Very Fast Transients in GIS

IEEE Transactions on Power Delivery, Vol. 4, No. 1, January 1989

V e r y F a s t T r a n s i e n t s i n GIs

I N T E R N A L V F T

t r a v e l l i n g waves i n s i d e G I s

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E X T E R N A L V F T

t r a v e l l i n g waves and r a d i a t i o n o u t s i d e GIs

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J . Meppe l ink , K. D i e d e r i c h BBC Brown B o v e r i L t d .

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D u r i n g s w i t c h i n g o p e r a t i o n s o r e a r t h f a u l t s i n a SF - g a s - i n s u l a t e d swi t c h g e a r ( G I S ) v e r y f a s t t r a n s i e n t s ( V f T ’ s ) o c c u r and s t r e s s t h e equ ipment i n G I s , adjacent. equipment, a i r - i n s u l a t e d swi t c h g e a r (AIS) and i n secondary equ ipment .

The d i f f e r e n t t y p e s o f v e r y f a s t t r a n s i e n t s a r e c l a s s i f i e d and t h e i r c h a r a c t e r i s t i c parameters a r e summarized based on measurements i n a 800-kV-SFp-gas- i n s u l a t e d p i l o t i n s t a l l a t i o n and a 420-kV-G S i n s e r v i c e .

The measurements show t h a t t h e VFT’s a t t h e i r o r i g i n i n a G I S a r e c h a r a c t e r i z e d by a s t e e p f r o n t h a v i n g 4-7 n s r i s e t i m e f o l l o w e d b y a monof requent o s c i l l a t i o n o f some MHz caused by t h e e x t e n s i o n o f t h e s u b s t a t i o n . A f t e r a VFT has passed a b u s h i n g i t s shape and a m p l i t u d e w i l l be changed. Two t r a n s i e n t waves a r e observed o u t s i d e t h e GIS. The e l e c t r o m a g n e t i c f i e l d between t h e e n c l o s u r e and e a r t h r e s u l t s i n a t r a n s i e n t e n c l o s u r e v o l t a g e o f up t o some 10 kV and a t t h e same t i m e i n a r a d i a t i o n o f an e l e c t r o m a g n e t i c f i e l d f r o m t h e e n c l o s u r e w i t h a m p l i t u d e s o f some 10 kV/m. The e l e c t r o m a g n e t i c wave g u i d e d by t h e overhead l i n e w i l l r e s u l t i n an o v e r v o l t a g e w i t h a m u l t i f r e q u e n t o s c i l l a t i o n o f some 10 t o 100 kHz depending on t h e parameters o f t h e overhead l i n e and t h e a d j a c e n t equipment, e .g . a t r a n s f o r m e r . I n t h i s paper e s p e c i a l l y t h e t r a n s i e n t e n c l o s u r e v o l t a g e and measures t o reduce i t a r e ana lysed.

Keywords : g a s - i n s u l a t e d s u b s t a t i o n , f a s t t r a n s i e n t s , t r a n s i e n t e n c l o s u r e v o l t a g e

1. I n t r o d u c e

I n G I S v e r y f a s t t r a n s i e n t o v e r v o l t a g e s (VFT’s ) a r e g e n e r a t e d d u r i n g d i s c o n n e c t o r o r b r e a k e r o p e r a t i o n o r b y an e a r t h f a u l t . I n t h e p a s t a l o t o f e x p e r i e n c e has been o b t a i n e d w i t h t h e s e VFT’s /9/, /13 / . T h e r e f o r e t h e r e s e a r c h e f f o r t s have been c o n c e n t r a t e d on t h e i n - v e s t i g a t i o n o f t h e e f f e c t s o f t h e s e v e r y f a s t t r a n s i - e n t s i n G I S and a d j a c e n t equ ipment . Repor ted a r e i n - s u l a t i o n f a i l u r e s i n GIS /9/, b u s h i n g f a i l u r e s 1131, resonances i n connected t r a n s f o r m e r s w i t h c o n s e c u t i v e i n s u l a t i o n f a u l t s and m a l f u n c t i o n o f e l e c t r o n i c d e v i c e s /12/, e . g . o f p r o t e c t i v e r e l a y s .

A c c o r d i n g t o t h e IEC- and ANSI-Standards G I S equ ipment has t o be d e s i g n e d f o r t h e B I L as t h e w o r s t case c o n d i t i o n . Up t o now t h e r e a r e no Standards w i t h r e s p e c t t o VFT’s. On t h e o t h e r hand VFT’s were known i n t h e p a s t and t h e i r consequences were t a k e n i n t o

86 WM 114-1 by t h e ti.:t.:E S u b s t a t i o n s Committee of t h e L E V ? ‘r’ower C n g i n e c r i n g S o c i e t y f o r prc’sentnt ion a t t h e

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n;;, TTeig York , ’Jeur Y o r k , 988. l a n u s c r i p t

!lade a v a i l a b l e for

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O R I G I N O F V E R Y F A S T T R A N S I E N T S

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o e a r t h f a u l t d u r i n g on s i t e t e s t i n g o r i n

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I V E R Y F A S T T R A N S I E N T S I

I TRANSIENT ENCLOSURE I VOLTAGE

OVERVOLTAGES

VFTO between

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MAGNETIC VOLTAGES .........................................

TEMF 1 FTO 1 e n c a p s u l a t i o n

a d j a c e n t s t r e s s o f s t r e s s of i n s u l a t i o n

i n G I s i n s u l a t i o n secoi

i n G I s

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a c c o u n t d u r i n g t h e deve lopment o f G I s . The t e c h n i q u e t o measure VFT’s i s f a r deve loped and computer s i m u l a - t i o n s o f VFT’s a r e u n d e r s t u d y /8/. The VFT i n d u c e d breakdown mechanism has a l s o been i n v e s t i g a t e d / 1 5 / . T h e r e f o r e , more q u a n t i t a t i v e u n d e r s t a n d i n g o f VFT’s i n G I S w i l l l e a d t o o p t i m i z e d GIS d e s i g n . The purpose o f t h i s paper i s t o g i v e a q u a n t i t a t i v e u n d e r s t a n d i n g o f VFT’s based on measurements i n a 8 0 0 - k V - p i l o t - G I S and a 420-kV-GIS i n s e r v i c e . A c l a s s i f i c a t i o n o f t h e d i f - f e r e n t k i n d s o f VFT’s s h a l l g i v e an o v e r v i e w . The o r i g i n o f VFT’s and t y p i c a l parameters o f i n t e r n a l and e x t e r n a l VFT’s, e s p e c i a l l y t h e t r a n s i e n t e n c l o s u r e v o l t a g e a r e d e s c r i b e d i n d e t a i l .

R e s u l t s o f VFT-phenomena o u t s i d e a GIS measured u s i n g a n e w l y deve loped p o t e n t i a l f r e e e l e c t r i c f i e l d sensor w i t h an adequate f r e q u e n c y c h a r a c t e r i s t i c / 4 / w i l l be p r e s e n t e d . V o l t a g e measurement i n s i d e t h e GIS was made u s i n g a c a p a c i t i v e v o l t a g e d i v i d e r w i t h a bandwid th o f 445 MHz / 2 1 / .

2. C l a s s i f i c a t i o n o f VFT’s

A c l a s s i f i c a t i o n o f VFT’s i s g i v e n i n F i g . 1. I t shows t h a t d i f f e r e n t phenomena can be r e l a t e d t o s w i t c h i n g o p e r a t i o n s i n S F 6 - g a s - i n s u l a t e d s w i t c h g e a r c a u s i n g a f a s t v o l t a g e s t e p w i t h a r i s e t i m e i n t h e range o f 4 t o 7 ns a t t h e l o c a t i o r : o f t h e source . The

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1.5 r 1.5 I

0 200 400 600 ns 1000 “0 2 4 6 8 p s l 0 a. t - b. t -

0- 0 1 2 3 4 m s 5

C. t -

disconnectorOS I F / A

F i g . 2: D isconnec to r i nduced v e r y f a s t t r a n s i e n t o v e r v o l t a g e i n a 110-kV-GIS measured w i t h d i f f e r e n t t i m e d e f l e c t i o n s /18 / . a. s teep v o l t a g e t r a n s i e n t b. o s c i l l a t i o n o f t h e G I S w i t h some MHz c . o s c i l l a t i o n o f t h e t r a n s f o r m e r and t h e

d i r e c t l y connected G I s w i t h some 10 kHz

observed phenomena a t d i f f e r e n t d i s t a n c e s f rom t h e o r i g i n o f t h e VFT can be d i v i d e d i n t o i n t e r n a l and e x t e r n a l VFT’s and may e x h i b i t d i f f e r e n t shapes, because o f r e f l e c t i o n s o f t r a v e l l i n g waves g i v e n by t h e l a y o u t o f t h e i n s t a l l a t i o n /8 / and damping e f f e c t s i n s i d e and o u t s i d e o f t h e G I s . The e f f e c t s o f VFT’s on equ ipment used i n s u b s t a t i o n s a r e a l s o ment ioned i n F i g . 1.

