Choice of criteria for rejection of insulating components in high-voltage surge equipment

A systematic approach to rejection of insulating components in high-voltage surge equipment is developed on the basis of changes in surge strength. Data on the effect of pretesting or aging on the ability to withstand single surges and repetitive surges, respectively, were gathered in experiments with polyethylene insulation. Lots of 40 specimens were subjected to single ramp pulses of positive polarity, breakdown occurring on the average after (0.6 to 1) times 10 to the minus 6th power seconds at an electric field intensity of 226 kV/mm. Lots of 200 specimens were subjected to repetitive pulses with the electric field intensity stepwise decreased from 181 to 75 kV/mm. The data was evaluated in terms of fitting empirical distributions of rejects, a composite Weybull distribution later reduced to a simple linear one, and curves of surge strength as a function of aging. The results indicate that the critical points where those curves bend, criteria for rejection, should be determined on the basis of 3-5 different surge amplitudes within the (1.1-1.5) v nominal range of voltages. It is then necessary and sufficient to terminate preliminary tests at those critical points and to measure the single-surge strength margin. One can now calculate and use the optimum electric field intensity and Weybull distribution exponent for rejection tests.