Synergistic effect of SF6 and N2 gas mixtures on the dynamics of electrical breakdown

This paper discusses the efficiency of a moderation effect in a mixture of SF₆ and N₂ gases, in case of the application of impulse voltages with varying rise times. The moderation in question pertains to the slowing down of free electrons through nonelastic collisions with molecules of N₂ gas, which increases the probability, i.e. the effective cross-section, for the formation of negative ions of SF₆. Variable parameters included relative mixture composition, pressure, inter-electrode gap, shape of the electric field, and the manner of electrode surface processing. By applying statistical analysis to experimentally obtained values of the impulse breakdown voltage random variable, and presenting the results as impulse characteristics, it was established that a positive synergetic effect of adding N₂ gas into the mixture was achieved at smaller impulse speeds. This effect was lost at greater impulse speeds. The obtained results were explained by examining the relative ratio of the time constant characterizing the process of moderating the spectrum (statistical distribution of energy) of free electrons, to the characteristic time of elementary electric discharge processes in gases.