Voltage-time characteristics of SF/sub 6/ and SF/sub 6/-N/sub 2/ under lightning and steep-fronted impulses

Voltage-time characteristics of SF/sub 6/ gas and an SF/sub 6/-N/sub 2/ gas mixture with a mixing ratio of 50/50 were measured in a 50/20-mm-diameter coaxial electrode system with and without a spacer under both lightning impulse and steep-fronted impulse voltages. A nonoscillating steep-fronted impulse with a risetime as short as 50 ns was derived from a 1.2/60- mu s lightning impulse by means of a wave-front-steepening gap, and was measured using a compact resistive type voltage divider, with the measuring system response time being less than 5 ns. Experiments showed that the breakdown voltage under the steep-fronted impulse was lower than that under the lightning impulse for the same time to breakdown in both SF/sub 6/ and the SF/sub 6/-N/sub 2/ gas mixture, but it was still higher than the 50% lightning impulse breakdown voltage used for insulation co-ordination. For the case with a spacer, breakdowns in both SF/sub 6/ and SF/sub 6/-N/sub 2/ were greatly scattered compared with gas insulation without any spacer.<
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