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Chapter 1I n t r o d u c t i o n

1 .1 Gen e r a lW ith t he inc r eas ing dep end ence on e l ec t r i c i t y supp l ie s , i n bo th deve lo p ing anddev e lop ed coun t r i e s, t he n eed t o ach i eve an accep t ab l e l eve l o f r e li ab il it y , qua l i tya n d s a f e ty a t a n e c o n o m i c p r i c e b e c o m e s e v e n m o r e i m p o r t a n t to c u s t o m e r s . A f u r-t he r r equ i r em en t i s t he s a f e ty o f t he e l ec t r i c i t y supp ly . A p r i o r i t y o f any sup p ly sy s t emi s t ha t i t ha s bee n w e l l de s igned and p rop e r ly ma in t a ined i n o rd e r t o l im i t the nu m bero f f au lt s tha t m igh t o ccur .

As soc i a t ed wi th t he d i s tr i bu ti on ne two rks t hem se lves a r e a nu m ber o f anc i l l a rysys t ems t o a s s is t i n m ee t i ng t he r equ i r em en t s f o r s a f ety , r e l iab i l i ty and q ua l i t y o f sup -p ly . Th e m os t im por t an t o f t hese a r e t he p ro t ec t i on sys t ems w hich a r e i n s ta l led t o c l ea rf au l ts and l im i t any d am age t o d i s t ri bu t ion equ ipmen t . A m ong s t t he p r i nc ipa l causeso f fau l t s a re l ightn ing d i scharges , the de te r iora t ion of insula t ion , va nda l i sm, and t reeb ranch es an d an ima l s c on t ac t i ng t he e l ec t r i c i ty c i rcu i ts . Th e m a jo r i t y o f f au lt s a re o fa t r ans ien t na tu r e and can o f t en be c l ea r ed wi th no l oss o f supp ly , o r j u s t t he shor t e s to f i n t e rrup t i ons , whe reas pe rm ane n t f au l ts can r e su l t in l ong e r ou t ages . I n o rde r t oavo id dam age , su i tab l e and r e l iab l e p ro t ec t i on shou ld be i n s ta l led on a ll c i rcu i t s ande l ec tr i ca l equ ipmen t . P ro t ec t i ve r e l ays i n i ti a te t he i so l a ti on o f f au l ted s ec t ions o f t hene tw ork i n o rde r t o m a in t a in supp l i es e l s ewhe re on t he sys t em. Th i s then l eads t o animp rove d e l ec t r i c i ty s e rv i ce wi th be t t e r con t i nu i t y and qua l i t y o f supply .A p rope r ly co -o rd ina t ed p ro t ec t i on sys t em i s v i t a l t o ensu re t ha t an e l ec t r i c i t yd i s tr i bu ti on ne tw ork can ope ra t e wi th in p r e se t r equ i r emen t s f o r s a f e ty fo r ind iv idua li tems o f equ ipm en t , s t a f f and pub l i c , and t he ne tw ork ove ra l l . Au tom a t i c ope ra t i onis neces sa ry t o i so l a t e fau l ts on t he n e twork s a s q u i ck ly a s pos s ib le i n o rde r t o min -i m i s e d a m a g e . T h e e c o n o m i c co s t s a n d t h e b e n e f it s o f a p r o t e c t i o n s y s te m m u s t b econs ide red in o rde r to a r r i ve a t a su i tab l e ba l ance be twe en t he r equ i r em en t s o f t hescheme and t he ava i l ab l e f i nanc i a l r e sources . I n add i t i on , m in imi s ing t he cos t s o fnond i s t r i bu t ed en e rgy i s r ece iv ing i nc r eas ing a t t en ti on .

