DETERMINATION OF THE MICROSCOPIC FIELD ENHANCEMENT FACTOR β OF PRESTRESSED VACUUM INTERRUPTER CONTACTS

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HAL Id: jpa-00219178

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Submitted on 1 Jan 1979

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DETERMINATION OF THE MICROSCOPIC FIELD ENHANCEMENT FACTOR β OF PRESTRESSED

VACUUM INTERRUPTER CONTACTS

K. Fröhlich, W. Widl

To cite this version:

K. Fröhlich, W. Widl. DETERMINATION OF THE MICROSCOPIC FIELD ENHANCEMENT

FACTOR β OF PRESTRESSED VACUUM INTERRUPTER CONTACTS. Journal de Physique

Colloques, 1979, 40 (C7), pp.C7-407-C7-408. �10.1051/jphyscol:19797199�. �jpa-00219178�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6rnent au n07, Tome 40, JuiZZet 1979, page

C7-

407

DETERMINATION

ff

THE

MICROSCOPIC

FIELD ENHANCEMENT FACTOR f~

OF PRESTRESSED VACUUM INTERRUPTER CONTACTS

K. F r a h l i c h , W. Widl.

Chair o f Switchgear, TechnicaZ U n i v e r s i t y o f Vfenna.

A b s t r a c t - The m i c r o s c o p i c f i e l d enhancement which a l s o c a l c u l a t e d and d e t r a c t e d t h e c a p a c i t i v e f a c t o r 6 and t h e f i e l d e m i s s i o n a r e a a o f v a r i o u s l y gap c u r r e n t . F i n a l l y t h e m i c r o s c o p i c f i e l d enhance- p r e s t r e s s e d vacuum i n t e r r u p t e r c o n t a c t s were i n v e s - ment f a c t o r 6 and t h e f i e l d e m i s s i o n a r e a a were t i g a t e d u s i n g t h e Fowler Nordheim e q u a t i o n . Addi- p r i n t e d o u t .

t i o n a l l y t h e c o r r e s p o n d i n g breakdown v o l t a g e v a l u e s

were c o n s i d e r e d . S t a t i s t i c a l l y e v a l u a t e d r e s u l t s RESULTS AND DISCUSSION

a r e shown and d i s c u s s e d . 1 . D e t e r m i n a t i o n o f 6.a a f t e r v a r i o u s s w i t c h - INTRODUCTION i n g o p e r a t i o n s - T a b l e 1 shows t h a t t h e s u r f a c e o f b o t h t h e v a l v e c o n t a c t s and t h e Cu c o n t a c t s a r e i n - The c o n t a c t s u r f a c e m i c r o s t r u c t u r e o f a vacuum

i n t e r r u p t e r i s s t r o n g l y a f f e c t e d by t h e t y p e o f t h e p r e c e d i n g s w i t c h i n g o p e r a t i o n . T h i s c a u s e s a b a d l y d e f i n e d d i e l e c t r i c w i t h s t a n d s t r e n g t h o f t h e c o n t a c t gap l e a d i n g t o a n u n r e l i a b l e i n s u l a t i n g b e h a v i o u r o f open vacuum i n t e r r u p t e r s .

For t h i s r e a s o n a n i n v e s t i g a t i o n on a s t a t i s t i - c a l b a s i s was s t a r t e d r e c e n t l y i n o r d e r t o l e a r n a b o u t t h e r e l e v a n t phenomena i n f l u e n c i n g t h e b r e a k - down performance of a n open vacuum i n t e r r u p t e r gap

[ll .

V a r i o u s l y p r e s t r e s s e d c o n t a c t s were t e s t e d by v a r i o u s l y shaped v o l t a g e c u r v e s i n a l l combinations.

B e s i d e a n i n s i g h t i n t o t h e d i e l e c t r i c w i t h s t a n d be- h a v i o u r o f t h e gap t h e r e s u l t s a l s o y i e l d e d c o n c l u - s i o n s a b o u t t h e s u r f a c e m i c r o s t r u c t u r e and t h e i n i - t i a t i n g breakdown mechanisms.

