Impulse waveform effects on negative point breakdown phenomena in SF6

The authors investigate the impulse waveform effects on the initiation, propagation, and stabilization of gas discharges over a wide range of pressures in SF₆. Impulses are generated by a Fletcher type generator. By changing the internal pulse forming components, 0.0075/80- and 0.6/80-μs surges can be generated and applied to a 0.25-mm-radius stainless steel point forming a 5-mm point/plane SF₆ gap. Discharge development is recorded by a collimated photomultiplier with quartz optics forward focused to view the whole gap. Extensive studies of the patterns of light emission have indicated that three distinct modes of discharge development exist. Although all of the modes identified are present at 0.4 MPa under a lightning surge application, the mode incidence is very different, the preponderance depending both on pressure and on risetime. Mode A is always present and results in breakdown in the majority of cases for both fast and slow pulse application. However, the stabilized mode (Mode B) is highly dependent on pressure and only present for the slow rising impulses. Mode C is present only at high pressure (above 0.3 MPa) and occurs independent of impulse risetime