Characterization of the Span Factor for the Assessment of the Lightning Performance of High-Voltage Transmission Lines

An advanced procedure based on Monte Carlo method is applied to determine the so-called span factor for lightning performance evaluations of typical 138-kV-, 230-kV-, and 500-kV transmission line with realistic distributions of tower-footing grounding impedance. The procedure assumes the Hybrid Electromagnetic Model to determine overvoltages across insulator strings, the Disruptive effect method to assess backflashover occurrence and Love's attractiveness model to represent the distribution of lightning strikes along the line. A slight variation of the span factor was found for realistic distributions of tower-footing impedance along the line. Considering the same attractiveness for towers and shield wires, values of the span factor of about 0.8, 0.56, 0.37 were found, respectively for the 138-kV, 230-kV, and 500-kV transmission lines.