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KILOVAR BRIEFS Date NOVEIIBER , 1924 FROM THE McGRAw-EDIsoN POWER SYSTEMS CAPACITOR PLANT GREENWOOD, SOUTH CAROI-INA O v e r v o l t a g e s on C a p a c i t o r Banks - Issue 1 De-energization Transients The u s e o f s h u n t - c a p a c i t o r b a n k s f o r v o l t a g e c o n t r o l a n d p o w e r - f a c t o r improvement has i n c r e a s e d a t b o t h t h e t r a n s m i s s i o n and t h e d i s t r i b u t i o n voltage levels. The c r i t i c a l n e e d f o r r e l i a b i l i t y i n t h e s e i n s t a l l a t i o n s j u s t i f i e s careful evaluation o f overvoltage steady-state, harmonic, l i g h t n - i n g - s u r g e , and s w i t c h i n g - s u r g e p r o t e c t i o n on t h e c a p a c i t o r banks t h e m s e l v e s and t h e r e s t o f t h e system. Steady-state and harmonic o v e r v o l t a g e s c a n n o t be e f f e c t i v e l y l i m i t e d by surge arresters; these c o n d i t i o n s m u s t b e c o n t r o l l e d v i a s y s t e m d e s i g n a n d m o n i t o r i n g schemes. L i g h t n i n g and s w i t c h i n g s u r g e o v e r v o l t a g e s c a n be l i m i t e d by s u r g e arresters. - - I n t h e past, surge a r r e s t e r s were a p p l i e d t o p r o t e c t c a p a c i t o r banks a g a i n s t o v e r v o l t a g e s ; however, t h e gap-type s u r g e a r r e s t e r s o f t e n c o u l d n o t w i t h s t a n d t h e subsequent c a p a c i t o r discharge. The p o s s i b i l i t y o f e x c e s s i v e a r r e s t e r . d u t y h a s r e s u l t e d i n many c a p a c i t o r i n s t a l l a t i o n s w i t h o u t a r r e s t e r protection. New M O V g a p l e s s s u r g e a r r e s t e r s c a n w i t h s t a n d s i g n i f i c a n t l y h i g h e r c a p a c i t o r d i s c h a r g e e n e r g i e s a l l o w i n g t h e renewed a p p l i c a t i o n o f a n o l d surge p r o t e c t i o n concept. T h i s a r t i c l e addresses one p a r t i c u l a r l y s e r i o u s t y p e of s w i t c h i n g restriking of a switch following capacitor bank overvoltage de-energization. W h i l e modern s w i t c h g e a r i s designed t o be r e s t r i k e f r e e , r e s t r i k e s do i n f a c t o c c u r , and s h o u l d be c o n s i d e r e d as a d e s i g n contingency. - To u n d e r s t a n d t h e s e r i o u s consequences o f a r e s t r i k e o f a c a p a c i t o r switching device, i t i s important t o analyze t h e t r a n s i e n t overvoltages t h a t a r e i m p r e s s e d o n a c a p a c i t o r bank f o l l o w i n g normal d e - e n e r g i z a t i o n o f t h e b a n k , shown i n f i g u r e 1. A p r o p e r l y o p e r a t i n g s w i t c h i n g d e v i c e w i l l I n t h e case o f a c a p a c i t o r always c l e a r t h e c i r c u i t a t a c u r r e n t zero. bank, t h e c u r r e n t z e r o of t h e s w i t c h w i l l a l w a y s o c c u r c o i n c i d e n t w i t h t h e v o l t a g e b e i n g a t c r e s t v a l u e o n t h e c a p a c i t o r bank. T h a t i s , t h e v o l t a g e i s always 90 degrees o u t o f phase w i t h t h e c u r r e n t . When t h e s w i t c h i n t e r r u p t s t h e c i r c u i t a t c u r r t n t zero, a c r e s t v o l t a g e o f one p e r u n i t i s Power Systems Division McGraw-Edlson Company Post Offlce