry p1'Ot<'Ction2. Bockup prote:tkln

The primary prote:tkln Is \h. first line 01 defence .nd is reponolble to protK1 oJlth.;. pow,., .r.t.m elementsfrom .u the typos 01 f.uJl3. ",. "'aup prok-ctl"" romn",to play only when tlw primary ptOlKlion (aIls.

Fig. 1.2o"trllpplng .ones In pnmlry rellylng

It can be seal from the Fig. 1.2 that the dn:uit brttbn .re located in thec<ll1JleCUO<\Sto each power system element, This provision II\AMs it possible todisronnec:t only the ""ulty element from 1M system. OcassioNlly for economy in thenumber of circuit breaUn. • br~ak"r ~.." th~ two adjooent ...ctlon. moy be"""tted but In that ColSC! both the power system >eetions .'" required to bed...."""~ (or the !~ilurein elth~rof lhe two.

E.>dt protl!(li..., zone 4. certain prot«tive scheme 0.114ead> scheme has number01 protective ',-stems..

, J88

•

~~ ----~ _:~~~~~-~J-~-~-~:-~;--{-~-:-[J-~-l~---~~~~-~~~~~~-~-~-~:-t:----ptote<;bCltl .,.. ... ... ... r .. ...... .... -, :

[ :!;c~J • II'.tf~ •-r- __ ....._"__I_ .............. !. __ ~_ ...... __ J_""'_ --_ : ,!----l:---T tiOf\ ....

ptoClICi1iorI _'- -- - -- J_ -- -- r-.'. 8' ' •~_ " ,. l

'~hONt--~' ...................' ~..............._'JlIf()tt(IOn

,..-_ ... __ ......•G}-......-- __

-.....----:-

1·.Prottctlo" Ind Swlt~"'

(5 • I)

The various faults which can occur assodated with a generaeor can be clasSfied ~

1. Stator faull$ : "".... fntJlts ~t('d WIth the ,.talor or tbe 8eT'K'~tQr.

2. Rotor foulill : Tbe (.,,11. olSSOC'dtN"Ith the rotor of thl' 8O"""'lor.) Abnormn! nlonlll(l; ('()nditiol16 ThfS, "",'Iud,-._. .ltlulb4,·r (.1 Olbn(.rml.\1 condlbOn.\

which ",ay ",-"CurIII practlce, (rom ...hkh the ~ener••or mu,! b. prol«ledLt1us discuss Ihese faults in dClnil

5.2 Generator Faults

1M ge..erators used in the power ">"""" are the alternators which prodcce \'..,!ugh ;I.e voltages. The protection of gmeo-.tOI'S is very much complex due 10 thefolJo\o.ing reasons.

1. The generators a~ very large rn;ldlU"k.':t pf'\Jd\lcing \'l'ry 'llgh YOltpg,~ .Jnd ;l~

ecenccred to bu..o.rs.~ V.rious other t'<Juipmcnts are .1"")'$ .ssoci.tro wilh the generators. Such

equipmenrs a", prime movers. ""attnon $)'$Iem., voltage regulators, coolingsystems etc. Thu. protection of gmt'r.tors must consider the presence 010-"'her equlpm...,,, .110.

l. The gcncrftton; ..re \fCry W!S,ly* (Olopt'M4"'C dnd V(''')f Import~I't ("ttllt In I PO"{(!I'sy.lem. The protection 6Ch""'" must be such th.1 It should not shul 011 thegl.."eriStC)ra; tb lilt a~ ~ibl(". The ehut off t;('n('r~ltt'r' rl--"IJll ill a PO'vetohort3ge.

AU these {actors make the design 01 protection scheme for the generator. ,..,'much complex.

6eihr'(' $t'ld)'tng tt,c ",.1ri,'ll.\ prol«tiQn )('ilcmt'$ tor theSet'~rl'torsl let ~ d~\'~nou~ftlults whicll (-._,,\OC'Cur aM<JCi.lte<i ....-ith t~ gt"I\{'ri1'or~.

5.1 IntroductIon

Generator Protection

ll1.3 Stltor Inler-TumFaults'The coils used in the alternators llI"l' generally multitum coils. So short citcuil

berwe..... the turns of 0". coil may occur which I. caUed an inter-tum laull. lbis floul.<'<OJ" due to <urre"' .IIrS''S with high v.lue of (L dl/cJt) v<,hllSC.(" ... the tumo. BUt,f "'" rod. u-cd Me 'Ingle tum then this fault an not o<'('ur. Hence(or the largemachl_ of the order of 50 MVA and m<ft. It is • normal practice to lise single IUmcoUt 8ul in IOOmecountries, multllum coUt lite very commonly used where pl'Olrdlon~inst mter-turn {BullS is must.

U 1.2Phas. to Phtu F .The phase to ph fa,,11$means short =11 between two p..... e winding>. Such

faults are uncommon because the Insulation used between the coils of different phasesin .. slot is large. But once phase to arth nult occurs, due to the over heating phaseto ~ fault also rna)' occur. This ,.ult is likely to occur at the end connections ofthe armature windings which are ov~ting parts outside the slots. Such a nullcauses severe arcing wilh ""1' "ISh tempentu ..... Thi. may leAd to melting of <opperand fire iI the insulation is not fire resiSW\~

Sl.1.1 Phase to Eitth FaillaTht-<e (aul ts molnly occur In tho .1tNItu~ slots. The f.u]'" are dd"llCfOUS and con

(AU'k· W\o'enodamitg<- to the CXpt:nslve machine. Th~ fault cureens I~ (hal\ 20 A cau...'ienegltgib'" burning o( core II machine i. tripped quickly. aut if the fault curren .. a~higtI. severe burning of $t••or cor. can taU plact. This may lead to the requirement 01..."Ioong rhe laminations which is very cootIy and tim. consuming. So to avoid thedam.>IlCdue to phase to earth faul.... sq>OTill:, sensitive earth lault prol<dlon is~ry lor .he senOrA")"; ,.tollgwlth lhe Nrthing ,.,1...........

