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BREAKDOWN OF SPHERE AND SPHERE/PLANE GAPS STRESSED WITH D.C. VOLTAGE ; THE
EFFECT OF ”CONDITIONING”
T. Allibone, J. Saunderson
To cite this version:
T. Allibone, J. Saunderson. BREAKDOWN OF SPHERE AND SPHERE/PLANE GAPS
STRESSED WITH D.C. VOLTAGE ; THE EFFECT OF ”CONDITIONING”. Journal de Physique
Colloques, 1979, 40 (C7), pp.C7-425-C7-426. �10.1051/jphyscol:19797207�. �jpa-00219187�
JOURNAL DE PHYSIQUE CoZZoque C7, suppldment au n07, Tome
40,J u i z z e t 1979, page
C7- 425BREAKDOWN OF SPHERE AND SPHERE/PLANE GAPS STRESSED W I T H DC. VOLTAGE
;
THE EFFECT OF nCONDITIOMNGnT.E. Allibone and J.C. Saunderson.
EZectricaZ Engineering Department, The c i t y University, London,
U . K .I n t r o d u c t i o n
Very l i t t l e i n f o r m a t i o n has been published on t h e behaviour of sphere-gaps s t r e s s e d with D.C.
v o l t a g e s , and none on t h e behaviour of s p h e r e l p l a n e gaps, and t h e a u t h o r s have found no p r e c i s e guid- ance a s t o t h e treatment which should be accorded t o t h e s u r f a c e of spheres b e f o r e using them f o r v o l t a g e measurement.
Bowers and Kuntke (1937) made t h e f i r s t measurements of t h e sparkover v o l t a g e s of a 1 m d i a sphere-gap s t r e s s e d w i t h D.C. On n e g a t i v e
p o l a r i t y t h e sparkover c h a r a c t e r i s t i c coincided w i t h Meador's (1934) n e g a t i v e impulse v o l t a g e
c a l i b r a t i o n of t h e l m sphere-gap, and on p o s i t i v e p o l a r i t y i t l a y some 3% below Meador's p o s i t i v e impulse v o l t a g e c a l i b r a t i o n , both v a l u e s being lower t h a n t h e A.C. c a l i b r a t i o n t h e n quoted i n t h e V.D.E. s p e c i f i c a t i o n . Bowers
e t
a 1 made t h e i n t e r e s t i n g o b s e r v a t i o n t h a t , on p o s i t i v e p o l a r i t y , sparkover can, i n f r e q u e n t l y , t a k e p l a c e over f a r g r e a t e r d i s t a n c e s t h a n t h e normal; e.g. a t 800 kV a t t h e 12% g r e a t e r d i s t a n c e ; a t 900 kV, 30%g r e a t e r and a t 1000kV, 50% g r e a t e r t h a n normal.
Udo and Watanabe (1968) c a l i b r a t e d a 0.75 m d i a sphere-gap w i t h D.C. and found t h a t r e s u l t s agreed w i t h t h e p r e s e n t I E C c a l i b r a t i o n of such a gap under impulse v o l t a g e s (IEC 52-1960) b u t made no r e f e r e n c e t o d i s p e r s i o n of sparkover v a l u e s .
Colombo, Mosca and Motta (1972) c a l i b r a t e d 0.5 m d i a , 1.0 m d i a and 3.0 m d i a sphere-gaps with D.C.; t h e i r c a l i b r a t i o n f o r t h e 1 m s p h e r e s l a y some 5% below t h a t of Bowers e t a 1 on p o s i t i v e D.C.
and t h e s t a n d a r d d e v i a t i o n from t h e mean of 20 d e t e r m i n a t i o n s a t each gap s e t t i n g r o s e t o 2.5% f o r a spacing S = 0.5D. They showed t h a t f o r t h e 3 m s p h e r e s spaced a t 0.5 m, a f t e r a p e r i o d > 1 day w i t h o u t use, sparkover began a t 90% of t h e normal
S.O.V. and r o s e t o 98% o n l y a f t e r some 50 s p a r k s . For t h e 1 m s p h e r e s , l i k e w i s e spaced a t 0.5 m,a small number of "anomalous" s p a r k s o c c u r r e d a t v o l t a g e s below t h e mean by v a r i o u s amounts a s low a s 6 times t h e s t a n d a r d d e v i a t i o n .