Box 1224 Greenwood, SC 29648 t r a p p e d on t h e c a p a c i t o r u n i t s . T h i s w i l l remain t h e r e i n t h e form o f a DCvoltage which i s g r a d u a l l y d i s s i p a t e d through t h e i n t e r n a l discharge resistors o f the capacitor units. These r e l a t i o n s h i p s a r e i l l u s t r a t e d i n F i g u r e 2. the transient recovery voltage F i g u r e 3 i s an o s c i l l o g r a m of i m p r e s s e d upon one phase o f t h e s w i t c h i n g d e v i c e f o l l o w i n g s u c c e s s f u l interruption of the circuit. It i s important t o note t h a t here t h e i n i t i a l r a t e o f r i s e o f t h e r e c o v e r y v o l t a g e i s q u i t e low. This i s characteristic A c a p a c i t o r bank i s a c t u a l l y one o f o f switching i n a capacitor circuit. t h e e a s i e s t t y p e s o f c i r c u i t s t o i n i t i a l l y i n t e r r u p t because o f t h i s l o w r a t e of r i s e of t h - recovery voltage. NORMAL DE-ENERGIZING OF CAPACITOR BANK msn UI NORMAL TRANS1ENT RECOVERY VOLTAGE ACROSS SWITCHING DEVICE FIGURE 2 A f t e r the c i r c u i t i s interrupted, the recovery voltage gradually b u i l d s up u n t i l 180 degrees l a t e r , o r a p p r o x i m a t e l y i n e i g h t m i l l i s e c o n d s , t h e t r a n s i e n t r e c o v e r y v o l t a g e a c r o s s t h e s w i t c h i n g d e v i c e r e a c h e s a peak value o f two per u n i t . Some s w i t c h e s may n o t b e a b l e t o w i t h s t a n d t h i s o v e r v o l t a g e a n d w i l l b r e a k down. I f t h e breakdown o c c u r s a f t e r 90 degrees, t h e phenomena i s c a l l e d a r e s t r i k e . I f i t o c c u r s b e f o r e 90 degrees, t h e phenomena i s c a l l e d a r e i g n i t i o n . When a r e s t r i k e o c c u r s , c u r r e n t w i l l f l o w i n t o t h e c a p a c i t o r b a n k through t h e switching device. The c u r r e n t w i l l b e s i n u s o i d a l i n shape and have t h e n a t u r a l f r e q u e n c y o f t h e c i r c u i t , which i s n o r m a l l y i n t h e range o f 200 t o 8 0 0 h e r t z . The m a g n i t u d e o f t h i s c u r r e n t i s h i g h e r t h a n t h a t o f t h e 6 0 h e r t z p o w e r c u r r e n t t h a t was f l o w i n g i n t o t h e c a p a c i t o r bank p r i o r t o the f i r s t interruption o f the circuit. F o r t h e w o r s t case r e s t r i k e , t h e s y s t e m v o l t a g e i s e q u a l b u t o p p o s i t e i n s i g n t o t h e DC v o l t a g e t r a p p e d o n the capacitor unit. The c a p a c i t o r v o l t a g e w i l l a t t e m p t t o r e a c h t h e s y s t e m v o l t a g e , b u t b e c a u s e o f t h e m i n i m a l amount o f d a m p i n g i n t h e c i r c u i t , t h e v o l t a g e w i l l o v e r s h o o t and, i n f a c t , a p p r o a c h t h r e e p e r u n i t a t t h e same t i m e as t h e r e s t r i k e c u r r e n t r e a c h e s i t s n e x t n a t u r a l c u r r e n t z e r o . Many successfully clear the c i r c u i t at t h i s point switching devices w i l l and t r a p a v o l t a g e o f t h r e e p e r u n i t DC o n t h e c a p a c i t o r s . T h i s e f f e c t i s shown i n F i g u r e 4. While capacitor u n i t s are designed t o withstand t h i s t y p e o f o v e r v o l t a g e , i t i s s t i l l n o t recommended t h a t t h e c a p