5.2.1 Stator Fault$The stator (Dults means (o"lts nssodoted wilh the three ph.lSe armoture windings

01 .he ~.,...tor. These (.ults ore mainly due to the insulation (ailure o( the annat""'"~nding$. The m.in type8 o( stator f.ults "".

I l'h.liIC to I'olrthf.ul"2. l'ho5(' to phAse (.ults

3. Inter-turn (aul", involving turns of ...me phase winding.

The most important and common fault IS phase to earth fault. The other twO arenot ,,~et).common \\'hile inter-tum fault is ""'elY difficlLlt to detect.

Gen."tor PrOlOClio<tp,~" Ind SwflA:hgelr

5.2.3.1OvtrtoldingOu .. to the rontinuous overloading. lhe O\-emeoling of Ihe stato< results. This mal'

lr..,.,,_ lhe winding "'m~rolure. If this tt>m~r"ur. rtse ",cuocJs certain limit theinsu"'t"", 01 1M wlndins may get cUrNgfd The dCIj"('<' 01 ovm""dlng dodd es tM"fh.'C" .nd tem~r.tu .... ri,." The ovemtfTnlt proleelloll I~ gc·n.r.lIy ",I to v~" h.gh

5,2,3 Abnormal Running ConditionsIn practice there are number of simanoes in which g(,'f1erator is subjected to some

.boormal running rondi~ons, The protecnon must be provided .S"inst th<.' abrom,.1",1f1dltions ~ obnonnol ("nditlo,,-, .nclud e.

I Ovt,loodl"82. Overspe...',bns3. Unbalanced loading

4. Overvoltage

5. Failure of prime mover

6. ~ of ""cilation (Field failure)

,. Cooling 'Y'IMn failure

5.2.2 Rotor Fautllll'tw rotor of an alwmator is generally> ';'!d wlndl"K as ~t of the .Itonloltu,.

are of rotating field type Tbe field ",..,dmS is made lip of number of tu~ So theconductor to earth faults and short circu.1 between the turns of the field windmg. areIh. <""'"'<MIll' <l«Umng f.ullS with R.'>f'«I 10 ,. ,<.>t'Ir, 'I II""" f,.ull> • re eau se'<1 due 10the ~"\'ere mccNtnica1 and tkcrmnl stresses. .lC11ng on Il)Cfield,", tnd,ng insul ...tion..

The rfeld \yinding is generalty not grounded 311d hence ~inglc line to ground bultdoes "'" give any fault current. A second fault to earth ",III short arruit the part ofthe field Winding and may there by produce an unsymmetrical field system, Such anunsymmetrical system gives rise to lhe unbalanced forces On the rotor and results ine""" pressure on the bearings .Ind the '<halt dI5l0rli,)II, if such " f,ult is not eteared"(If) ft.lrl)!. So it il very liluch t,cce~ry to MluW Ihe (').Ip,tl.'n(c' (1/ tl"C n~, \lC('\lrr\"I\4,,·"I II,.· ~.rth fault '" that corrective me.. um can be '3kc'l\ bc fore {<'ClInd I,IUlt "''CU"'

~ unb.,lanced I... dlng on the getWRtOf is responsible to produce the n.g.h,(·st<IIk!J1« currmts. Tbese currents produce. rotAting magn.lic field which rota! es inopposite din.-Won 10 th.1 of rolor magnetic field, Due to this field. tbere is ind.-lemf In the rotor winding. This causes cverbeatmg of the rotor,

ROtor earth bult ~"n.,tl"('th,ln..lnd rotor b."rt\P'!l'ahJrc jJ,,-iic3t0I'5 are the ("$.._~tial .andare provid~'d to large 'aliOj; generalors,

G.".rator ProtltCllon5·'Protection and Swltc:hU"",

5.2.3.4 OvtNOttage

TI,,, overvoltages .'" basic.U), due to the overspeedlng 01 generators. Anotherre.",n tor the 0\ Of'ol"'g"" i. the roulty opmttion of vol~agc regulat<m. Not onl)' t.t..11\tl·nl.;,1ovcrvouages an' d~lt\g"rou.sbut .atmospheric SUfS" v-oIta.gescan also reach tolhe gcncr,tlurs. Such atmospheric surge "oItagcs are gt.~~ by dirt'C't lightingstrokes to the a_I lines of high VOltageS)'litem. Inductively and capacirively, thesesurges can get lransferred to the genentor. To protect the ga..... tors from surgevohages, IhI' surge arresters and surge capocitors aee often used

At the time 01 ~tnk,"g "C,'(ISS thl' cont>CUof cirt'Uit breakn.. ttw tr~nslcnl 0\'"

".h.,S''S get s..,cr.t<.'d Such ¥u'Kc" ..... wled twlll'hlng .u,~ ... nd "d., be IImil,'(! by

5.2.3.3 Unbal.nced Lotdinslrhe unb"i.r>ttd lo.Jding uf the goner.tor ... ult. In lhe amllatootl of "''8-'llv.

)t~llh,'n('('currents. lh..'!'!It.-- (\.Ir~t\ts produce thr rotatl"g rn;\gnebC' fit"Jd. 11,ls: rot...ringm,'gnetic field robles al the synchronous speed with respect to rotor. The dirE'Ctionofrotation 01 this m.>gneticAcid is oppo$ile to thai of rotor. H~ effecIively tile ",Ialivespeed between the two is double the synchronous speed.

Thus the e.m f. j;...., Induced, tIoving double the ncrn ....1 frequ"ncy. In the rotor,.,.,nding The ",,'UI>l1D1lcurrents due 10 !he indu~ e.m.I.• ~ resplln,iblc to overheatlh(' wttll' winding ~~ w.~1 os rotor lIampln&lo. COI1tJnuousunbalAnced IUlld moe. tNn10'~, of th,' rnted lo>cI causes IrrlDo!'ndous hI'''ing which b dominonl 'ncaS<' ofcvluidrlcal rotor of truboaltt'mators..

The reasons for the unbalanced load condilions are,

Occurrence of unsymmetrical f.ults near the generating slation.

