A steep-front square pulse voltage generator, which has a rise time of 16ns, a dumping factor less than 2.5% after 10
μs duration and a peak value of 200kV, is successfully used to survey the
V-
t characteristics in nanosecond range. The
V-
t characteristics of CF
3I, SF
6, CF
3I/N
2 and SF
6/N
2 gas mixtures under non-uniform field gaps by using the steep-front square pulse voltage. When the field utilization factor are measured less than 0.38, the sparkover voltage of CF
3I is lower than that of SF
6. On the contrary, when the field utilization factor is more than 0.38, the sparkover voltage of CF
3I is higher than that of SF
6. The corona inception voltage calculated by using Schumann's criteria and compared with measured one. The
V-
t characteristics for a uniform field gap in CF
3I/N
2 gas mixture containing N
2 gas of 40% are equivalent to those in pure SF
6 gas. The synergism for appears when CF
3I gas is mixed with N
2 gas.
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