Lightning performance criteria for electric power systems

In the paper the author has examined the parameters which influence the number of lightning strikes terminating on electric power lines and, having made assumptions, has proceeded accordingly to apply them to a variety of transmission and distributions lines worldwide, thanks to the co-operation of some 30 electric supply authorities who, in answer to a questionnaire, supplied basic parameters and performance data at short notice, covering over 100 lines totalling nearly a quarter million kilometres in length. Regression analysis was used to compute correlations between sustained outage rates, lightning flash density and system voltage, thus establishing median expressions for these relations which could be used to compare the relative performance of lines. Secondly, a parameter was devised to indicate the lightning protection efficiency (LPE) of a system and median values were computed, indicating a general tendency to improve with increasing lightning flash density. Whilst the sustained outage rate was considered to be the main criterion, in so far as the lightning performance of a line affected the community at large, trip-out rates, including successful reclosures, were important to the engineering designer, as representing the risk of damage from lightning flashover. Although this was not a question addressed to the utilities, some returned this data, to which was added the extensive data compiled for the CIGRE survey by Whitehead in 1974. It was thus possible to formulate a relationship between this trip-out rate and the sustained outage rate, leading to the calculation of what could be termed the lightning flashover efficiency (LFE).