Breakdown of Rod-Plane Gaps in SF6 Under Positive Switching Impulses

The effect of the front of impulse voltages on the breakdown of nonuniform gaps in SF6 has long been reported to be unsystematic. The present study provides an explanation to this phenomenon by examining the breakdown probability distributions of rod- plane gaps in SF6 over a wide range of gas pressure and under different impulse wave forms. The unsystematic effects of the impulse front are experimentally confirmed and are attributed to the coexistence of two main aspects of discharge-corona stabilization which increases the breakdown strength and whose effect disappears beyond a certain gas pressure, and the statistical formation delay which is responsible, under impulse voltages, for the creation of a voltage range of uncertainty where breakdown may or may not occur. By considering the "impulse breakdown threshold", rather than the 50% breakdown voltage, the effects of statistical formation are deleted and the systematic effect of corona stabilization is singled out. The work applies this analysis to SF6 gaps with different degrees of field nonuniformity.