W h e n p r o v i d i n g p r o t e c ti v e d e v i c e s o n a n y s u p p l y n e t w o r k t h e f o l lo w i n g b as icp r inc ip l e s m us t app ly . On t he oc cur r en ce o f a f au lt o r abnorma l cond i t i on , t he

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2 Protection o f electricity distribution networksEHV)c

MajorHVsubstation

HV/MV

HV network

i tMV (Er-overheadnetwork

MVtdjacentnetwork

ruraldistribut

~ HV/MV MV

Vca~ ? ~ M urban distribution

V/LVLV board

O Generator Circuit reaker ~ Feed erTransformer / Disconn ector ~ F us e

[] Customer

Figure 1.1 EH V/H V/M V/L V netw ork arrangements (reproduced fro m ElectricityDistribution N etwo rk Design)

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Introduction 3

pro t ec t i on sys t em mus t be capab l e o f de t ec t i ng i t immedia t e ly i n o rde r t o i so l a t ethe a f f ec t ed sec t ion , t hus pe rmi t t i ng t he r e s t o f t he pow er sys t em to r ema in i n s e rv i ceand l im i t ing t he pos s ib i li t y o f dam age t o o the r equ ipmen t . D i sconn ec t i on o f equ ip -m e n t m u s t b e r e s tr ic t e d t o t h e m i n i m u m a m o u n t n e c e s s a r y t o i s o la t e th e f a u lt f r o m t h es y s te m . T h e p r o t e c t io n m u s t b e s e n si ti v e e n o u g h t o o p e r a t e w h e n a f a u l t o c c u r s u n d e rmin imum fau l t cond i t i ons , ye t be s t ab l e enough no t t o ope ra t e when i t s a s soc i a t ede q u i p m e n t is c a r r y in g th e m a x i m u m r a te d c u r r en t , w h i c h m a y b e a s h o r t -t im e v a l u e . I tmu s t a l so be f a s t enough t o o pe ra t e i n o rde r to c l ea r the f au l t f r om the sys t em qu i ck lyt o m i n i m i s e d a m a g e t o s y s t e m c o m p o n e n t s a n d b e r e l i a b l e i n o p e r a t i o n . B a c k - u ppro t ec t i on t o cov e r t he pos s ib l e f a i l u r e o f t he m a in p ro t ec t i on is p rov id ed i n o rde r t oimp rove t he r e l i ab i li t y o f t he p ro t ec t i on sys t em. W hi l e e l ec t rom echan i ca l r e l ays cans ti ll be fo und i n so m e u t i li ti e s, t he t ende ncy i s t o r ep l ace these b y m ic ropro ces so r andnum er i ca l re l ays , pa r t i cu l a r ly i n the m ore com plex p ro t ec t i on a r rangem en t s .

1 .2 B a s i c p r i n c i p l e s o f e l e c t r ic a l s y s t e m sT h e p r i m a r y a i m o f a n y e l e c tr i c it y s u p p l y s y s t e m i s t o m e e t a ll c u s t o m e r s ' d e m a n d sf o r en e rg y . P o w e r g e n e r a t io n i s c a r r ie d o u t w h e r e v e r it a c h i e v e s th e m o s t e c o n o m i cse l l i ng cos t ove ra l l . The t r ansmi s s ion sys t em i s u sed t o t r ans f e r l a rge amoun t s o fene rgy t o ma jo r l oad cen t r e s , wh i l e d i s t r i bu t i on sys t ems ca r ry t he ene rgy t o t hefu r t hes t cus tomer , u s ing t he m os t approp r i a te vo l t age l eve l . Wh ere t he t r anspor t o fve ry la rge amou n t s o f po w er ov e r l a rge d i s t ances is i nvo lved , an ex t r a h igh v o l t age( E H V ) s y s te m , s o m e t i m e s t e r m e d m a j o r o r p r i m a r y t r a n sm i s s io n , is r e q u ir e d . S u c hsys t ems ope ra t e i n the 300 kV p lus r ange , t yp i ca l va lues be ing 400 , 500 and 765 kV.

