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Busbar protect ionby L.C.W.Frerk

13.1 His tory of the deve lopm ent o f busbar protect ionU p to t h e m id 1 9 3 0 s , n o w id es ca l e e f fo r t s h ad b een m ad e to p r o t ec t b u s b a r s o n aun i t bas i s . A lso there was r e luc tance in a r r ang ing one p ro tec t ive equ ipmen t to causes imu l t an eo u s t r ip p in g o f a la rg e n u m b er o f c i r cu it s .

Befo re the Br i t i sh Gr id Sys tem was bu i l t in the ear ly 1930s , many under tak ingsran i so la ted f rom ad jacen t ones , and so the power ava i lab le fo r busbar f au l t s waso f ten r e la t ive ly smal l , and damage due to these f au l t s was genera l ly no t ex tens ive .

B y th e l a te 1 9 3 0 s , t h e B r it is h P o w er S y s t ems w er e ex ten s iv e ly i n t e r co n n ec ted ,wi th a consequen t increase in f au l t power .

A n u m b er o f b u s b a r f au l ts o ccu r r ed ab o u t t h is t ime , b u t d u e to th e i r r e la t iv e lys low c learance f rom the sy s tem b y ov ercu r ren t an d ear th - fau l t r e lays , cons iderab ledamage resu l ted , espec ia l ly in indoor s ta t ions .

These f au l t s l ed to e f fo r t s be ing m ade to p rodu ce busbar p ro tec t i on in such af o r m th a t i t co u ld b e w id e ly ap p l i ed w i th o u t i ts e l f b e in g a f u r th e r h aza r d to t h es y s t e m .

C o n s t r u c t io n o f t h e B r it is h 2 7 5 k V s u p e rg r id s y s t em b eg an in ab o u t 1 9 5 3 , b yw h ich t ime s t an d a r d p r in c ip le s o f b u s b a r p r o t e c t io n h ad b een ad o p ted f o r o u td o o rswi tchgear a t the h igher vo l tages . A t th i s t ime the emphas is was p laced on theav o id an ce o f u n w an ted o p e r a t io n s i n o r d e r t o g iv e m ax im u m s ecu r i ty o f s u p p ly .W i th th e i n t r o d u c t io n o f 4 0 0 k V s u b s t a t i o n s i n t h e 1 9 6 0 s , t h e t r an s i en t s t ab i l i t yo f g en e r a to r s b ecame th e mo r e imp o r t an t co n s id e r a t io n an d th i s l ed to a ch an g e o femphas is so tha t f as t opera t ing t imes and r e l iab le opera t ion wou ld be ob ta ined fo r af au lt o ccu r r in g w i th in t h e p r o t ec t ed zo n e , w h ich in t h i s cas e w o u ld b e th e b u s b a rsand swi tchgear .13 .2 Gen era l cons id era t ions

13 .2 .1 Th e bas ic ph i loso phy o f busba r protect ionBusbar p ro te c t ion is tha t co m bina t ion o f c .t . s and r e lays w h ich a re used to de tec t


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82 Busbarprotectionfaults occurring within the busbar zone of a busbar substation and which initiatestripping of all those circuit breakers, the opening of which is necessary to isolatethese faults. In Fig. 13.2.1A which shows a double busbar substation with examplesof typical circuits, the dotted line encloses the busbar zone but for simplicity c.t.sare not shown. Where unit protection is employed there may be one or more zonesof busbar protection within the overall zone shown in Fig. 13.2.1A, the boundaryof each being the circuit breakers (or the c.t.s thereof) connecting that zone to thetransmission system, generators, transformers and to other zones of busbarprotection. The circuit breakers themselves are included in the respective zones sothat a fault within a circuit breaker causes operation of the busbar protection. Thisensures interruption of all infeeds of fault current in the same way as for faults

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Generator

F i g . 13 .2 .1A A double busbar substation showing {dotted) the zone covered by busbarprotection

occurring on the busbars themselves or on bus selector disconnectors, busbarsection disconnectors and the connections between these items.

Although the protection of other zones of switchgear, such as the corners ofmesh substations, often uses similar principles they are not considered as busbarprotection within the scope of this chapter.


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Busbar prote ction 831 3 . 2 . 2 E a r t h -f a u lt p r o t e c t i o n v e r su s p h a s e a n d e a r t h -f a u l t p r o t e c t i o nI n som e subs t a t i ons d i vi d ing wa l ls , wh i c h m a y be pa r t o f the bu i ld i ng s tr uc t u r e f o rindoor swi tchgear , o r s t ee l p l a t e s sepa ra te the th ree phases one f rom anothe r . Ino t he r de si gns o f swi t chge a r, e a c h pha se c on duc t o r is supp o r t e d w i t h i n a n e a rt he dtubula r chamber . Such ins ta l l a t ions a re commonly re fe r red to a s 'phase -segrega tedswi t c hge ar ' a nd p r ov i de d t ha t t he s e g re ga ti on is m a i n t a ine d t h r o ug ho u t the busba rzone , i t fo l lows tha t a l l na tu ra l ly occur r ing busbar fau l t s mus t be be tween one o rm ore phases and ea r th . Th i s be ing the case , the n , fo r lower vo l t age sys tems thew hole ins t a l la t ion need on ly be p ro tec ted aga ins t ea r th fau l t s . Three -phase fau l t s onphase-segregated sw i tchgear ca n, h ow eve r , s ti ll occu r i f the bus bars are energised ine r r o r w i t h s a f e t y e a r t h s c onne c t e d . I n t he se c i r c um s t a nc e s t he sum o f t he f a u l tcur ren t in the th ree phases (which is the ne t cur ren t f low ing to ea r th ) w ou ld besmal l and un l ike ly to opera te the re l ay(s ) o f an ea r th - fau l t p ro tec t ive sys tem. I t i sfo r th i s reason tha t phase and ea r th - fau l t p ro tec t ion i s app l i ed to phase -segrega tedm eta lc lad swi tchgear on the supergr id sys tem . A t lower vo l tages the cho ice wi l l bein f luenced by the add i t iona l cos t o f the phase and ea r th - fau l t p ro tec t ive sys tems.

Wi th open t e rmina l swi tchgear i t was o r ig ina l ly cons ide red adequa te to p ro tec taga ins t ea r th fau l t s on ly , because exper i ence has shown tha t mos t fau l t s on busbarsand swi tchgear had invo lved ea r th . T he con sequen ces o f re ly ing on back -up pro-t ec t ion to c l ea r the few phase fau l t s tha t d id occur were , however , su f f i c i en t lyd r a s t i c t o c a use a c ha nge o f po l i c y a nd f o r m a ny ye a r s now busba r p r o t e c t i onrespon ding to phase - to .phase fau l ts a s we l l a s ea r th - fau lt s has been rega rded asessen ti a l. The on ly ex cep t ion s occu r a t tho se 33 kV , 11 kV and 6~5 kV su bs ta t ionsw here the jus t i f i ca t ion fo r p rov id ing busb ar p ro tec t io n is m arg ina l and the s imp les tposs ib le sys tem is usua l ly f i t t ed .

Non-phase -segrega ted meta lc l ad swi tchgear shou ld , fo r obv ious reasons , a l so bep r ov i de d wi t h busba r p r o t e c t i on wh i c h wi l l ope r a t e f o r pha se f a u l t s a nd e a r t hfaul t s .

The method of ach iev ing ea r th - fau l t on ly o r phase - and ea r th - fau l t p ro tec t ion i sdesc r ibed l a t e r on in th i s Ch apte r .

13 .3 The c learance o f busb ar fau l t s by no n-u n i t c i rcu i t p r o t e c t i o n1 3 . 3 . 1 B a c k - u p o v e r c u r r e n t a n d e a r t h -f a u l t r e l a y sF or the reasons g iven in Ch apte r 8 , fau l t s cl ea red by ba ck-up ov ercur ren t and ea r th -fau l t p ro tec t ion wi l l usua l ly be nondi sc r imina t ive , caus ing unnecessa ry sp l i t t i ng o ft he sy s t e m w h i c h m a y le ave som e se c ti ons w i t h in su f fi c ie n t ge ne r a t ion t o m e e t t heload , l ead ing to widespread load shedding by vo l tage reduc t ion an d qu i t e poss ib lyt he d i sc onne c t i on o f som e c onsum e r s .

Re l i ance on back-up pro tec t ion to c l ea r busbar fau l t s i s nowadays conf ined torad ia l sys tem s a t the lower d i s t r ibu t ion vo l tages o f 11 kV and 6-6 kV wh ere thee c onom i c s a re suc h t h a t e ve n t he s im p l e s t f o r m o f de d i c a te d busba r p r o t e c t i on c a n


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84 8usbar protect ionr a re l y b e j u s t i f ie d . W h e re p e r m i t t e d b y t h e s u b s t a t i o n c o n f i g u r a t i o n , so m e i m p r o v e -m e n t i n t h e s e c u r i t y o f s u p p l ie s ca n b e a c h i ev e d b y d e s i g n in g t h e p ro t e c t i o n s o t h a t ,a s a f i r s t s t e p , r e l a y s o p e ra t e t o t r i p a l i m i t e d n u mb e r o f c i r c u i t b r e a k e r s s o a s t os p li t t h e b u s b a r s i n t o s e c t i o n s , e a c h w i t h i t s o w n i n f e e d a n d a s a s e c o n d s t e p t o t r i pt h o s e i n f e e d s s ti ll c a r ry i n g f a u l t c u r r e n t a f t e r a t i m e < l e l a y o f , s a y , 0 4 s . W h e re amo re d i s c r i mi n a t i v e s c h e me i s n e c e s s a ry i t w i l l u s u a l l y b e o f t h e f r a me -e a r t h t y p edesc r ibed in Sec t ion 13 .4 .2 .13 .3 .2 Dis tance protectionF ro m C h a p t e r 9 i t w a s s e en t h a t f e e d e r s f i t t e d w i t h d i s t a n c e p ro t e c t i o n h a v e a t i mec h a ra c t e r i s t i c a s s h o w n i n F i g . 1 3 . 3 A . T h i s me a n s t h a t t h e d i s t a n c e p ro t e c t i o n f i t t e da t e n d A o f t h e f e e d e r A B w i l l , f o r a f a u l t F o n t h e b u s b a r s a t B , o p e n t h e c ir c u i tb r e a k e r a t A in t h e Z o n e 2 t i m e , w h i c h w i ll b e s e t b e t w e e n 0 -4 a n d 1 .0 s .

W h e re a l l f e e d e r s c o n n e c t e d t o a b u s b a r s t a t i o n a r e p ro t e c t e d b y 3 -z o n e d i s t a n c ep ro t e c t i o n , t h i s c a n , i n s o m e c ir c u m s t a n c e s , p ro v i d e a l i m i t e d d e g re e o f b u s b a rp r o t e c t i o n . I f , h o w e v e r , s o m e o f th e c i rc u i ts c o n n e c t e d t o t h e b u s b a rs c o n c e r n e da re g e n e ra t o r s , o n e mu s t c o n s i d e r h o w t h e s e w o u l d b e t r i p p e d fo r a b u s b a r f a u l t .T h e o n l y g e n e ra t o r p ro t e c t i v e s y s t e ms w h i c h o p e ra t e fo r f a u l t s o n t h e h i g h -v o l t a g ec i r c u i t b r e a k e r o r t h e r e mo t e s i d e t h e re o f a r e e a r t h - f a u l t a n d / o r o v e rc u r r e n t a n dn e g a ti v e p h a s e - s eq u e n c e p ro t e c t i o n . T h e s e r e l a y s w o u l d a ll b e u n a c c e p t a b l y sl o w inc l e a r i n g b u s b a r f a u l t s a n d , d u e t o t h e d e c r e m e n t o f t h e g e n e r a t i o n c o n t r i b u t i o n t ot h e f a u l t c u r r e n t , o v e rc u r r e n t a n d e a r t h - f a u l t r e l a y s ma y e v e n f a i l t o o p e ra t e a t a l l .F u r t h e r m o r e , t h e n e g at iv e p h a s e -s e q ue n c e p r o t e c t i o n o n l y o p e r a t e s fo r u n b a l a n c e dfa u l ts . T h e re fo re d i s t a n c e p ro t e c t i o n c a n n o t b e re li ed o n t o c le a r b u s b a r f a u l tsw h e re g e n e ra t i o n i s c o n n e c t e d t o t h e s a me b u s b a r s .

