In this paper a methodology is presented to quantify the year-to-year variability of the lightning outage rate of transmission lines based on the stochastic and climatological variations in lightning incidence. The stochastic variations can adequately be described by the Poisson distribution, and the year-to-year variation in the lightning ground flash density by a Log-Normal distribution function. Based on data from ALDIS and EUCLID it is suggested that the standard deviation of Ln(Ng) typically falls between 0.3 and 0.6 and a median of 0.423.

The EN 50341‑1 provides procedures for establishing internal and external clearances for overhead lines. Internal clearances are selected in a way that an acceptable outage performance is achieved, and the external clearances are then coordinated with the internal clearances to ensure that no flashovers occur to objects in the vicinity of the line. In the standard, this coordination is carried out by assuming that lightning overvoltages will be limited by flashovers of the insulator strings. There are, however, cases where this approach may be too conservative, e.g., when long insulators are needed to obtain an acceptable power frequency flashover performance with insulator pollution. In this paper, a method is presented for determining the external clearances on overhead lines based on a statistical evaluation of the representative lightning overvoltage as defined in the EN 50341‑1.

Three distinct data sets are analyzed in terms of probability of flashes having subsequent strokes with higher peak currents than the first stroke in the flash. This type of flashes may have some implication in the shielding failure rate of transmission lines. For three distinct data sets we obtained about 30 % of flashes with a ratio I sub /I 1 >1. For about 10 % of the flashes the peak current of a subsequent stroke is more the two times the peak of the first stroke, and for about 3 to 4 % of the flashes we obtained a ratio I sub /I l is grater then 3.