An expression of critical pressure in SF6-filled point/plane gaps

A new method for calculating low probability impulse breakdown voltages is put forward, which consists of three limiting lines: the streamer breakdown line, the low probability impulse breakdown level and the streamer corona onset line in different gas pressure ranges. The streamer breakdown line and the streamer corona onset line can be calculated using the basic knowledge of SF₆ gas insulation. If the critical pressure P_c is known, then the low probability impulse breakdown level can be predicted, because the streamer corona onset voltage and the low probability impulse breakdown voltage are of almost the same value at P_c. So the expression of P_c is very useful for insulation design. In this paper, different point/plane gaps are tested, and an expression of P_c is derived based on the experimental results. Compared with the measured values for given gaps, the calculated values have a relative error no more than 2%