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( ~ = C i r c u it -b r e a k e r s t r i p p e d Z - Distance p r o t e c t i o n e q u i p m e n tF i g . 1 3 . 3 B Lim ited busbar prote ct ion by distance pro tect ion

F ig . 1 3 .3 B s h o w s h o w th e p r o t e c t io n o f b u s b a rs can b e a f f o r d ed b y d i s tan cep r o tec t io n w h e r e t h e f o r eg o in g co n s id e r a t io n s d o n o t ap p ly . H e r e t h e i n f eed tof au lt F o n b u s b a r B f r o m th e f o u r f eed e rs is i n t e r r u p ted b y th e o p e r a t io n o f t h e i rd i s t an ce p r o t ec t io n a t t h e r emo te en d s a t s u b s t a t i o n s A an d C .i t i s a l so to be no te d tha t the w ho le o f the bu sbar s a t s ta t ion B a re c leared byr emo te d i s t an ce p r o t ec t io n , a l t h o u g h i t i s o n ly n eces s a r y f o r t h e b u s s ec t io n , tw ofeeder c i r cu i t b reaker s and one t r ans fo rmer c i r cu i t b reaker to opera te to c learf au l t F .

On a c lose ly in te rcon nec ted sys tem such as the 132 kV B r it ish Gr id , wh ereth e r e a re a n u mb er o f s h o r t f eed e r s o f i n s u ff ic i en t l en g th to p e r mi t p r o t ec t in g b yd i s tan ce p r o t ec t io n , u n i t p r o t ec t io n h as t o b e u s ed . S u ch u n i t p r o t ec t io n r em a in ss tab le fo r busba r f au l t s , and so the bac k-up ove rcu r ren t r e lays , w i th the i r s lowopera t ing t ime , have to be r e l ied upon to c lear the f au l t .

B e t t e r d i s c r im in a t io n m ay b e p o ss ib l e b y a ls o f it t in g d i s t an ce p r o t ec t io n to b u scoup ler and sec t ion swi tches and poss ib ly genera to r s a t l a rgo dou b le busb ar sub -s ta t ions , bu t per fec t d i sc r imina t ion cou ld s t i l l no t be guaran teed under a l lc i r cu ms tan ces .F r o m th e f o reg o in g , i t is ap p a r en t t h a t n o n - u n i t t y p e s o f p r o t e c t io n s u ch a so v e r cu r r en t , e a r th f au l t an d d i s t an ce can n o t p e r f o r m ad eq u a te ly f o r b u s b a r f au l t s


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86 Busbarprotectiono n a l a r g e in t e r co n n ec ted s y s t em. C o n s eq u en t ly , u n i t f o r ms h av e b een d ev e lo p edto en s u r e s h o r t c l ear an ce t imes an d co r r ec t d is c r im in a t io n f o r s u ch f au lt s . Th e o n lyp r e s en t d a y ap p l i ca t io n f o r d i s tan ce p r o t e c t io n a s a lim i t ed f o r m o f b u s b a r p r o t ec -t i o n is o n d i s t r i b u t io n s y s t em s a t 3 3 k V .1 3 .4 U n i t s y s t e m s o f b u s b a r p r o t e c t i o n f o r meta l c l ad d i s t r i b u t io n s w i t ch g ea r1 3 . 4 .1 Ge ne r a l c o ns i d e r a t i o nsT h e m a i n r e q u i r e m e n t s o f u n i t p r o t e c t i o n w h e n f i tt e d t o p r o t e c t b u s b a rs ar e t h a t i tm u s t :( a ) Have a sho r t op era t in g t im e , espec ia l ly w here f au l t levels a re h igh , in o rd er tomin imis e d amag e to t h e s w i t ch g ea r an d to a ss is t s y s t em s t ab i l i ty .(b ) Be cer ta in to ope ra te on in te rna l f au l t s . Busbar f a u l t s a r e r a re , espec ia l ly sof o r i n d o o r me ta l c l ad eq u ip men t s , a s t h ey a r e l e s s a f f ec t ed b y a tmo s p h e r i c p o l -lu t io n e t c . O n ly b y v e r y ca r e fu l d es ig n an d r eg u la r co m p r eh en s iv e r o u t in e t e s ti n g o fth e b u s b a r p r o t ec t io n can th e d es i r ed r e li ab il i ty b e ach iev ed .( c ) Re m ain s tab le du r ing a ll ex te rna l fau l t s . S ince m an y m ore f au l t s occ u rex te r n a l ly t o b u s b a r s t h an in t e r n a l ly , b u s b a r p r o t ec t io n i s ca l l ed u p o n to s t ab i l i s em a n y m o r e t i m e s t h a n t o o p e r a t e . T h e m a x i m u m v a lu e o f e x t e r n a l f a u lt c u r r e n to ccu r s w h en a f au l t i s ad j acen t t o t h e b u s b a r an d m ay b e eq u a l t o t h e r u p tu r in gcap ac i ty o f t h e s w i t ch g ea r. O n th e o th e r h an d , t h e cu r r en t a t w h ich th e b u s b a rp r o t e c t io n is req u i r ed to o p e r a t e o n an in t e r n a l f au l t m ay o n ly b e o n e f i f t ie th o fth i s va lue . The p ro tec t ion in th i s in s tance i s thus sa id to r equ i re a s tab i l i ty f ac to ro f a t l eas t 50 .

( d ) D i s c rimin a te co r r e c t ly , t h a t i s d ec id e o n w h ich sec t io n o f t h e b u s b a r s t h ef au l t h a s o ccu r r ed , an d th en t r i p r ap id ly o n ly th o s e c i r cu i t b r eak e r s co n n ec ted toth a t s ec t io n . I t i s s o me t im es n eces sa r y to t r i p th e r em o te en d s o f s o me c i rcu i ts , b u tth is d e p en d s o n th e l o ca t io n o f t h e cu r r en t t r an s f o r me r s i n t h e s w i t ch g ea r an d isdescr ibed in de ta i l l a te r .( e ) B e imm u n e f r o m m a lo p e r a t io n . S in ce b u s b a r p r o t e c t io n h as t o t r i p a la rg en u m b e r o f c i rc u it s, i t is m o s t i m p o r t a n t t h a t i t d o e s n o t d o s o w h e n t h e r e is n o t a nac tua l f au l t o n the b usba r . T hus , bes ides r equ i r ing a h igh s tab i l i ty f ac to r , as d i scussedin i t em ( c ) t h e eq u ip men t an d c i r cu i t r y s h o u ld b e a s f a r a s p o s s ib l e immu n e to t h ee f f ec t o f f au l t s i n w i r in g , au x i l i a r y s w i t ch es an d h u man e r r o r s . Th es e ad d i t i o n a lp recau t ions aga ins t malopera t ion a re descr ibed in de ta i l l a te r .T h i s S e c t i o n d e a l s w i t h f r a m e - e a r t h b u s b a r p r o t e c t i o n w h i c h i s c o m m o n l y a p p l i e dto meta lc lad swi tchgear a t vo l tages up to 11 kV. At h igher d i s t r ibu t ion vo l tages , the


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8usbar protection 87b u s b a r p r o t ec t io n u s e s t h e s ame p r in c ip le a s t h a t f o r t r an s mis s io n s u b s t a t i o n s d ea l twi th in Sec t ion 13_5 .1 3 . 4 . 2 F r a m e e a rt h s y s t e m sW ith m e ta l c lad s w i t ch g ea r , a v e r y s imp le f o r m o f b u s b a r ea r th - fau l t p r o t ec t io n canb e ach iev ed b y l i g h t ly i n s u l a t in g f r o m ea r th a l l t h e me ta l f r amew o r k . Th e f r ame-w o r k is t h e n c o n n e c t e d t o e a r t h a t o n l y o n e p o i n t a n d a c u r r e n t tr a n s f o rm e r f it te do v e r th i s co n n ec t io n . A ty p ica l p h y s i ca l a r r an g em en t is s h o w n in F ig . 1 3 . 4 . 2 A an dthe m ain cab le g land in F igs . 13 .4 .2C a nd 13 .4 .2D. I t is seen tha t there a re tw oea r th b a r s , w h ich r u n th e l en g th o f t h e s w i t ch b o a r d . O n e i s ca l l ed th e s w i t ch g ea rb o n d in g b a r an d in t e r co n n ec t s each cu b ic l e o r f r am ew o r k . W h ere th e c ir cu itb r eak e r s a re w i th d r aw ab le , e .g . t r u ck ty p e s w i tch g ea r , t h e m o v in g p o r t i o n m u s ta ls o h a v e a h e a v y c u r r e n t e a r t h c o n n e c t i o n t o t h e b o n d i n g b a r t o p ro v i d e a p a t h f o ran y ea r th f au l t o ccu r r in g w i th in t h e c i rcu i t b r eak e r i ts e lf . Th e s eco n d ea r th b a r ,k n o w n a s t h e cab le - sh ea th ea r th b a r , i s l ig h t ly in s u l a ted f r o m th e s w i t ch g ear cu b ic l eo r f r am ew o r k an d p r o v id es a d i r ec t ea r th co n n ec t io n f o r t h o s e cab le s h ea th s w h ichrequ i re to be e ar the d ; th i s wi l l no t be used fo r every c ir cu i t because , fo r exa m ple ,s in g le -en d o r m id p o in t e a r th in g m ay b e s p ec i f ied . A ll c ab le s w i th an ea r th ed s h ea th ,i r r espec t ive o f the po in t o f ea r th ing , r equ i re an in su la ted cab le g land to ensu re tha tcu r r en t f r o m an ea r th f au l t w i th in t h e s w i t ch b o a r d can o n ly f l o w to ea r th v i a t h e

B u s.b ar ( o n l y o n e p h a s e s h o w n )

Swi t chgea rh{}nd ing bar

ind ica t ion I nsu la t ed~ '- ' ~ " ' - J II g ~ . d f o rr " 7 ~ ' t ' ~ " 7 " , , - I I I aux i l i a r y~ Z..~.Z_..~q [ [ ~ F rame-ea rth cab lec u r r e n t t r a n s f o r m e r

' " ~ f ou nd a t i onC i r cu i t s ide I ~ / ~p lug / socke t W i thd rawab leand m a in c i rc u i t b reaker Cab le shea th I ' a r thcab le g land (one on ly sho wn ) ea r th bar e lec t rode

F ig . 13 .4 .2A Cub ic leswitchgear arranged fo r frame-leakage pro tectio n


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88 8usbar prote ctionS w i t c h g e a r f r a m e w o r k S w i t c h g ea r

Local infeed Itransformer

|

3 4 IF

A ll cable glands Cable shea thinsulated from earth barswitchgear

RI R2- - - - - I Qo l c ~ 1- . ? -

O I C ~ 2===r" To trip coils0 1 0 - - 3- r - ' . , ~ - - 4

F i g . 1 3 . 4 . 2 B Frame~arth busbar protection without c h e c k

~ R I

s w i t ch g ea r b o n d in g b a r an d f r ame ea r th - f au l t cu r r en t t r an s f o r mer w h i l e , f o r cab lef au lt s , an y c u r r en t r e tu r n in g d o w n th e s h ea th f l o w s d i r ec t ly t o ea r th an d n o t v ia t h ecu r r en t t r an s f o r mer .Th e b o n d in g b a r , e a r th b a r , t h e i n t e r co n n ec t io n b e tw een th em an d a l l t h eb o n d in g co n n ec t io n s r e f e r r ed to ab o v e mu s t b e ad eq u a te t o ca r r y t h e r a t ed s h o r t -t im e cu r r en t o f t h e s w i t ch g ear f o r t h e s p eci fi ed d u r a t io n , w h ich is u s u a l ly t h r ees eco n d s .

