drozhzhin a.p.*, korobeynikov s.m.**, teslenko v.s.*

Experimental Studies of Breakdown Initiation in Dielectric Liquids with the

Help of Cavitation Bubbles

Drozhzhin A.P.*, Korobeynikov S.M.**, Teslenko V.S.*

* Lavrentiev Institute of Hydrodynamics, SB RAS, Novosibirsk, 630090, Russia

** Novosibirsk State Technical University, 20, K. Marx Ave., Novosibirsk, 630092, Russia

5th EHD Workshop - SFE2004 2

Abstract

The possibility of breakdown initiation in liquid dielectrics with the help of bubble clusters was considered. The bubbles inside the interelectrode gap were created by focusing bipolar shock-acoustics wave. Electromagnetic generator or multispark discharge generator was the source of the wave. Sharp effect of electric breakdown voltage decrease in the case of cavitation in interelectrode gap was shown experimentally. The electric strength of polyfluorochlorocarbon liquid (PFCC) decreased approximately five times when cavitation took place.


Model of breakdown initiation with the help of bubbles


Computer simulation


Experimental modeling with the help of air bubbles. Anode

Breakdown initiation on anode is accompanied by bubble elongation, appearance of supersonic streamers, its’

disappearance, growth of second set of streamers and breakdown of the gap.


Experimental modeling with the help of air bubbles. Cathode

Breakdown initiation on cathode is accompanied by bubble elongation, transformation in

“mushroom” form, growth of subsonic streamer as a “bush”- like structure.


Experimental set-up

1 – glass; 2 – outer electrode; 3 – electrolyte; 4 – dielectric film;

5 – perfluorochlorocarbon liquid or transformer oil;6 – inner electrode (acoustic radiator); 7 – slits; 8 – HV electrodes; d = 0-100 um; L - gap distance.


Electrical circuit


Polyfluoroclorocarbon liquid H-(CF2)nCl

The main properties:n = (610);

Density - 1.9 g/cm3;

Boiling point - 120 C;

Freezing point -10 C;

Dielectric permittivity - 2.02.5;

Specific resistance - 1012 Ohmcm;

AC electric strength in standard cell - 200 kV/cm.


Breakdown in oil (U = 8 kV)


No breakdown in oil (U = 7 kV)


Results and discussion: fluorocarbon liquid

Without cavitation Ubr = 10 kV After appearance of cavitation bubble

clusters Ubr = 2 kV

p ~20 Torr Е~ 20 kV/cm, Е/p ~ 100 Vcm-1Torr-1

PD ~ 3 Pam

•Close to the minimum of Pashen’s curve Breakdown initiation is difficult.


Results and discussion: transformer oil

Without cavitation – no breakdown at U = 8 kV After appearance of cavitation bubble clusters Ubr = 8 kV at the probability ~ 0.05

p ~0.01 Torr Е~ 80 kV/cm, Е/p ~ 107 Vcm-1Torr-1

PD < 0.001 Pam Left, “vacuum” branch of Pashen’s curve.

Breakdown initiation is difficult.


Conclusion

The method of cavitation bubble formation with the help of focusing shock-acoustic waves in dielectric liquid for the purposes of switching applications is developed;

It is shown experimentally in case of perfluorochlorocarbon liquid that cavitation cluster leads to low electric stress breakdown at voltage that is 5 times less than breakdown voltage of homogeneous liquid;

The effect of breakdown voltage was weaker in transformer oil.

drozhzhin a.p.*, korobeynikov s.m.**, teslenko v.s.*

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