A Study of the Critical Pressure in SF6/N2 Mixtures for Positive Point Plane Systems with Impulse Voltages

A dual photomultiplier system and a field probe have been used to study the optoelectrical phenomena in commercially pure SF6 and mixtures of SF6 with oxygen-free N2, under impulse voltages in a point-plane gap. The critical pressure has been estimated using an extrapolation technique, and the results have been compared with those of other recently published work. An attempt has been made to establish the different regimes of operation with regard to leader formation both prior to and after streamers cross the gap. This has been done with particular emphasis on the d/r ratio and the field variations with increment in voltage. The cloud model has been shown to be inoperative under impulse voltage conditions such as reported in this work. Comparisons have been made of the critical pressure determined under dc conditions with any available results obtained using impulse voltages. It has been shown that there is no simple relationship linking the radius of the anode to the critical pressure. An attempt is made to explain the large discrepancy between the calculated, and the experimentally determined values of Vi, the corona onset voltage for SF6.