Streamer and Breakdown Characteristics of Dielectric Liquids Under Impulse Waveforms With Different Tail-Times

Impulse waveforms have been widely used in the studies of streamer and breakdown phenomena in dielectric liquids. In this article, both experiments and analyses were carried out to investigate the effects of impulse tail-time on streamer and breakdown characteristics of a mineral oil and a synthetic ester liquid. A range of positive impulse waveforms with the same front-time of
$0.8~ \boldsymbol {\mu } \textrm {s}$
but different tail-times from 8 to
$3200~ \boldsymbol {\mu } \textrm {s}$
were considered. Needle-plane electrodes with a needle tip radius of 10
$\boldsymbol {\mu } \textrm {m} $
and a gap of 10 mm were used. It was found that reduction of tail-time leads to an increase of 50% impulse breakdown voltage, while the instant breakdown voltage and the time-to-breakdown remain constant. On this basis, an analytical method is developed to estimate the 50% impulse breakdown voltage under waveforms with various tail-times. The method is valid when streamers remain of second mode and in divergent fields. It was also observed that some “self-regulation” by branching explains the constancy of streamer propagation velocity when the applied voltage is increased.