Statistical Analysis of Flashover Voltage of Solid Insulators Immersed in Insulating Oils and Quantification of the Ionization Rate Under LI Stress

This article deals with an experimental investigation of the flashover voltage occurring at the end of the propagation of creeping discharges on some solid insulators immersed in mineral oil (MO) and palm kernel oil methyl ester (PKOME). The solid insulators investigated are: hardened glass (HG), porcelain (PO), and simple glass (SG). The experiments are conducted under positive lightning impulse voltage. The analysis methods are based on a statistical and probabilistic approach. The normality test of the flashover voltage is done by the Anderson–Darling (AD) statistic, the Kolmogorov–Smirnov (KS) test, and the Ryan–Joiner (RJ) statistic. The statistical analysis of the radial coverage of discharges is presented. Quantification of the ionization rate generated by the discharges is also assessed using a pixel-counting method. The experimental results show that the flashover voltage follows the normal distribution. The Weibull distribution at 1% risk shows that for a square solid insulator of 5-mm thickness and 120-mm sides, the flashover voltage is 45 kV for HG, 56 kV for PO, and 60 kV for SG when immersed in PKOME. These results are 48, 58, and 61 kV, respectively, when immersed in MO. It is also shown that the radial coverage of the discharge is greater on HG than on PO and SG. Similarly, it is greater on PO than on SG. The results also show an increase in the ionization rate of 9.4%, 24.6%, and 25.9%, respectively, for solid insulators with permittivities of 5, 5.7, and 7.3.