Transient Impulse Breakdowns of SF6 Gas in Inhomogeneous Electric Fields

Abstract

In this paper, we describe the dielectrics and predischarge developments of gaseous SF6 stressed by nonoscillating and oscillating impulse voltages in inhomogeneous fields caused by a metallic protrusion. The breakdown voltage-pressure curves and the formative time lags to breakdown were measured in the gas-pressure range extending from 0.05 to 0.3 MPa. The predischarge processes were simultaneously investigated through measurements of the current induced by moving space charges and photomultiplier records. The predischarge propagates into the gap with a stepwise leader, and the basic physical mechanisms leading to electrical breakdown are the same for nonoscillating and oscillating impulse voltage waveforms. While for the other, the quickly changing macroscopic electric field due to space charges in the gap affects the leader propagation under the oscillating impulse voltage waveform. In the case of negative polarity, the predischarge growth processes at the increasing and decreasing parts of the oscillating wave tail are not very similar. It is inferred that the breakdown at the decreasing part of the oscillating impulse voltage originates from ion depletion due to recombination.