Sensitivity analysis of lightning performance calculations for transmission lines and substations

Analysis has been made of the influence of various lightning parameters on the computed risk of line and substation insulation failure. The analysed lightning current parameters are the mean value and the standard deviation of the current magnitude, as well as the mean value and the standard deviation of current wavefront steepness in an assumed log-normal frequency distribution. The influence of the lightning channel surge impedance on the computer risk of insulation failure is also investigated. The whole analysis is performed for two different equivalent circuits. In the first case, the line outage rate due to the backflashes is predicted. In the second case, the number of strokes to a tower top in front of a gas-insulated substation with a cable entrance, causing one transformer insulation failure, is estimated. The purpose of the paper is to analyse the sensitivity of the lightning-outage-rate prediction method to some uncertainties of the most important lightning parameters. It was found that the mean value and the standard deviation of the lightning current magnitude have the most important influence on the computed risk of insulation failure. On the other hand, the lightning current steepness as well as lightning channel surge impedance have small influence on the computed outage rate.