2 Ihe r.llur(' 01 l'muil breaker n(!Ar!he gcncroting SI"tion in '*aring .lll the tim...pll.,"tt"!fI.

Neg."vc "'quence proeectlcn i. imporunt to prevent dangt'f'OUSsltuMlun. due 10

rI<'!latll'e sequence a.rr('fl13 which ore ~_ of unbalanced load conditions.

5.2.3.2 OvtfSpeed"mg

In case of h\'dr~uJic generators 4 suddm lOll. of I"ad results in ovcrspeeding ofrIw generator. This I> because Ih., waler flow 'I) Ihe lurbine cannol be stl'Pped orredueed Inst.mlly. c;."ootraUy a IU~ IS provided 10 pr~~1 Ille OVCSlxoedillJ;.8111if th.", is any 14ull in Ihe II"bin~ SO\'M>O( th"n the dangerous overspC<.'dingmayIo,k. place. H~ it IS oecessary 10 su~ the working 01 nut"". governor and.,k. some corrective measures ;r there is >OInt' fault in the governor

\ dille hence COI'\tln\.KlU~ove....I,~dR 0( lesa. value thur\ Ih~ tM.'1t;ng cannot bf· ....·nh('" byO\~n:urn!fltprotecuon

Generltor ProtectkH1PYotlellon Incl Sw1tcha-Ir

5.2.3.7Cooling SYlltm FailureTIw 1.11".. 01 roollng Sy5tfm .Iso c&u>cs severe o" erheoling to nS<' the

k'mper.lure above .. r. limit. This may 1".><110 in8ul.Hon r.ilu..." ...."sinS some otherfaults 10 occur. Tbe thermocouples .,.. r<S1SWlce thermometers are used in ia'll"IIt.lChint:s to sense the temperature. The coctf(tlVe measures 31'\" b\l_tn whenever ~t""'»('r'N" exceed. the limll

A~rt trem the ."'lYC domin~nl .b-.nol condit,on •. >OtnC (001<11110""may at>!whK'h Are rJI~ h, J-,racticc. Stich conditions are, Wn)11J; ~)'''l\Chronl.eatlon. \(,c..J

5.U.6 Loss of Excita1lonThe ~ or •• citalil>nOr reduced ..obtion is possible due 10 the field failure I~

"I,""ng 0( lield winding or due 10 shorl circuil in Odd or due I" some (.ull ,nt.,"'\t,ltl'f ~Y"\'m.

Such loot Ilr ,')(d"'lloII '.8,,11:. in 1_ 01 .)nchrolll.n, wlthin ., .<CURd and IIu>eousn 1M Inere""" in speed of Ih~ g....... tor, SilK.. power tnput to the m",:h"",remains same. the generalor starts working as an induction generator, drawing 1Mreactive power from the bus. The machinr starts drawing an exC'il{ngcurrent from the5}__ which is equal to lhe (uD load r..ted value. This leads to Ill<!overheating (}(the .tal<>< winding and the rotor bod>' due 10 induced currents, The I""" of c>(.labOO1TI4)'.l"" tead to the pole sUpping condItion whtch resuits in Ih. voltage rNII(tlnn forthe ""!pul ebovc half 'N;> ".Ied load

Loss of excttation should not pet5ist (or long and corrective measures li:kedlscnnnection 01 alternator should be taken immediately. For this • tripping schemecan be used which can trip the generator circult breaker immediately when t~ is ~~Id(.ilure.

5.U.$ flilure of Prime MOYerThe failure 01 prime mover results in motoring operation of synchronous

gcncmuor. The generator draws active power from the network and continues to runat .ynduonous speed as " synchronous motor This moy lead to dangerous rnedw\Jc,31condibOnS if 3UO,_.,'ed to persist (or more! th.ln twe."t)' SL-C()llcis. Thc scrlous ()\"l-rhColh_ng

0( .t... "Nm turbine bl.d"" may result. To prevent this Ih,· reverse power protl'Ct1on0ch''''N by dlrectio ....ll power relays is used,

I"" uses 01 modem <irtuit breakers R-C surge suppressors abo help in ~uangswildting surges. Anol"", situation. whm 1M I1lInsienlovervohages are g.""rated. iswhm the .res a", groundl-d. During amng grounds. Ihe tr.n$ienl voltages ha,'ing.R\plitudes IIvo ~",<!'lm".... Ihnn lhe _I line 1(' neu"JI P'·.k ompliluM .,"'gt'Tltr~h.'CISuch tr~nsl('nt voltages .1\' d""llerou.. ""d C.lI' .... reduc.....;1 b)' u<i11j\re5dW>CC •• rthlng.

5·5

Fig. 5.2

____ I- ...__ - __ ...__ ,

_lIal.... y

I,"'"

,/ ~ I,V F!: V

"- - - - . I,• ,

,---'j .......... ,, ,. .,...., ,

fig. 5.1Sup!"'" the ('\I1Tt'I1t ) nUW8 thtough the primor)' 01 C.T, I\) IIw externol f."lt If

tlt4" I,v" C.1'.4 av"ot' ~,rrwr..uo, them no CUrt"·,,t will now chrouSh the 1'('1:'t)',,\d II",mains mOP<''''tiv<,.

Bul now if an internal fault 0Ct'UJ'$ at point X as shown in the Fig. 5.2. the currentflows through me fault from both the sides. The primary CUJn'[l1s are I, and I, whiJf,h,· secondary currents are i, and it. 50 the current flowing ~h the "'lay will be11 .. It- E\Cf' some CUtTentnowing out 01 one side while a urge current entering the<>Ihe'"de w'" rA""" til<' di(IN.nU.1 current Such A current Is _poru!iblc 10 t'l"'t~lett\~,...lav

Gerw .....~. ~ C T. AI

t IV IV , &t!n.,""'.

......... t, ,, ,, ,, ,, ,~---:--_, .