R e lay R 1 f ed f r o m th i s c . t . w o u ld d e t ec t an y f au l t t o t h e me ta lw o r k o f t h eswi tchgear as show n in F ig . 13 .4 .2 B .R e lay R 1 co u ld th e n o p e r a t e a mu l t i co n tac t d . c . rep ea t r e l ay R 2 to t r ip a ll t h e

c i rcu i t b r eak e r s co n n e c ted to t h e f au l ted b u s b a r a s s h o w n .C ar e h as t o b e t ak en to en s u r e t h a t t h e s w i t ch g ea r f r amew o r k h as n o o th e re a r t h c o n n e c t i o n t h a n t h a t t h r o u g h t h e c u r r e n t t r a n s f o r m e r . T h e u s u a l c o n c r e t e


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Busbar protection 89foun da t ion s used for sw i tchgear a re adeq ua te fo r insu la tion f rom the genera l bo dyof the g roun d prov ided the ho ld ing do w n bo l t s are c lear o f any re in forcing rods .Breaks must be made in any vent i la t ing pipes or ear thed condui ts car rying_secondary wir ing , and bo th main and auxi l ia ry cab les mus t have insu la ted g lands .A typ ica l m a in cab le g land wi th an insu la ted ba r r ie r is show n in F ig . 13 .4 .2C.I t i s poss ib le to check th i s g land insu la t ion on ly by d isconnec t ing the ea r thcon nec t ions , since b o th s ides o f i t a re no rm al ly conne c ted to ea r th . F or reasons ofsa fe ty , th i s would requ i re an ou tage of the comple te swi tchboard . A more e labora teg land which can read i ly be te s ted wi thout b reak ing the main ea r th connec t ions i ssho w n in F ig . 13.4 .2D. Here , the in sula t ion is in tw o par ts w ith a m eta l is land layerbe tw een . N orm al ly , th i s laye r i s jo ined to the m ain swi tchgear m e ta lw ork by a l inkconnec t ion . The cab le g land i s te s ted by opening th i s l ink and the insu la t ionmeasured between the is land layer and the cable sheath , and a lso be tween the is landlayer and the swi tchgear me ta lw ork . I f bo th insu la ted sec t ions are sou nd , then thecab le mus t be insu la ted f rom the swi tchgear me ta lwork .

Switchgearbondin~ baI r f J ( ) l I S w i t c h g e a rm e t a l w o r k

e a r t hc o n n e c t i o n~ RI

Gland insulation

! , l " 1| -l .t | ~ , ~ Wiped joint- -" - ( . ' a b l e s h e a th

e a r t h b a r C a b l e l e a d s h e a t h

F i g . 1 3 . 4 . 2 C Simple nsulated cable gland

The e lem enta ry scheme of busbar p ro tec t ion sho wn in F ig . 13 .4 .2B suf fe r s f romthe d isadvantage tha t i t m ay op era te fo r spu r ious cur ren ts th rou gh the f rame-ea r thor c . t . , which , fo r example , cou ld be produced by insu la t ion fau l t s on secondaryc i rcu i t wi r ing on the swi tchgear . Also , inadver ten t ope ra t ion of re lay R1 by , say , amechanica l b low would cause a l l the switches to t r ip . This l imita t ion can beovercom e by f i t ting an add i t iona l r e lay R3 w hich is ca lled a check re lay , a s show nin F ig . 13 .4 .2E.


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90 Busbarprotect ionSwitchgearbonding bar

l"rame~orkearthconnection

~ R ILink

Double insulation

island layer

Fig . 13 .4.2D

Cable sheathearth barDouble insulated cab le gland

I f t h e i n fe e d t o t h e s w i tc h g e a r is f r o m a l o ca l t r a n s f o r m e r t h e n t h is re l a y R 3 c a nb e f e d f r o m a c . t . o n t h e t r a n s f o r m e r n e u t r a l c o n n e c t i o n . O n l y a n e a r t h f a u l t i n t h es w i tc h g e a r w i ll c a us e o p e r a t i o n o f b o t h R 3 a n d R 1 .I f t h e r e a r e n o s u i t a b l e t r a n s f o r m e r n e u t r a l s a t t h e s t a t i o n , t h e n a c o r e - b a l a n c ec . t. f i t te d a r o u n d t h e i n c o m i n g c a b l e c o u l d b e u s e d t o s u p p l y R 3 . I f m o r e t h a n o n ec a b le ( o r l o c a l t r a n s f o r m e r ) w e r e f e e d i n g t h e s u b s t a t i o n , c .t .s m u s t b e f i t te d a r o u n de a c h c a b le ( o r n e u t r a l ) a n d p a r al le l ed t o e n s u r e o p e r a t i o n s h o u l d o n e o f t h e c a b le s( o r th e t r a n s f o r m e r s ) b e o u t o f se r vi ce .(a) S ing le busbar sw i tchboar d w i th one sec t ion c i rcu i t b reaker: S o f a r o n l y as in g le s e c t i o n s w i t c h b o a r d h a s b e e n c o n s i d e r e d . I f a s e c t i o n c ir c u i t b r e a k e r is i n -e l u d e d , t h e n , w h e n a n e a r t h o c c u r s i n o n e s e c ti o n , t h e b u s b a r p r o t e c t i o n c a n b ea r ra n g e d t o o p e n o n l y t h e b u s s e c t i o n a n d f e e d e r a n d t r a n s f o r m e r c i r c u it b r e a k e r so f t h a t s e c t i o n t o i s o la t e c o m p l e t e l y t h e f a u lt f r o m t h e p o w e r s y s t e m . T o d o t h is ,t h e f r a m e w o r k o f t h e b u s s e c t i o n c i r c u i t b r e a k e r m u s t b e i n s u l a t e d f r o m t h ea d j a c e n t s w i t ch g e a r o n b o t h s id e s a s s h o w n in F i g . 1 3 . 4 . 2 F . T h e t h r e e f r a m e w o r ks e c t i o n s a r e t h e n s e p a r a t e l y e a r t h e d , w i t h a r e l a y f e d f r o m a c . t . o n e a c h e a r t hc o n n e c t i o n . T h e c h e c k r e l a y R 4 i s s h o w n i n t h i s c a s e f e d f r o m a c . t . o n t h e l o c a lt r a n s f o r m e r n e u t r a ls . T h e d . c . t ri p p i n g c i rc u i ts a r e s o a r ra n g e d t h a t t h e i n a d v e r t e n to p e r a t i o n a n y o f o n e a . c . o r d . c . r e la y w i l l n o t c a u s e c i r cu i t b r e a k e r t r i p p i n g .


11/68

Busbar protec t ion 91

R 3

l I " l ] !i i -

i |i ~ I I l l . . ,

1 2 4 R I

L o c a l i n f e e dt r a n s f o r m e r

' ! 11_oIIIIIIIJ

R 1 R 2I

R4 I/ o ..l_ c~ - - - - ? . ~ 1

" - " ? d . ~ 2, ' - ' ~ I O ' 3

~ 4

@T o t r i p c o i l s

Fig . 13 .4 .2E Frame-earth busbar pro tection with neutral checkF r o m t h e s e c i r c u it s i t w i ll b e s e en t h a t t h e b u s s e c t io n c i rc u i t b r e a k e r a lw a y s

t r i p s o n a b u s b a r f a u l t , a n d t h a t f o r t h e f a u l t F a i n t h e b u s s e c t i o n i ts e l f, e v e r yc i r c u i t b r e a k e r c o n n e c t e d t o t h e b u s b a r s m u s t b e t r i p p e d . T h i s i s n e c e s s a r y b e c a u s et h e r e la y R 2 w i ll o p e r a t e f o r f a u lt F a o n t h e p r i m a r y c o n n e c t i o n o r c o n t a c t s o nt h e s id e c o n n e c t e d t o t h e L . H . b u s b a r s as w e l l a s f o r a f a u l t in a s i m i la r p o s i t i o n o nt h e r i g h t -h a n d s id e , t h a t is i t c a n n o t d i s c r im i n a te b e t w e e n t h e t w o , a n d s o t h ew h o l e s w i t c h b o a r d is m a d e d e a d .(b) D ou ble busbars: I n t h e c a se o f s w i t c h b o a r d s f i t te d w i t h d o u b l e b u s b a r s ,b e c a u s e o f t h e g r e a t d i ff i c u l ty o f in s u l a ti n g b e t w e e n m a i n a n d r e s er v e b u s b a rc a sin g s , i t is n o t p r a c t ic a l t o d i s c r im i n a t e b e t w e e n f a u lt s o n o n e b a r a n d t h e o t h e r .T h e r e f o r e i t i s n e c e s s a r y t o e m p l o y t h e s a m e p r i n c i p l e a s f o r s i n g l e b u s b a r s . I tf o l lo w s t h a t t h e f a u l t F ~ s h o w n in F i g . 1 3 . 4 . 2 G w o u l d t r i p a ll f e e d e r s w i tc h e s


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9 2 Bu s b a rprotectionI nsu l a t i on ba r r i e r s

B u s s e c t i o n

R 2 | R 3

+ , - - ~ . . . . . . . . .

- ~ + E + + +~_!~+++.T o t r i p c i r c u i t s T o t r i p c i r c u i t so f 1 , 2 , 3 a n d o f 1 , 2 , 3 , 4 , S , 6b u s s e c t i o n a n d b u s s e c t i o n

IT o t r i p c i r c u i - t so f 4 , 5 , 6 a n db u s s e c t i o n

F ig . 1 3 .4 .2 F Typ ica l fram e~ar th p ro tec t i on fo r two sect ions o f busbars


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Bus section

Busbar protection 93

. - # . , ,

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Insulation barriers

))

Fig. 13.4.2G Frame-ear th pro tect io n fo r doub le busbar swi tchbo ardse lec ted to the le f t -hand s ide o f the m ain bar , the bus sec t ion swi tch , and a llf eeder s se lec ted to the r eserve bar . Aux i l ia ry swi tches on the busbar se lec to r s wou ldb e co n n ec ted to ach iev e th i s .(c) Performanceo f frame-earth p rotection: I n F ig . 1 3 . 4 . 2 E th e f r ame ea r th . f au l tr e lay R 1 an d th e n eu t r a l ( o r co r e -b a l an ce ) ch eck r e lay R 3 a r e u su a l ly b o th a t t r ac t eda r m a t u r e t y p e s a n d t h e i r o p e r a t i n g t i m e s t o g e t h e r w i t h R 2 d o n o t u s u a l l y e x c e e d8 0 ms . I t i s t o b e n o ted th a t t h e ch eck r e l ay w i l l o p e r a t e f o r ea r th f au l t s b o th o nthe busb ar s and a l so fo r a ll ex te rna l f au l t s near to the s ta t ion . I f the in s ta l la tion iso f s u f f i c i en t imp o r t an ce , a f u l ly d i s c r im in a t iv e ch eck s ch eme can b e ach iev ed b yf i t t ing ex t ra c . t .s to every f eed er . T h is a r r ang em en t i s descr ibed in de ta i l in Sec t ion1 3 . 5 an d w o u ld r e s p o n d o n ly to f au l t s a t t h e s u b s t a t i o n .

