V-t Characteristics of SF₆ Gap Disturbed by a Needle-Shaped Protrusion under Oscillating Transient Overvoltages

Abstract

This paper presents the V-t characteristics of SF₆ in inhomogeneous field for oscillating transient over-voltage and non-oscillating impulses under positive and negative polarities. The experiments were performed by using a hemisphere-plane arrangement with a needle-shaped protrusion, in the gas pressure range from 0.05 to 0.3 MPa. The predischarge current and the luminous signals were measured by a current shunt and a photomultiplier, respectively. The minimum breakdown voltage with a needle protrusion of 0.5mm in diameter is less than that with a needle protrusion of 1.0mm in diameter. The dependence of breakdown voltage on the diameter of a needle-shaped protrusion is appreciable in the negative polarity and low gas pressure conditions. The formative time-lag to breakdown is influenced by the gas pressure, and the difference between the time-lag for positive and negative polarities increases with the increase of the gas pressure.