Comparison of the minimum breakdown level of SF6 mixtures in inhomogeneous gaps under steep wave front impulse and double-exponential impulse

The minimum breakdown level of SF₆ mixtures in inhomogeneous gaps under steep wave front impulse and double-exponential impulse was studied experimentally. The leader guiding voltage increases, whereas the streamer guiding voltage decreases with air content ratio in SF₆/air mixtures at constant total pressure. The fact that in pure SF₆V_i(min) coincides with V_step(min) and in SF₆/air mixtures V_i(min) exceeds V_step(min) shows that, unlike in pure SF₆, the corona stabilization effect is always operative with the 4/1500-μs impulse. For mixtures in which breakdown takes place as a streamer breakdown, V_i(min) may exhibit a peak which is nonexistent in the case of V_step(min). In applications where fast transients occur, possible SF₆/air mixtures should be in the high-SF₆-content and high-pressure region in the corresponding leader-to-streamer transition graph, the form of which varies with gap nonuniformity. This would ensure that the minimum breakdown voltage of the mixture increases with air content for both slow- and fast-rising impulses