1 3 .5 U n i t s y s t e m s o f b u s b a r p r o t e c t i o n f o r t ra n s m i s s io n s u b s t a t io n s1 3 . 5 .1 G e n e r a l c o n s i d e r a t i o n sTh e r eq u i r emen t s o f u n i t p r o t ec t io n a s s e t o u t i n S ec t io n 1 3 . 4 . 1 ap p ly eq u a l ly t o


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94 Busbarprotect ionb u s b a r p r o t ec t io n emp lo y ed a t t h e h ig h e r s w i t ch g ea r v o l t ag es i n c lu d in g th a t o nth e s u p e rg r id s y s t em , w h e r e t h e u n i t f ea tu re is ach iev ed b y co m p ar in g th e cu r r en t sin each c i r cu i t co n n ec ted to t h e p r o t ec t ed b u s b a r .

A n u m b e r o f s y s te m s w e r e d e v el o p e d b y p r o t e c t i o n r el a y m a n u f a c t u r e rs w h e nth e in s t a ll a t io n o f u n i t b u s b a r p r o t e c t io n w as f o u n d to b e n ecess a r y . Mo s t o f t h e seem p lo y e d a bi a s f ea tu r e t o ach iev e th e r eq u ir ed s t ab i li t y an d i t w as n o t u n c o m m o nto u s e a cu r r en t s u m m at io n t ech n iq u e to en ab le ea r th f au lt s an d p h as e f au lt s t ob e d e t ec t ed b y a s in g le p h as e r e l ay . H o w ev e r , a s co n f id en ce in t h e h ig h - imp ed an cec i rcu la t ing cu r ren t d i f f e ren t ia l p ro tec t ion p r inc ip le increased , i t was used in the UKto th e ex c lu s io n o f o th e r s y s t ems . A l th o u g h s o me o ld e r s u b s t a t i o n s a r e s t i l leq u ip p ed w i th o n e o r o th e r o f t h e ea r li e r d e sig n s, t h ey a r e n o lo n g e r f i t ted o n n ewswi tchgear and a re there fo re no t descr ibed here ; r eader s r equ i r ing in fo rmat ionth e r e o n a r e r e f e rr ed to t h e f ir s t ed i t i o n o f th i s b o o k an d to t h e b ib l io g r ap h y .

H ig h - imp ed an ce c i r cu l a t in g cu r r en t d i f f e r en t i a l p r o t ec t io n w as in t r o d u ced in.Chap ter 4 dea l ing wi th p ro tec t ive t r ans fo rm ers . T he p r inc ip le is o f su f f ic ien ti m p o r t a n c e , h o w e v e r , t o w a r r a n t a f u r t h e r e x p l a n a t i o n h e r e , be g in n i ng f r o m t h eb as ic p r in c ip l e s o f t h e c i r cu l a t in g cu r r en t s y s t em .

13 .5 .2 C u r r e n t b a l a n c e u s in g c i r c u l a t in g c u r r e n t pr incip leA v e r y s imp le f o r m o f u n i t p r o t ec t io n o f a c i rcu i t can b e ach iev ed b y co m p ar in gth e cu r r en t s en t e r in g i t w i th t h o s e l eav in g , t h e c i rcu i t b e in g h ea l th y i f t h e y a ree q u a l, a n d f a u l t y i f t h e y d i ff e r b y m o r e t h a n a c e rt a in a m o u n t .

I n o r d e r t o u n d e r s t an d th e p r in c ip le m o r e ea s ily , t h e ap p l i ca t io n to a s h o r ts ing le phase f eeder car ry ing load cu r ren t wi l l f i r s t be cons idered , and F ig . 13 .5 .2As h o w s th e tw o eq u a l r a t i o c . t . s s o co n n ec ted to each o th e r t h a t t h e i r s eco n d a r ycu r r en t s f l o w in o p p o s i t e d i r ec t io n s i n t h e r e l ay R co n n e c ted ac r o ss th em .

I f t h e c . t. s w e r e p e r f ec t , t h e i r s eco n d a r y cu r r en t s w o u ld b e id en t i ca l in b o thm ag n i tu d e an d p h as e an g le , b u t a s n o tw o c . t. s p e r f o r m ab s o lu t e ly a li k e , t h en l ett h e tw o a lm o s t eq u a l c .t . s eco n d a r y cu r r en t s cau s ed b y th e l o ad c u r r en t b e I a and I t ,.

E n d A E n d B | T o l o a dF e e d e r c u r r e n t ~ m | o r_ _f au lt

l a _ b

R e l a y c u r r e n t is p h a s o r d i f f e r e n c e (1 a - l b ) A m p e r e s

F1

Fig. 13.5.2A Circulat ing curre nt pro tect io n (external faul t )


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Busbar prote ction 9 5T h e i n s t a n t e n e o u s v a l u e s o f c . t. c u r r e n t s a r e s h o w n d u r i n g t h e p e r i o d o f t i me

w h e n t h e h a l f -c y c l e o f p r i m a ry c u r r e n t is fl o w i n g f ro m A t o B . A s t h e s e f l o w ino p p o s i t e d i r e c t i o n s t h ro u g h t h e r e l a y , t h e r e s u l t in g r e la y c u r r e n t w o u l d b e t h e i rd i f f e r e n c e ( Ia - Ib ) .

I f a f a u lt e x t e r n a l t o f e e d e r A B w e r e t o o c c u r a t F ~ b e y o n d e n d B o f th e f e e d e r,t h e c . t . s e c o n d a ry c u r r e n t s la a n d Ib w o u l d f l o w e x a c t l y a s fo r t h e l o a d c u r r e n t sc o n s id e r e d a b o v e , b u t t h e y w o u l d b e i nc r ea s ed in v a lu e m a n y t im e s . F o r th i s f a u l ta t F l , r e l a y R m u s t n o t o p e r a t e a n d s o t h e c . t. d i f f e r e n c e c u r r e n t ( Ia - I b ) f l o w i n gt h r o u g h t h e r e l ay m u s t n o t e x c e e d t h e v a lu e o f t h e c u r r e n t n e e d e d t o o p e r a t e t h ere l a y . T h i s c o n d i t i o n m u s t r e m a i n s a ti sf ie d u p t o t h e la rg e st v a lu e s o f t h ro u g hc u r r e n t t h a t c a n f lo w i n t o a n y f a u l t j u s t b e y o n d e n d A o r e n d B .

End A End B

t F 2

t 1 'a ~ la /~ R lbFig. 13.5 .2B

Relay cu r rent i s phasor su m (1 a + Ib) Am pere sCircu la t ing cur rent p ro tect ion ( in te rna l fau l t )

F i g . 1 3 . 5 .2 B s h o w s a d i s t r i b u t i o n o f i n s t a n t a n e o u s v al u es o f c u r r e n t w h e n af a u l t o c c u r s i n s i d e t h e p ro t e c t e d c i r c u i t a t F 2 . H e re i t i s s e e n t h a t t h e r e l a y n o wreceives the s u m o f t h e c . t . s e c o n d a ry c u r r e n t s a n d t h e r e l a y mu s t b e d e s i g n e d t oo p e ra t e r a p i d l y fo r t h i s v a l u e o f c u r r e n t . I f n o i n f e e d t o t h e f a u l t e x i s t e d a t e n d Bt h e n t h e r e l a y c u r r e n t w o u l d b e o n l y I a a n d t h i s mu s t a l s o p o s i t i v e l y o p e ra t e t h ere la y d o w n t o t h e l o w e s t v a lu e o f fa u l t c u r r e n t l ik e l y t o e x i st o n t h e p r i m a rys y s te m , th a t is d u r i n g m i n i m u m p l a n t c o n d i t i o n s .1 3 . 5 . 3 C o n n e c t i o n s f o r c i rc u l a t i n g c u r r e n t b u s b a r p r o t e c t i o nF o r b u s b a r p r o t e c t i o n t h e a p p l i c a ti o n o f t h e c i rc u l at in g c u r r e n t p r in c i p le is s h o w ni n F i g . 1 3 . 5 . 3 A fo r a b u s b a r w i t h fo u r c i r c u i t s c o n n e c t e d t o i t . T h e c . t . s f o r t h i sp ro t e c t i o n a re s h o w n f i t t e d o n t h e f e e d e r s id e o f t h e c i r c u i t b r e a k e r s so t h a t t h ep ro t e c t e d z o n e i n c l u d e s al l c i r c u i t b r e a k e r s a s w e l l as t h e b u s b a r s .

The s ing le phase sys tem i s s t i l l cons ide red fo r s impl ic i ty , and the c . t . s a re con-n e c t e d in p a ra ll e l w i t h th e r e la y R . F o r t h e f a u l t a t F 1 o n f e e d e r Da n a s s u me d p r i m a ryf a u lt c u r r e n t d i s t r i b u t i o n is s h o w n , f r o m w h i c h i t w i ll b e s e e n t h a t t h e s u m m a t i o no f t h e c . t . s e c o n d a ry c u r r e n t s r e s u l t s i n z e ro c u r r e n t i n t h e r e l a y R a n d t h e re fo ret h e b u s b a r p r o t e c t i o n d o e s n o t o p e r a t e .


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96 Busbarprotection

, ,, !, o o o / , ( " I !

T 5 0 A T 4 , o ~ 1 0 0 0 A.- " 1 " 1 ( 1 5 0 0 A )

R e l a y c u r r e n t = 1 . 0 - 0 . 2 5 - 0 . 4 5 - 0 . 3 - 0 A

F i g . 1 3 . 5 . 3 A Ci rcu la t i ng cu r ren t busbar p ro tec t i o n s tab il is i ng on an ex te rna l f a u l t

T h e f a u l t i n th i s ca se w o u l d b e c le a r ed b y t h e f e e d e r p ro t e c t i o n o n c i rc u i t D .I f t h e f a u lt w e re o n t h e b u s b a r s a s s h o w n b y F 2 i n F i g . 1 3 . 5 .3 B t h e n fo r t h e

s a me p r i m a ry f a u l t c u r r e n t v a lu e s as fo r t h e e x t e rn a l f a u l t , t h e r e l a y c u r r e n t w i l lnow be 1 .5 A and so i t wi l l ope ra te to t r ip the c i rcu i t b reakers A , B , C and D toisola te the faul t .

F 2 ( I S 0 0 A )

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R e l a y c u r r e n t = 0 . 2 5 + 0 . 4 5 + 0 . 3 + 0 . 5 = 1 . 5 A

F i g . 1 3 . 5 . 3 B C i r c u la t in g c u r r e n t b u s b a r p r o t e c t i o n o p e r a t in g o n a n i n te r n a l f a u l t

(a) Single busbar arrangement: F or s imp l ic i ty , the app l ica t ion is f ir s t cons ide redo n a s i n g l e -p h a s e b a s i s . I n o rd e r t o r e d u c e t h e r i s k o f w ro n g o p e ra t i o n o n t h ro u g hfa u lt s , t h e p r in c i p le o f h a v in g t w o i n d e p e n d e n t l in e s o f d e fe n c e is u s e d , as w a s d o n ein t h e f r am e e a r t h s y s te m s . T h e s e t w o s e p a ra t e f o r m s o f p r o t e c t i o n h a v e b o t h t oo p e ra t e b e fo re t r i p p i n g ca n t a k e p l a c e , a n d t h e y a r e s h o w n i n F i g . 1 3 . 5 .3 C .

