Analysis of the cumulative probability distribution of the stroke angle in lightning incidence to three-phase overhead transmission lines

Lightning leaders follow a random behavior when approaching ground surface, so the usual assumption of a vertical stroke is not realistic. This paper is aimed at analysing the sensitivity of the shielding failure flashover rate (SFFOR) of three-phase overhead transmission lines to a probability distribution of the stroke angle, i.e. both vertical and non-vertical strokes. A modified Electrogeometric Model (mod-EGM), derived from a probabilistic computation of the exposure area of phase conductors is described. A computational code is developed in MATLAB, along with IEEE Flash algorithm implementation. Sensitivity analyses are carried out for a waist-type transmission tower. Results show that considering particularly vertical leaders may lead to underestimated values for flashover rates.