2 . 1 I n t e r n a l VFT

As a t y p i c a l example, t h e VFT measured u s i n g a c a p a c i t i v e v o l t a g e d i v i d e r d u r i n g d i s c o n n e c t o r opera- t i o n i n a 110-kV-GIS /18/ a r e shown i n F i g . 2. Measure- ments o f t h e same phenomenon w i t h t h r e e d i f f e r e n t t i m e d e f l e c t i o n s show t h e t y p i c a l waveform o f t h e VFT i n a GIS a t t h e source s i d e o f t h e d i s c o n n e c t o r . The v e r y f a s t f l a s h o v e r i n SF6 genera tes a s teep v o l t a g e impu lse hav ing a t y p i c a l r i s e t i m e o f 4 . . . 7 ns ( F i g . 2 a ) . T h i s p u l s e t r a v e l s w i t h i n t h e G I S and w i l l be f r a c t i o n e d and r e f l e c t e d . A f t e r some p s a monof requent o s c i l l a t i o n on t h e source- and l o a d - s i d e o f t h e d i s c o n n e c t o r can be observed. Approx ima te l y 100 ps a f t e r t h e b e g i n n i n g o f t h e f l a s h o v e r t h e a r c i n t h e d i s c o n n e c t o r e x t i n g u i s h e s because t h e c u r r e n t g e t s t o sma l l / 18 / . The l o a d s i d e i s t h e n d i sconnec ted b u t a t rapped charge remains and t h e t r a n s f o r m e r on t h e source s i d e o s c i l l a t e s w i t h the p r o p e r f requency i n t o t h e s teady s t a t e c o n d i t i o n /18 / . As a r e s u l t t h e i n t e r n a l VFT i s o f a complex v o l t a g e shape w i t h a f a s t r i s e t i m e f o l l o w e d by m u l t i f r e q u e n t o s c i l l a t i o n s i n t h e range o f some MHz and some kHz. The d e s c r i b e d VFT shows a r e p e t i t i o n r a t e o f abou t 200 per d i s c o n n e c t o r o p e r a t i o n . Due t o r e f l e c t i o n s and r e - f r a c t i o n s t h e y have a subs ta t i on -dependen t o s c i l l a t i n g waveshape w i t h a magn i tude up t o 1 .5 p .u . under c a p a c i t i v e s w i t c h i n g /8 / and up t o 2.4 p .u . i n t h e case o f phase o p p o s i t i o n /19/. T h e r e f o r e t h e y neve r reach t h e B I L o f t h e s u b s t a t i o n . The w i t h s t a n d v o l t a g e o f SF6 i n s u l a t e d sw i t chgear under VFT’s i s r e p o r t e d t o be h i g h e r o r equa l t o t h e l i g h t n i n g impu lse w i t h s t a n d v o l t a g e /9/, /20 / . I n t h e case o f i r r e g u l a r i t i e s t h e w i t h s t a n d v o l t a g e f o r VFT’s decreases depend ing on t h e s i z e o f t h e i r r e g u l a r i t i e s /9/. The e f f e c t may be u t i l i z e d t o i d e n t i f y i r r e g u l a r i t i e s d u r i n g o n - s i t e t e s t s w i t h f a s t t r a n s i e n t s .

2.2 E x t e r n a l VFT

An i n t e r n a l l y genera ted VFT p ropaga tes as a

F i g . .3: S i m p l i f i e d e x p l a n a t i o n o f t h e t r a n s i e n t e n c l o s u r e v o l t a g e (TEV) a. p r o p a g a t i o n o f t r a v e l l i n g waves caused

by d i s c o n n e c t o r o p e r a t i o n 0 i n s i d e c o a x i a l bus d u c t @ on overhead t r a n s m i s s i o n l i n e @ between ground and e n c a p s u l a t i o n

b . r e p r e s e n t a t i o n o f a . f o r VFT w i t h d i s c r e t e

c . s i n g l e l i n e d iagram f o r c a l c u l a t i o n of U k e l emen t s

based on t h e assumpt ion o f i n f i n i t e l o n g 1 i n e s

t r a v e l l i n g wave th rough t h e bus 0 and reaches t h e bush ing where a p a r t causes t h e t r a n s i e n t r i s e o f t he e n c l o s u r e v o l t a g e @ and a p a r t p ropagates a long t h e overhead t r a n s m i s s i o n l i n e @ and may s t r e s s t h e a d j a c e n t equ ipment ( F i g . 3a) / l / .

2.2 .1 . T r a n s i e n t e n c l o s u r e v o l t a g e (TEV)

VFT’s i n s i d e and o u t s i d e a G I s a r e w e l l unders tood by t r a v e l l i n g wave t h e o r y . R e f l e c t i o n s a t s h o r t b ranches a r e most c r i t i c a l , because these r e f l e c t e d waves can sum up a t o t h e r l o c a t i o n s t o h i g h amp l i t udes 181.

F o r t h e i n t e r p r e t a t i o n o f t r a v e l 1 i n g waves o u t s i d e t h e e n c l o s u r e a s i m p l i f i e d e q u i v a l e n t c i r c u i t can be used ( F i g . 3 ) . I n t h e i d e a l i z e d F i g . 3a t h e overhead t r a n s m i s s i o n l i n e i s d i r e c t l y connected t o t h e G I s . I f t h e d i s c o n n e c t o r DS f l a s h e s o v e r a t r a v e l l i n g wave, hav ing 0.5 p .u .magn i tude, p ropagates towards t h e end o f


225

t h e bus d u c t . When t h e t r a v e l l i n g wave reaches t h e end o f t h e bus duc t , where n o r m a l l y t h e bush ing i s l o c a t e d , an e l e c t r o m a g n e t i c f i e l d occu rs o u t s i d e t h e bus d u c t caus ing t h e t r a n s i e n t r i s e o f t h e e n c l o s u r e v o l t a g e . T h i s e f f e c t can be e x p l a i n e d u s i n g t h e e q u i v a l e n t c i r c u i t F i g . 3b where t h e bus d u c t as w e l l as t h e t r a n s m i s s i o n l i n e and t h e e n c a p s u l a t i o n a r e r e p r e s e n t e d b y LC-elements. The e q u i v a l e n t c i r c u i t ( F i g . 3b) can f u r t h e r be s i m p l i f i e d ( F i g . 3c ) f o r t h e e s t i m a t i o n o f t h e w o r s t case peak irk o f any TEV u s i n g t h e c h a r a c t e r i s t i c impedances o f t h e SF bus (Zi) . o f t h e e n c l o s u r e above e a r t h ( Z ) and 6,f t h e overhead t r a n s m i s s i o n l i n e (ZF) . The F o l l o w i n g e q u a t i o n does n o t i n c l u d e t h e s u b s t a t i o n response f a c t o r wh ich g i v e s t h e r a t i o between peak a .c . - vo l tage and maximum VFTO :

‘k ‘k = - ‘0 Zk+ Z i t ZF

W i t h t h i s s imp le e q u a t i o n t h e a m p l i t u d e o f t h e TEV irk can be e s t i m a t e d and i n a d d i t i o n t h e p r i n c i p l e measures t o reduce t h e TEV can be g i ven . As an example t h e TEV irk o f a 420-kV-GIS connected t o an overhead l i n e w i l l be c a l c u l a t e d . F o r such an arrangement t h e f o l l o w i n g va lues can be assumed:

v7 U. = v u m = 330 kV

zk = 60P ... 20Q Z i = 60P

ZF = 270Q

There fo re :

= -50 kV 17 kV ’k ‘k = - “0 Z k t zi+ ZF

The TEV irk i s in t h e range o f some 10 kV. The r a t i o i s i n t h e range o f 30 % t o 10 %. I t can be i n f l u e n c e d remarkab ly by t h e v a l u e o f zk wh ich shou ld be as low as p o s s i b l e / 3 / .

I n p r i n c i p l e t h r e e p o s s i b i l i t i e s e x i s t t o reduce zk and hence t h e TEV a m p l i t u d e irk:

o Inc rease o f t h e d iamete r o f t h e e n c a p s u l a t i o n . T h i s i s n o t p r a c t i c a l .

o Reduc t ion o f t h e h e i g h t o f t h e bus above e a r t h . T h i s i s p o s s i b l e , b u t n o t always p r a c t i c a l .

o S h o r t c i r c u i t o f t h e impedance Z between t h e p o i n t s B and C ( F i g . 3 ) . T h i s i s \he most p r a c t i c a l s o l u t i o n . Because o f t h e h i g h f r e q u e n c i e s i n v o l v e d t h e s h o r t c i r c u i t shou ld be pe r fo rmed w i t h a low induc tance e .g . by t h e me ta l s t r u c t u r e c a r r y i n g t h e bush ing . Bu t i n any case such a c o n n e c t i o n a c t s as a s h o r t inhomogeneous t r a n s m i s s i o n l i n e .

To demonst ra te t h e a p p l i c a b i l i t y o f t h i s s i m p l i - f i e d c i r c u i t t o e x p l a i n t h e TEV a t e s t a r rangement c o n s i s t i n g o f a SF6-bus and a bush ing was b u i l t up ( F i g . 4 ) . F i g . 4a shows t h e ar rangement and F i g . 4d t h e e v a l u a t e d r e s u l t s o f fik/uO a t d i f f e r e n t p o i n t s .