O n e g ro u p o f c . t. s ( a ) ( b ) a n d ( c ) f o rm s a c i rc u l a ti n g c u r r e n t s y s t e m c o v e r i n gzone l and a s imi la r g roup (d ) (e ) (O covers zone 2" these a re ca l l ed d i sc r imina t ingz o n e s . A t h i rd g ro u p o f c . t .s m a d e u p o f (g ) (h ) ( j ) ( k ) c o v e rs b o t h z o n e s 1 a n d 2 ,tha t i s the whole o f the busbars , and i s used fo r a check sys tem. Thus fo r a fau l tF in z o n e 1 , t h e d i s c ri mi n a t i n g z o n e 1 r e la y R I w i ll o p e ra t e t o g e t h e r w i t h t h e


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S e c t i o n 1B us s e c t i o n

8usbar protection 97

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s y s t e m

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C h e c k s y s t e m I 1

Circulat ing cu rren t dupl icate l ine o f defence busbar protect ionch eck s y s t em r e l ay R 3 to t r i p a ll c ir cu it b r eak e r s i n zo n e 1 , t o g e th e r w i th t h e b u ssec t ion c i r cu i t b reaker .To ap p r ec i a t e t h e p u r p o s e o f em p lo y in g a ch eck s y s t em co n s id e r n o w th eco n d i t i o n o f a h ea l th y s y s t em ca r r y in g n o r ma l l o ad cu r r en t s . I f o n e o f t h e l ead sto , s ay , c . t . ( a ) w e r e t o b eco me o p en - c i r cu i t ed , t h en r e l ay R I w o u ld h av e a cu r r en tf lowing in i t due to the lack o f the c . t . cu r ren t ( a ) ba lanc ing o u t those o f c . t .s (b )an d ( c ). Th i s cu r r en t i n R 1 m ay n o t b e l ar ge en o u g h to o p e r a t e i t , b u t w h en anex te rn a l f au l t occ u r s th e cu r re n t cou ld w el l be la rge eno ugh to cause R1 to p icku p . T r ip p in g o f zo n e 1 w o u ld , h o w ev e r , s ti ll b e p r ev en ted b y th e f ac t t h a t t h ech eck s y s t em r e l ay R 3 w i ll n o t h av e o p e r a t ed .

The method o f u s ing separa te check and d isc r imina t ing sys tems a l so cover sag a in s t t h e i n ad v e r t en t o p e r a t io n o f an y o n e r e l ay d u e , f o r ex amp le , t o v ib r a t io n ,o r o p e r a ti o n b y h a n d .(b) D ouble busbar arrangement: Fig . 13_5 .3D shows the a r rangemen t r equ i red tocover du p l ica te busba r s , a s ing le phase sys tem s ti ll be ing cons idered a t th i s stage tohe lp in more eas i ly demons t ra t ing the p r inc ip les invo lved .Here , fou r d i sc r imina t ing r e lays a re r equ i red , one fo r each main and r eservesec t ion o f b usbar s . I f f eeder A on the le f t -hand s ide o f the d rawing were se lec ted toth e r es e rv e b u s b a r , t h en th e au x i l i a ry s w i t ch o p e r a t ed b y th e s e l ec to r w o u ld c lo s e ,so se lec ting i ts d i sc r imina t ing c . t . to the r eserve zone 1 d i sc r imina t ing sys tem , i ff eeder B we re se lec ted to the m ain busbar , the n the r eserve zone 1 d i sc r imina t ings y s t em w o u ld o n ly h av e tw o c . t . s co n n ec ted to i t , n ame ly th o s e o f f eed e r A an dthose o f the bus coup ler (b ) . Main zone 2 wou ld on ly have two c . t . s se lec ted toi t, n am e ly f eed e r B an d th e c . t. ( c ) o f th e b u s s ec t io n .

I n o r d e r t o m a k e o n e o f t h e t w o f in es o f de f e n ce i m m u n e f r o m d e f e c t s w h i c hm igh t a r i se in aux i l ia ry swi tches , none a re f i t t ed in the chec k sys te m , w h ich there -


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9 8 B u s b a rpro tec t ionS e c t i o n I S e c t i o n 2

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Busbar pro tec t ion 99c i r c u it s t o e n s u r e t h a t o n ly t h e c i r c u it s s e l e c t e d t o a p a r t ic u la r zo n e a re t r ip p e d .S i n c e b o t h t h e b u s c o u p l e r a n d b u s s e c t i o n m u s t b e t r i p p e d f r o m t w o z o n e s t h e yn e e d t w o t r ip p i n g r el a y s.

T h e c h e c k r e l a y c o n t a c t i s c o n n e c t e d i n t h e c o m m o n n e g a t i v e l e a d o f t h ein d iv id u a l t r ip p in g r e la y s , w h i l s t t h e d i s c r im in a t in g r e la y c o n t a c t s a r e in t h e p o s i t i v el e a d . T h i s e n s u r e s t h a t n o t r ip p in g w o u ld o c c u r i f a p o s i t i v e s u p p ly w e r ein a d v e r t e n t ly a p p l i e d t o t h e w ir in g o f a n y t r ip r e la y , a s , f o r e xa m p le , a t p o in t P o nf e e d e r A i n F i g . 1 3 . 5 3 E . T h i s c o u l d h a p p e n b e c a u s e t h e l e a d s t o t h e d i s c o n n e c t o ra u x il ia r y s w i t c h e s m a y b e o f c o n s i d e ra b l e l e n g t h a n d p a s s t h r o u g h s e ve r a l j u n c t i o nb o x e s .

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Fig. 13.5.3E C i r c u l a t ing c u r r e n t p r o t e c t ion f o r d oub l e busbars (d .c .. c ir c u i ts ) ; i nd i v id ua lc i rcu i t - t r ipp ing re la ts


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100 Bu sb ar rotect ion

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To trip To trip To trip To tripfeeder bus bus feeder

A couple r section BFig. 13.5.3F Circu la t ing current pro tect ion for doub le busbars (d .c . c i rcu i ts ) /mul t icontacttripping relays

The ind iv idua l c i rcu i t - t r ipp ing re l ays a r rangement shown in F ig . 13 .5 .3E has theadvan tage tha t r egu la r p rov ing t e s t s o f c i rcu i t t r ipp ing can read i ly inc lude p rov ingt he t r i pp i ng f r om t he busba r p r o t e c t i on r e l a y T . W i t h t he a l t e r na t i ve a r r a nge m e n to f m u l t i c on t a c t t r i pp i ng r e l a ys shown i n F i g . 13 .5 .3 . F , c a r e f u l c ons i de r a t i on ha sto be g iven be fore opera t ing such re l ays , in o rde r to ensure tha t dur ing ' t r ip t e s t ing 'on ly the des i red c i rcu i t is t r ipped .(c) Protection fo r ea rth faults: I f on l y e a r t h - f a u l t p r o t e c t i on o f t he busba r s i srequ i red , then the c . t. s can be con nec ted as show n in F ig . 13 .5 .3G wh ere it wi l l beseen tha t only one re lay e lement i s requi red s ince the c . t . s of a l l phases arepara l le led together .

I f the outgoing c i rcui t s a re cables (e i ther three core or three s ingle cores) , anal ternat ive arrangement of c . t . s i s to f i t a s ingle core-balance c . t . on each c i rcui t ,a s show n i n F i g . 13 .5 .3G.

W i t h t h i s f o r m o f e a r t h f a u l t p r o t e c t i on , a s w i t h f r a m e e a r t h - f a u l t p r o t e c t i ondesc r ibed ea r li e r, t he check fea tu re m ay be by a re l ay fed f ro m the sum of thecur ren t s in the neu t ra l s o f the loca l t r ansform ers con nec ted to the busba rs . Thed i sadvan tage o f th i s a r rangem ent how ever , is tha t th i s re lay wi ll op e ra te fo r ex te rna lea r th fau l t s. Al so , the re l ay wi ll no t op era te i f t he re i s no n eu t ra l cu r ren t due to theloca l t r ansformers be ing ou t -o f - se rv ice . The most re l i ab le check a r rangement i scur ren t ba lance as shown in F ig . 13 .5 .3C.(d) Protection fo r phase and earth faul ts: F ig . 13 .5 .3H shows the p r inc ip le o fsuch a cur ren t ba lance sys tem w hich i s, i n e f fec t , th ree separa te sys tem s, eachcover ing one phase an d jo ined toge the r a t th e s t a r p o in t o f each se t o f c .t . s to savei n m u l t i c o r e s . F o r s i m p l i c i t y , pa r t ( a ) shows on l y t wo p r i m a r y c i r c u i t s c onne c t e dto the th ree -phase b usbars . F or the b lue to yc ll c 0v busb ar fau l t F sh ow n, b o th b lue


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Fig. 13 .5.3G Protection for earth faults


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102 Bu sba r ro tection

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J J - j

l i I

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F i g . 1 3 . 5 . 3 H Com bined phase and e ar th - fau l t p ro tec t i on


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Busbar pro tection 103an d y e l lo w r e lay s w i ll o p e r a t e . I n t h e ev en t o f a b lu e p h as e - to - ea r th f au l t , as s h o w na t F i n p a r t ( b ) o f F ig . 1 3 . 5 .3 H th en o n ly th e b lu e e lem en t w i ll o p e r a t e .

13 .5 .4 T h e i n f l u e n c e o f c . t . p e r f o r m a n c e o n t h r o u g h . f a u l t s tab i l i tyTh e c i rcu l a tin g cu r r en t p r in c ip le can b e e x t en d ed f o r ap p l i ca t io n to v a r io u s s u b -s t a t i o n co n f ig u r a t io n s w i th m u l t ip l e s ec t io n s o f sin gle o r d o u b le b u s b a r s w i t ch g ea r ,b u t b e f o r e d o in g th is a p r ac ti ca l l im i t a t i o n to t h e s y s t em d es c r ib ed s o fa r m u s t b ee l im i n a te d . T h e re w o u l d b e n o p r o b l e m w i t h s u c h a s y s t e m if t h e o u t p u t o f t h ecu r r en t t r an s f o r mer s w as a f a i t h f u l r ep r o d u c t io n o f t h e p r imar y cu r r en t a t a l lt im es . H o w ev e r , o n e m u s t a l lo w f o r th e f ac t t h a t f au l t cu r r en t m ay , a s ex p la in edin C h ap te r 4 , co n ta in a t r an s i en t d . c . co mp o n en t cu r r en t w h ich can cau s e s a tu r a t io no f t h e cu r r en t t r an s f o r m er co r e s an d d i s to r t i o n o f t h e s eco n d a r y cu r r en t , e f f ec tsw hich can o n ly be avo ided b y increas ing the co re s ize subs tan t ia l ly . S ince the c . t .sw h ich a re co n n e c ted to g e th e r t o f o r m a c i rcu l a tin g cu r r en t zo n e w il l e ach g en e r a llyb e ca r r y in g d i f fe r en t m ag n i tu d es o f cu r ren t an d h en ce d i f f e r en t m ag n i tu d es o f d eco m p o n e n t , t h e d eg r ee o f d i s to r t i o n w i ll v a r y f r o m o n e c .t . t o an o th e r , w i th th er e s u l t t h a t ( f o r an ex t e r n a l f au l t ) t h e s eco n d a r y cu r r en t w i l l n o t s u m to ze r o an dthere wi l l be an u nba lanc e ( sp il l) cu r ren t in the r e lay . I f the m agn i tude o f the sp il lcu r r en t ex ceed s th e r e l ay s e t t i n g f o r s u f f i c i en t t ime , o p e r a t io n o f t h e r e l ay w i l lo ccu r an d th e p r o t ec t io n w i ll b e u n s t ab l e . M an y ea r ly ap p l i ca t io n s o f ci rcu l a tin gcu r r en t p r o t ec t io n u s ed r e lay s w i th a r e la t iv e ly s h o r t o p e r a t in g time an d th eseo ccas io n a l ly o p e r a t ed in co r r ec t ly f o r an ex t e r n a l f au l t . Th e r e l ay s w er e f r eq u en t lyo f t h e a t t ra c t e d a r m a t u r e t y p e o f fa i rl y l ow i m p e d a n c e w i t h a n o m i n a l o p e r a ti n gt im e o f ab o u t 1 0 0 m s . A t t h is t im e i t w as n o t c lea rly u n d e r s to o d w h y th e re l aymis b eh av ed in t h i s w ay . I t w as f o u n d , h o w ev e r , t h a t i f a r e l ay w i th a mu ch lo n g e ro p e r a t in g t im e w as u s ed , f o r ex am p le an in d u c t io n d is c r e l ay i t d id n o t - o p e r a t e o nex te r n a l f au l t s . I t i s n o w k n o w n th a t t h i s w as b ecau s e i t p r o v id ed t ime f o r t h e d cc o m p o n e n t t o d e c a y a n d f o r s t e a d y s t a t e c o n d i t i o n s t o b e c o m e e s t a b l i s h e d .U n f o r tu n a te ly , t h e l o n g fau l t c l ea ran ce t im es r e s u lt in g f r o m th e u se o f s u ch r e lay sw er e f o u n d to b e u n accep tab le an d , f o r a t im e , bi a sed s y s t em s w er e co n s id e r ed tob e th e o n ly s o lu t io n .