I n a d d i t i o n t o t h i s r e s e a r c h t h e m i c r o s c o p i c f i e l d enhancement f a c t o r 6 and t h e e m i s s i o n a r e a a were d e t e r m i n e d a f t e r p r e s t r e s s i n g t h e c o n t a c t s by v a r i o u s t y p e s o f s w i t c h i n g o p e r a t i o n s . The r e s u l t s a r e p r e s e n t e d i n t h i s p a p e r and y i e l d more i n s i g h t i n t o t h e s u r f a c e phenomena o f a vacuum i n t e r r u p t e r u n d e r a c t u a l c o n d i t i o n s .

PROCEDURE The t e s t s were c a r r i e d o u t w i t h

- Rogowski p r o f i l e shaped c o p p e r o r t u n g s t e n e l e c - t r o d e s i n a n e x p e r i m e n t a l chamber a t t o r r [ e l e c t r o d e d i a m e t e r 40 mm, gap l e n g t h 5 mml - a commercial vacuum i n t e r r u p t e r v a l v e ( 1 5 . 5 kV,

600 A , 1 2 kAl; 5 mm g a p length;, Cu c o n t a c t s New c o n t a c t s were c o n d i t i o n e d by 100 d c a r c s [IOD A, 500 rns] i n o r d e r t o produce c l e a n and w e l l d e f i n e d s u r f a c e s . Then t h e c o n t a c t s w e r e s t r e s s e d by s e r i e s o f one o f t h e f o l l o w i n g s w i t c h i n g o p e r a t i o n s : - no l o a d c l o s e - o p e n o p e r a t i o n w i t h o u t a r c i n g INLI - no l o a d c l o s e - o p e n o p e r a t i o n a f t e r o n e c o n d i t i o n -

i n g d c a r c o f IOOA, 1 0 0 m s INLC o p e r a t i o n 1

-

no load c l o s i n g f o l l o w e d by a 1 0 0 A d c o p e n i n g a r c of 100 m s o r 1000 m s d u r a t i o n

- no l o a d c l o s i n g f o l l o w e d by 50 Hz, 5 kArms i n t e r - r u p t i o n ( 9 m s a r c d u r a t i o n )

F i f t e e n s e c o n d s a f t e r e a c h s w i t c h i n g o p e r a t i o n a v o l t a g e p u l s e was a p p l i e d a c r o s s t h e o p e n gap ris- i n g w i t h 20 h / m s o r 0.1 kV/ms r e s p e c t i v e l y . The 20 kV/ms p u l s e s w e r e i n c r e a s e d u n t i l b r e a k d o w n o c c u r e d w h e r e a s t h e maximum v o l t a g e of t h e 0 . 1 kV/mspulses was l i m i t e d by t h e s u p p l y t o v o l t a g e v a l u e s below t h e breakdown v o l t a g e .

During t h e t e s t s t h e f i e l d e m i s s i o n c u r r e n t (FEC) 2 100 nA was measured. The c o r r e s p o n d i n g Fowler Nordheim (FN) c u r v e w a s p l o t t e d b y a computer

f l u e n c e d by t h e p r e c e d i n g s w i t c h i n g o p e r a t i o n i n t h e same way. A f t e r no l o a d c l o s e - o p e n o p e r a t j o n s p r e - ceded by a ID0 A d c a r c INLC) t h e mean v a l u e s B w e r e h i g h e s t c o n n e c t e d w i t h l o w e s t mean breakdown v o l t a - g e s VB. ( A l l B- a n d a v a l u e s w e r e c a l c u l a t e d u s i n g t h e F N e q u a t i o n a t a n a s s u m e d work f u n c t i o n of 4.6 eV.1

A 1 0 0 A , I 0 0 rns d c a r c d e c r e a s e s 6 c a u s i n g a n i n c r e a s e o f

vB.

T h i s e f f e c t i s more s i g n i f i c a n t , i f t h e a r c d u r a t l o n i s e x t e n d e d t o 1 s e c . ( a s s h o w n f o r Cu c o n t a c t s ) . 6 and VB v a l u e s i n d i c a t e d s u r f a c e smoothening by 5 kArms i n t e r r u p t i o n a r c s . Repeated no l o a d s w i t c h i n g (NLI d e c r e a s e d 6 below t h e c o r r e s - ponding v a l u e s f o r NLC o p e r a t i o n . A l l t h o s e t e n d e n - c i e s c o n c e r n i n g t h e breakdown v o l t a g e s a g r e e w i t h [I).