A t t h e b e g i n n i n g o f t h e bus a low v o l t a g e s t e p U. was a p p l i e d . The s t e p g e n e r a t o r was connected t o t h e bus th rough a c a b l e and a c o n i c a l adap to r t o t h e d i a - meter o f t h e bus r e s u l t i n g i n a c o n s t a n t impedance. The i n j e c t e d l ow v o l t a g e s t e p U. w i t h a r i s e t i m e o f <1 ns i s shown i n F i g . 4b. The v o l t a g e between e n c a p s u l a t i o n and e a r t h was measured u s i n g a c o n v e n t i o n a l c a p a c i t i v e d i v i d e r w i t h a r i s e t i m e o f 4 ns . The r e s u l t i n g TEV Uk

a.

1000

-1000

-2000 -400 0 100 200 300 ns 500 0 100 200 300 ns 500

t - t - b. C .

0.3 c I I I I

I I

measuring point: 0 0 0 @ d.

F i g . 4: Exper imen ta l t e s t s e t up f o r t h e i n v e s t i g a t i o n o f TEV e f f e c t s i n a 800-kV-bus d u c t i n c l u d i n g a 800-kV-outdoor bush ing a. t e s t s e t up b . i n j e c t e d i n t e r n a l l ow v o l t a g e impu lse c . e x t e r n a l t r a n s i e n t e n c l o s u r e v o l t a g e a t

p o i n t a w i t h o u t e a r t h c o n n e c t i o n E d. measured r a t i o o f i n t e r n a l f a s t t r a n s i e n t

v o l t a g e and e x t e r n a l e n c l o s u r e v o l t a g e a t d i f f e r e n t p o i n t s - w i t h o u t e a r t h c o n n e c t i o n E -.- w i t h e a r t h c o n n e c t i o n E

a t p o i n t a w i t h o u t e a r t h connec t ion E a t t h e end o f t h e bus i s g i v e n i n F i g . 4c. As can be seen t h e amp l i t ude o f t h e TEV 6 i s abou t 24 % o f Uo, t h e f requency i n t h e range o f 5 hHz, showing t h a t t h e i n i t i a l r i s e o f t h e VFT o f 1 ns i s reduced c o n s i d e r a b l y . T h i s i s t y p i c a l f o r TEV’s.

2.2.2 T r a n s i e n t e l e c t r o m a g n e t i c f i e l d s

I n a d d i t i o n t o TEV’s e l e c t r o m a g n e t i c f i e l d s a r e r a d i a t e d f r o m t h e e n c l o s u r e and s t r e s s secondary equ ipment . E s p e c i a l l y when s o p h i s t i c a t e d computer- c o n t r o l l e d equ ipment i s used a consequent p l a n n i n g of e l e c t r o m a g n e t i c c o m p a t i b i l i t y i s r e q u i r e d /16/ .

2.2.3 F a s t t r a n s i e n t o v e r v o l t a g e s a t a d j a c e n t equipment

The second f o r m o f e x t e r n a l v e r y f a s t t r a n s i e n t s a r e t r a v e l l i n g waves on connected overhead t r a n s m i s s i o n l i n e where t h e a d j a c e n t equ ipment may be s t r e s s e d by f a s t t r a n s i e n t o v e r v o l t a g e s . I n t h e case o f t r a n s -


226

f o rmers exposed t o these FTO, resonance phenomena can occu r . I n a des ign s t u d y t h e resonance behav iou r o f t h e t r a n s f o r m e r has t h e r e f o r e t o be ana lysed /13 / .

3 . F i e l d measurements o f v e r y f a s t t r a n s i e n t o v e r v o l t aoes

3 . 1 Measur ing dev i ces

Up t o now t h e measurement o f o v e r v o l t a g e s i n sub- s t a t i o n s was pe r fo rmed by v o l t a g e d i v i d e r s /13 / , / 2 / . F o r t h e measurement o f TEV’s s p e c i a l h i g h impedance v o l t a g e d i v i d e r s were used / l / , / 7 / wh ich were connec ted t o t h e p o i n t s o f i n t e r e s t . Because o f t h e i r p r i n c i p l e , t r a v e l l i n g waves w i l l occu r i n t h e measur ing appara tus .

The measurement o f TEV’s shown i n t h i s paper was pe r fo rmed u s i n g a s p h e r i c a l e l e c t r i c f i e l d sensor a p p l i e d f o r v o l t a g e measurement /4 / , / 5 / .

3 . 1 . 1 The s p h e r i c a l e l e c t r i c f i e l d sensor

The e l e c t r i c f i e l d sensor i s c o n s t r u c t e d as a s p h e r i c a l c a p a c i t i v e p robe f o r t h e measurement o f two components o f t h e e l e c t r i c f i e l d s t r e n g t h . To p e r f o r m measurements a t a r b i t r a r y p o i n t s e s p e c i a l l y i n t h e v i c i n i t y o f h i g h v o l t a g e s t h e d a t a t r a n s m i s s i o n f rom t h e sensor i t s e l f t o t h e r e c o r d i n g i n s t r u m e n t i s b u i l t w i t h an ana log f i b e r o p t i c d a t a l i n k . The measur ing p r i n c i p l e i s based on t h e measurement o f t h e i n f l u e n c e d charges on t h e i n s u l a t e d measur ing areas a t t h e s u r f a c e o f t h e sensor . These charges a r e a measure o f t h e f i e l d s t r e n g t h E ( t ) i n t h e c e n t e r o f t h e sensor . The f i b e r - o p t i c d a t a t r a n s m i s s i o n l i n k has a bandw id th o f about 25 MHz wh ich cor responds t o a r i s e t i m e o f abou t 15 ns. The lower c u t o f f f requency i s abou t 10 Hz and t h e dynamic range i s 46 dB.

To i n s t a l l t h e r e c e i v e r i n s t r u m e n t i n s i d e a measur ing c a b i n e t t h e o p t i c a l l i n k can be connected a t t h e r e c e i v e r s w i t h o p t i c a l connec to rs . The ad jus tmen t o f t h e d a t a l i n k can be done w i t h a t e s t g e n e r a t o r connected f o r t h i s purpose t o t h e s p h e r i c a l sensor . The power supp ly i n t h e sensor i s c o n t r o l l e d w i t h a t r i g g e r u n i t v i a i t s own o p t i c a l channe l . The measured d a t a can be reco rded w i t h a f a s t s t o r a g e o s c i l l o s c o p e o r a t r a n s i e n t r e c o r d e r w i t h h i g h samp l ing r a t e . The sensor i s p l a c e d a t t h e measur ing p o i n t w i t h i n s u l a t i n g m a t e r i a l and has t h e r e f o r e no i n f l u e n c e on t h e s i g n a l t o be measured.

The d e s c r i b e d e l e c t r i c f i e l d sensor can be used f o r v o l t a g e measurement i f t h e sensor i s c a l i b r a t e d a t

-------------------

conductor

F i g . 5 : Vo l tage c a l i b r a t i o n o f t h e s p h e r i c a l e l e c t r i c f i e l d sensor w i t h a . c . - v o l t a g e

t h e measur ing p o i n t w i t h a known v o l t a g e U,. Tha t means, t h e known v o l t a g e U g i v e s t h e f i e l d s t r e n g t h E and t h e n any unknown v o l i a g e u ( t ) can be de te rm ine2 w i t h t h e measured f i e l d s t r e n g t h E ( t ) . T h i s l eads t o

uC u ( t ) = E ( t ) = k * E ( t ) C

w i t h t h e c a l i b r a t i o n f a c t o r k , depend ing o n l y on t h e geomet r i ca l a r rangement . Because o f t h e l ower c u t o f f f requency o f 10 Hz, t h e c a l i b r a t i o n f a c t o r k can be de te rm ined w i t h a . c . - v o l t a g e s o f 50 Hz.

The main advantage o f t h i s sensor and i t s a p p l i c a - t i o n as a measur ing d e v i c e f o r v o l t a g e measurement i s i t s smal l s i z e , t h e d iamete r i s 40 mm, t h e p o t e n t i a l f r e e d a t a t r a n s m i s s i o n and t h e d i s t o r t i o n - f r e e connec t ion t o t h e e l e c t r o d e o f i n t e r e s t , e.g. t o t h e e n c a p s u l a t i o n o f a G I S d u r i n g a TEV measurement.

F o r t h e TEV measurement a s p e c i a l c a l i b r a t i o n method was deve loped ( F i g . 5 ) . An a .c . - vo l tage w i t h a f requency o f 50 Hz and a c a l i b r a t e d a m p l i t u d e o f 10 kV i s a p p l i e d t o a copper f o i l su r round ing t h e GIS encap- s u l a t i o n ve ry c l o s e b u t i n s u l a t e d f r o m i t th rough a P V C - f o i l . By t h e measurement o f t h e e l e c t r i c f i e l d s t r e n g t h E, w i t h t h e s p h e r i c a l f i e l d sensor t h e c a l i - b r a t i o n f a c t o r k = U c / E i s de termined. T h i s copper f o i l i n f l u e n c e s t h e impetance and hence t h e TEV s i g n a l n e g l i g i b l e , because i t means o n l y a sma l l d i s c o n t i n u i t y i n t h e impedance.

