Critical radius phenomenon and mechanism for SF₆ gaps under very fast transient and lightning impulse voltages

Abstract

Insulation-related accidents of gas-insulated switchgear, induced by very fast transient overvoltage, have become increasingly serious in high-voltage power systems. A steep-fronted impulse voltage test equipment is developed for studying the discharge characteristics and mechanisms in the SF 6 gap. The influence of the electrode radius on the breakdown characteristic of a fixed-distance SF 6 gap is studied. It is found that discharge behaviors of the SF 6 gap show the so-called critical radius effect. With decreasing electrode radius r, the 50% breakdown voltage of the gap decreases till a critical radius R cr is reached, and then remains practically constant for further reduction of r. R cr increases with decreasing gas pressure and wave front time. Calculation results show that R cr is physically close to the length of the filamentary streamer zone L for negative voltages, whereas for positive voltages, R cr is close to the length of the homogeneous streamer zone l . The critical radius phenomenon is related to the streamer-precursor-leader discharge process in SF 6 gas, which is discovered and explained for the first time in this paper by the proposed critical charge criterion of leader breakdown.