Leakage currents on lightly polluted 275 kV glass disc insulator strings in conditions of light wetting

The research described here forms part of a larger study into the underlying reasons for a significant number of unexplained flashovers on Eskom's 220, 275 and 400 kV lines. Deductions made so far suggest that the flashovers could be the result of the combined effects of light wetting, light pollution and condensation on glass insulator strings. To evaluate this hypothesis further, leakage currents flowing on naturally polluted 275 kV glass cap and pin insulator strings (I and Vee) were monitored for about a year on a test tower in Highveld ambient conditions. The currents remained below 1 mA for most of the time, but were found at times to increase during periods when the relative humidity exceeded 75 to 80%. No flashovers occurred, but the leakage current patterns observed provide new insights into the possible mechanisms which could lead to flashover in low pollution, light wetting conditions.