Voltage-time characteristics of SF6 and SF6-N 2 under lightning and steep-fronted impulses

Voltage-time characteristics of SF₆ gas and an SF₆ -N₂ 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-μs 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 SF₆ and the SF ₆-N₂ 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 SF₆ and SF₆-N₂ were greatly scattered compared with gas insulation without any spacer