Effect of combined AC and lightning-induced overvoltages on the risk of MV insulator flashovers above lossy ground

Abstract

Soil resistivity is one of the prevailing parameters associated with the flashover of line insulation in medium voltage (MV) overhead lines due to nearby lightning strikes. This paper investigates the influence of lossy ground to determine the level of lightning-induced overvoltages and the corresponding risk of insulator flashovers. Experiments are performed in the high voltage laboratory to evaluate the flashover characteristics of a 24-kV pin-type insulator under combined AC and lightning-induced overvoltages. Based on experimental results, a modified Gaussian cumulative distribution function has been employed to estimate the probability of single-phase, two-phase and three-phase flashover of insulators based on the assumption of ground resistance close to zero. Monte Carlo simulations are performed on Rusck's and Darveniza's models to obtain the probability density function of peak lightning-induced overvoltages for both ideal and lossy grounds, respectively. The results indicate that lossy ground increases the risk of insulator flashovers. The risk indices are further aggravated with increasing soil resistivity and become significantly high for soil resistivity above 3000Ωm. The advantage of the proposed method is its simplicity and ability to distinguish between single-phase and multi-phase flashover faults with increasing soil resistivity. Thus, the proposed model can be successfully employed for insulation coordination studies of MV overhead lines.