Particle-Initiated Breakdown Characteristics and Reliability Improvement in SF6 Gas Insulation

Particle-initiated breakdown characteristics in SF6 gas are investigated. Breakdown voltages are greatly influenced by particle diameter especially for AC voltages, and for very fine metallic wire particles are much higher than those for moderately thick ones. The breakdown stress of a gas gap is obtained as a function of particle length and can be estimated fairly well by assuming a corona stabilizing zone. For a spacer, the electric stress which is parallel to its surface dominates surface breakdowns and the relation between the stress and particle length is experimentally determined. In order to increase breakdown voltages, the effects of both the dielectric coating on a high-voltage electrode and spacer shape are examined. Finally, the effectiveness of the dielectric coating on a grounded electrode is demonstrated to prevent particle levitation.