I n a d d i t i o n t o t h e a p p l i c a t i o n f o r TEV-measurements t h e s p h e r i c a l e l e c t r i c f i e l d sensor can be a p p l i e d f o r t h e measurement o f o v e r v o l t a g e s o u t s i d e t h e G I S f o r example a t a t r a n s f o r m e r bush ing . The c a l i - b r a t i o n o f t h e sensor can be done w i t h t h e v o l t a g e t r a n s f o r m e r a t t h e a . c . - o p e r a t i n g v o l t a g e .

3.1 .2 The c a p a c i t i v e p robe

F o r t h e measurement o f VFT’s i n s i d e t h e G I S a c a p a c i t i v e p robe w i t h an upper c u t o f f f requency o f 445 MHz was used / 2 1 / . The c a l i b r a t i o n o f t h i s c a p a c i t i v e p robe i s a l s o pe r fo rmed by an a . c . - v o l t a g e . The probe i s connected t o t h e o s c i l l o s c o p e v i a an impedance c o n v e r t e r and a c o a x i a l c a b l e .

3 . 2 . T e s t arrangements

The d e s c r i b e d measur ing methods were a p p l i e d t o two t e s t a r rangements . The f i r s t one was a 800-kV-SF6- g a s - i n s u l a t e d p i l o t i n s t a l l a t i o n a t t h e h i g h v o l t a g e l a b o r a t o r y o f BBC, Z u r i c h , S w i t z e r l a n d . The second one was t h e 420-kV-GIS, K u p f e r z e l l , Energ ie -Versorgung Schwaben AG, Germany.

3.2 .1 R e s u l t s w i t h t h e 8 0 0 - k V - p i l o t i n s t a l l a t i o n

The open a i r v iew o f t h e 8 0 0 - k V - p i l o t i n s t a l l a t i o n i s shown i n F i g . 6 . The d i s c o n n e c t o r and o t h e r G I S - components a r e i n s t a l l e d i n s i d e t h e b u i l d i n g . The SF6- bus d u c t i s connected t o a bush ing . A SF6- inSUla ted z i n c - o x i d e su rge a r r e s t e r i s l o c a t e d beyond t h e bush ing . The TEV was measured v e r t i c a l l y beyond t h e SF bus d u r i n g a CLOSE-operat ion o f t h e d i s c o n n e c t o r w i t h an a .c . - vo l tage o f 800 / f l kV rms. F i g . 7 shows t h e r e s u l t s o f t h e TEV-measurement w i t h t h e s p h e r i c a l e l e c t r i c f i e l d sensor a t d i f f e r e n t d i s t a n c e s f r o m t h e bus. A t a d i s t a n c e o f 9 cm an e l e c t r i c f i e l d o f 57 kV/m was measured w i t h a main f requency o s c i l l a t i o n o f 10 MHz ( F i g . 7 ) . I n a d i s t a n c e o f 5 m f r o m t h e bus t h e e l e c t r i c f i e l d i s abou t 5 kV/m, b u t h i g h e r f r e q u e n c i e s can be observed i n t h e o s c i l l o g r a m . The e l e c t r i c f i e l d


227

1 encapsulation

2 bushing 3 surge arrester 4 building 5 earthmesh

M measuring point

F i g . 6: 8 0 0 - k V - p i l o t i n s t a l l a t i o n

reduces w i t h about 1 / R , as can be expec ted f r o m t h e o r y ( F i g . 7 ) . I n F i g . 8 measurements a r e shown w i t h t h e e l e c t r i c f i e l d sensor a t t h e same d i s t a n c e R f r o m t h e bus ( 9 cm), b u t l o c a t e d i n s i d e and o u t s i d e t h e b u i l d i n g . The b u i l d i n g c o n s i s t s o f a m e t a l l i c mesh connected t o t h e e n c a p s u l a t i o n o f t h e bus and t o ground. The damping e f f e c t o f t h e m e t a l l i c mesh can be seen ( F i g . 8 ) . A d d i t i o n a l l y F i g . 8 p r e s e n t s t h e measured VFT i n s i d e t h e G I S a f t e r a d i s c o n n e c t o r o p e r a t i o n . The VFT a t t h e source has a s teep f r o n t o f 4 t o 7 ns /15 / . A t some d i s t a n c e f r o m t h e d i s c o n n e c t o r

7 0 1 I

, I I \ i o k t -

ioons t - \*-=- 2

" I

0 1 1 2 3 4 m 5

R- RGIS

F i g . 7: Measured magni tude o f t h e v e r t i c a l e l e c t r i c f i e l d component beyond t h e 800-kV-SF6-gas i n s u l a t e d bus d u c t versus d i s t a n c e d u r i n g a d i s c o n n e c t o r o p e r a t i o n a t s e r v i c e v o l t a g e 800 / fl kV

E E

1 kVlm t 1

SOkVim

F i g . 8: VFT measurement i n s i d e and o u t s i d e o f t h e b u i l d i n g and t h e G I s d u r i n g a CLOSE-operation o f t h e d i s c o n n e c t o r a t U,,, = 8 0 0 / f l k V

t h e s teep f r o n t o f t h e VFT i s reduced and r e f l e c t i o n s and r e f r a c t i o n s r e s u l t i n g f r o m t h e i n t e r n a l s t r u c t u r e can be observed ( F i g . 8 ) .

3.2.2 R e s u l t s w i t h t h e 420-kV-GIS " K u p f e r z e l l "

Measurements w i t h t h e measur ing d e v i c e d e s c r i b e d above have been per fo rmed i n t h e 4 2 0 - k V - s u b s t a t i o n KUPFERZELL o f t h e Energ ie -Versorgung Schwaben AG (EVS), S t u t t g a r t , West Germany. The G I s KUPFERZELL i s con- n e c t e d t o t h e 420-kV-West European power t r a n s m i s s i o n network and i s l o c a t e d i n t h e s o u t h western p a r t o f Germany. To genera te v e r y f a s t t r a n s i e n t s i n t h i s s u b s t a t i o n t h e second bus d u c t was s w i t c h e d on w i t h t h e d i s c o n n e c t o r s w i t c h . Measurements have been per fo rmed a t d i f f e r e n t p o i n t s i n t h e system. F i g . 9 shows t h e arrangement o f t h e s u b s t a t i o n and t h e l o c a t i o n of measur ing p o i n t s 1 and 2. The o p e r a t e d d i s c o n n e c t o r was i n s i d e t h e b u i l d i n g . F o r t h e o v e r v o l t a g e measurement a t t h e t r a n s f o r m e r bush ing t h e s p h e r i c a l e l e c t r i c f i e l d sensor was p l a c e d near t h e t r a n s f o r m e r a t a d i s t a n c e f r o m t h e t o p o f t h e bush ing o f about 3 m. The ca- l i b r a t i o n o f t h e sensor f o r t h i s measurement was done w i t h t h e a . c . - o p e r a t i n g v o l t a g e U, = 420 k V . F i g . 10 shows t h e r e c o r d e d s i g n a l s . I t can be observed t h a t a t t h e t r a n s f o r m e r o v e r v o l t a g e s o f about 170 X ( 1 . 7 P . u . ) o f t h e o p e r a t i n g v o l t a g e were measured. The f r e q u e n c i e s o f t h e o s c i l l a t i o n s a r e about 10-20 MHz.

The main aim o f t h e measurements per fo rmed i n KUPFERZELL was t o measure t h e t r a n s i e n t e l e c t r i c f i e l d s o u t s i d e t h e e n c l o s u r e caused by d i s c o n n e c t o r o p e r a t i o n i n a r e a l s u b s t a t i o n . The e l e c t r i c f i e l d s were measured a t d i f f e r e n t d i s t a n c e s f r o m t h e e n c a p s u l a t i o n i n s i d e t h e b u i l d i n g ( F i g . 11). The b u i l d i n g has no s p e c i a l s c r e e n i n g hence t h e damping o f t h e t r a v e l l i n g wave coming f r o m t h e bush ing i s s m a l l . The a m p l i t u d e o f t h e f i e l d s t r e n g t h nearby t h e e n c a p s u l a t i o n i s about 20 k V / m . A rough c a l c u l a t i o n l e a d s t o a t r a n s i e n t e n c l o s - u r e v o l t a g e o f about 70 kV. From t h e o s c i l l o g r a m s i t can be deduced t h a t t h e a m p l i t u d e o f t h e e l e c t r i c f i e l d s t r e n g t h i s reduced w i t h i n c r e a s i n g d i s t a n c e t o t h e en- c a p s u l a t i o n b u t i t does n o t v a n i s h near t h e e a r t h system. A t l a r g e r d i s t a n c e s t h e f requency i n c r e a s e s f r o m 10-20 MHz near t h e e n c a p s u l a t i o n t o 30-40 MHz near t h e e a r t h because o f m u l t i p l e r e f l e c t i o n s i n t h e e a r t h i n g


228

1” - I”, F i g . 9: SF6-swi tchgear Kup fe rze l l /EVS:

arrangement, measur ing p o i n t s 1,2

system ( F i g . 1 1 ) . T h i s i s comparable t o t h e r e s u l t s o f t h e measurements w i t h t h e 8 0 0 - k V - p i l o t i n s t a l l a t i o n . I n t h a t case h i g h e r f r e q u e n c i e s can a l s o be observed i n t h e o s c i l l o g r a m taken a t a l o c a t i o n near t o t h e e a r t h ( F i g . 7 ) .

t - b. t - a.