A b",ic d,rferen~1 protection schI..",., used lor the S""""'tors is shown in theFi~5.1 It Is known Itt.t Ihc dlff.rmtbl I't'ay operates when tt... phosor diff.rerta of,\,~ or more similar eie<"t'tcal quatltities ftCe(Ids a pl'cdctermi.ned value,

5.3 Basic Differentlal Protection Scheme for Generators

",.rhe"''''8, I•• luge in hydrogen circuli, mouture in the &"""r.1O<winding, oxys~n IIIpure water cucun, \ibraHons, bearing eurrents, exeesstve bearing leml"',atute ere,

<O<In ... IO' Prottc:tJon5·eProt.c:tlon Ind SwilchllM'

R.ttf.lnlng 001

Fig. 5.3 ,.. ree nt.ge dltf_tlal protecllon sclMme

The f.ult occurs at point X and the primary currents in the Olt'Uit are I, and I,.The C.T. secondary rum.'I'Its are I, and I,. TN> current flOWing tluough the operatingroil of the ""I.), I, I, - I" WhU" rlw current Rowing through the rOSlIi1ining coli of tlw

...J.y i5 C' ; i,) .Th .. i. bee••UI!C the ~ting coil I. oonnected to the midpoint 01

th~restraining coil.

Thus if the num~ of turns of the restraining coil Is N then i. flows through ~

d n N " N I)N." I, Ow~ through remalninll Z·The IOUI .mp.rc IUITl$ ..... T +"2- I.e. N

I, CT. ,/ C.T .,".... "....

V

F:':v

" , .... - ... - ... -_ ... _ ...."

, •, I •, Operating, 1,-,> I coil, •I-I -I1 '_/ ________ '_'" ........... rotoy

5.4 Baalc Percentage Differential Protection Scheme for GenoratorsThj~ protect!oo schtome is atso (.11<'<1b,ased dllferrnUl1 p_ion scheme. TN>

FiS"5.3 shows tlw connections 01 the percentage dlfle1'C11ti.1 relo)', in such. prolL',:tionscheme.

Thus tht ~l.y CUrrC'l1II. prop"rbOnili to the ph.J!Or d,JI_ bet",,,,,, Ito..currollls ""tering and "'.vinS the prol\'(1('d orcutt and 'f 1M dilferential CUrrC'I1t.,.",<.'<.'<15 the pickup value, the rcl.y operate.

This b•• ic differential $Chemehas loIl~'ing drsadvantages .t This ~itcuitoper.tes In<lccuratelywith helvy external I.utb-

2 The C T.. may saturate "lid "US<' w""1"nl $('C"ond.ry (,\UTents and th<­d;!fcrcll«' 01 _ondary currents _y approach the podcup value 10op;:ratc therelay un~rily.

Th""" dis"dvant~ ore overcome in the percentage dilrercnWI reLly.

o.ne,.'tor ProtoCi1onProtecllon and SwitehilMf

t.CI U.) dl~u)'i lhe "t,'I,11I9()f M"on·Pnn: prol4."('th.,l' Khe~ Illr ti't' 1),r4,,,,,·ph~.ltern>lor .1.tOr Wlnd",ss,5.5 Men·Price Protection of Alternator Stator Windings

ThIS IS most commonly used protedlon scheme lor the al12mafor stator windings.Thc scheme is al<o called biased dilf""",tt.ll protecuon and pe<ttntagc dlffcl'\'lltialpeorecrlcn.

I" Ihl. ,,"'11\0<1,th,< curr"ntil "' t.... two ....d. of Ihe p ...,t«I«I seelIon .f\! ~.....'<I"slOg currem traroslorrnel'l>.11,. wires CON1«1lng relay coils to Ihe current "",*former""",,,,d.uil'S .1'" calltd pilOt wires.

Under norm.aJ condItiOns. when there is no ({lull ill the- W\ndings. the currents inlhe pilot wires led (rom C.T. secondarl ... ore equal. The diCt\ormhal current I. - I,through the opcnbng colls of the ..... Y IS ZA.'rO. H.'no: Ihe ",by i. lncperativc and~)-stcmis said to be- balanced.

\\'hen fault 0CX\1J'$ inside the pf'Oll!Cted section of the .totor windings, thedilferential current il - i1flows through the operating coils 0( 1M eclay. Due 10 thisCUTn'nl,the "'lay Clpft'ates, This trips t.... &mentor circuit brnker to isolate the nultysection. The field IS ,~, dlscoenected and is dlleharged Ihrough • suitable impC!dancw.

The Fig. S5 .00",< • sclwmaUc .rrongcmt'lll of MCr7·Pri« prOC«tiOIlscheme lor a.)tllr connected altern.lIOr

FIg. 5.4

2

-.._I

~c:u ...ntThe Opl'r.llting characteristic:s

of .u<h a b,.S<"d differential~l;,).lS shown in the> Fig, 5.4,The (h~'r;,cte">lieot ~\.\18 t),,,tt,c.:pr III 10\0,1 currents, rano ofdll(e-rentiJ11oper.:lng current to." t'mgc rcstrainmg C'Um"nt is .1

h"'" per«nt.lj\e H,,",,c the",I ••) i, e••lled II.... I"",,·olltog.dlfr""'nt",l rei.)'

This bask percentagedi({e,:,~ntialprctection schemeforms the NsIS of thepr.lctic.lly very commonty usedper(,'II"'gc difft'l'""tt.l1 prolectioll lICh<me lor alternator St.l.Ot wlooing_. This popuLlr"""nw i. known d> BLI...-d di(lercnll.ll pl'OU.'<tlonor M..."·Pri,,, prot,'Ction

l"" ~ I, ) This is ... It',.,.j os the flow 01 CUl'Ttflt ("' ; I, ) through the en tire res ,r.Inlng

(0.1

o.n.,.,o, P,_'on$.,Protoelion and Swilel>geor

I, , ~,-""-IftyFIg. 5.7 R•• trlcted _ fault pro~tIon

Consider IMI earth fault occurs on p~ 8 duo to breakdown of its insulation 10•• rUt. •• shown In the Fig 5.7. Th. fAult CWTtml I,will now Ihroul!h the core. I•...".,

8

y

m • rK'\_ .... _J~. • • ..~• I-N

--'rx-,. • V.. I r'm.0 - • I

--l~'- .~../" I,...Jv

I, ' ~ Ear1h faM.;1t •• - I---_ ..... __ .......

