Tall-Structure Lightning Return-Stroke Models

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From the point of view of electrical engineers, the return stroke is the most important event in a lightning flash, as it causes most of the destruction and disturbance in electrical and telecommunication networks. In order to determine these fields, a lightning return-stroke (LRS) model must be constructed. Simultaneous measurement of the lightning current, its associated electromagnetic field, and lightning channel characteristics are necessary for the quantitative evaluation of lightning return-stroke models, and for the construction and evaluation of new models. This experimentally-based verification and development method will enhance our understanding of the LRS process, help in the formulation of models that can be used more successfully in lightning detection algorithms, and permit the development of more sophisticated measures for lightning protection. The CN Tower, with its extreme height, presents one of the best options in the world to carry out this investigation. By 1991, five measurement stations were in operation to simultaneously record all relevant lightning parameters, including the return-stroke current at the tower and its generated electromagnetic (EM) field 2 km north of the tower. Over the past several years, the CN Tower and the attached lightning channel have been represented by a series of cascaded uniform transmission line (TL) segments. In this model, current reflections at the front of the continuously growing return-stroke channel were taken into consideration. Reflection coefficients at CN Tower's three major structural discontinuities were determined from the measured current. A quantitative evaluation of this model was carried out by comparing the EM field, computed by applying a particular LRS model and that measured 2 km away from the tower. Although this comparison verified the merits of this TL model, the use of uniform TL segments is clearly an approximation since the distance to ground of each level of the lightning current path (the elevated object and the channel) is different. The applicant is planning to model the lightning current path as a series of non-uniform transmission lines, each having its characteristic impedance that varies with height.