H i g h v o l t a g e ( H V ) n e t w o r k s t r a n s p o r t l ar g e a m o u n t s o f p o w e r w i t h i n a p a r ti c u la rr eg ion and a r e op e ra t ed e i t he r a s i n t e r conn ec t ed sys t ems o r d i s c re t e g roups . B e lowthe t r ansmi s s ion sy s t em the r e can be tw o o r t h r ee d i st r ibu t i on vo l t age l eve l s to ca t e rf o r t h e v a r i e ty o f c u s to m e r s a n d t h e i r d e m a n d s . I n g e n e ra l , th e m e d i u m v o l t a g e ( M V )ne tw orks and l ow vo l t age (LV) ne tw orks a r e ope ra t ed as rad i a l sys tems .

F igure 1.1 i ll u s tr a te s t he i n t e r re l a t ion o f t he va r i ous ne tworks . Th e H V ne t -w o r k s a r e s u p p l i e d f r o m E H V / H V s u b s t a t i o n s w h i c h t h e m s e l v e s a r e s u p p l i e d b yin t e r- r eg iona l E H V l ines . H V /M V t r ans fo rm ing subs ta t ions s i tua t ed a round each HVn e t w o r k s u p p l y in d iv id u a l M V n e tw o r k s . T h e H V a n d M V n e tw o r k s p r o v i d e s u p-p l i e s d i r ec t t o l a rge cu s tomer s , bu t t he vas t m a jo r i t y o f cus tomer s a r e co nne c t ed a tl ow vo l t age and supp l i ed v i a MV/LV d i s t r i bu t i on subs t a t i ons and t he i r a s soc i a t edne two rks , a s show n in F igure 1 .2 .

1 .3 P r o t e c t io n r e q u i r e m e n t sT h e p r o t e c t i o n a r r a n g e m e n t s f o r a n y p o w e r s y s t e m m u s t t a k e i n t o a c c o u n t t h efo l l owing bas i c p r i nc ip l e s :1 . Re l iab i li ty : the ab i l ity o f the pro tec t io n to opera te cor rec t ly . I t has two e lem ents -dependab i l it y , w h ich is the ce r t a i n ty o f a co r r ec t ope ra t i on on t he occ ur r enc e o f

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4 Protection o f electricity distribution netw ork s

tGeneration

Generat ion

8

E H V

E H V / H V

Figure 1.2

H V

v J M v f tiConsumption

M V / L V f

M V

L V

ConsumptionBlo ck schem atic o f transmission an d distr ibution system s (reproducedfro m E lectricity Distr ibution N etw ork Design )

.

a fau l t , and secur i ty , which i s the ab i l i ty to avoid incor rec t opera t ion dur ingfaults.Speed : m in imum ope ra t i ng t ime t o c l ea r a f au l t i n o rde r t o avo id damage t oe q u i p m e n t .

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Introduction 53 . S e l e ct iv i ty : m a i n t a i n i n g c o n t i n u i ty o f s u p p l y b y d is c o n n e c t i n g t h e m i n i m u m

s e c t i o n o f t h e n e t w o r k n e c e s s a r y t o i s o l a t e th e f a u l t.4 . C o s t: m a x i m u m p r o t e c t i o n a t t h e l o w e s t c o s t p o s s i b l e .S i n c e i t is p r a c t ic a l l y i m p o s s i b l e t o s a t is f y a ll th e a b o v e - m e n t i o n e d p o i n t s s i m u l -t a n e o u s l y , i n e v i t a b l y a c o m p r o m i s e i s r e q u i r e d t o o b t a i n t h e o p t i m u m p r o t e c t i o ns y s t e m .