~1ingcoil

I l/-: I,"

Eomog ............. nce ~.""': I-

*_ ...... _ ....__ ...... ,•C. T.

C.'tll... lIv MM'1·l'rice prutl'Cllon b.,!Ied on ClrrulAllIlS eur .....nt prln<lplc provid .....thrprutll('tu,)f1 dgltln.s.. int"n'dl e.lrtl, rdult~. But 1« large scnl'rntor'S, liS Ih<.'8c J ......('0).11\, an~cLb0r\.i11pml'("("liol' !i(.:hclne c..Uud restricted .....rth (:a'IJI rrot'''('CIOI'l I~ provided

"'hen the neutral IS .olldly grounded then the generator gets completely protectedagamst earth faults, But when neutral is grounded through earth resistance, t~ the."'tor windings gel p.rtl~' protected against earth faullS. The p.1t,:."'tagc of wmdingrl'Ok<b.'<i depends on Ih. vdlue of •• rthing resistance and the relay setting.

In this scheme. tho "31". of eurth reslstal1(\'. relay set ling. curr~nt rating o( ""rth~nce must be co",·tullyselccted. The earth (Aul", are ra",' near the ne"tr.1 poot! .,t.I'w ~\Jibge of neutra1 ptlint with respect IQ ean" is vef)' less. But when eanh fauttoccu .... ", ... Ih< nculTal poinl lhen the insuffocienl voltage 0<1'0&$th. fault drives veryI"" fault current than the pick up current 01 ~Llyroil. Hen«' lhe relay coil """"ins",,,p.... nve Thus lS to 20'11.winding from the """lr.1 side rem.ins unprotected in this~ H.~ if Is <011<'<11\.",lrlcl<'<I•• rth I.ull prot('rlll'"' It I. usual pr.ffi(C' toprok'Cl as"" of the winding

The reslriclcd .,rth f~ull protection !iC~ ... hown In tl....Fill. 5.7.

5.6 Restricted Earth Fault Protection of Generator

4 It does not l'O."lu;rc (Urr<1I1 luR>iorm..... wuh air I:"P' or speclal b.llonang(""11'0.'5.

GeMrator ProtectJon5· II~ and Swlt<;hg, ..

for 10'\' ($blanc{' 1(, the Y(ill~ of R is o;;l:l(:clOO su(l\ lh,)t fuJI 100"tdC1lnmt pt\SSCSIh""'gh tlw ".UI"'. for a 1,,11line to .......' ...1voltage V.

In medium resistance R, the earth I.ult current Is limited 10 aboul lOOA lor lullhne II>neutral voltage v. lor" 60 MW machine.

In hlSh roslSlan(\! R, the •• rth fault cu"",nl Is IImilt'd to about 10 A. Tim I. u8<!d(or ddtribution tran(((lrmcrs l\nd 8'-'1"K"I"'".tor"'lr.,ns{ofrncr unlb.

Cc.lf'ISidcr IN- (',\rth!'<'SlSt4""" R used to limn earth(,ault C'Urn!Ot as shown in theFig. 5.8-

The Y.lu~ 0( the resblanceR lim!.. 1M earth I,ult cumml.

I( 1M""I.UI~ K I~vory.n'.U I.e. lhe neutr.1 I. almostsolidly grounded.tht-n the (aultcurrent is very high. BUI highFault currents are not desirableI"""", small R is not preferredlor the large machines.Fig. 5.8

/~R~ ~B

R

5.6.1 EtftK:t of Earth Resistance on % of Winding Unpt'otectedlei us sc:>e tll~cifl..'(tof earth ~nce on the w~o( ~ winding \-\'hkh I"emai~

u rtprol\."(t{'CI ..

0( m...:h,n,·to ~.nh ~nd C<l'np"'1~tM .,.lIh through 'h<- ~.'thinK """".nce. The C.T._dMY curret" I. flows through ~ operaling coli And I~ re-tncted .~rth Inulo",I.) (oil of the diff~t<"ti"1 prol«'t1on. 'The selting 01 restricto!d earth laulo relay and'!dIJng of overcurrent rol.1Yare indqlmdenl of each other. Under UlIS s«ondary("Ur~nt I., ~ rel.1Y upcr;l~ to trip the arcuit breaker. Tth: vcncgc Vt-, H ~\Ir(lcl('nttodnve Ih.." en'lugl1 (,\ult current II when the fAllit pornl ), ~ '\\\''')' (R)tl' tI,~n_~'ltral""lnl

If the f.1UItpoinl , is nearer ro tht- neutral point then the voltage Vb. is small and001 sufficienl to drive enough fault current I,. And for this I~ relay cannot operate.Thus pan 0( the \\'inding from the IWUtraJ point remains unprotected, To overcomethis. if rel,,)' St!tbng i.s chosen "ery 10'\\' to make it sensinve to low rault currents, then\\."-"'~ ()Ix·r.,tloo of rel••y 1'r\ny I"f'iUII 11\(' relay ('1\" opt'r01tco undt'r the conditions ()(he.\ Y through I~ult~. '""c(ur.'t~ C T s.... lur4hon (I( C.T> tIC Hl'1lt:\lpra<1kaUy IS% 01wtnd.ng (rolll Ih< "<,,tr.I' po.nt i> ~I unp"I1C<""1.pro'«linll tilt! ....""unong 8S'Y. oft"" wlndong ~&'I""I pho$~ tl) eonh 1.111t~.

G.n.rator ProteCtion5 ·12P,olKtion and Swltchgea,

'The un .... trictod •• rth (.ult prot..'CtlOn USO • ,,,,ldu.lIy connected eatth ('ult ®y.It """"ists of th ......C.T .s, one In e.,,,, phaSt'. 'The ."«"'0"')' windings of these CT ••• reco>M«ted In parao.l. n", earth (.ult rtloy i. ronn ected .,"", .. the second •.ri6 whichcom .. " res idu.1 current. The scheme .. ohown In the Fig. 5.9.

