Statistical Evaluation of Critical Flashover and Withstand Voltage for Small Airgap

Insulation co-ordination and the risk of failure for insulation can be predicted if critical flashover (CFO) and insulation withstand voltages are known. The paper aims at using the statistical method of evaluating the critical flashover and the withstand voltage for an airgap distance of 10cm. The work was carried out with a point-plane electrode (needle point) arrangement using air as the dielectric medium for the given gap. Ten shots were allowed in each test and the breakdown probability was found. From the experimental values the critical flashover or the V50%, and the percentage breakdown probabilities were found. The withstand VN at 10 percent was estimated from the cumulative distribution function. From the knowledge of the withstand voltage and the statistical over voltage at 90 percent probability, it was possible to estimate the risk of failures if desired for the given insulation. Keywords—Breakdown probability, critical flashover voltage, cumulative distribution function, dielectric medium, Gaussian distribution, risk of failures