Breakdown behavior of SF6 gas in nonuniform electric field under transient impulse voltages

This paper describes the dielectric characteristics and the predischarge development mechanism of SF6 gas in a nonuniform electric field. The breakdown voltage-gas pressure (V - p) characteristics were measured under positive and negative transient impulse voltages, and the predischarge development was investigated with special respect to the applied voltages. The testing electrode system had a coaxial cylindrical electrode geometry with a needle protrusion, and the predischarge current was measured by a shunt. As a result, the dependence of the V - p, characteristics on the polarity of the applied voltage was known to be appreciable. The leader step time, which is a measure for the energy input required to leader propagation, was approximately a factor of 10 higher for the negative polarity than for the positive polarity. It was confirmed that all the predischarges in positive and negative polarities developed in stepwise leader extension and that the dielectric strength of SF6 gas in a nonuniform electric field was closely associated with the predischarge development mechanisms.