Wht-n th.... I. no 1.1I1t,under norm.1 conditions, vector sum o( the th_ li.wrurmt~ is zero, HMlCYthe vector sum o( the th"'" t!{'(.,)ndnl')'(urrmtlO b ."0 ,eto

5.7 Unrestricted Earth Fault Protection

•.4.7631 a Jr'''' (0. 1512)'

22.1>875 • ,2 _0.02281>

r • 22.6646

r = 4.76070

This is the earthing resistance required,

..

.. (2)

Z • r + i(rea,tln<r 0( unprotected winding)

Z • r+ J (0.1512) nIZI = Jr'+(O 15121!

Equ.Jting (I) and (2),

Now

.. (I)= 4.76310

• 200 AZ • tmpcdanee O(ltred to Ilw (~uh

v 952.6279·- .i 200

i lit Fault C\lrl"(.'f'Il

V e voltoS" indU«>d 11'1 unprotected winding

~ I':,' V s O.IS·6350.8S29

= 952.6279 V

= O.ISI2l)

• (<Y. of lIl1pl'O(eeoed WInding) , (Xl

15• 100,1.0063

Generator PrOlectlol'l5 ·15~tIol'1 and Switcng.ar

tn practlee for ..... 11 r.tins allemotcB. the neutral rnd. 01 the three p""- are(QIUl<.'Ctedto a single pornt. lienee it i._ pClI5SI'bleto intmducr C.T. in each phase onneutral side os tequim! in Mer.·Price pt'O«!Ction. In such cases, the balanced earthfault protection an be used.

5.8 Balanced Eal1h Fault Protection

i,•• i~,•I.... 0

The sum 01 tht- thr<'C C\lrr('T\'S it Midu.al currt"nt IK. whkh is lero und,,·, nC)C'1'11a1OOfldIII"ns.

TIle earth fault relay IS connected In sud'o • way that the reldual current Ilowsthrough the relay operating coil. UnO.!<._."..I condition, residual cur rent is zero so(('lay does not carry O1Ily current and is mopentive. However in presence of earth faultcu"di.lon, the bal.tnce gets disturbed and the residual current I.. is no more ?era. IIthiS eurrent is ~ ,han the pickup val"" 01 the •• tth f.ult relay, the r('lay ~all..'Sand opms the elrnnt b,,·.kcr through trlpping of Ih,' trip drcuit.

In the scheme .shoW" In the flig. 5.9, the ~rthfault at ony location near or awayfrom the loc.llon of C.T." con cause the reidual current, H........ he protected :rone bnot definite. Such. scheme is hence callod unrestricted earth f.u1t proIeclion.

Rg.$.9Unrestrlc:l8d .-Ih fault p_So if I", I,., and I.., are C.T. secondary currents then under normal conditions we

an write,

.,...... ~

[..L-,_ IRa -'" 'o"',,.c""')

L : To .,,,,,"",.

e

y

R• • ,.."....

• • IV• • -'",• •N • • "'"• • IV• • -'"• •• , rx»:

R v• ,... _------- -'""-=- ------- '...- , ,-

C.TGGn.,"Qf

,tatOt winding

Gen. rotor Protoctl ....Protection and SwltchoN<

0\ cocphng tr;,)J'I~()rmcr15connected betwt"en tM earll, .lnd the (·.l_rthi~~RIC' In neutral 10 g:rollttfO.lcircuit. The primary or the coupling trtlJ\S(omler is e--'Citcodt>~ceded ~igll"lcurrenr source. Th.b <XJded signa} current has a fn.~lu(,:,\C'y0( J2..~HLTtus current is connnuously injected into the generator stator winding through ~sec,.,dary 01 the coupling transformer.

During the no"".1 eondinon the sigW current in;c.:ted inlO the stator flowsIhrough stray upaotAn"" 01 the S,onenlor and dlr<'<'Ily coonedlod system, Bul whon... rth I"utt 0«"". Ilw ",,'y copadt.1'ICt' Is b~. 'l'hl. 1~4s<)' Ih. ltl(lt'UtOI'lngcumont Thl, Inor.... I. ml'osured by • tnt'"ufinll device. Ot!p<'T1dlng upon thl$me_rement nn Imm,odi••tc rortt'(tlve ocbon ISuken.

nus scheme gtves the p,Oll'CNon 01 IS to 20% or st~tor winding from lhe neutr~1.-...J~. lhe portion which Is unprote<'led by Mer.-l'rlce pn,t«tion. The remalfling!",,1I01l 15 proteeted by Men-Price p~ Overall 100':1>o( .Mlnr wind'"l! gets~cd dgnitlst earth 1,lulls.

Fig. 5.11 10C1'1.ntth fault prolKllon

II

R

- -,,- ------t-----

A. seen uptil! now, no prolecticln 5Cheme is In n position 10 give <empkoterrot«hon to Ih~ ,IA'M of generator '&ainsl earth (auIIS. The maximum p_l .....>d>~ ed ,$ uPI(! !IS 10 'lO')" lrom lh~~ dltcus.ocd "rtill now.

801 tn modrm dllY8 II Is ","",'bl e 1(1 provld~ 100% "Mlh (,1\,11prol«IW .. to thestalOf of IIw &fnCr~lor. II USI'S ~ coupling IT.\nslormrr and 1m' coded sign.1 current,the .chem. i. shown," the Fig. S.lI

5.9 100% Earth Fault Protection

Gener.tor P_1ion5·1.ProtlKUon and Switchgeor

Fig. 5.12 Intortum fault prolfCllon

C.T St

+---__ 8

s,C.T.

~ v

C.T.CT

C.T.

The: Men·Pntt pn)tC<'tIOJ'l system gi\e5 prorectlon a~aj'''$t ph.. ..se to pha~ fa\llt»and ea"" faults. It does not give prote<tion .pinst Intertum foults. TIle inferrum (aullis a ""on circuli betw_ the tums 01 the same phase windUlg. Thus the currentptodua.-d dU4! co &ud\ fAult Is u locat oC"t"Uit eurren and " dOt..~not .,ffeet th~CUrrl'1"ltscnt<rinf( .nd leaving the winding al the two ends, wh('r'(' eToS are loesred. H<onccM.'r~·Prke protection C3nnot give protedlOn a&""'51Interturn f.ults.