13 .5 .5 B a s i c p r in c i p l e s o f h i g h - i m p e d a n c e c i r c u l a t in g c u r r e n t b u s b a r p r o t e c t i o n :s t a b i l i t yTh e r eq u i r em en t s o f h ig h -imp ed an ce c i rcu l a tin g cu r r en t p r o t e c t io n a r e , i n p r in c ip l e ,t h e s ame w h e th e r i t i s ap p l i ed to a tw o - en d ed zo n e , s u ch a s a s h o r t i n t e r co n n ec to r ,o r t o t h e mu l t i - en d ed zo n es o f b u s b a r p r o t ec t io n . Th e n o tab le d i f f e r en ces o ccu r i nt h e s t ab i l it y l ev el an d th e i n c reas ed c o m p lex i ty o f b u s b a r p r o t ec t io n r eq u i r ed top reve n t the un w an ted t r ipp ing o f the severa l c i rcu i t b reake r s in one zon e , e .g . dueto th e i n ad v e r t en t o p e r a t io n o f a sin gle r e l ay .Th e d i s ad v an tag es o f c i r cu l a t in g cu r r en t p r o t ec t io n u s in g lo w imp ed an ce r e l ay sw er e d es c r ib ed ea r li e r, t h e m a in o n e b e in g th r o u g h f au l t i n s tab i l i ty d u e to c . t .


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104 Bu sb ar rotect ions a t u r a t i o n . I t w a s f o u n d t h a t t h is c o u l d b e o v e r c o m e b y t h e c o r r e c t c h o i ce o f c .t .sa n d r e la y c i rc u it c o m p o n e n t s , in w h a t h a s b e c o m e k n o w n a s h i g h . im p e d a n c ec i rc u l at in g c u r r e n t p r o t e c t i o n .C o n s i d e r a s y s t e m t o p ro t e c t a z o n e h a v e o n l y t w o c i r c u i t s c o n n e c t e d t o i t .T h e mo s t o n e ro u s c o n d i t i o n s fo r s t a b i l i t y a r i s e w h e n :( i ) M a x i m u m f au l t c u r r e n t e n t e r s t h e z o n e t h ro u g h o n e c i r c u it a n d l ea v e s t h ro u g ht h e o t h e r t o a f a u l t j u s t o u t s i d e t h e z o n e , as s h o w n i n F i g . 1 3 . 5 .5 A .

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C.T. secondary winding ItS" ~ _ . . .L . I L~ C .T. turnsresistance = Rct ~ . ~ _ . _ i ~!~ ratio TI F i1 ,

Fig. 13 .5 .5 A H ig h mpedance scheme

( ii ) O n e c .t ., s a y t h a t o n c i r c u i t B , s a t u r a t e s c o m p l e t e l y d u e to a s s y m m e t ryo f t h e f a u l t c u r r e n t , w h i l e t h e o t h e r c . t . d o e s n o t e n t e r s a t u r a t i o n a t a l l, a n dm a i n t a in s it s o u t p u t a s a f a it h f u l r e p r o d u c t i o n o f t h e p r i m a r y c u r r e n t .I n C h a p t e r 4 t h e e q u i v a l e n t c i r c u i t o f a c . t . w a s d e s c r i b e d a n d t h i s c a n b e u s e d t oi l l u s t r a t e t h e c o n d i t i o n s e x i s t i n g i n t h e s y s t e m b e i n g c o n s i d e re d , a t w h i c h p o i n t i ts h o u ld b e n o t e d t h a t t h e c a l c u l at e d p e r f o r m a n c e o f th e p r o t e c t i o n c a n o n l y b eo b t a i n e d w i t h t h e d e s ir e d r e l i a b il i ty i f th e c . t. s a re o f t h e l o w r e a c t a n c e t y p e a sd e f i n e d i n B S 3 9 3 8 . T h i s b e i n g t h e c a s e , t h e p r i ma ry a n d s e c o n d a ry l e a k a g e r e a c .t a n c e s c a n b e o mi t t e d f ro m t h e e q u i v a l e n t c i r c u i t , w h i c h i s s h o w n i n F i g . 1 3 . 5 . 5 B .B e c au s e th e c . t. o n c i rc u i t B is s a t u r a te d b y t h e d . c . c o m p o n e n t o f t h e p r i m a r yc u r r e n t i ts m a g n e t i s in g b ra n c h m a y b e a s s u m e d , p e s s i mi s t i c a ll y , t o h a v e z e roi m p e d a n c e . H e n c e i t i s v a lid t o i n s e r t t h e c o n n e c t i o n PQ. I n t h e ab s e n c e o f as o c o n d a ry e a n . f ., t h e c . t . on feeder B wi ll behave as a re s is to r hav ing a va lue equ a lt o t h e r e s i s t a n c e o f t h e s e c o n d a ry w i n d i n g .

T h e e q u i v a l e n t c i r c u i t c a n b e fu r t h e r s i mp l i f i e d t o t h a t s h o w n i n F i g . 1 3 . 5 . 5 C b y


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|11 ~ I

i' A RCT

5gZR L A

2 ~RL B3~

w i t ]

8usbar protection 105

RCT

i iI II I

/~ I Zero seconda rye .m.f , due toI sa tura t ion

N o n - s a t u r a t e d c . t . '1Q iagnetising and Satu rated c. t .core Joss branchF i g . 1 3 . 5 . 5 B Equivalent circuit of high-impedance schemem ak in g th e f o l lo w in g a s s u m p t io n s :( a ) F o r t h e c . t . o n c i rcu i t A , t h e imp ed an ce o f t h e mag n e t i s in g b r an c h an d th e

res i s tance o f the co re lo ss b ranch a re h igh r e la t ive to the o ther pa ths th roughwh ich the secon dary cu r ren t can f low ( i. e . the c . t . is per fo rm ing a lm os tp e r f ec t ly ) .

(b) Since, in gen eral , al l c . t . s ins ta l led for the bus bar pr o te ct i on o f a g iven sub-s ta t ion wi l l be ide n t ica l , the i r sec ond ary wind ing w i ll have the same res i s tance ,shown in the equ iva len t c i r cu i t as Rct .

The seco ndary cu r ren t f rom the c . t . on c i r cu it A wi l l d iv ide be tw een the r e layc i r cu i t and the seco nda ry wind ing o f the c . t . on c i rcu i t B . I f, as we sha ll subs eque n t lyf ind to be the case , the r e lay c i r cu i t r es i s tance i s h igh compared wi th the loopcom pr is ing the sa tu ra ted c . t . and i t s connec t ions , the la t te r can be cons idered toca r r y th e w h o le o f t h e cu r r en t . Th e r e s u l t an t v o l t ag e d r o p ap p ea r s ac r o s s X Y an dtherefo re across the r e lay c i r cu i t and th i s mus t be in su f f ic ien t to opera te the r e lay i f

Perfect c.t .

RCT+~,' _ _

4.5FZ

24A

F i g . 1 3 . 5 . 5 C Simplified equivalent circuit

p

S R

R

5 . 5 ~

S a t u r a t e d c . t .


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106 Bu sba r rotectionthe p ro tec t ion i s to r emain s tab le . The lead bu rdens wi l l near ly a lways d i f f e r due tovar ia t ions in the d i s tance f rom the r e lay to each th ree-phase o f c . t. s and a re show nas R L A and R L B .

A s imi la r ca lcu la t ion m us t be m ade assuming tha t the c . t . on c i rcu i t A sa tu ra tesand that on B behaves as a per fect c . t . Whichever vol tage is the h igher is refer red toas the s tab i l i ty vo l tage o f the sys tem . ( In th i s in s tance , fo r the va lues o f res i s tanceg iven in F ig . 13 .5 .5C , the fo rm er wi ll be h igher ) .

The re lay c ircu i t is shown as the re lay operat ing co i l , R , and a ser ies res is tor , SR.I f the r e lay i s o f the cu r ren t -opera ted type wi th ad jus tab le se t t ings , i t s r es i s tancem u s t b e a s ce r ta in ed f o r t h e s e t ti n g to b e emp lo y ed . Th e s e lec t io n o f t h e cu r r en tse t t ing i s di scussed in Sec t ion 13 .5 .6 . A lm os t invar iab ly , the p rod uc t o f se t t ingcur ren t and the co r respon d ing r es i stance is less than the s tab i l i ty vol tage re fe r redto above and the va lue o f the r es i s to r SR i s ca lcu la ted so tha t the p roduc t o f there lay se t t ing cu r ren t and the sum o f r e lay r es i s tance and s tab i l i s ing r es i s to r (SR) isequa l to , o r s l igh t ly g rea te r than , the s tab i l i ty vo l tage V . I t i s apparen t f rom th ishow the s tab i li s ing r es i s to r beca m e so -ca l led .

Summar is ing the s tab i l i ty ca lcu la t ion , the s tab i l i ty vo l tage mus t be de te rminedf r o m:

V = 1F T ( R L + Ro t ) 13 .5 .1an d , f o r r e l ays w i th cu r r en t ca l ib r a tio n ,

I S (R R + R S R ) " ) V s 13.5.2while , for re lays cal ibrated in vol tage"

set t ing ) V 13 .5 .3

w h er e :f au lt cu r r en t i n p rima r y amp sse t t ing o f r e lay in amps

Rct = secondary wind ing r es i s tance o f c . t . a t 75 CR L = R L A ,R L B = res is tance o f leads be tw een re lay and the c . t . o f

c i r cu i t s A o r B ( to ta l fo r the loop)RR ~-" r es is tance o f r e lay co i lR S R = res is tance of s tab i l is ing res is tor


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Busbar prote ction 107T = t u r n s r a t i o o f c . t .Vs = s t a b i l i t y v o l t a g e

L e t u s t a k e , a s a n e x a m p l e , th e c i r cu i t o f F i g. 1 3 . 5 .5 B w i t h r e s i st a n c e s a s s h o w n ,a c . t . r a t i o o f 12 00 0 t u r n s a nd a m a x i m u m f a u lt c u r r e n t o f 5 0 k A .S u b s t i t u t i n g i n e q n . 1 3 .5 .1 t o o b t a i n t h e s t a b i l i ty v o l ta g e , f i r s t ly a s s u m i n g

s a t u r a t i o n o f t h e c . t . o f c i r c u it A :5 0 0 0 0V s = (2 + 2 - 5 )2 0 0 0

= 112-5 Va n d t h e n f o r s a t u r a t i o n o f th e c . t. o f c ir c u i t B "

5 0 0 0 0Vs = (3 + 2 -5 )2 0 0 0

= 137 .5 VI f t h e r e la y t o b e u s e d is o f a t y p e c a l ib r a t e d i n c u r r e n t a n d h a v in g a b u r d e n a ts e t ti n g o f 0 . 5 V A w e c o u l d s e l e c t o n e h a v i n g a s e t ti n g r a n g e o f 2 0 t o 8 0 % o f 1 Aa n d u s e t h e 0 . 5 A t a p , a t w h i c h t h e i m p e d a n c e i s:

V A 0 . 5I s ~ 0.52 - 2 1 2

U n l e ss t h e r e la y h a s a l o w p o w e r f a c t o r , t h is m a y b e t a k e n o n t h e c o il r e s is t an c eR R ; o t h e r w i s e t h e re si st iv e c o m p o n e n t , R n s h o u l d b e d e t e r m i n e d .R e a r r a n g i n g e q n . 1 3 . 5 . 3 t o g i v e R S R ,

zsR S R ~ - - - _ R gI s137 .5. . . . . . 20-5

273 ~ 2A l t e r n a t i v e l y , a s e t ti n g r a n g e o f 1 0 - 4 0 % o f 0- 5 A c o u l d h a v e b e e n c h o s e n a n d t h ec a l c u l a t i o n b a s e d o n t h e u s e o f t h e 2 0 % (0 -1 A ) t a p , i n w h i c h c a s e t h e s t a b i li s in gr e si st o r s h o u ld h a v e a m i n i m u m v a lu e o f ( 1 3 7 5 - R R ) I 2 .


