Division Director´s Message

Near industrial and coastal areas contamination is a frequent cause of insulator flashover, most cases of which result in lengthy outages. The insulators are washed regularly to avoid flashover. Insulator washing is an expensive and time-consuming procedure. The frequency of washing can be reduced if the washing is initiated when flashover is imminent. Several researchers have attempted the prediction of flashover from the analyses of leakage current. This letter presents the results of applying level crossings to the analysis of the probabilistic dynamics of the leakage current envelope. The problem was formulated in terms of the mean crossing rate at specified thresholds and mean time spent above these thresholds. Sequential analysis of the envelope process until complete flashover shows that: the risk of flashover is increased with increase in the levels of, and frequency of large excursions of the leakage current envelope; at specified high levels (large peaks) of the envelope, the frequency of finite crossings of and the time duration above these levels play an important role in determining ensuing flashover. The notion of LCA of the envelope is introduced and a measure of the LCA is proposed. This measure is related to the mean distance between successive crossings of the envelope at the prescribed level. In the case of artificially contaminated insulators at 0.15 ESDD, a mean duration of 100 milliseconds above 92 mA could cause flashover.