1 .4 P r o t e c t i o n z o n e sT h e g e n e r a l p h i l o s o p h y f o r t h e u s e o f r e l a y s i s t o d i v id e t h e s y s t e m i n to s e p a r a tez o n e s , w h i c h c a n b e i n d i v i d u a l ly p r o t e c t e d a n d d i s c o n n e c t e d o n t h e o c c u r r e n c e o f af au l t, i n o r d e r t o p e r m i t t he r e s t o f th e s y s t e m t o c o n t in u e i n s e r v i c e w h e r e v e r p o s s ib l e .I n g e n e r a l a p o w e r s y s t e m c a n b e d i v i d e d i n t o p r o t e c t i o n z o n e s - g e n e r a t o r s ,t r a n s f o r m e r s , g r o u p s o f g e n e r a t o r tr a n s f o r m e r s , m o t o r s , b u s b a r s a n d l in e s . F i g u r e 1.3s h o w s a s y s t e m w i t h d i ff e r e n t p r o t e c t i o n z o n e s . I t sh o u l d b e n o t e d t h a t t h e z o n e so v e r l a p a t s o m e p o i n t s i n d i c a t i n g t h a t, i f a f a u lt o c c u r s i n th e s e o v e r l a p a r e a s , m o r et h a n o n e s e t o f p r o t e c t i o n r e l a y s s h o u l d o p e r a t e . T h e o v e r l a p i s o b t a i n e d b y c o n n e c t i n gt h e p r o t e c t i o n r e l a y s t o t h e a p p r o p r i a t e c u r r e n t t r a n s f o r m e r s a s i ll u s t ra t e d i n F i g u r e 1 .4 .

1 .5 P r i m a r y a n d b a c k - u p p r o t e c ti o nA l l t h e e l e m e n t s o f t h e p o w e r s y s t e m m u s t b e c o r r e c t l y p r o t e c t e d s o t h a t th e r e l a y s o n l yo p e r a t e o n t h e o c c u r r e n c e o f a f a u lt . S o m e r e l a y s , d e s ig n a t e d a s u n it t y p e p r o t e c ti o n ,o p e r a t e o n l y f o r f a u lt s w i t h i n t h e i r p r o t e c t io n z o n e . O t h e r r e l a y s d e s i g n a t e d a s n o n - u n i tp r o t e c t i o n , a r e a b l e t o d e t e c t f a u lt s b o t h wi t h i n a p a r t i c u l a r z o n e a n d a l s o o u t s i d e i t,u s u a l l y i n a d j a c e n t z o n e s , a n d c a n b e u s e d t o b a c k u p t h e p r i m a r y p r o t e c ti o n a s as e c o n d l in e o f d e f e n c e . I t is e s s e n t i a l th a t a n y f a u l t i s is o l a t ed , e v e n i f t h e a s s o c i a t e dm a i n p r o t e c t i o n d o e s n o t o p e r a te . T h e r e f o r e , w h e r e v e r p o s s ib l e , e v e r y e l e m e n t i n t h ep o w e r s y s t e m s h o u ld b e p r o t e c te d b y b o t h p r i m a r y a n d b a c k - u p r e l ay s .1.5.1 Primary protectionP r i m a r y p r o t e c ti o n s h o u l d o p e r a t e e v e r y ti m e a n e l e m e n t d e t e c ts a f a u lt o n th e p o w e rs y s te m . T h e p r o t ec t io n e l e m e n t c o v e r s o n e o r m o r e c o m p o n e n t s o f t he p o w e r s y s t e m ,s u c h a s e l e c t r i c a l m a c h i n e s , l i n e s a n d b u s b a r s . I t i s p o s s i b l e f o r a p o we r s y s t e mc o m p o n e n t t o h a v e v a r i o u s p r i m a r y p r o t e c t i o n d e v i ce s . H o w e v e r , th i s d o e s n o t i m p l yt h a t th e y a ll h a v e t o o p e r a t e f o r t h e s a m e f a u l t, a n d i t s h o u l d b e n o t e d t h a t t h e p r i m a r yp r o t e c t i o n f o r o n e i t e m o f s y s t e m e q u i p m e n t m i g h t n o t n e c e s s a r i l y b e i n s ta l le d a t th es a m e l o c a ti o n a s th e s y s t e m e q u i p m e n t ; i n s o m e c a s e s i t c a n b e s it e d in a n a d j a c e n ts u b s t a t i o n .1.5.2 Back-up protectionB a c k - u p p r o t e c t i o n i s i n s t a l l e d t o o p e r a t e w h e n , f o r w h a t e v e r r e a s o n , t h e p r i m a r yp r o t e c t i o n d o e s n o t w o r k . T o a c h i e v e t h is , th e b a c k - u p p r o t e c t i o n r e l a y h a s a s e n s i n g

