Transmission line sag influence on lightning stroke probability

Shielding failure in a transmission line occurs when lightning bypasses the ground wires and strikes the phase conductors. Most of the probability calculation methods for these failures are electro-geometric and empirical and they refer to the line as being horizontal. The purpose of this research is to examine the influence of changes on the line sag and the tension on the lightning stroke probability. A mathematical model for calculating the shielding failure rate and the expected stroke rate on ground wires based on an electro-geometric model and on a known probability density function of the stroke flows in the area was developed in this research. The results show that, for single ground wire towers, an increase in the sag (i.e. a decrease in the tension parameter) of the ground wire, relative to the phase conductors, increases the stroke rate on the phase conductors in long spans. This research proposes an analytical calculation method that considers several factors, among them the sag and the tension along the span between adjacent towers, the catenary equation and the electromagnetic theory.