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108 Bu sb ar rotect ionI f a r e lay ca l ib ra ted in vo l tage w ere used in th i s app l ica t ion , i t wo u ld m ere ly be

necessary to se lec t a se t t ing equa l to , o r g rea te r than , 137 .5 V . One design o f such are lay has se t t ings in 25 V s teps f rom 25-175 V, and in th i s case the 150 V se t t ingw o u ld b e u s ed .

1 3 . 5 . 6 Bas ic p r inc ip les o f h igh - impedance c i r cu la t ing cu r ren t busbar p ro tec t ion :o p e r a t io n

O n e o f t h e m a in r eq u i r em en t s t o en s u re r el iab le an d f as t o p e r a t io n o f t h is t y p e o fp ro tec t io n on in te rna l f au l t s is tha t a ll cu r ren t t r ans fo rm ers m us t have a knee -po in tvo l tage w h ich is a t l eas t tw ice the se t t ing vo l tage o f the r e lay c i r cu i t . F o r theexam ple ju s t cons ide red , th ere fo re , the c . t .s shou ld have a knee -po in t vo l tage o f a tleast 275 V (o r 300 V in the case wh ere the r e lay is se t to 150 V) .

Th e max imu m in t e r n a l f au l t cu r r en t w i l l u s u a l ly b e th e s ame a s t h a t f o r t h es tab i l i ty cond i t ion and i f we look a t the cu r ren t f lowing in the secondary c i r cu i t( as shown in F ig . 13 .5 .6A) i t i s seen tha t the c . t . a t tempts to d r ive 25 A th rough there lay c i r cu i t . S ince , in one so lu t ion to the p rev ious example , the r es i s tance o f therelay c ircu i t was 1375 I2 (and could be h igher for a vol tage cal ibrated re lay) i twou ld appear tha t the vo l tage across the r e lay c i r cu i t cou ld be 34 -4 kV o r more .Due to the l imi ta t ion imposed by i t s co re en te r ing sa tu ra t ion on each ha l f . cyc le , noprac t ica l c . t . cou ld p roduce a vo l tage o f th i s va lue bu t i t cou ld p roduce sp ikes o fvery h igh vo l tage a roun d the in s tan ts o f ze ro f lux as exp la ine d in the ch ap te r o nc. t .s and v. t .s .A fo rmula due to Mathews ( see B ib l iog raphy) g ives a r easonab le approx imat ion toth e p eak v o l tag e p r o d u ced u n d e r t h e s e co n d i t i o n s :

Vpk = 2 ~ 1 3 .5 .4w h er e :

V.k is the peak va lue o f the d i s to r ted vo l tage wave fo rmv: is the r .m . s , va lue o f the vo l tage th a t wo u ld appe ar i f the c . t .

d id no t sa tu ra tevk is the k nee-p o in t vo l tage o f the c . t .

F o r ex am ple , Vf is the vo l tage o f 3 4 4 kV a l r eady re fe r red to and Vk is say 275 V ,ju s t m ee t in g th e r eq u i r em en t o f tw ice th e s e t t i n g v o lt age .

H e n c e ,Vpk = 8.7 kV


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8usbar prote ction 109To p r o t e c t t h e c . t. s , t h e s eco n d a r y w i rin g an d th e r e lay f r o m d am ag e d u e to s u ch ah ig h v o l t ag e , a n o n l in ea r r e s is to r i s co n n ec ted ac r os s p o in t s X Y as s h o w n in F ig .1 3 . 5 . 6 A i f t h e p eak v o lt ag e w o u ld ex ceed 3 k V . Th i s re s i s to r i s o f t h e t y p e w h ichreduces in va lue as the vo l tage across it increases and i s se lec ted w i th charac te r i s t icsw h ich l im i t t h e v o l tag e to b e tw een 1 k V an d 2 k V . Th e n o n l in ea r r e si s to r s u s ed fo rth i s p u r p o s e co n ta in a l arg o p r o p o r t io n o f s i li con ca rb id e o r s im i l a r m a te r ia l man u -f ac tu r ed a s d is c s. Th e p r o p e r t i e s o f t h e m a te r i a l, co u p led w i th t h e d im en s io n s , g iv ethe r equ i red charac te r i s t ics .

[ ]50 kA j k 0

A f

ii

I t , s ,N o n - l i n e a rr es i s t o r ! 3 2 $ ~

~ 2 5 A R 50f~

, , , , I ,

l . 1 8F i g . 1 3 . 5 . 6 A Connect ion of vo l tagedimi ting d e v i c e

Th e v o lt ag e i s ap p li ed b e tw een th e tw o c i r cu la r f aces o f t h e d is c b y co n n ec t io n sw h ich en s u re co n tac t w i th t h e m a te r ia l o v e r t h e w h o le o f each a r ea.The r es i s to r cu r ren t and app l ied vo l tage a re r e la ted by the fo l lowing express ion :

V = klO 13. 5.5

where /3 var ies f rom 0 .2 to 0 .25 and k i s con t ro l led by the d imens ions .Fo r non l inear r es i s to r s u sed fo r h igh - impedance p ro tec t ion , k usua l ly has a va lue

b e tw een 2 0 0 an d 1 0 0 0 . B ecaus e th e m an u f ac tu r in g p ro ces s is s u ch th a t t h e r e a relarge to le rance on bo th g and k , it is an advan tage to spec i fy the l imi t s o f vo ltage a tt h e m a x i m u m c u r r e n t a n d a ls o th e m a x i m u m v a lu e o f/3 .

Th e r e l a ti o n s h ip in eq n . 1 3 .5 .5 o n ly ap p l ie s t o i n s t an t an eo u s v a lues o f cu r r en tand vo l tage and there fo re i f the cu r re n t is s inuso ida l the vo l tage w avefo rm wi l l no tb e . F o r t h i s an d a n u m b er o f o th e r r ea so n s ( t h e r ma l r a t in g , f o r ex am p le ) ca l cu l a t io n sf o r t h e s e l ec t io n o f s u i t ab l e n o n l in ea r r e s i s to r s f o r b u s b a r p r o t ec t io n s h o u ld b es u p p o r t ed b y th e r e s ul ts o f t e s ts o n a ty p ica l i n s t a ll a t io n .

A n eces s a r y p a r t o f t h e d es ig n o f h ig h - imp ed an ce c i r cu l a t in g cu r r en t p r o t ec t io ni s t h e ca l cu l a t io n o f t h e f au l t s e t t i n g , s o me t imes r e f e r r ed to a s p r imar y o p e r a t in gcur ren t (p .o . c . ) .


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110 8usbar prote ctionA n i n t e rn a l f a u l t , w h o s e m a g n i t u d e is j u s t s u f f i c ie n t t o c a u se o p e ra t i o n o f t h e

re lay , wi l l r e su l t in a vo l tage be tween X and Y (F ig . 13 .5 .6A) equa l to the se t t ingv o l t a g e o f t h e r e l a y c i r c u i t . T h e c u r r e n t s w h i c h , a t t h i s v o l t a g e , p a s s t h ro u g h e a c hc o m p o n e n t a n d a ll t h e c . t. ma g n e t is i n g a n d c o re lo s s b r a n c h e s w i ll , w h e n a d d e dt o g e t h e r a n d r e f e rr e d t h r o u g h t h e i d ea l c . t. , e q u a l t h e f a u l t se t t in g i n p r i m a ry a m p s .T h e r e l e v a n t c o mp o n e n t s a n d p a r t s o f t h e c . t . e q u i v a l e n t c i r c u i t s a r e s h o w n i n F i g .13 .5 .6B and i t wi l l be seen tha t an add i t iona l re s i s to r , R a i l , h a s b e e n a d d e d i npara ll e l w i th the re lay c i rcu i t . Th i s shu n t s e t t ing res i sto r is f i t t ed w he n i t is requ i redt o r a i s e t h e f a u l t s e t t i n g a b o v e t h a t w h i c h w o u l d o t h e rw i s e o b t a i n . R e a s o n s fo rdo ing th i s a re g iven in Sec t ion 13 .6 .10 .

F ig . 1 3 . 5 . 6 B

T u r n s trati~ T

[ ] . . . ~ " ], , -RLA RLB2 _ R s

, , I L IEquivalent circuit for in ternal fault condit ion

The pr imary fau l t s e t t ing i s ca lcu la ted f rom"

where" & = ( h i e + rlR + [SR "t" ]NLR + IsH ) + T 1 3 . 5 . 6

n = nu m be r o f c . t. s in pa ra ll e lle = exc i t ing cur ren t o f each c . t . a t re lay c i rcu i t s e t t ing vo l tageIR = re lay cur ren t a t s e t t ingINLR = cur r en t in non l inea r re s i s to r a t re lay c i rcu it s e tt ing vo l tage

1St = shu n t s e t t ing res i sto r cur r en t a t re lay c i rcu i t s e t t ing vo l tageI SR = superv i s ion re lay cur re n t a t m a in re lay se t t ing vo l tage


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Busbar pro tect io n 111r = n u m b e r o f r e la y s in p a r a ll e lT = c u r r e n t t r a n s f o r m e r t u r n s r a t io .

I f t h e r e l a y s h a v e a r a n g e o f c u r r e n t s e t t i n g s , i t i s f r e q u e n t l y p o s s i b l e t o a v o i d t h en e e d f o r a s h u n t s e t t i n g r e s i s t o r b y s e l e c t i n g t h e r e l a y s e t t i n g t o g i v e t h e r e q u i r e df a u l t s e t t in g . W h e r e , h o w e v e r , t h e r e is a v a r ia b l e n u m b e r o f r e la y s in s e rv i ce d e p e n d -in g o n t h e r u n n i n g a r ra n g e m e n t s o f t h e s u b s t a t i o n , f o r e x a m p l e fo r b u s b a r p r o t e c t i o nd i sc r imina t ing zones , i t i s usua l t o se l ec t a s l ow a r e l ay cur r en t s e t t i ng as poss ib l e i no r d e r t h a t t h e f a u l t s e t t i n g d o e s n o t v a r y t o o w i d e l y a n d t o f i t a s h u n t s e t t i n gres i s tor .

I t is u s u a l t o a s s u m e t h a t :( i )

(i i)l e is in phase w i th t he r e s i s t i ve cur r en t sINLR is s i n u s o id a l w i t h a p e a k v a lu e c a l c u l a t e d f r o m e q n . 1 3 . 5 . 5 .