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6 Protection of electricity distribution networks

TII

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Figure 1.3 Protection zones

B u s b a r ) r o t e c t io n

L i n e p r o t e c t io nFigure 1.4 Overlap ofprotection zones

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Introduction 7e l e m e n t w h i c h m a y o r m a y n o t b e s im i l a r t o t h e p r i m a r y p r o te c t io n , b u t w h i c h a l s oinc ludes a t ime-d e l ay f ac i li ty t o s l ow d ow n the ope ra t i on o f t he r e l ay so a s t o a l l ow t imefo r t he p r imary p ro t ec t i on t o ope ra t e f ir st . On e r e l ay can p rov id e back-up p ro t ec t i ons i m u l ta n e o u s l y to d i ff e r e n t p i e c e s o f s y s te m e q u i p m e n t . E q u a l l y th e s a m e e q u i p m e n tc a n h a v e a n u m b e r o f d i f fe r e n t b a c k - u p p r o t e c t i o n re l a y s a n d i t is q u i te c o m m o n f o ra r e l a y to a c t a s p r i m a r y p r o t e c t i o n f o r o n e p i e c e o f e q u i p m e n t a n d a s b a c k - u p f o ranother .

1 .6 D i r e c t i o n a l p r o t e c t i o nAn impo r t an t cha rac te r i s ti c o f som e types o f p ro t ec t i on i s the i r capac i t y t o be ab l e t ode t e rm ine the d i r ec t ion o f t he f l ow o f pow er and , b y t h is m eans , t he i r ab i l it y to i nh ib i topen ing o f t he a s soc i a t ed sw i t ch wh en t he f au l t cu r r en t f l ows i n the op pos i t e d i r ec t ionto t he se t t ing o f t he r e lay . Re l ays p rov ided w i th t h is ch a rac t e ri s ti c a r e imp or t an t i np ro t ec t i ng mesh n e tworks , o r where t he re a re va r ious gene ra t i on sources , w hen f au l tcu r r en t s can c i r cu l a t e in bo th d i r ec t ions a rou nd t he m esh . I n these ca ses , d i r ec ti ona lp r o t e c t io n p r e v e n t s th e u n n e c e s s a r y o p e n i n g o f s w i tc h g e a r a n d t h u s i m p r o v e s t h esecur i t y o f t he e l ec t r i c i ty supp ly . O n p ro t ec t i on schem a t i c d i ag rams t he d i r ec ti ona lp ro t ec t i on i s u sua l l y r ep re sen t ed by an a r row unde rnea th t he appropr i a t e symbol ,i nd i ca ti ng t he d i r ec t ion o f cu r r en t f l ow fo r r e l ay op e ra t ion .E x a m p l e I . I C h e c k o n c o r r e c t o p e r a t i o n o f p r o t e c t io nUsing t he po w er sys t em show n in F igure 1 .5 , exam ples a re g iven where t he re has beenincor r ec t ope ra t ion o f p ro t ec t i on and t he a s soc i a t ed b reake r s , l e ad ing t o t he o pe ra t i ono f b a c k - u p p r o t e c t io n t o i so l a te t h e f a u l t f ro m t h e s y s te m , f o l l o w e d b y a n e x a m p l e

F - - ~ - - - 'm r -- - - - F - ~I l i " t I 'i / i i

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8 Protection o f electricity distribution netw orkso f c o r r e c t r e l a y o p e r a ti o n , w i th a f in a l e x a m p l e o f u n n e c e s s a r y r e l a y o p e r a ti o n . T h ed i re c t io n a l p r o t e c t i o n i s in d i c a t e d b y t h e a r r o w s b e l o w t h e c o r r e s p o n d i n g b r e a k e r s .