No w r i t e r s have commented upon t h e occurrence of t h e anomalous sparkover v o l t a g e s s i g n i f i c a n t l y
above
t h e mean v a l u e .Such anomalous h i g h v a l u e s were r e p o r t e d by Vibholm and Pedersen (1974) when a p p l y i n g 1.2/50ps impulses of n e g a t i v e p o l a r i t y t o a 0.45 m rod/rod gap, using b r a s s e l e c t r o d e s . When t h e e l e c t r o d e s were cleaned r e p e a t e d l y throughout t h e t e s t , a w e l l
defined Gaussian d i s t r i b u t i o n curve of sparkover was o b t a i n e d , t h e V 50o/,value of which was 17%
higher
than t h a t o b t a i n e d a f t e r t h e e l e c t r o d e s had been "conditioned" w i t h 3000 s p a r k s . Copper e l e c t r o d e s behaved s i m i l a r l y , b u t aluminium and s t a i n l e s s s t e e l e l e c t r o d e s showed an o p p o s i t e e f f e c t ( d e t a i l s were n o t g i v e n ) . One of t h e p r e s e n t a u t h o r s (Allibone and Dring 1979) h a s re- peated t h i s work and found a s t r o n g e f f e c t with b r a s s e l e c t r o d e s s t r e s s e d w i t h s w i t c h i n g impulses of n e g a t i v e p o l a r i t y . The p r e s e n t a u t h o r s(Allibone and Saunderson 1979) have found extremely l a r g e e f f e c t s with a v a r i e t y of rod/rod e l e c t r o d e s of d i f f e r e n t m a t e r i a l s , t h e gaps being s t r e s s e d w i t h D.C.
I n t h i s paper t h e behaviour of s ~ h e r e l s p h e r e and s p h e r e l p l a n e gaps has been s t u d i e d , t h e s u r f a c e s of t h e l a t t e r having been cleaned o r , a l t e r n a t i v e l y , ' c o n d i t i o n e d ' .
Apparatus
The D.C. g e n e r a t o r i s of t h e Cockcroft-Walton type w i t h 7 s t a g e s of v o l t a g e m u l t i p l i c a t i o n and i t h a s an o u t p u t c a p a c i t a n c e of 0.005
UF
a tlo6
v o l t s . It i s connected t o t h e sphere-gap and s p h e r e / p l a n e gap through a non-inductive r e s i s t a n c e oflo6
ohms.The whole i s corona-free except f o r &he gap under t e s t . For v o l t a g e measurement a 10 ohm
r e s i s t a n c e p o t e n t i o m e t e r b r i d g e s t h e whole g e n e r a t o r .
Voltage c a l i b r a t i o n
The D.C. g e n e r a t o r was c a l i b r a t e d up t o 300 kV a g a i n s t an e l e c t r o s t a t i c v o l t m e t e r (Waterton 1976) a c c u r a t e t o ? 0.25%; t h i s i n t u r n agreed t o w i t h i n 0.25% with t h e v o l t a g e recorded by t h e r e s i s t a n c e v o l t m e t e r and w i t h t h e 0.75 m sphere-gap a t 300 kV.
C a l i b r a t i o n by t h i s v o l t m e t e r up t o 700 kV agreed with BSS 137 on both p o l a r i t i e s t o w i t h i n 1.5% on n e g a t i v e and 3% on p o s i t i v e p o l a r i t y .
R e s u l t s
(1) Sphere-gap breakdown. F i g . 1 i l l u s t r a t e s a d i f f i c u l t y encountered i n t h e measurement of D.C.
v o l t a g e s . The 0.75 m s p h e r e s had been cleaned a s normally recommended and, spaced a t 0.3 m, were used t o c a l i b r a t e t h e s e t f i r s t on n e g a t i v e and t h e n on p o s i t i v e p o l a r i t y . Sparkover f i r s t occurred some 16 times a t v o l t a g e s below t h e v a l u e of
-
660 kV, t h e BSS f i g u r e c o r r e c t e d f o r a i r d e n s i t y on t h e day of t h e t e s t . Even a f t e r reaching-
660 kV t h e r e were s e v e r a l abnormally lowArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797207
5 0
1
I I I Ilo number of sparks. 4-*
Fig 1. Sequence of sparkover voltages of a 0.3m gap between spheres o f 0.75111 dia.
... Negative polarity, xxxxx positive.
breakdowns. On p o s i t i v e p o l a r i t y t h e r e were some s p a r k s a t 5% below t h e a n t i c i p a t e d v a l u e b u t i n g e n e r a l a t a l l gap s p a c i n g s t h e r e were few low v a l u e s , a r e s u l t d i f f e r e n t from t h a t quoted by Bowers e t a l .
(2) SphereIplane breakdown. Spheres of 0.125
m,
0.25 m. and 0.5m
diameter have been used: f i r s t t h e y were cleaned by t h e accepted methods and then, a f t e r t e s t , t h e y were extremely thoroughly cleaned w i t h a nylon pad of t h e k i n d used i n t h e home (e.g.a brand named S c o t b r i t e ) .