F i g . 10: Overvo l tages a t t h e 420-kV- t rans fo rmer bush ing a f t e r a d i s c o n n e c t o r o p e r a t i o n i n t h e G I s (measur ing p o i n t 1 o f F i g . 9 ) a. t i m e d e f l e c t i o n 500 n s / d i v b. t i m e d e f l e c t i o n 50 n s / d i v

4. Conc lus ion

The measurements pe r fo rmed show t h a t t h e s p h e r i c a l e l e c t r i c f i e l d sensor i s an adequate measur ing d e v i c e t o measure o v e r v o l t a g e s and TEV’s i n s u b s t a t i o n s .

S w i t c h i n g o p e r a t i o n s i n a SF6-gas - insu la ted - sw i t chgear l e a d t o v e r y f a s t t r a n s i e n t phenomena, wh ich can be s u b d i v i d e d i n t o i n t e r n a l and e x t e r n a l v e r y f a s t t r a n s i e n t s . These VFT’s s t r e s s t h e equ ipment i n G I S as w e l l as t h e secondary equipment.

The o v e r v o l t a g e s i n s i d e t h e G I S have r i s e t imes i n t h e range o f 4 t o 7 ns . Due t o r e f l e c t i o n s and r e f r a c t i o n s t h e y have a s u b s t a t i o n des ign dependent o s c i l l a t i n g waveshape w i t h a magn i tude up t o 1,5 p.u. under c a p a c i t i v e s w i t c h i n g /8 / and up t o 2,4 p.u. i n case o f phase o p p o s i t i o n /19 / .

Two k i n d s o f e x t e r n a l v e r y f a s t t r a n s i e n t s can be observed. The one wh ich l e a d s t o o v e r v o l t a g e s a t t h e a d j a c e n t equ ipment . The magn i tude o f t hese o v e r v o l t a g e s i s i n t h e range o f up t o 2 p .u w i t h f r e q u e n c i e s o f about 10 MHz. The o t h e r phenomenon i s t h e so c a l l e d TEV w i t h amp l i t udes o f 10 % t o 30 % o f U. and aga in w i t h f requenc ies o f 10 t o 20 MHz.

The TEV can be c o n t r o l l e d b y adequate e a r t h i n g o f t h e bush ing . An a d d i t i o n a l way t o reduce t h e TEV e f f e c t i v e l y i n s i d e t h e b u i l d i n g c o n t a i n i n g a G I s i s t h e e a r t h i n g o f t h e i ncoming busses a t t h e e n t r a n c e i n t o

.3m

F7”

F i g . 11: E l e c t r i c f i e l d o u t s i d e t h e SF6-encapsu la t i on ( p o i n t 2) i n d i f f e r e n t d i s t a n c e s ( 2 I - 2’”) f r o m t h e e n c a p s u l a t i o n

t h e b u i l d i n g . Thus t h e t r a n s i e n t v o l t a g e o f t h e encap- s u l a t i o n o f a G I S i n s i d e a b u i l d i n g can be reduced t o accep tab le va lues .

Acknowledgement

The a u t h o r s wou ld l i k e t o thank t h e Energ ie - Versorgung Schwaben AG f o r a l l o w i n g t h e per fo rmance of measurements i n K u p f e r z e l l and e s p e c i a l l y M r . Haug f o r h i s h e l p d u r i n g t h e measurements.

5. References

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T r a n s i e n t Ground P o t e n t i a l R i s e i n Gas I n s u l a t e d S u b s t a t i o n -Exper imen ta l s t u d i e s . I E E E Trans . on

/2/ N . F u j i m o t o , E . P . D i ck , S . A . Boggs, G.L. Fo rd :

PAS, PAS-101, 1982, pp. 3603-3609

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/5 / K. Feser , W . P f a f f , G. Weyre ter , E . Gockenbach: D i s t o r t i o n - F r e e Measurement o f H igh Impu lse Vo l tages . IEEE /PES, 1987 W i n t e r Mee t ing New Or leans 87 WM 177-9


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/ 7 / G.Newi: A High-Impedance, Nanosecond R i s e Time Probe f o r Measur ing H i g h V o l t a g e Impu lses . IEEE Trans . on PAS, Vol PAS-87, 1968. pp. 1779-1789

No.TH0152-9.

/ 8 / K . Feser , H.A. M a i e r : C o n s i d e r a t i o n s r e l a t e d t o t h e d i e l e c t r i c f i e l d t e s t i n g o f GIS. I n S.A. Boggs, F.Y. Chun, N. F u j i m o t o (Eds) Gas I n s u l a t e d S u b s t a t i o n s - Techno logy and P r a c t i c e . Pergamon Press , New York, 1986

L. Menten: I n s u l a t i n g b e h a v i o u r o f SF6 w i t h and w i t h o u t S o l i d I n s u l a t i o n i n c a r e o f F a s t T r a n s i e n t s . CIGRE P a r i s , 1986, Paper No. 15-07

500 kV b u s h i n g connected t o gas i n s u l a t e d s w i t c h g e a r . Paper p r e s e n t e d a t t h e 5 0 t h Annual I n t e r n a t i o n a l Conference o f Dob le C l i e n t s , A p r i l 1983

/9 / W . Boeck, W . Taschner, J. Gorab lenkov , G.F. Luxa,

/ l o / A.J. D i r e n e , M.P. Leme, P . C . Neves: F a i l u r e s o f a

/11/ F.Y. Chu and S.A. Boggs: G a s - i n s u l a t e d s u b s t a t i o n r e l i a b i l i t y surveyed. T r a n s m i s s i o n and D i s t r i b u t i o n , J a n u a r y 1983

/ 1 2 / S.M. Harvey : C o n t r o l w i r i n g t r a n s i e n t s and e l e c t r o m a g n e t i c c o m p a t i b i l i t y i n GIS. I n S.A. Boggs, F.Y. Chun and N. F u j i m o t o (Eds) Gas I n s u l a t e d S u b s t a t i o n s - Techno logy and P r a c t i c e . Pergamon Press , New York, 1986.

/13/ N. F u j i m o t o , S.A. Boggs: C h a r a c t e r i s t i c s o f GIS D i s c o n n e c t o r - I n d u c e d S h o r t R i s e t i m e T r a n s i e n t s I n c i d e n t on E x t e r n a l l y Connected Power System Components. IEEE PES W i n t e r Meet ing , New Or leans , 1987, Paper No. 87 WM 185-2

/ 1 4 / S.A. Boggs, N. F u j i m o t o : Techn iques and

K u r t F e s e r was b o r n on December 10, 1938 i n G a r m i s c h - P a r t e n k i r c h e n , F.R. Germany. From 1958 t o 1963 he s t u d i e d E l e c t r i c a l E n g i n e e r i n g a t t h e T e c h n i c a l U n i v e r s i t y o f Munich, f i n i s h i n g w i t h t h e "D ip lom- I n g e n i e u r " . A f t e r a y e a r w i t h Brown B o v e r i and C i e AG i n Mannheim, Germany, he j o i n e d t h e h i g h v o l t a g e i n s t i t u t e o f t h e U n i v e r s i t y o f Munich.

I n 1970 he r e c e i v e d h i s D r . - I n g . f r o m t h e U n i v e r - s i t y o f Munich, and i n 1971 he j o i n e d H a e f e l y & C i e AG, Base l , S w i t z e r l a n d as c h i e f deve lopment e n g i n e e r f o r h i g h v o l t a g e t e s t equ ipment . From 1980 he was r e s p o n s i b l e a t H a e f e l y & C i e f o r c a p a c i t o r s , h i g h v o l t a g e t e s t equ ipment and a c c e l e r a t o r s and was as d i r e c t o r member o f t h e e x e c u t i v e b o a r d o f H a e f e l y . D u r i n g 1977 t o 1980 D r . F e s e r was member o f t h e b o a r d o f D i r e c t o r s o f American H i g h V o l t a g e T e s t Systems, A c c i d e n t , Mary land.