The e v a l u a t i o n of t h e FEC i m m e d i a t e l y b e f o r e breakdown ( c r i t i c a l prebreakdown c u r r e n t i ) , s h o w s t h a t i depends on b o t h

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Merely a f t e r 5 kA i n t e r r u p t i o n a and 1 y i e l d e d u n e x p e c t a d l y low v a l u e s i n d i c a t i n g t h a t 5 k,8 a r c s produced s m a l l e r e m i s s i o n s i t e s t h a n 100 A dc a r c s a t c o m p a r a b l e c h a r g e t r a n s f e r . I n some c a s e s t h o s e s m a l l e r s i t e s were d e s t r o y e d by t h e a p p l i e d v o l t a g e c a u s i n g a s t e p i n t h e FN c u r v e .

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and 6 m a n i f e s t e d by a nega- t i v e c o r r e l a t i o n f a c t o r p ( 6 , V I . A f t e r 5 K A i n t e r - r u p t i o n , however, p(B. V B ) was p o s i t i v e f o r t h e Cu B c o n t a c t s i n d i c a t i n g t h e s u p e r p o s i t i o n of a n i n i t i - a t i n g mechanism which i s n o t governed by t h e F E p h e - nomenon. I n t h i s c a s e t h e peak v a l u e o f t h e a r c c u r r e n t was c l o s e t o t h e t h r e s h o l d o f anode s p o t f o r m a t i o n and i n c r e a s i n g t h e l i k e l y h o o d of p a r t i c l e breakdown [I]. When 5 kA were i n t e r r u p t e d by t h e v a l v e t h e a r c c e r t a i n l y was i n a d i f f u s e mode b u t

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797199

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a c c o r d i n g t o [I] p a r t i c l e breakdown i s l i k e l y t o o . T h i s may e x p l a i n t h e l o w e r p v a l u e compared t o t h a t

a f t e r t h e 1 0 0 A d c a r c . F u r t h e r e v i d e n - c e f o r t h i s a s s u m p - t i o n i s g a i n e d by t h e f a c t t h a t i n some c a s e s t h e FN c u r v e s w e r e e x a c t l y i d e n t i - c a l b e f o r e a n d a f t e r breakdown, i . e . no main e m i s s i o n s i t e was i n v o l v e d i n breakdown e x c l u d i n g a FE i n i t i a t e d me- PULSE M A S E R N e v e r t h e l e s s t h e f a c t t h a t t h e c r i t i c a l m i c r o s c o p i c F i g . 1 : Change o f B a n d a d e -

field

i C G V ,g

was p e n d i n g o n t h e number o f a p -

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

t i o n s and i n d e o e ~ d - - -

e n t o f t h e B v a l u e i n d i c a t e s t h a t brea-kdown m$rrly i s i n i t i a t e d by FE a t a c o n s t a n t mean FE c u r r e n t d e n - s i t y

1 .

~ S m ~ a r a b l e m e a s u r e m e n t s w i t h t u n g s t e n c o n t a c t s y i e l d e d

B

v a l u e s o n e o r d e r o f m a g n i t u d e less t h a n f o r Cu e l e c t r o d e s . I n t h i s case t h e p r e b r e a k d o w n c u r r e n t c u r v e showed s i g n i f i c a n t m i c r o d i s c h a r g e pul- s e s w h i c h c a u s e d breakdown b e f o r e t h e FEC r e a c h e d a c r i t i c a l v a l u e .

v -

l o r i g i n a l v a l u e p r o -

2 / ,o-12

m 2

duced by t h e s w i t c k 2 . Changes o f B,a d u e t o t h e a p p l i e d v o l t a g e - I f a s e r i e s o f f i v e c o n s e c u t i v e v o l t a g e p u l s e s Li3.l kV/msl w e r e a p p l i e d i n i n t e r v a l s o f 4 min.

