CRITICAL AVALANCHE SIZE FOR IMPULSE BREAKDOWN IN QUASI-UNIFORM FIELD SF6 GAPS

A method of calculation has been developed for determining the average voltage-time (V-T) characteristics of quasi-uniform field gaps subjected to high impulse voltages. The method consists of evaluating the test voltage which will produce a critical electron avalanche in the gas gap, the size of which is a function of the time to breakdown. An empirical relationship determined from experimental data obtained at IREQ defines the critical avalanche size which would cause breakdown of the gap at a time Tf, the measured time to flashover. Comparison of the calculated average V-T curves with experimental data for various gap configurations shows reasonable agreement. The error margin was found to be about 10–15% over the range of times to breakdown from 0.2 to 2.0 μs and for gas pressures up to 1.065 MPa.