In single tum generalor, there is no question of interrurn faults but in multitumgenerators, the intertum (ault protection is necessary. So such interrum protection isprovided for multitum generators such as hydroelectric generators These gener.torsM' e double Winding armatures. Thi> means, each phose wi"ding is dlvidl'd into twoh..I,,,,. due 10 the 'ery he,IVY currents which Ihey hDYl' to catry Thl, .plilhl1g of'InSIe PMOCwinding lilto two Is ortYant.lgoou, In p"ovldlng intftnlm (aull protectimh.I .urn hydrwlec1tk ttClwrJIol'$.

TIle fig. 5.12 shows the intenuen f.u1t proloction scheme uS«! (or the 80"" .. torWith double winding armatures.

5.10 Stator Protection Agaln.t Interlum Fault.

G4tn.,.to( Protections .,.p,OIecll ..... nd Switcll9ttar

Tha_. rotor circuit ,'If the alternator k not eilrttwd and d.c, volta8~ i, Imposed on it.And bence single ground Cault in rotor does not cause circul.ting current to flowthrough the rotor (i""uiL Hence single ground mult in rotor does nol caUS<!anydarNjte to it. ilul .ingle ground r.ult QI15<$ an tncrease in tht> stress to ground ill\lther poilU!. in the fidtf Winding whm voI~ .re induced in the rotor due 10tnns.£l1b. Th"s the prob.b,lity of 5«'Ond ground foult inc reases

lf the second ground f.ult oceues then pitn of the rotor winding is bypassed andthe currents in tht remaining portion increase abruptly. This causes the unbatanee 01""'* circuit and hen", the mechanical and tht-rrnal stresses on the rotor. Due to t:his,rotor ma), gel damaged. Sometimes dama~ of bearings and bending o( rotor shaftbkcs place due to the vibr.lions. Hen« the rotor must be protected ugainsl earthC.ult

Method 1 : In Ihis m"thoel • hlJlh ~ioI~ I. ",n",'('t,'<I ac""," th" rotor nl'CU1l ItIs pfO\'lded with cenrre top and the ct11tre tap point 15 connected to th~ sn>undt!\rough a sensltlve eann (oult relny lISohown in the Fig. 5.13.

5.11 Rotor Earth Fault Protoctlon

The scheme u..." <r,,,.' differenlinl ptmC'lple. Eoch ph ... (If the gene.oh" Is doubly"ounJ and split into two parts S, and St as shown in the Fig. 5.12. The currenttnnsfcrmors arc CMnl,(tcd in the two parallel paths of th~ •• ,h phase windins- The~"t'Ond...til'S (Ii lhc.~(1.lrree" 'tDns(ormrrs are ("I"Q!IS conntC'ttd. The current tr"n,-f('lln:nft'5,'vor~ on drc\ll~llnR cur",n! principle The relay Is connecl~ """"'" the ~4."OnJl«tc."ti $e('(lod.trll·s or ",c curn. ..,' trd&Wrmt..,..

Under Ilor,"al 0lx·,.ltUlS ~'On\.1JtiQOS, ~'henthe t\'\'() patlls 3n_-'!tOlll\d tht-.., Nrmltsm tht> two parallel paths S, and S, ore equal Hence cur","'" in the ~'COnd.n..s qf tht>current transformers are also equal. TI-.e serondary current Rows round th. loop andIS <.l.mt' at oil the points. Hence no current Dows through the relay and the """y is""""""b"e.

II tht' short circUli b developed ben.-""" the adjacent tum. 01 the part 5, 01 the",,,,,lang say 'hon (\",,'nt, Ihrough 5, and S, no longer ,,'main >-,me Thus Itnl.~J

(\Jrrt"Ob \v111be i"du~N:f til 'he S4.'conda.rirrsof the current tr3nsfomltl'~. TN> diftM--rw:e01 theie currents flows through the relay R. Relay then dOS<'S its contacts to trip thedrewl breaker which isolates the gonerator from the system.

~ an intertum (."It protection 5)~ten\ ill extremely sensitive bUI it can bearplo.-.l I" th" g.:ncr.lm" h,'''ing doubly W(lund arm.h,res

o.n ... tor ProQction5-20Pnrtectlon and Swltchg."

-lCFill. 5.15 los. of udlatlon ch.roc~

Ihe "'1ui\·.~t &_,.Ior impedance locus trace... path from first quadrant or R·Xd,agro.m 10 the lourth quadrant. The dlSlancz relay Is used which covers the portion 01the fourth quadrant wbere impedance locus path exists. Thus when the impedancetakes value in the region covered by the relay characteristics, the relay operates. Tht.­rel.y operates whtn generator (irsl ."''''' to sUp poles, Then "Lay trlp$ Ihe (itld ciKUilbreaker And II ctl"l(ON'U'<1' th~ gen... tor from tl\(' 8)'l'lem, too """on the c,cJtJhon ISn.'S.'I""'od dud b«ome normnl, II,c S~.tor can thOI\ be rettlmed to ICrvlc.'(' 11\l\tAntJy.

'YO--

5,12Protection Against Loss of ExcltlltlonThe los~ or ''':'~'''on or the lI""""'ato<~y tesult in the """ or synchronl>m ,.00

:-1,~t"ly illcrca...e In .he 8,·ner"t'or speed, "Jlw machinto starts bMaVlng as an illductlongt'llt?r.lfQr. It ~Jr.Jwnfe.)ctivf" power from the system which is undesirAble. The 106$ of.,cilation may lead '0 the pole slipping condJtion, Hence ptoh!Ction .goins' 1_ 0{e.<itation must be provided,

Ihe prot,,<tion L< pro\~ded ulling dim:tlonaI distnnce ty~m.)' with the gentrotor",rmlna'"

\\-thell the-re ts. ~s of excitation, thr equivalent gent-rator impedan~ varits AndI,.,,,,, a curve as shown in the Fig. 5.1S. This Fig. 5.15 shows the loss of excibotioncharacteristics alortg',;th the relay operating characteristics, on R·X diagram.

