ANALYSIS OF THE BEHAVIOR OF DISTRIBUTION INSULATORS AGAINST OSCILLATORY LIGHTNING OVERVOLTAGES

Abstract

Overhead power distribution lines are often exposed to lightning surges, which have usually a great impact on their overall performance. Such overvoltages depend on many lightning and network parameters, as well as on the electrical characteristics of the soil, so that their waveshapes vary widely and may differ substantially from the standard lightning impulse voltage, with which electric equipment insulation is tested against lightning overvoltages. The standard lightning impulse voltage consists in an unipolar impulse with front time of 1.2 µs and time to half-value of 50 µs (1.2/50 µs waveshape). As the voltage withstand capability of insulation depends not only on the surge amplitude but also on its waveshape, the analysis of the lightning performance of a given power line demands the knowledge of the behavior of the line equipment, especially the insulators, when subjected to lightning surges. A preliminary investigation based on theoretical and experimental data has shown that oscillatory overvoltages induced on overhead distribution lines by nearby lightning strokes may not be uncommon. Although different models have been proposed for predicting the strength of insulation subjected to non-standard impulses, none of them apply to the case of distribution equipment subjected to oscillatory lightning impulses. Therefore, this research project aims at: evaluating the occurrence of oscillatory lightning overvoltages and identifying their typical characteristics; obtaining experimental data relevant to the breakdown characteristics of 15 kV insulators; and developing a suitable mathematical model for representing the behavior of these insulators when subjected to oscillatory lightning impulses.