Insulating characteristics of SF6 and SF6/N 2-mixtures for very fast transient voltages (VFT)

The time lags in SF₆ and SF₆/N₂ mixtures for gaps with strong nonuniform field distribution, stressed by very fast transient voltages (VFTs), were investigated. The shape of the applied voltages was used to simulate the voltages that can occur by switch operation in gas-insulated substations (GISs). Sinusoidal voltages with different frequencies of up to 20 MHz were superimposed on a voltage step. The maximum amplitude was 160 kV_pp. It is shown that the dielectric strength is not only a function of the frequency of the VFT; all the other parameters of the gap, i.e. the gap distance, the pressure of the gas, and the admixture of N₂ to SF₆, have to be taken into account. It was found that, under certain conditions for a 20-MHz VFT, the dielectric strength is higher than for a 8-MHz VFT. In most of the experiments, however, increasing the frequency will decrease the breakdown voltage