I n A p r i l 1982 he j o i n e d t h e U n i v e r s i t y o f S t u t t g a r t as head o f t h e power t r a n s m i s s i o n and h i g h v o l t a g e i n s t i t u t e . P r o f . F e s e r i s a member o f IEEE, VDE and CIGRE, cha i rman o f CIGRE WG 33-03 " H i g h v o l t a g e t e s t t e c h n i q u e " and a u t h o r o f a b o u t 100 papers .

i n s t r u m e n t a t i o n f o r measurement o f t r a n s i e n t s i n g a s - i n s u l a t e d s w i t c h g e a r . IEEE Trans . on E l . , E l - 19, No. 2, 1984, p . 84

J. K i n d e r s b e r g e r , R. Morrow, W . Zaengl , M. Zwicky, J. Gal 1 i m b e r t i , S . Boggs: Inhomogeneous f i e l d breakdown i n GIS - t h e p r e d i c t i o n o f breakdown p o s s i b i l i t i e s and v o l t a g e s . IEEE PES W i n t e r M e e t i n g 1987 New Or leans , WM 192-8

/16/ J. Meppe l ink , H. Remde: E l e c t r o m a g n e t i c C o m p a t i b i l i t y i n GIS S u b s t a t i o n s . Brown B o v e r i Review No. 9, 1986

/ 1 7 / F . F r e i s i n g e r , M. Muhr, A. D i e s s n e r , H. Schenner: F i e l d Measurements o f F a s t T r a n s i e n t V o l t a g e s i n t h e 420 kV-GIS Wien Sued. CIGRE-Symposium, Wien, 1987, 5.-7. Mai

/15/ W . Wiegar t , L . Niemeyer, F. Pinnekamp, W . Boeck,

/18/ D . Kon ig , G. Imgrund, C . Neumann, K . Maatz, L. Schiweck: Vorgange be im S c h a l t e n k l e i n e r k a p a z i t i v e r Strome m i t S F 6 - i s o l i e r t e n , m e t a l l - g e k a p s e l t e n T r e n n s c h a l t e r n i m 110 kV-Netz und i h r e S i m u l a t i o n i m Hochspannungslaborator ium. E l e k t r i z i t a t s w i r t s c h a f t 85, 1986, H.4 pp. 131-138

/19/ A. E d l i n g e r and a l . : D i s c o n n e c t o r s w i t c h i n g o f Charg ing C u r r e n t q i n M e t a l E n c l o s e d SFg-GaS I n s u l a t e d S w i t c h g e a r a t EHV. CIGRE P a r i s , 1984, Paper No. 13-14

/ 2 0 / J.P. Reynders, J . M e p p e l i n k : C h a r a c t e r i s t i c s o f D i s c o n n e c t S w i t c h T r a n s i e n t s and C o n t r o l o f t h e Consequent E l e c t r o m a g n e t i c I n t e r f e r e n c e . Open Conference on EHV T r a n s m i s s i o n Systems and M e e t i n g s o f S tudy Commit tees 22 and 23, 19-21 October 1987, Johannesburg, South A f r i c a

/ 2 1 / J. M e p p e l i n k , P. H o f e r : Des ign and C a l i b r a t i o n o f a H i g h V o l t a g e D i v i d e r f o r Measurement o f Very F a s t T r a n s i e n t s i n Gas I n s u l a t e d S w i t c h g e a r . 5. I n t . Symposium on H i g h V o l t a g e E n g i n e e r i n g , 1987, Braunschweig, Paper No. 71.08

Wolfgang R . P f a f f was b o r n i n S t u t t g a r t , F.R. Germany, on May 20, 1955. He r e c e i v e d t h e D i p 1 . - I n g . f r o m t h e U n i v e r s i t y o f S t u t t g a r t i n 1982.

Since 1982 he i s w i t h t h e power t r a n s m i s s i o n and h i g h v o l t a g e i n s t i t u t e o f t h e U n i v e r s i t y o f S t u t t g a r t , S t u t t g a r t , Germany.

H i s c u r r e n t r e s e a r c h e s a r e concerned w i t h t h e EMP, e l e c t r i c f i e l d c a l c u l a t i o n s and measur ing prob lems i n h i g h v o l t a g e t e c h n o l o g y . M r . P f a f f i s a member o f VDE.

Jan Meppe l ink was b o r n on A p r i l 2 4 , 1950 i n Fed. Rep. o f Germany. He r e c e i v e d h i s D i p 1 . - I n g . i n 1978 and h i s D r . - I n g . i n 1984 f r o m T e c h n i c a l U n i v e r s i t y o f B e r l i n . S i n c e 1984 he i s head o f H i g h V o l t a g e and EMC E n g i n e e r i n g i n BBC Brown B o v e r i L t d , . Z u r i c h , S w i t z e r l a n d .

K a r l J o s e f D i e d e r i c h was b o r n on November 16, 1949 i n Fed. Rep. o f Germany. He r e c e i v e d h i s D i p 1 . - I n g . i n 1979 and h i s D r . - I n g . i n 1985 f r o m Aachen T e c h n i c a l U n i v e r s i t y . S i n c e 1985 he i s Member o f t h e Development Dept . f o r H i g h V o l t a g e S w i t c h g e a r w i t h BBC Brown B o v e r i L t d . , Z u r i c h , S w i t z e r l a n d .


230

Discussion

N. Fujimoto and G. L. Ford (Ontario Hydro Research, Toronto, ON, Canada): The paper gives an overview of very fast transient (VFT) phenomena in CIS, reviews the mechanisms of some aspects of VFT and presents some measurements in actual CIS. We are encouraged that the results and conclusions presented in the paper, in general, support our own data and conclusions published some years ago (paper reference [ 1]-[3], discussion reference [I]). Although we agree with the general conclusions of the paper, we identified some specific inconsistencies in the paper which should be addressed. We ask the authors to comment on these points and, where appropriate, clarify or elaborate on their point of view as presented in the paper.

Section 2.2.1 is a discussion of transient enclosure voltage (TEV), which is also known as transient groundrise (TGR) or transient ground potential rise (TGPR). in this section, an equation is given which relates the TEV voltage, uk to the peak of the phase-to-ground operating voltage, U,. This equation implies, as also indicated in previous publications, a specific relationship between an internally propagating traveling wave incident on the bushing and the voltage coupled onto the enclosure as TEV. This relationship is based on traveling wave effects as indicated by the dependencies on the various characteristic impedances. The equation, as presented in the paper, apparently assumes the statement in the previous paragraph that the initial transient is a traveling wave of 0.5 pu. This assumes that the surge impedances on either side of the disconnector (DS) are equivalent, that there are no trapped charges, and the DS intercontact voltage is 1 pu. However, this is not always true. For instance, for a DS at a “T” junction, the impedance on one side of the DS is half of that on the other and the traveling wave generated on the higher impedance side has a magnitude of 67 percent of the DS intercontact voltage. If the intercontact voltage is 2.0 pu, which is possible under some conditions such as phase opposition switching, then the traveling wave will have a magnitude of 1.33 pu. The equation for Uk must then be adjusted by a factor of 2.67. In addition, the experiment described in Fig. 4 injects a pulse of magnitude U. between the phase conductor and ground. In this case, a traveling wave of 1 pu (1 pu = UO) is generated and the equation should be adjusted by a factor of 2. The equation is better stated as a relationship between the TEV magnitude and the magnitude of the traveling wave, Ul which is easily accomplished by adding a scaling factor of 2. The equation then becomes

In the example which follows in the paper, the equation is used to estimate TEV magnitudes. The value of the characteristic impedance of the CIS enclosure above earth, Z,, is stated to be in the range of 60-20 0. However, for a CIS enclosure of radius r and a height-to-center h, the characteristic impedance can be calculated with an equation found in most electromagnetic textbooks.

Zk-60 . cos k l ( h / r )

For 500-kV class GIS, r is usually in the range of 25-30 cm. Therefore, for a relatively low height of about 1.5 m, z k is in the range of 150 Q. To achieve the 20-60 0 suggested by the authors, the CIS bus needs to be just a few centimeters above the ground. Otherwise, a low value of z k could be interpreted as a composite impedance which takes into account the influence of ground leads, support structures, etc. However, this point is not made in the paper.

In the concluding remarks of Section 2.2.1, an observation is made that the initial rise of the TEV is “considerably reduced” from that of the internal VFT. Although the measurements presented in the paper support this statement, we caution against the following statement that this effect is “typical.” Our own measurements of TEV (paper reference [2]) indicated a much faster risetime in the range of 10 ns. The reasons for this discrepancy can be several, including the geometry and construction of the bushing assembly (for instance, gas bushing or condenser type), the bus support structures, and ground leads. In addition, the graph shown in Fig. 4(d) could be misunderstood. The TEV distribution along the bus is shown to have a maximum at some point midway on the bus. This is presumably caused by some standing wave phenomena resulting from the limited dimensions of the laboratory setup. Measurements in actual GIS (paper reference [2]) indicate clearly, as predicted by theory, that TEV originates at the bushing and is generally highest at that point. Because the transmission mode along the enclosure is lossy, TEV magnitudes are less severe as the distance from the bushing increases. Our own measurements in GIS with an enclosure about 1 m above grade indicate an attenuation in the range of 0.3 db/m for transient propagating away from the bushing.

The authors mention in the conclusions that TEV can be controlled with “adequate earthing of the bushing,” although a discussion of “adequacy” is lacking in this context. More robust grounding leads, for instance, might only slightly reduce TEV levels because the ground lead, no matter of what size, is a vertical transmission line with finite transit time and relatively high surge impedance for ns-risetime transients. As discussed thoroughly in the paper’s references [1]-131, effective grounding requires the use of very short leads or grounding arrangments with very low surge impedances. The connection of CIS busses to an earthing mesh incorporated in the wall of station buildings, as discussed in the paper, is an example of a low-surge impedance ground connection. Such a connection can be effective in reducing the propagation of TEV into the station building if properly implemented. Some of the parameters of such an implementation are discussed at length in the paper’s reference [3] and discussion reference [ 11.

