1. 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

2. 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.

3. 5th EHD Workshop - SFE2004 3 Model of breakdown initiation with the help of bubbles

4. 5th EHD Workshop - SFE2004 4 Computer simulation

5. 5th EHD Workshop - SFE2004 5 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.

6. 5th EHD Workshop - SFE2004 6 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.

7. 5th EHD Workshop - SFE2004 7 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.

8. 5th EHD Workshop - SFE2004 8 Electrical circuit

9. 5th EHD Workshop - SFE2004 9 Polyfluoroclorocarbon liquid H-(CF 2 ) n Cl The main properties: n = (6  10); Density - 1.9 g/cm 3 ; Boiling point - 120  C; Freezing point -10  C; Dielectric permittivity - 2.0  2.5; Specific resistance - 10 12 Ohm  cm; AC electric strength in standard cell - 200 kV/cm.

10. 5th EHD Workshop - SFE2004 10 Breakdown in oil (U = 8 kV)

11. 5th EHD Workshop - SFE2004 11 No breakdown in oil (U = 7 kV)

12. 5th EHD Workshop - SFE2004 12 Results and discussion: fluorocarbon liquid Without cavitation U br = 10 kV After appearance of cavitation bubble clusters U br = 2 kV p ~20 Torr Е~ 20 kV/cm, Е/p ~ 100 V  cm -1  Torr -1 P  D ~ 3 Pa  m Close to the minimum of Pashen’s curve Breakdown initiation is difficult.

13. 5th EHD Workshop - SFE2004 13 Results and discussion: transformer oil Without cavitation – no breakdown at U = 8 kV After appearance of cavitation bubble clusters U br = 8 kV at the probability ~ 0.05 p ~0.01 Torr Е~ 80 kV/cm, Е/p ~ 10 7 V  cm -1  Torr -1 P  D < 0.001 Pa  m Left, “vacuum” branch of Pashen’s curve. Breakdown initiation is difficult.

14. 5th EHD Workshop - SFE2004 14 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.