( w i t h o u t any b r e a k d o w n ) B was d e c r e a s e d e s p e c i a l l y a f t e r no l o a d - s w i t c h i n g [NLC). F i g . 1 shows t h e mean v a l u e s

B

and a o f s i x t e s t series a c r o s s t h e p u l s e number. Each p u l s e was s w i t c h e d o f f when t h e FEC i n c r e a s e d t o 1 mA. The c o r r e s p o n d i n g FN c u r v e s s t a r - t e d a s s t r a i g h t l i n e s a t low c u r r e n t s a n d w e r e b e n - d e d t o w a r d l o w e r c u r r e n t v a l u e s a t h i g h e r v o l t a g e s i n d i c a t i n g a g e n e r a l d e c r e a s e o f B d u r i n g t h e t e s t .

- .

i n g o p e r a t i o n . F i g . 2 : I n f l u e n c e o f t h e resi-

DC v o l t a g e a p - d u a l g a s p r e s s u r e on B and a .

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b e t w e e n t h e 4th a n d t h e 5th p u l s e c a u s i n g 1 mA FEC y i e l d e d no f u r t h e r s i g n i f i c a n t d e c r e a s e o f t h i s s a - t u r a t i o n v a l u e .

T h i s r e s u l t shows c l e a r l y t h a t c h a n g e s o f t h e i s o l a t i n g p e r f o r m a n c e of a vacuum g a p c a n b e c a u s e d by t h e a p p l i e d v o l t a g e e v e n i f no breakdown o c c u r s . 3 . I n f l u e n c e o f p r e s s u r e o n B a n d a

-

If t h e o r e s s u r e was i n c r e a s e d t o 1 0 - 5 t o r r w i t h 0, o r N 7

I I I I I I I I

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s c r i b e d i n d e t a i l it i s e v i d e n t t h a t t h e x h a n g e s of a,B a r e c a u s e d by s u r f a c e l a y e r g r o w t h . The a u t h o r s s u p p o s e t h a t b e c a u s e o f t h i s s u r f a c e l a y e r t h e work f u n c t i o n i s i n c r e a s e d L2] y i e l d i n g l o w e r c a l c u l a t ' e d

v a l u e s a t t o r r . I t i s n o t y e t c l e a r why t h i s e f f e c t o c c u r s o n l y a f t e r a r c i n g and n o t a f t e r no l o a d s w i t c h i n g , b u t i t i s assumed t h a t t h e a r c i n - c r e a s e s t h e s u r f a c e l a y e r g r o w t h i n some way.

A l s o u n s t e a d y f l u c - t u a t i o n s w e r e s u p e r - imposed e s p e c i a l l y a t h i g h e r FEC v a l u e s s h o w i n g a g e n e r a l i n s t a b i l i t y o f t h e e m i s s i o n s i t e s .

A f t e r a t l e a s t f o u r p u l s e s t h e B v a l u e s a p p r o a c h e d a s a t u r a t i o n v a l u e w h i c h was n e a r l y i n ?

d e p e n d e n t o f t h e

CONCLUSIONS

Vacuum i n t e r r u p t e r Cu c o n t a c t s show a c l e a r c o r r e l a t i o n b e t w e e n t h e breakdown v o l t a g e and t h e microscopic f i e l d , e n h a n c e r n e n t f a c t o r B . T h i s i n d i - c a t e s a m a i n l y f i e l d e m i s s i o n g o v e r n e d breakdown mechanism. N e v e r t h e l e s s t h e p r o b a b i l i t y o f a p a r t i c - l e breakdown i n c r e a s e s a f t e r i n t e r r u p t i o n of h i g h e r c u r r e n t s e v e n f o r d i f f u s e arcs.

The s u r f a c e m i c r o s t r u c t u r e and t h e e m i s s i o n be- h a v i o u r d e p e n d s s t r o n g l y on t h e p r e s t r e s s o f t h e ' c o n t a c t s . O b v i o u s l y t h e amount a f c h a r g e t r a n s f e r by t h e a r c a s w e l l a s t h e number o f c a t h o d e s p o t s i n f l u e n c e t h e a and B v a l u e s .

The e x t r e m l y h i g h B v a l u e s e s p e c i a l l y p r o d u c e d b y no l o a d s w i t c h i n g c a l l i n q u e s t i o n t h a t t h e a s s u - med work f u n c t i o n o f 4 . 6 eV i s r e p r e s e n t a t i v e f o r i n t e r r u p t e r c o n t a c t s p r e s t r e s s e d a t a c t u a l c o n d i t i - o n s ; p a r t i c u l a r l y a s t h e r e s u l t s f o r i n c r e a s e d r e - s i d u a l p r e s s u r e g a v e t h e i d e a t h a t t h e w o r k f u n c t i o n i s i n f l u e n c e d by t h e c o n t a c t s u r f a c e l a y e r .

