Influence of the Distribution of Lightning Strikes Along the Span of Transmission Lines On Their Backflashover Rate: The Span Factor

Abstract

The effect of the distribution of lightning strikes along the span on the performance of transmission lines was assessed following an advanced methodology, based on a Monte Carlo approach. First, considering typical features of 138-kV and 230-kV transmission lines, systematic simulations were performed, using an electromagnetic model to determine overvoltages and the disruptive effect method to assess backflashover occurrence. The influence of five variables was analyzed (tower-footing impedance, span length, waveform of the lightning return stroke current, insulation withstand and model of lightning attractiveness). The results indicated that a span factor is able to properly express the effect of the strikes distribution in the span and revealed trends: larger span factor with increasing tower-footing impedance and longer span length, smaller span factor with increasing critical flashover overvoltage and larger span factor for those models that assume larger attractiveness of the transmission line tower. Considering realistic distributions of tower-footing impedance along the line, a very narrow dispersion of the span factor was found. Assuming the same attractiveness for towers and shield wires, values of the span factor of about 0.8, 0.56, 0.53, and 0.37 were found, respectively for the 138-kV, 230-kV, 345-kV and 500-kV transmission lines.