Breakdown of non-uniform field gaps in transformer oil under surge voltages of various front durations

It has been well established that breakdown voltage of highly asymmetrical gaps in air with the smaller electrode positive generally decreases with increasing wavefront duration up to a certain value and thereafter it increases. The most severe surge voltage causing flashover to occur at minimum value has been reported to be within a range of 50–300 μsec time to peak.
^1,2,3
The dependence of the breakdown voltage on wavefront duration has been attributed by some investigators to the effect of space charge resulting from the impulse corona earlier on the voltage wavefront.