a c i t o r s b e r o u t i n e l y subjected t o t h i s stress. Proper surge a r r e s t e r p r o t e c t i o n would l i m i t t h i s v o l t a g e t o 2 p e r u n i t . Additional restrikes o f the switch are possible since the i n i t i a l recovery voltage across t h e switching device i s again q u i t e low f o l l o w i n g i n t e r r u p t i o n of t h e f i r s t r e s t r i k e c u r r e n t . T h i s e f f e c t i s shown i n F i g u r e 5 where i t can be seen t h a t f o l l o w i n g a second r e s t r i k e , t h e magnitude o f t h e c u r r e n t i s even h i g h e r and t h e v o l t a g e r e v e r s e s p o l a r i t y f r o m a v a l u e It i s of 3 p e r u n i t t o a value o f 5 p e r u n i t i n t h e o p p o s i t e d i r e c t i o n . i m p o r t a n t t o n o t e t h i s phenomena c a n b e d e s t r u c t i v e n o t o n l y t o t h e d e v i c e s w i t c h i n g t h e c a p a c i t o r banks, b u t t o t h e c a p a c i t o r bank i t s e l f and o t h e r components i n t h e system. DE-ENERGI Z I NG CAPACITOR BANK WITH RESTRIKING SWITCHING DEVICE FIGURE 4 DE-ENERGIZING CAPACITOR BANK WITH MULTIPLE RESTRIKING SWITCHING D E V I C E FIGURE 5 F i g u r e 6 shows t h a t p r o p e r l y a p p l i e d M O V a r r e s t e r s w i l l : ; ; n i t t h e v o l t a g e s on t h e c a p a c i t o r banks from exceeding a p p r o x i m a t e l y 2 p e r u n i t f o r any number o f r e s t r i k e s . MOV a r r e s t e r s w i l l a l s o r e d u c e t h e magnitude o f t r a n s i e n t r e c o v e r y v o l t a g e s s h o u l d a r e s t r i k e o c c u r and, t h e r e f o r e , p r o t e c t t h e s w i t c h i n g d e v i c e a s w e l l a s o t h e r cornk ) n e n t s i n t h e s y s t e m . MOV ARRESTERS LIMITI~G OV ERVOLTAGES DUE TO RESTRIKES RECOrlMENDED . ARRESTER CONNECTION FIGURE 6 FIGURE 1 The m a g n i t u d e s o f t h i s t y p e o f t r a n s i e n t o v e r v o l t a g e v a r y , d e p e n d i n g upon w h e t h e r t h e c a p a c i t o r bank b e i n g s w i t c h e d i s c o n n e c t e d grounded o r u n g r o u n d e d wye. T h e b a s i c p h e n o m e n a i s t h e same i n e i t h e r c a s e . McGraw-Edison recommends t h a t t h e s u r g e a r r e s t e r s b e a p p l i e d f r o m l i n e t o ground on t h e c a p a c i t o r s i d e o f t h e s w i t c h . F i g u r e 7 shows t h e recommended a r r e s t e r c o n n e c t i o n p r a c t i c e s f o r a grounded-wye c a p a c i t o r bank. S y m p t o m s o f r e s t r i k i n g s w i t c h i n g d e v i c e s may i n c l u d e s w i t c h a n d / o r c a p a c i t a r f a i l u r e s , s p u r i o u s f u s e operations, f i a s h o v e r s o f i n s u l a t o r s , and c o u n t s on s u r g e a r r e s t e r s l o c a t e d i n t h e s u b s t a t i o n . T h i s a r t i c l e addressed one aspect o f c a p a c i t o r s w i t c h i n g , s p e c i f i c a l l y r e s t r i k i n g of a switch following de-energization o f a g r o u n d e d wye c a p a c i t o r bank. A f u t u r e i s s u e o f KILOVAR BRIEFS w i l l e x p a n d o n t h e s u b j e c t o f c a p a c i t o r bank s w i t c h i n g , d e a l i n g w i t h a r e s t r i k i n g s w i t c h o n u n g r o u n d e d wye b a n k s , d e l a y e d r e s t r i k e s a n d t h e e f f e c t s o f r e s t r i k e s o n t h e capacitors.