VARIATION OF CORONA INCEPTION AND SPARKOVER WITH POSITIVE IMPULSE VOLTAGE IN A ROD/PLANE GAP : EFFECT OF NEGATIVE SMALL ION VARIATION

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VARIATION OF CORONA INCEPTION AND

SPARKOVER WITH POSITIVE IMPULSE VOLTAGE IN A ROD/PLANE GAP : EFFECT OF NEGATIVE

SMALL ION VARIATION

N. Allen, T. Allibone, D. Dring

To cite this version:

N. Allen, T. Allibone, D. Dring. VARIATION OF CORONA INCEPTION AND SPARKOVER WITH POSITIVE IMPULSE VOLTAGE IN A ROD/PLANE GAP : EFFECT OF NEGATIVE SMALL ION VARIATION. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-423-C7-424.

�10.1051/jphyscol:19797206�. �jpa-00219186�

JOURiVAL DE PHYSIQUE CoZZoque C7, strppZdment au n07, Tome 40, J u i Z l e t 1979, page C7- 423

VARIATION OF CORONA INCEPTION AND SPARKOVER W I T H POSITIVE IMPULSE VOLTAGE IN A ROD~PLANE GAP : EFFECT OF NEGATIVE SMALL ION VARIATION

N.L. Allen, T.E. Allibone and D. Dring.

Department o f EZectricaZ and EZectronic Engineering, The U n i v e r s i t y o f Leeds, LEEDS LS2 SJT, U . K .

INTRODUCTION

The t i m e s t o i n c e p t i o n o f corona and sparkover i n a r o d / p l a n e gap a r e a f f e c t e d b y t h e p r o b a b i l i t y o f occurrence o f c r e a t i o n o f a f r e e e l e c t r o n c l o s e t o t h e anode ( 1 ) - ( 4 ) . No s y s t e m a t i c measurements have been r e p o r t e d i n which t h e sequence o f events d u r i n g i n d i v i d u a l impulses has been s t u d i e d a t a number o f v o l t a g e s t e p s between Uw and UIOO w h i l e d e t e r m i n i n g t h e sparkover c h a r a c t e r i s t i c s . T h i s paper d e s c r i b e s measurements i n w h i c h r a d i o a c t i v e sources have been used t o v a r y t h e i o n d e n s i t y . EXPERIMENTAL ARRANGEllENTS AND PROCEDURE

The work was c a r r i e d o u t u s i n g a 2cm d i a hemi-

c a n t l y reduced a t c o n s t a n t v o l t a g e b y i n c r e a s i n g t h e n e g a t i v e i o n d e n s i t y up t o 1030 c c -1

.

b ) I n c e p t i o n t i m e s were o n l y m o d e r a t e l y reduced w i t h i n c r e a s i n g v o l t a g e a t c o n s t a n t backgraund i o n d e n s i t y . For t h e two h i g h e s t d e n s i t i e s ( z 5200 C C - ' ) Ti was g e n e r a l l y reduced.

DISCUSSION

The most i m p o r t a n t f e a t u r e o f t h e r e s u l t s i s t h a t w i t h t h e background n e g a t i v e s m a l l i o n d e n s i t - i e s w i t h i n t h e range 150 c c - I t o 1030 c c - I , t h e r e s p h e r i c a l l y - e n d e d brass rod, 60cm above a l m d i a was no s i g n i f i c a n t change i n e i t h e r t h e corona p l a n e . Impulse v o l t a g e s (+ 13/2000 u s ) were i n c e p t i o n time, a t a g i v e n v o l t a g e , o r i n t h e

a p p l i e d . sparkover c h a r a c t e r i s t i c s . Thus, day-to-day v a r -

Background ion densities were varied by using a t i o n s i n t h e s m a l l i o n d e n s i t y i n t h e atmosphere, lmCi, l4mCi and lOOmCi c~~~~ sources each p l a c e d which a r e w e l l w i t h i n t h e s e l i m i t s , w i l l have n o in its lead container level with the Diane and 50cm e f f e c t upon sparkover c h a r a c t e r i s t i c s . They can- from t h e a x i s of t h e r o d / p l a n e gap. A c o l Timated ^"Ot the ( 6 ) where ^Occas- y - r a y beam was p o i n t e d upwards p a r a l l e l t o t h e i o n a l s p a r k o v e r c h a r a c t e r i s t i c s w i t h n a t u r a l back- e l e c t r o d e a x i s , w i t h o u t i m p i n g i n g upon t h e e l e c - ground a r e s i m i l a r t o t h a t o f c u r v e ( e ) , s i n c e t h e t r o d e s u r f a c e s . The s m a l l i o n d e n s i t i e s were n a t u r a l i o n d e n s i t y i n t h i s l a b o r a t o r y has never sampled 60cm f r o m t h e a x i s and were measured w i t h a exceeded ^Q, 400 c c - l .

Gerdien i o n c o u n t e r ( 5 ) . Corona i n c e p t i o n t i m e s (Ti) "ere o b t a i n e d u s i n g a p h o t o m u l t i p l i e r u n i t .

Sparkover c h a r a c t e r i s t i c s were determined f o r each background i o n d e n s i t y b y a p p l y i n g 20 impulses a t any one v o l t a g e s e t t i n g a t i n t e r v a l s o f 20 sec.