The behaviour of t h e 0.25 m s p h e r e l p l a n e gap i s shown i n Fig. 2. The sparkover v o l t a g e s on t h e two p o l a r i t i e s l i e between t h o s e of a r o d I p l a n e gap measured i n t h e l a b o r a t o r y : t h e s e a r e shown a s t h e two s l a n t i n g l i n e s . Very wide s c a t t e r was en- countered on b o t h p o l a r i t i e s : v a l u e s a r e s h o w by t h e v e r t i c a l l i n e s . On p o s i t i v e p o l a r i t y s c a t t e r diminished a s t h e gap i n c r e a s e d t o 1.5 m but on n e g a t i v e p o l a r i t y i t remained h i g h up t o 700 kV.
F i g . 2. Sparkover of a 0.25 m s p h e r e / p l a n e t o t a l range and mean p o s i t i v e s.o.v.,
9
sphere a s normally cleanedx
sphere thoroughly cleanedrange of n e g a t i v e s.o.v., sphere a s
I
normally cleanedn sphere thoroughly cleaned.
gaps, though no gap has so f a r been t e s t e d w i t h more t h a n 50 s p a r k s .
It has not been p o s s i b l e t o examine t h e s u r - f a c e t o seek a cause f o r t h i s improved performance b u t a t l e a s t i t can be s a i d t h a t normal d e g r e a s i n g , t h e n t h e use of f i n e emery and t h e n chamois l e a t h e r i s not s u f f i c i e n t t o s t a b i l i s e t h e s p h e r e f p l a n e sparkover mechanism.
Leader-stroke o b s e r v a t i o n s . In t h e D.C. corona, n e g a t i v e p o l a r i t y , between sphere and p l a n e a t 1 m spac?ng i t i s p o s s i b l e t o s e e l e a d e r - s t r o k e s c r o s s - , i n g almost h a l f of t h e gap from t h e cathode. I n a d d i t i o n , mid-gap s t r e a m e r s branching both ways, a s f i r s t recorded by one of t h e a u t h o r s (Allibone and Meek 1938) can be s e e n a t l o c a t i o n s some 30% below t h e sphere.
References
Bowers A. and Kuntke A. 1937 "A 3 - m i l l i o n v o l t D.C.
Generator". Z e i t . f u r Tech. Phys.
5
209-219.Meador J . R . 1934 " C a l i b r a t i o n of t h e sphere-gap".
Trans. Am. I n s t . Elec. Eng.
52
942-948.Udo T. and Watanabe Y. 1968 "D.C. high-voltage spark-over c h a r a c t e r i s t i c s of gaps ahd i n s u l a t o r s t r i n g s " IEEE PAS-87 266-270.
Colombo A., Mosca W. and Motta G. 1972 "Performance of sphere and rod/rod gaps under h i g h D.C.
v o l t a g e s " IEEE PAS-% 501-509.
Vibholm S. and Pedersen A. 1974 "Factors a f f e c t i n g breakdown v o l t a g e d i s t r i b u t i o n f o r a r o d / r o d gap i n a i r " IEE I n t . Conf. Gas Disch. London 434- 437.
A l l i b o n e T.E. and Dring D. 1979 " I n f l u e n c e of e l e c t r o d e shape and c o n d i t i o n on t h e humidity c o r r e c t i o n f a c t o r f o r n e g a t i v e p o l a r i t y spark- over of r o d l p l a n e gaps". 3rd I n t . Symp. H-V Engineering Milan (paper 0135).
A l l i b o n e T.E. and Saunderson J . C . 1979 "Sparkover of r o d l p l a n e and rod/rod gaps s t r e s s e d with D.C.
v o l t a g e s : e f f e c t s of c o n d i t i o n i n g " 3rd I n t . Symp. H.V. Engineering Milan (paper 0225).
Waterton F.W. 1976 "A 300 kV e l e c t r o s t a t i c v o l t - meter" J o u r n a l of P h y s i c s E. S c i e n t i f i c Instruments
9
647-650.Allibone T.E. and Meek J.M. 1938 "Development of t h e spark d i s c h a r g e f t Proc. Roy. Soc.
937
246- 268.A f t e r t h e thorough s c o u r i n g w i t h nylon a l l sparkover v a l u e s were i n c r e a s e d and t h e s c a t t e r r a r e l y exceeded 1%. The w i t h s t a n d v o l t a g e in- c r e a s e d markedly; a 0.5 m gap w i t h a breakdown r a n g i n g from +260 t o 370 kV now withstood +390 kV f o r a one-minute t e s t and continued s p a r k i n g d i d n o t cause t h e breakdown v o l t a g e t o f a l l ,