Ihe eJrlh flull ,d~y$"re inslanta_ in opt'<>lIon and are ",n"..-t,-d 10 on.I.rm cireutt (or iOOkdtion and to take the proper acuon, ThIs is because, • singleground (a"lt does 001 require lin inunedia~ action of isolating the generator.

o.n .. olo, Prot.c:tlon5·22Ptot.,llon and Swllchgt ....

Fig. 5,16 Nog.dve pI\u. sequ.".,. .. ley

A

5.13 Negative Sequence Relay.Tho! negnlh'" ~1.)'1o "''' stso called phase unb.II,,,,,,' ,.c1ay> bee••usc these <cl.ys

provide protection .gainst negative sequon<e component 01 unb3Ianc:ed cum!flts.~istin!l due to unbolanred loads or pn.....pN<e l.lIln., Th,· unboL>l\ced cUlTCllts .....d~n&et\)\_LSlroln Gener••tors ."In'limotors pool"t 01 view ns th,,~ olrrC!nb C"....ln C.'US('o\'erhc'lms. Neg.thc sequence relays .. " &<....,r.lly u",'<.1 10 gIve protO<'tion COgl"l\("rators ...nd morers 3g<1in)tunba'Jf\C'~ currents,

A negative sequence relay has. filter circuit which is operative only for negative5<l<Juenc:<! cemponents, Low order of Over current also call cause dangerous situationshence e negative sequence reilly hos low current scrrlngs. 11'1"earth relay providesprot«t.on lor ph.5t> In .orth f,ul! but not lor phose to ph.lst> fault. A ""!t,live"'<Iu"1IO!",I.y pmvid .....protection 81;.;""1 """"" to ph"." inult. whod! .'" rl'Spons.bleto produw 11(1Iollv('S<"<luNWCc'O.npO~'I\b

llw FIS- 5.16 .bt.",') tl~ &chematlc ~rr.lng~"fnl.'f'I'01 ,t,"gl\ll\'l' phaK ~"C(\I~flC't'r~14y

Genttllor Prot.etion5·23P_ion Ind Swltthv-ar

Flg. 5.22 Induction type n~lv. s.quence relay11", eentral limb 01 ul'l'''' ""goet camtS 1M primary whkh h,•••• ""'" lOp Dee

10 Ihis, Ihe primary winding has Ihree Inmlnal I. 2, and J. 1'IIe section )·2 IS,"'erglztd from .he s<.'('ondnryof an ou>lJ.i.uytr.nslormer 10 R·phase. llle section 2·3IS diroctly energtud from the Y-phase currenl

The .uxillory Ir.nsfonncr is • sp«iol device havln)\ an air gap In ilJl INgneticcln:uJl Wllh the twlp 01 Ihls, the ph•se Ingle between tis prirn.,ry ~nd se<und.ry esn

..-,.~- -...-\0

~ I"~:.--r,&4

Ft~

F~

E ':! - I l-I '.1 ~ ... .-t...._ ...

lJ _

i- = -"-~Wl~

••

5.13.1 Induction Ty.,. Negative Sequence RolAYAnQlhcr mmnu",') u.....t "~8.,tlv~""'I"""'" ",I.y '0 Inducli"n .ype I.. C'OI\Mruct"'"

b, sumlar 10 Ihal of Induction type 0'''- currml ,..13Y.The sch."",b< diagram 01 IhbI)~of n.'lay ts shown in 1M "ig. 5.22.

FIg. 5.21 0.'1. conn..:lJon01C.T.•

To nl4kc the n.-lAy M-oru.llive to onl)·neg.tive sequence currents b)' making IIlJ'lOf'f!mtive under the in_nuencc 0# zerosequence currcms i5 possilile bvconn«tins lhe cum.ont tmM(ONnC'J' tnd"'~ as shown In Itw Fig 5.21. Und~rdelta connection of cut'r"ent trnns-f()m\(,N~no zero SCiillencecurrent can (10\" in th~nen.-ork.

y C T

I7r'y C T

nr

1c. T.

I

a.rwr.'or Prot..:,"",$·21Protection.nd Swll~

% S • pe'''''''lage asymmetry

I,.. ~ N~<Jtive sequ~ CUf'm\t

I iIif R._'tl.~currt!f1t

~xlOOI

~I 5 •

Fiy. 5.2S Induction typo ~ and •• ro Hqwnc... lay5.14 Protection Against Unba.lanced Loading

\\'twl\ II,,) load on the geocraror bt;:a).l1e5 unbalaneect, neg3tivc.· phase sequencecurrents flow. Tile ncg.,li". sequence COInpcl<lellls produce a rotating magnetic freldwhich rotates at syncl\tonous speed in • d.itection opposite to the direction of rotorfield. Hence effectively the relative speed betwoen the two is double the synchronous'f'l,,'<l Til". double (""Iu<ncy currents are induced fn the rotor. These currents co_S<'\'ere h••'tlng ,,( the ",lor and can ClI_ d&mllgc to lilt rotor. Ttw unbalanced ~IalorrurnY1t~ nbo C'rtUM.')l'vttrc vibr.ttiOfl.!fAnd hc':o1tins of Amlor. tlenc~ It Iii nccl'Ssary topro. Ide Ihe IIcl(ah't !l<'<jU""","prolection to the ll<>neratoNiag~ln't the unbal.1l1Cl'dto;,d conditinl'ls.

The neg. live ph.", sequence fill.... a""'gw1th the OVtrcurmtt reloy pmvides the""""""ry P'lltl'(tion .'g~il'\St the unbaI.J11Cl'dIo;r.da.

Tht· ",flltlvl' .symmetry 01 • three pNSf: gcrx....lor '" Sivj!Rby tor ",tin ,,( negative><'1lU'1Wrcurrent 10 th,' r.ned current, M.thttnatlc.llly It C.1" be ~p,,_d M.

- ••

J ~ --.1=

E M EI

~IT c--

-r-'

1= '- ,

The schematic arrangement of indudkln type neg.ti\·~and zero sequence reI.y 1$

shown in the I'ig. 5.25

Gentrator ProlKtlo<lProtection and Swllchvear