RESULTS

I n c e p t i o n t i m e s t o t h e f i r s t corona p u l s e a r e g i v e n i n T a b l e 1, and t h e sparkover c h a r a c t e r i s t i c s i n F i g 1 ( a ) - ( e ) were d e r i v e d f r o m t h e same s e t o f r e s u l t s . The background n e g a t i v e i o n d e n s i t y was measured w i t h no impulses a p p l i e d , and t h e f o l l o w - i n g t r e n d s a r e e v i d e n t :

A t t h e h i g h e r d e n s i t i e s , v ~ h e r e th e corona i n c e p t i o n t i m e s ( a n d hence s i z e of corona) were s i g n i f i c a n t l y reduced, t h e s l o p e o f t h e s p a r k o v e r c h a r a c t e r i s t i c i n c r e a s e d . The c o n d i t i o n f o r t h e i n c r e a s e i n s l o p e r e q u i r e s a background d e n s i t y o f n e g a t i v e i o n s between 1030 and 5200 c c - l .

N e g a t i v e I o n s and Corona I n i t i a t i o n

I t i s g e n e r a l l y assumed t h a t corona i s i n i t i - a t e d b y e l e c t r o n detachment f r o m a n e g a t i v e i o n near t h e t i p o f t h e r o d . However, t h e n e g a t i v e

i o n d e n s i t y i m m e d i a t e l y b e f o r e each i m p u l s e would be much l o w e r t h a n t h e background v a l u e s q u o t e d here, due t o c o m b i n a t i o n w i t h t h e l a r g e excess of a ) Average i n c e p t i o n times, and t h e e a r l i e s t

p o s i t i v e i o n s produced b y p o s i t i v e corona ( 5 ) and i n c e p t i o n t i m e s o f t h e range, were n o t s i g n i f i -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797206

sparks. Since r e l i a b l e negative i o n d e n s i t i e s ~ e n a r d i > r e s group ( 2 ) . Minimum f i e l d s f o r corona c o u l d n o t be measured d u r i n g t h e experiments, an i n c e p t i o n a r e t h e s u b j e c t o f f u r t h e r work i n argument adopted e a r l i e r ( 7 ) was used t o estimate progress i n t h i s l a b o r a t o r y .

t h e i r value knowing t h a t the p o s i t i v e i o n d e n s i t y REFERENCES b e f o r e each impulse was % 10 times n a t u r a l back-

1. Waters R T, Jones R E and Bulcock C J; Proc

ground. The value estimated was o f the order IEE, 1965,

112,

one-tenth o f background. Thus, t h e presence o f 2. Les ~ e n a r d i s r e s Group; E l e c t r a , 1973, No 23, t h e r a d i o a c t i v e sources r a i s e d t h e negative i o n pp 55-157

d e n s i t i e s d u r i n g t h e experiments, b u t o n l y when a 3. Fieux R and H u t z l e r B; Proc I n t Symp on High Voltage Technology, Munich 1972 pp 320-325 background o f 5200 c c - I e x i s t e d were they s u f f i c i - 4. Somervi le and Tedford J ; Proc 5th e n t t o change t h e corona and sparkover character- I n t IEE Conf on Gas Discharges, L i v e r p o o l ,

i s t i c s . 1978, pp 250-253

5. A l l e n N L, A l l i b o n e T E and D r i n g D; Proc IEE, Table 1 shows t h a t w i t h the l a r g e s t background 1977,

124,

i o n d e n s i t i e s , t h e e a r l i e s t i n c e p t i o n times t e n d 6. A l l i b o n e T E and D r i n g D; I E E Colloquium on towards l i m i t i n g values and hence, minimum f i e l d s . P r o t e c t i o n o f E l e c t r i c a l Systems, Hay 1974 This may be compared t o the r e s u l t s o f H u t z l e r 7. A l l e n N L, A l l i b o n e T E and Dring D; Proc 5 t h

I n t IEE Conf on Gas Discharges, L i v e r p o o l , (8) and Fieux and H u t z l e r (3), where a 12 cm gap, 1978, pp 92-95

having a r o d o f 2 mm dia., was subjected t o 8. H u t z l e r B; Proc Xth I n t Conf on Phenom. i n enhanced negative i o n concentrations from a neigh-

b o u r i n g n e g a t i v e corona discharge. The f i r s t

"pre-onset" corona was r e p o r t e d t o be suppressed by the enhanced n e g a t i v e i o n concentration, and was replaced by an e a r l i e r avalanche which f a i l e d t o develop a streamer. However, a c t u a l i o n con- c e n t r a t i o n s i n the gap were n o t estimated.

I n t h e present work, a t r a n s i t i o n from a streamer t o an avalanche c o u l d n o t be observed.

However, the tendency towards a l i m i t i n g corona i n i t i a t i o n time and minimum f i e l d a t > 80,000 c c - I suggests t h a t avalanches a r e n o t s i g n i f i c a n t before t h i s time. Taking a simple expression ( 1 ) f o r t h e a x i a l f i e l d a t t h e t i p o f t h e rod, E = V/2r, t h e e a r l i e s t corona wi t h a1 1 t h r e e impulse v o l t - ages s t u d i e d occurred a t a f i e l d o f 38 kV cm-'.

This may be compared w i t h minimum values o f t h e order 30 kV cm-' f o r i n c e p t i o n r e p o r t e d by t h e Les

a & b

'I1!

E W

>

0

!x Y

<

n

1 1

250 275 300 325 Impulse: 13 ps t o c r e s t , 2000 vs decay t o h a l f

SPARKOVER VOLTAGE, kV value: 20 shots a t each v o l t a g e

FIG 1 SPARKOVER CHARACTERISTICS

a ) 150 i o n s c c - l ( n a t u r a l ), b ) 270 i o n s c c - I (ImCi), c ) 1030 i o n s cc-1 (14mCi), d ) 5200 i o n s cc-1 (lOOmCi), e) > 80,000 i o n s cc-1 (100mCi i n r o d )