Owen Farish

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.

Effect of Conducting Particles on AC Corona and Breakdown in Compressed SF6

This paper describes studies of the effects of conducting particles on breakdown phenomena in compressed SF6 under alternating voltage conditions. Results with free particles of different shapes and materials are compared with those obtained with fixed particles for both parallel-plane and coaxial electrode geometrics. Calculations of the particle motion and of the energy in microdischarges between particle and electrodes are used to suggest mechanisms by which breakdown is initiated. It is shown that the breakdown voltage, which can be as low as 10% of the uncontaminated value, is markedly dependent on particle shape, size and material, and that the nature of the motion in alternating fields is important in establishing conditions for free- particle-triggered breakdown.

Particle initiated breakdown in compressed SF 6

It is knownl1–3 that metallic particle contaminants can drastically reduce the breakdown voltage of compressed gases, but relatively little is known of the dependence of this effect on the test conditions, and of the breakdown mechanisms involved.

Motion of spherical particles and AC breakdown in compressed SF 6

Previous papers1,2 have described the effect of particles on corona and ac breakdown in compressed SF 6 , and have outlined the calculations for the motion of metal spheres under the influence of the alternating field. In this paper the calculated particle motion is discussed in detail, and comparisons are made with the observed bounce height and corona produced by spherical particles. Further breakdown measurements have been obtained with a 150 mm ID, 250 mm OD coaxial system in SF 6 , and the results are considered in terms of the particle motion.