Particle-Initiated Breakdown Between Coaxial Electrodes in Compressed SF6

The influence of conducting particles on the ac breakdown of compressed SF6 has been investigated for two coaxial systems having inner electrodes of diameter 75 mm and 150 mm and a 250 mm diameter outer electrode, using voltages up to 450 kV rms at pressures up to 1.8 MNm?2 (18 atm.). For filamentary particles of 0.1 mm or 0.4 mm diameter wire, there was a maximum breakdown voltage at 0.4 MNm?2 (4 atm.) which became more pronounced with increasing particle length and decreasing particle diameter. No maximum was observed with spherical particles but for each system there was a critical sphere size which gave the lowest breakdown voltage at a given pressure. There was also a slight density effect at the lower pressures, aluminum spheres producing lower breakdown voltages than spheres of brass or steel. Tests with wires and spheres fixed to the inner conductor, to simulate breakdown initiated by field enhancement at particles migrating to the inner, gave ac and impulse results which differed from the free-particle ac breakdown voltages. Calculations have been made of the motion of spherical particles and of the energy in microdischarges at the electrodes; the particles can make excursions lasting several cycles and this may be important in creating optimum conditions for breakdown.