Circumstantial evidence seems to indicate that the “startle” effect from unexpected shocks is probably the greater concern for safety rather than the direct physiological hazard. With careful design, TEV voltages can be reduced to acceptable levels. Although previous research has addressed the question of acceptability for problems related to electromagnetic interfer- ence [2], the adequacy of fundamental physiological knowledge to define “acceptability” from a personnel safety point of view has been the subject of practical concern [3]. Can the authors suggest acceptable levels in terms of peak TEV voltage, transient body current, TEV energy, or some other relevant parameters and their basis in terms of documented scientific research?

References “Study of fault-produced overvoltages as related to grounding practices for SF6-insulated substations,” Canadian Electrical Associa- tion Reserach Report 071T101, Mar. 1982. S. Harvey, “Control wiring transients and electromagnetic compati- bility in GIS,” in S. A. Boggs, F. Y. Chu, and N. Fujimoto (Eds.) Gas-Insulated Substations, Technology and Practice, Pergamon Press; New York, 1986. G. L. Ford and L. A. Geddes, “Transient ground potential rise in gas- insulated substations-Assessment of shock hazard,” IEEE Trans. PowerApp. Syst., vol. PAS-101, no. 10, Oct. 1982.

Manuscript received February 18, 1988.

Ernst Gockenbach (E. Haefely, Basel, Switzerland): The authors are congratulated for their contribution to the problem of very fast transients in GIS. Up to now the different Standards, e.g., ANSI, IEEE, IEC, VDE, and others recommend the lightning impulse voltage level as the basic insulation level. But very fast transients are well known in the past and all manufacturers of GIS have taken into account the consequences of fast transients during the development of GIS.

The measuring techniques of fast transients are well developed and computer simulations are under study, as already mentioned by the authors.

Due to the importance of very fast transients, the WG 03 high voltage test and measuring technique of Study Committee 33, “Overvoltages and Insulation Coordination,” of CIGRE is also dealing with the fast transients as a preferential subject. Within this year a contribution of the WG members will be prepared for the next meeting in September in which the origin of fast transients and its consequences and the different measuring devices and its calibration and measuring results will be described. The purpose of this CIGRE work is to give an overview on the present knowledge of fast transients and its measuring technique and to use it as a basis for future standards.

The authors have shown that with new measuring devices such as the spherical field probe and the capacitive probe, the fast transients can be measured inside and outside of a GIS. But in both cases the calibration of the measuring device is difficult and needs some precautions. For only electrical field or voltage shape measurement, the analog data transmission via fiber optic line should be calibrated or adjusted. For this purpose the receiver has a built-in calibrator which is battery-powered and can be used in the measuring position of the electrical field probe. For voltage amplitude measurements, an addithnal calibration is necessary which can be carried out at lower frequency, e.g., at 50 or 60 Hz. In case of fast transient measurements in a power system under service, the well known power frequency voltage can be used directly for amplitude calibration.

At least it would be very interesting to have the authors comment on the necessary bandwith for the measurement of very fast transients. Is a bandwith of about 25 MHz for the spherical electrical field probe sufficient or should the it be higher, up to 100 MHz, and can the existing measuring equipment be extended?

Manuscript received February 22, 1988


23 1

Our aim in this contribution is to support such measurement techniques and to present a practical application in a large opexational 765 kV GIS [l], [2]. In addition, using such measurements as a reference, we illustrate the complementary use of sophisticated computer simulations as an efficient means of determining overall VFT characteristics in extensive and complex GIS installations, (thereby taking account of the variety of switching permutations).

Figure 1 depicts a sample switching configuration in the 1 6 5 kV GIS concerned. (GIS bus lengths of more than 200 m are involved).

0

Leslie C. Campbell (University of Strathclyde, Glasgow, United King- dom): The authors have described a most useful arrangement to measure the transient E fields associated with disconnector operations, both inside and outside the GIS enclosures. In several European GIs installations the insulated but earthed section of GIS passing through an externally mounted current transformer has experienced sparking between its flanged ends and the flanges of other GIS sections adjacent to it. Would the authors indicate how their model, shown in Fig. 3, could be modified to represent this insulated section of GIS and also give typical values for c k and Lk?

Have the authors undertaken measurements of the field adjacent to such an arrangement and, if so, can they comment on whether it is the fast fronted internal waveform which results in the flashover between the insulated flanges, or the slower external waveform shown in Fig. 4(c) which is responsible?

Calculations carried out on a MESH-connected 420-kV substation have indicated that overvoltages of less than 3 pu can be expected, even when including 1 pu trapped charge [I]. These studies have identified transient voltages with frequencies in the 0.7-80-MHz range. A typical trace is shown in Fig. 1, where three distinct frequency components are evident. Have similar frequencies been obtained by the authors and do the external waves in general have lower or non-frequency components? Also, have the authors measured disconnector switch overvoltages greater than this and, if so, could they comment on the reasons for such large overvoltages?

... I 500 1000 1500 2000 2500 3000

nanoseconds

Id . 0 1 - 3 3 1 5 I 7 r--T I O

l1OZ

Tlnt I u 1

Fig. 1. Frequency components of GIS voltage transient.

I w C l O R

t

t 765 KV LINE

Fig. 1: Sample l ayou t in 765 kV GIS D: operated disconnect switch M: VFT measurement location

A capacitive VFT probe similar to that described by the authors was located at the point M. (In fact, a number of such probes have been installed permanently in this particular GIS).

Figure 2 depicts the resultant VFT characteristics recorded at this position, during operation of the disconnect. In this case, the VFT are relatively complex - due to the coupling across the open circuit-breaker grading capacitor, and because of the impedance transitions at the bushing interface to the overhead line.

000.

Reference

[I] J. Lewis et al., “Disconnector operations in GIS insulated substations. Overvoltage studies and tests associated with 420kV installa- tion.” To be published, CERE, August 1988.


G. MAUTHE, A.J. ERIKSSON, and J.GRANDL. (ABB Asea Brown Boveri AG, Zurich, Switzerland).

The authors of this paper have demonstrated the use of wide-band capacitive probes as means of characterising VFT during disconnect switching operations in sample GIS installations.

2ool I


232

By way of contrast, Figure 3 depicts the VFT characteristics computed at the same position in a computerised simulation of the above switching configuration.

? 200

aooi

i 2 1 I

0 . I 500 1000 1500 2000 2500 3000

nanoseconds

-2001

Fig. 3: Comparative computed VFT waveform

Once such a computer model is available, it is of course very simple to determine the related VFT characteristics at a variety of alternative locations in the GIs - where measurements may no longer be possible.

The computational models used foi this purpose have been developed in an evolutionary approach, which incllided basic circuit analysis and low voltage pulse response measurements on individual GIS components, as well as transfzr response studies in complex layouts - both analytical and empirical [2].

We have found that the combination of specialised measuring systems - such as those presented by the authors - together with modern computational procedures, provides an optimised and cost-effective approach to characterising VFT in practical GIs installations.

[l] Lohmann V . and Brinzer R. "World's first 800 kV GIS substation" Brown Boveri Review, Vol. 74 No 10, Oct. 1988

121 Grand1 J. Eriksson A., et al; "Studies of very fast transients (VFT) in a 765 kV subst at ion" CIGRE Paper 1988 33-12, Paris, Aug/Sept 1988.


J . Meppe l ink , K . D i e d e r i c h , K . Feser , W. P f a f f : The a u t h o r s a p p r e c i a t e t h e v a l u a b l e c o n t r i b u t i o n s p r o v i d e d by t h e d i f f e r e n t d i s c u s s o r s . T h e i r comments g i v e some f u r t h e r i n f o r m a t i o n about t h e s u b j e c t and i n a d d i t i o n an e x c e l l e n t o p p o r t u n i t y t o c l a r i f y some o o i n t s .

The c o n t r i b u t i o n s by N . F u j i m o t o , and G . L . F o r d a r e v e r y much a p p r e c i a t e d . I t i s c o r r e c t , t h a t t h e i n i t i a l v a l u e o f t h e a m p l i t u d e o f t h e t r a v e l l i n a wave induced by d i s c o n n e c t o r o p e r a t i o n i n a r e a l sub- s t a t i o n dspends on t h e g e o m e t r i c a l ar rangement . The i n f l u e n c e o f such s u b s t a t i o n - d e p e n d e n t r e l a t i o n s has

t o be taken i n t o account t h r o u g h t h e s u b s t a t i o n response f a c t o r ment ioned i n t h e paper .

The impedance o f one phase o f t h e G I S e n c l o s u r e above e a r t h can be c o n s i d e r e d as a s imp le l i n e above t h e e a r t h i n g system as done by t h e d i s c u s s o r s . But i n r e a l i t y each e n c a p s u l a t i o n o f t h e o t h e r phases have t o be cons idered. These reduce t h e impedance zk s i g n i f i c a n t l y . I n o u r paper ( F i g . 4 ) measurements a r e shown f o r a s i m p l e t e s t arrangeinent where t h e TEV a m p l i t u d e i s h i g h e r i n a c e r t a i n d i s t a n c e f r o m t h e bush ing . Such h i g h e r a m p l i t u d e s zan o c c u r a l s o i n r e a l s u b s t a t i o n s t h r o u g h t h e s u p e r p o s i t i o n o f t h e t r a v e l l i n g wave phenomena. TEV a m p l i t u d e s can be reduced s i g n i f i c a n t l y by "adequate e a r t h i n g " as e x p l a i n e d a g a i n by t h e d i s c u s s o r s .