T h e s e a s s u m p t i o n s s i m p l i f y t h e c a l c u l a t i o n a n d t e n d t o g iv e a s l ig h t l y p e s s i m i s ti cr e s u l t , t h a t is a h i g h e r s e t ti n g t h a n w i l l b e f o u n d b y i n j e c ti n g c u r r e n t t h r o u g h o n ec . t . a n d i n c re a s in g t h e c u r r e n t u n t i l t h e r e l a y o p e r a t i o n . T h e c a l c u l a te d s e t t i n g isa d e q u a t e f o r m o s t p u r p o s e s . T h e c . t . e x c i t in g c u r r e n t i s u s u a l l y o b t a i n e d f r o m t h em a n u f a c t u r e r ' s e s t i m a t e d c u rv e b u t c a n b e o b t a i n e d f r o m te s t c e r ti fi c a te s o rm e a s u r e m e n t s m a d e o n s i te .

1 3 . 5 . 7 E x t e n s i o n o f t h e b a s ic p r i n c ip l e s t o b u s b a r p r o t e c t i o nT h e b a s i c p r i n c ip l e s j u s t d e s c r i b e d a re a p p li c a b l e t o b u s b a r p r o t e c t i o n w i t h f e wc h a n g e s .I n t h e s t a b i l i t y c a l c u l a t i o n , t h e r e i s a l a r g e r n u m b e r o f c . t . s t o c o n s i d e r a n d t h el e a d r e si s ta n c e b e t w e e n t h e r e l a y a n d e a c h c . t . in t u r n m u s t b e c o n s i d e r e d in o r d e rt o f in d t h e c i r c u it w h i c h g iv e s t h e h i g h e s t s t a b i l i t y v o l t ag e . I f t h e c . t. s d o n o t a llh a v e t h e s a m e s e c o n d a r y r e s i s ta n c e , th i s s h o u l d a l so b e t a k e n i n t o a c c o u n t ; t h i so c c u r s c h i e f l y w h e n a s u b s t a t i o n is e x t e n d e d w i t h a d i f f e r e n t d e s i g n o f s w i t c h g e a ro r s e p a r a t e l y m o u n t e d p o s t - t y p e c . t. s . M o d e r n p r a c ti c e is t o c o n n e c t t h e c . t. s b yb u s w i r e r in g s as w i l l b e d e s c r i b e d l a t e r a n d t h e r e s is t a n c e b e t w e e n r e l a y a n d c . t .m u s t t a k e a c c o u n t o f t h e p a r a ll el p a t h s f o r m e d b y t h e t w o r o u t e s p r o v i d e d b y t h er ing.

W h e n c a l c u l a t i n g t h e f a u l t s e t t i n g t h e r e w i l l a l s o b e a l a r g e r n u m b e r o f c . t . s i np a r a ll e l c o r r e s p o n d i n g i n g e n e r a l to t h e n u m b e r o f c ir c u i ts c o n n e c t e d t o a g i v enz o n e . A l s o , a s c a n b e s e e n f r o m F i g . 1 3 . 5 . 9 A , t h e n u m b e r o f c . t. s i n p a r a ll e l a td o u b l e b u s b a r s u b s t a t io n s c a n d e p e n d o n t h e n u m b e r o f c i rc u i ts se le c te d t o a g iv e nb u s b a r . W i th b u s b a r p r o t e c t i o n i n p a r t i c u la r , i t is n o w c o m m o n p r a c ti c e to f i t m o r et h a n o n e r e la y p e r z o n e a n d t h e c u r r e n t p a ss e d b y t h e a d d i t io n a l r e la y s m u s t b ei n c l u d e d w h e n c a l c u l a ti n g t h e f a u l t s e t t i n g . F o r t h e re a s o n s g i v en i n S e c t i o n1 3 . 6 . 1 0 , c o n t r o l o v e r t h e f a u l t s e t ti n g is a l so r e q u i r e d a n d o n c e t h e r e l a y t y p e h a s


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112 Bu sba r ro tectionbeen dec ided and the c . t .s des igned , th i s i s ach ieved m ain ly by the se lec t ion o f thevalue for the shunt res is tor .13 .5 .8 Typ es o f h igh- im peda nce re laysA c om m on f o r m o f r e l a y u sed is t he a t tr a c t e d a r m a t u r e t ype wh ose c u r r e n t s e t ti ngm ay be a rou nd 30 m A , and i t has a re l a tive ly h igh s t ab i li s ing res i s to r con nec ted inse ri es w i th i t t o g ive i t t he vo l t age se t t ing de te rm ined by the s t ab i l i ty requ i rem ent s .I t s ope ra t ing t ime w i ll be l e ss than 60 ms a t twice the se t ting cu r ren t .

Anothe r fo rm of a t t r ac ted a rmature re l ay c i rcu i t i ncorpora tes a capac i to r inseries w i th the op era t in g coi l , w hich ren ders the re lay insensi tive to any d .c . vol tagew hich m ay be p resen t dur ing the f ir s t f ew cyc les o f fau l t cur ren t . The re l ay is g iventhe requ i red vo l tage se t t ing by the add i t ion o f l inea r and non l inea r re s is to rs , andi ts c i rcu i t is show n in F ig . 13 .5 .8A . T he use o f the l a t t e r ensures h igh ope ra t ingspeeds a t sma l l inc remen t s above the se t t ing vo l tage , s ince the cu r ren t r ises m orerap id ly than the vo l t age . Wi th th i s re l ay i t i s no t necessa ry to t ake in to account theposs ib i l ity o f a lm os t twice th e ca lcu la ted s inuso ida l a .c . cur ren t f lowing dur ing thefirst f ew cyc les .

Developed f rom this i s a s imi lar re lay in which the nonl inear res is tors arerep laced by ohm ic res is to rs .Be ca use o f t he d i s t o r te d wa v e f o r m o f t he c u r r e n t t h r ough t he re la y du r i ngs tab i l i ty and opera t ion , on ly ce r t a in re l ays a re su i t ab le fo r h igh4mpedancec i rcu la t ing cur ren t p ro tec t ion . For th i s reason , re l ays shou ld be se lec ted f rom typeswh i c h ha ve be e n t e s t e d i n c on j unc t i on wi t h t yp i c a l c . t . s t o de m ons t r a t e t ha t t he yg ive the pe r form ance p red ic ted b y ca lcu la t ion . In those t e s t s the cu r ren t o r the l eadbu r de n wi l l ha ve be e n i nc r e a se d un t i l t he sy s t e m be c om e s uns t a b l e , showi ng t ha tthe re is a fac to r o f sa fe ty in the ca lcu la t ions v a ry ing , fo r typ ica l re l ays , f rom 1-4 to2. From the ear l ie r ca lcula t ion of s tabi l i s ing resis tor va lue , or re lay vol tage se t t ing,i t appeared tha t the re lay w ould be jus t on the po in t o f ope ra t ing fo r a fau l t o fm a x i m um se ve ri ty a nd wi t h t he m os t a dver se o f o t he r c ond i t ions . H owe ve r , be -cause o f the pess imis t i c a ssumpt ions made in the ca lcu la t ion , th i s fac to r o f sa fe tyensures tha t the requ i red s t ab i l i ty i s ach ieved w i tho u t in t rodu c ing o the r fac to rs toincrease the se t t ing vol tage above the va lue given by eqn. 13.5 .1 .

Volts I*/5C -Plugsettingbridge

150 125 IO0 75 50 25- RelayA ~'~ A ~ ~'~ Linear Pcer~ing

, , , ' , ' I I " i - . _ oIN o n - li n e a r r e s is t o r C a p a c i t o r Ik j r - 7t ab i l i s i n g r e s i s t o r

F i g , 1 3 . 5 . 8 A Relay w i th capa c i tor and no nl inear s tabf li s ing resistors


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8usbar protec t ion 11313 .5 .9 Practi ca l h i sh- imp edan ce ins ta lla t ionsFo r t h e r ea s o n s a l r ead y g iv en , mo s t o f t h e h ig h - imp ed an ce s ch emes w h ich h av ebeen in s ta l led a re o f the phase and ear th - fau l t type , employ ing th ree-po le r e laysins tead o f the s ing le -po le re lays w h ich wou ld b e r equ i red fo r ea r th - fau l t on lyschemes . Th e th ree-po le r e lay scheme has the add i t ion a l advan tage o f enab l inglower ear th - fau l t se t t ings to be ach ieved fo r the sam e s ize o f sw i tchboa rd .The ear ly in s ta l la t ions em ploy ed on ly one r e lay fo r each check and d isc r imina t in8zone , as show n in F ig . 13 .5 .9A . The d isadvan tage o f busbar p ro tec t io n schemesem p lo y in g r e lay s co n t r o ll i n g th e t r ip p in g o f m o r e th an o n e c i rcu i t b r eak e r w as th a tth e p r o t ec t io n t en d ed to b e t ak en o u t o f s er vi ce mo r e o f t en th an w as n eces s a ry ,because o f the f ear o f caus ing mal t r ipp ing wh i le tes t ing . I t i s now recogn ised tha tthe consequ ence o f a busba r f au l t , w hen the busba r p ro tec t io n is no t in se rv ice , canbe con s iderab ly m ore se r ious than tha t o f a poss ib le m alop era t ion wh i le tes t ing . I tw as f r o m th e d es i r e t o r ed u ce th e r i s k s o f ma lo p e r a t io n to a m in imu m an d toe l imina te the d i f f icu l t ies assoc ia ted wi th tak ing the busbar p ro tec t ion ou t o f se rv icef o r n o r ma l co mm is sio n in g and m a in t en an ce p u r p o ses t h a t t h e s ch eme o f F ig . 1 3 . 5. 9 Bwas deve loped fo r 400 kV subs ta t ions . Main tenance o f th i s type o f in s ta l la t ion ,employ ing separa te a . c . r e lays fo r each c i r cu i t b reaker , i s very much eas ie r than i ti s w i th in s ta l la t ions employ ing on ly one r e lay per zone , and can be car r ied ou t wi ththe p ro te c t ion in se rv ice . The absence o f aux i l ia ry swi tches in the d . c . c i r cu it sr esu lt s in a s impler sch em e and there fo re one w h ich is less l ike ly to f a il to ope ra tew hen requ i red to do so . The schem e i s des igned in acco rda nce w i th the p r inc ip lesdescr ibed in S ec t ion 13 .5 .5 and 13 .5 .6 , and a l thoug h there a re now severa l r e lays inpara l le l per zone a sa t i s f ac to ry f au l t se t t ing can s t i l l be ob ta ined by us ing r e laysw i th a low cu r ren t a t se t t ing so tha t the to ta l cu r ren t r equ i red fo r the i r ope ra t ionis s t i l l low .

S ince ind iv idua l r e lay room s a re p rov ided a t B r it ish 400 k V d oub le busbarsubs ta t ions us ing open- te rmina l swi tchgear , tha t i s to say one room ad jacen t to eachc i rcu i t b reaker , the p e r c i r c u i t h igh- impedance r e lays and the assoc ia ted t r ip r e laysare accommodated there in . I t w i l l be seen tha t bus sec t ion and bus coup ler swi tchesrequ i re th ree h igh - impedance r e lays , nam ely one fo r the check zone , and two fo rd i s c rim in a t in g zo n es. E ach r e lay r o o m is eq u ip p ed w i th a 1 l 0 V b a t t e r y an d th i ssuppl ies the local re lays .

S in ce th e i n t r o d u c t io n o f t h is s y s t em w i th i ts p e r c i r c u i t r e lays , the need toach ieve r e l iab le , f ast c learance o f busbar f au l t s has assumed even g rea te r im por tan ce .Al though re lay f a i lu res have been smal l in number , i t was cons idered necessary toca te r fo r such a s i tua t ion by conne c t ing to the a . c . buswires per zone r e lays , theco n tac t s o f w h ich a r e co n n ec te

busbar protn

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