Computation of the lightning stroke probability and the effectiveness of the air termination

Abstract

The attractive volume concept is introduced because it characterises better the risk or effectiveness of the lightning protection systems than the protected volume concept. According to this concept the point of strike is determined in the orientation point characterised by the start of connecting leaders. The orientation distance is a function of the peak value of the lightning current and on this way its distribution function can be formulated, too. The expected annual frequency of the lightning strokes to an object is expressed by a general formula which contents a volume integral of the distribution density function of the orientation distances. these relations are expressed with formulas and so it is possible to solve the integrals with numerical methods by computer. In the calculations the principle of the minimal distances is applied but in some cases with modification according to the polarity of the lightning. Computer programs have been worked out to study the following problems. A vertical rod or a tower standing on flat country. A horizontal line over flat country which corresponds to a total transmission line. Horizontal conductor protected by one or two earth wires as characteristic parts of a high voltage transmission line. A flat surface with a network of horizontal conductors equivalent with a flat roof protected by an air termination network. It is demonstrated that the calculated annual frequencies are in good agreement with the observed values concerning towers or lines. With application of a constant protective angle the frequency of shielding failures raises to a higher degree than with a constant radius of rolling sphere, when the height of the line is going up. The results indicate the effect of the mesh size of an air termination network on a flat roof. It is demonstrated that neither the protective angle nor the rolling sphere methods yield a constant frequency of interception failure independent from other data.