S c i e n t i f i c r e s e a r c h on t h e e F f e c t s o f VFT-s on t h e human body a r e n o t known. Acceptab le e x i s t i n g va lues i n t h e s tandards ( v o l t a g e s , c u r r e n t s , e l e c t r i c o r magnet ic f i e l d s ) c o v e r t h e power f requency range as we1 1 as t h e microwave range.

A so c a l l e d "go o r no go" t e s t was per fo rmed by 3 . Meppel ink p u b l i s h e d i n / 2 / . I n t h i s t e s t t h e t e s t person exposed h i s body u s i n g an i r o n r o d i n h i s hand t o an e l e c t r o d e w i t h a v o l t a g e o f 30 kV o s c i l l a t i n g w i t h a f requency o f 1 M H z . C u r r e n t and v o l t a g e d r o p was measured d u r i n g t h e t e s t w h i l e t h e t e s t person was s t a n d i n g i n a c o n t a i n e r i n o r d e r t o measure t h e s t r a y c u r r e n t s , t o o . C u r r e n t s w i t h peak a m p l i t u d e s up t o 35 A and s t e a d y s t a t e c u r r e n t s o f some ampere were measured.

F u r t h e r more t h e d a i l y exposure o f humans t o e l e c t r o s t a t i c d i s c h a r g e s up t o 20 kV demonst ra tes t h e w i t h s t a n d c a p a b i l i t y t o f a s t t r a n s i e n t v o l t a g e s . From o u r p o i n t o f v iew no r i s k f o r t h e h e a l t h e x i s t s by t h e exposure t o TEV-s f o r human b e i n g s .

Some " s u r p r i s i n g " e f f e c t s may be t a k e n i n t o account f o r t h e employees i n s u b s t a t i o n s when t o u c h i n g t h e e n c a p s u l a t i o n i n case o f TEV. I n most o f t h e p r a c t i c a l s i t u a t i o n s r e p a i r o r maintenance t a k e s p l a c e on equipment l o c a t e d near g round where t h e magni tude o f TEV i s low compared t o t h e magni tude a t t h e equipment p l a c e d o u t s i d e t h e s w i t c h g e a r b u i l d i n g . I f employees a r e w o r k i n g a t t h e bus d u c t o r bush ing t h e p a r t i c u l i a r bus has t o be deenerg ized and t h e r e f o r e t h e e f f e c t o f TEV i s reduced s i n c e o n l y t h e TEV r e s u l t i n g f r o m f a r l o c a t e d o t h e r d i s c o n n e c t o r s can occur . Fur thermore t h e employees have t o use s a f e t y b e l t s o r s i m i l a r a i d s t o p r e v e n t f a l l i n g down. T h e r e f o r e i n most cases " s u r p r i s i n g " e f f e c t s w i l l n o t l e a d t o a severe a c c i d e n t .

E. Gockenbach ment ions some i n t e r e s t i n g h i n t s on p r e s e n t i n t e r n a t i o n a l e f f o r t s i n s t a n d a r d i s a t i o n e s p e c i a l l y f r o m t h e work o f C I G R E WG 33-03.

F o r t h e measurement o f t h e VFT phenomena i n s i d e t h e G I s a bandwid th up t o more t h a n 100 MHz /21! i s necessary t o r e c e i v e t h e whole i n f o r m a t i o n . F o r t h e measurement o f t h e c h a r a c t e r i s t i c TEV va lues l i k e t h e r i s e t i m e o r t h e main o s c i l l a t i n g f r e q u e n c i e s a meas- u r i n g system w i t h a bandwid th o f 25 MHz i s s u f f i - c i e n t . I n s p e c i f i c cases o s c i l l a t i o n s w i t h h i g h e r f r e q u e n c i e s b u t v e r y smal l a m p l i t u d e s can be measured (see F i g . 11 o r / 1 7 / ) . F o r t h e measurement o f these t r a n s i e n t s a s p h e r i c a l e l e c t r i c f i e l d sensor w i t h an analog bandwid th o f 100 MHz has been developed / l / .

R e f e r r i n g t o t h e q u e s t i o n s o f L . C . Campbell i n s u l a t e d p a r t s connected by s t r i p s can be taken i n t o c o n s i d e r a t i o n . The ment ioned model ( F i g . 3 ) has t o be m o d i f i e d a c c o r d i n g t o t h e d e t a i l e d c o n s t r u c t i o n . I n many p r a c t i c a l cases these p a r t s can be mode l led by


233

proposa l o f combin ing measurements and c a l c u l a t i o n s . Measurements can be pe r fo rmed f o r example w I LII

permanent ly b u i l t - i n c a p a c i t i v e v o l t a g e sensors .

E s p e c i a l l y f o r t h e i n v e s t i g a t i o n s o f t h e i n t e r n a l VFT-s t h e approach d e s c r i b e d i n t h i s d i s c u s s i o n i s a conven ien t s o l u t i o n because t h e m a i n l y c o a x i a l i n t e r n a l s t r u c t u r e o f t h e G I S can be mode l l ed adequate. As shown t h e v a l i d i t y o f t h e c a l c u l a t i o n s can be c o n t r o l l e d th rough measurements a t a few p o i n t s .

To c h a r a c t e r i z e t h e TEV phenomena i t i s much more c o m p l i c a t e d t o model t h e c h a r a c t e r i s t i c components w i t h t h e i r d i s t r i b u t e d impedances e s p e c i a l l y i n t h e e a r t h i n g system. T h e r e f o r e measurements w i t h adequate measur ing d e v i c e s l i k e t h e s p h e r i c a l e l e c t r i c sensor a r e good t o o l s t o s t u d y t h e phenomena and t o deve lop accep tab le models f o r t h e c a l c u l a t i o n .

i nduc tances . The c h a r a c t e r i s t i c va lues o f ck and Lk a r e depending s t r o n g l y on t h e s p e c i f i c des ign o f t h e GIS as ment ioned above. E s p e c i a l l y t h e e a r t h system e .g . t h e i nduc tance o f t h e suppor t s t r u c t u r e and t h e sc reen ing e lements have i n f l u e n c e on t h e amp l i t udes and f r e a u e n c i e s o f t h e TEV.

I n t h e equ ipment manufac tured by ABB t h e gap necessary f o r c u r r e n t t r a n s f o r m e r s i s b u i l t i n s i d e t h e e n c l o s u r e . The c o r e i s covered b y a c o n d u c t i v e s h i e l d . The i n t e r n a l gap can be des igned t h a t no s p a r k i n g occu rs . To p r e v e n t c u r r e n t f l o w th rough t h e i r o n vesse l o f t r a n s f o r m e r s i t i s o f t e n i n use t o i n s u l a t e t h e e n c a p s u l a t i o n . I n t h i s c3se s p a r k i n g m i g h t occu r caused b y i n t e r n a l VFT. K e t a l o x i d e a r r e s t e r s can be used t o p r e v e n t those sparks .

The c a l c u l a t e d magn i tude and f r e q u e n c i e s o f VFT d u r i n g d i s c o n n e c t o r o p e r a t i o n depends on t h e model- l i n g o f t h e i n d i v i d u a l G I S components and t h e assumed e l e c t r i c a l parameters as t r a p p e d charge, phase oppo- s i t i o n e t c .

From o u r e x p e r i e n c e we can c o n f i r m t h e t y p i c a l va lues f o r VFT-s ment ioned by C. L . Campbel l . I n p r a c t i c e t h e o v e r v o l t a g e s a r e n o r m a l l y l e s s t h a n 2 .4 ?.U. and t h e h i g h e s t f r e q u e n c i e s l e s s t h a n 100 MHz as a l s o r e p o r t e d i n many o t h e r p u b l i c a t i o n s . E x t e r n a l VFT- s have l o w e r f r e q u e n c i e s t h a n i n t e r n a l VFT- s .

G. Wauthe, A . J . E r r i c s o n , and J . Grand1 g i v e i n t e r e s t i n g a d d i t i o n a l i n f o r m a t i o n f o r a r e a l 765 kV G I S . The a u t h o r s agree c o m p l e t e l y t o t h e

Reference: /1/ W . P f a f f : Accuracy o f a S p e r i c a l Sensor f o r t h e

Measurement of Threed imens iona l E l e c t r i c F i e l d s . F i f t h I n t e r n a t i o n a l Symposium on H igh Vo l tage Eng ineer ing , Braunschweig 1987, Paper 32.05

i n s u l a t e d S u b s t a t i o n s , Techno logy and P r a c t i c e . Pergamon Press , New York, 1986, pp 161-162.

/2 / S. A . Boggs, F . Y . Chun, N . Fuj imot.0: Gas-

Manuscript received April 25, 1988.


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