T a b l e 1 . 1 s h o w s t h e b r e a k e r s t h a t f a i l e d t o o p e n a n d t h o s e t h a t w e r e t r i p p e d b yt h e p r i m a r y p r o t e c t i o n a n d b y t h e b a c k - u p p r o t e c ti o n .

Table 1.1 Relay~breaker operations fo r Ex am ple 1.1

Case Breakers Breakers Tripped b y Tripped b ythat that pr imary back-upoperated mal-operated protect ion protect ion

F I 1 , 2 , 4 3 4 1 , 2F 2 3 , 5 , 8 6 3 , 5 , 8F 3 10 - 10 -F 4 8, 11 8 |1 -

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I B a j o A n c h i c a y ~ L - - J r I 1 1 5 k V

i 7 '. - ] "q _ LI 2 1 Ii ~ ' 1 II II Iv / S I I

LI I I r l V i i ( . . . . .I I I | 1 I / I LI I - ~ 4 1 ~ - ~ I /I , I L E t l I Ip i I ~ k V Ii ! r I ~ I I ~ l ] iI I I 1 1 6 iI ; 1 7 1 I 1 8 I

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Figure 1.6 Sche ma tic diagram fo r Exercise 1.1

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9/9

Introduction 9Fo r f au l t F1 , t he p ro t ec t i on co r r ec t l y t r ipped b r eake r 4 t o op en on e end o f t he

f au l t ed feede r . W i th b r eak e r 3 f a il ing to op en , b r eake r s 1 and 2 w ere t r ipped b yback-up p ro t ec t i on t o s t op f au l t cu r r en t f l owing i n to t he f au l t f r om gene ra to r s G1and G2 . W i th f au l t F2, wh en b r eak e r 6 f a i l ed t o ope ra t e , the d i r ec ti ona l p ro t ec t i on onbreake r s 3 and 8 ope ra t ed t o open t he i ncoming f eede r s f rom the ad j acen t busba r s ,and t he back-up p ro t ec t i on on b r ea ke r 5 t ri pped t o s t op G3 f eed ing i n to the f au l t.

Fau l t F3 was co r r ec t l y c l ea r ed by t he t r ipp ing o f f eede r b r eak e r 10. Fau l t F4 wascor r ec t l y c l ea r ed by t he ope ra t i on o f b r eak e r 11, so t ha t t he t r ipp ing o f b r eak e r 8 wasincor r ec t. A ny f au l t cu r r en t f l owing a long i n t e r-busba r f eede r 7 -8 be fo re b r eak e r 11opened wou ld have been f rom 7 t o 8 . Re l ay 8 i s d i r ec t i ona l and ope ra t i on shou ldno t have been i n i t i a t ed fo r f l ows f rom 7 t o 8 . Thus t he f i r s t two cases i l l u s t r a t em a l -ope ra t i on f rom a dep endab i l it y po in t o f v iew, w i th t he l a st one i l lu s t r at ing m a l -ope ra t i on f ro m a s ecu r i t y s t andpo in t .

1 . 7 E x e r c i s e 1 .1

Fo r t he po w er sys t em a r r angem en t show n in F igure 1 .6 , comp le t e Tab l e 1 .2 , t ak ingin to accoun t t he ope ra t i on o f t he c i r cu i t b r eake r s a s show n fo r each f au l t c a se . P l easeno t e t hat , a s i n Exam ple 1 .1 , som e o f t he c i r cu it b r eake r s t ha t ope ra t ed m ay h avedon e so unneces sa r i ly .

Table 1.2 Relay~breaker operations for Exercise 1.1

Case Breakers Breakers Tripped bythat that primaryoperated ma l-operated protectionF 1 2 , 3 , 4 , 5F2 21, 22, 2324, 27F3 10, 11, 17, 19

2 ,5

Tripped b yback-upprotection