Influence of impulse waveshape on breakdown voltage of a nonuniform-field gap in compressed gases

Abstract

Investigations were made into the influence of the impulse waveshapes, gas pressure p and polarity on the breakdown voltage V s of a sphere-plane gap in compressed air and nitrogen. The pressure was varied from Olbf/in 2 gauge to 150lbf/in 2 gauge, the gap spacings from 6-35 mm to 102 mm and the risetimes of the impulse voltages from 2 • 1 /us to 189 /AS. For an impulse voltage of either polarity, certain gap lengths exhibited a critical-pressure effect, and the gap lengths and pressure at which this effect was observed depended on the risetime of the impulse voltage. Observations of the dependence of V s on the risetime showed that V s decreased with increasing risetimes, for risetimes of 16/xs or 40/AS, beyond which it increased again to reach a maximum at risetimes of the order of 100/u.s. For risetimes longer than 100/xs, V s again decreased with increasing risetime. This behaviour was similar for both nitrogen and air. Possible mechanisms are suggested to explain this behaviour. 1Introduction It is well known that the impulse breakdown voltage V s of a nonuniform field gap such as exists between a point and a plane is higher than that for steady-state voltage. While considerable data are available on V s for steady-state voltage, particularly in uniform electric fields, 1 * 2 very few investigations have been made into the influence of the impulse waveshape on V s in compressed gases. The earlier measurements of McConnell 3 in compressed air were confined to a single waveshape, 1-5 x 50/xs, while the data of Crouch and Whitman 4 were obtained in nitrogen at atmospheric pressure. The voltages used in their investigation had risetimes of 0-5-550/MS, and the time taken to reach 50% of the crest on a tail of a wave was 10-9000/xs.For a nonuniform-field electrode configuration, V s does not increase with pressure continually. A peak in the breakdown voltage/pressure characteristics (V s against p) has been observed at a pressure p m , and, for higher pressures up to a critical pressure p c , V s decreased. For pressures beyond p c , V s increased again. This effect was known as the 'criticalpressure effect', and was reported only for a positive polarity voltage. 5 " 7 Recently, Govinda Raju and Lakshminarasimha 8 have confirmed, in compressed air, the earlier results of Steiniger, 9 who showed that, in sulphur hexafluoride, the critical-pressure effect occurred both for positive and negative polarity voltages. Lakshminarasimha and Govinda Raju 10 have reported on the dependence of V s of a point-plane gap on the impulse risetime in compressed air.Because of our interest in compressed gases as a possible insulating medium, we have measured the impulse-breakdown voltage of compressed air and nitrogen and investigated the influence of the impulse waveshape, gas pressure and polarity on V s . The waveshapes used had risetimes of 2-1-189/as (1-67 x the time required to rise from 30% to 90% of peak value), and the time taken to decline to half the crest value was in the range of 1000/xs (±10%). Sphere-plane ga...