Calculation of backflashover outage rate of transmission lines: A discussion on traditional methodologies and recent advances

This work presents a discussion on the traditional methodologies presented by CIGRE and IEEE to calculate the lightning outage rate of high voltage transmission lines in terms of backflashover occurrence. The main aspects of such methodologies are reviewed. The simplifications and approximations adopted to determine the outage rate are discussed. Systematic simulations are developed considering a typical 138-kV line configuration. The obtained results in terms of peak overvoltage, critical current and backflashover rate are compared with those obtained by an advanced methodology composed by an accurate electromagnetic model (HEM) to calculate the overvoltage across insulator strings and an elaborate model (Disruptive effect - DE) to evaluate the flashover occurrence across insulators. Very distinct backflashover outage rates were presented. In general, CIGRE and IEEE methodologies yield large and lower outage rate, respectively, in comparison to those estimated by the advanced methodology. The results indicate the need of incorporating the recent advances of the area in the calculation of outage rate, aiming the improvement of the quality of the results given by traditional methodologies.