A s a l r e a d y assumed i n [I], c o n s i d e r a t i o n o f t h e c r i t i c a l f i e l d e m i s s i o n c u r r e n t y i e l d s a r o u g h q u a l i t a t i v e i n s i g h t i n t o t h e s u r f a c e m i c r o s t r u c t u r e o f t h e i n t e r r u p t e r c o n t a c t s ; E s p e c i a l l y when s w i t c h - i n g low c u r r e n t s o r p e r f o r m i n g no l o a d o p e r a t i o n s .

The breakdown v o l t a g e o f a vacuum i n t e r r u p h s r g a p i s s t r o n g l y i n f l u e n c e d by t h e h i s t o r y o f t h e c o n t a c t s a s w e l l a s by t h e a p p l i e d t e s t i n g v o l t a g e i t s e l f . M e a s u r e m e n t s w i t h o u t c o n s i d e r i n g t h o s e f a c t s c a n y i e l d r e s u l t s which a r e n o t r e p r e s e n t a t i v e f o r t h e i n s u l a t i n g p e r f o r m a n c e t h e b r e a k e r would h a b e u n d e r a c t u a l c o n d i t i o n s [3]

.

The o c c u r r e n c e o f m i c r o d i s c h a r g e s f o r t u n g s t e n e l e c t r o d e s n o t o b s e r v e d f o r Cu c o n t a c t s i n d i c a t e s t h a t t h e r e s u l t s o b t a i n e d h i t h e r k o - 7 may n o t b e g e - n e r a l l y v a l i d f o r o t h e r m a t e r i a l s . F u r t h e r i n v e s t i - g a t i o n i s n e c e s s a r y i n o r d e r t o r e v e a l t h i s m a t t e r .

ACKNOWLEDGEMENT

The a u t h o r s w i s h t o t h a n k P r o f . D r . Werner R i e d e r [ T e c h n . U n i v . V i e n n a 1 f o r t h e many u s e f u l d i s c u s s i o n s . The i n v e s t i g a t k o n was m a t e r i a l l y s p o n - s o r e d by t h e A u s t r i a n "Forschungsf8rderungsfonds d e r G e w e r b l i c h e n W i r t s c h a f t " , by t h e " A u s t r i a n Brown B o v e r i Comp., L t d . " a n d " M e t a l l w e r k P l a n s e e " (Aus- t r i a l .

REFERENCES

1. K . F r E h l i c h , On t h e S t e a d y S t a t e B r e a k d o w n V o l t a g e o f a Vacuum I n t e r r u p t e r , 1 n s u l . V a c . V I I I (19781 pp. € 6 - 1 . . .E6-14

2 . K.Ashok. K.Oimoff, The e f f e c t o f o x i d e s u r f a c e l a y e r s on t h e breakdown v o l t a g e i m metal-vacuum- m e t a l s a n d w i c h e s , J o u r n . o f M a t e r i a l s S c i e n c e 1 1 [ 1 9 7 6 ) , p p . 250.. . I 6 3

3 . F . B a t t i w a l a , U.Baumgart1, G.Stock. U n t e r s u c h u n g - e n beim E n t w i c k e l n und P r f i f e n d e r Vakuum-Leis- t u n g s s c h a l t e r 3AF, S i e m e n s Z e i t s c h r i f t H.12 ( 1 9 7 8 1 , pp. 661.. , 6 6 5

;he

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v a l ; e s d e c r e a s e d a n d ti v a l u e s i n c r e a g e d ( F i g . 2 1 . T h i s e f f e c t was s i g n i f i c a n t i f t h e c o n t a c t s w e r e p r e s t r e s s e d by a 1 0 0 A d c a r c e s p e c i a l l y f o r o x y g e n . However, a f t e r no l o a d o p e r a t i o n s [NLC) t h e i n f l u e n c e o f p r e s s u r e was n e g l i g i b l e .

Oue t o t h e s p e c i f i e d t e s t p r o c e d u r e n o t d e -