Electromagnetic Field Analysis of Lightning Return Stroke Current

Abstract

Accurate knowledge of the characteristics of lightning electromagnetic fields is needed for studying the effects of the potentially deleterious coupling of lightning fields to various circuits and systems, for achieving an efficient insulation design of electric power networks and for determining electromagnetic compatibility requirements o telecommunication systems. In the first step, lightning analysis requires the development of return stroke models, which are able to reproduce the electromagnetic field signature similar to that generated by natural return stroke. In this thesis, the lightning return stroke is, firstly, assumed as a current pulse originating at ground level and propagating along the channel of perfect electrical conductor. Then the effects of channel resistance and inductance both on the current and the electromagnetic fields are investigated. The results show that the inclusion of channel resistance and inductance will change the prediction of the model in such a direction that they will come closer to the experimental observations. Reviewing different models, electromagnetic model is used in the present study. For the accurate analysis of the transient electromagnetic field around a three-dimensional (31)) conductor system, application ol electromagiietic modeling codes are more appropriate. Among many available codes, Numerical Electromagnetic Code (NEC-2) based on the 4 method of moments is chosen for the above purpose since it has been widely and successfully used in analyzing thin-wire antennas. The effect of ground electrical parameter on the channel current and remote electromagnetic fields is deduced in the present thesis for several grounds. With different methodologies different input data are involved in the lightning analysis: one of them is related to the lightning current itself. The effect of different current wave-shapes is also investigated in this work. Experimental observations and theoretical investigations have shown that the presence of an elevated strike object (such as a tall tower) could affect lightning current and their radiated electromagnetic fields substantially. lherefore, an analysis of tower current and remote electromagnetic fields radiated by lightning return strokes to tall towers is presented taking two towers of different heights. In the work, both the slow-front and steep-front wave-shapes, corresponding respectively to typical first and subsequent return strokes, are adopted as injected current wavefonii. Because, the subsequent return strokes that are characterized by lower current peaks but higher front steepness and return stroke speed may result in higher field peaks. It is shown in particular that the presence of a tower tends, in general, to increase substantially the electric and magnetic field peaks and their derivatives. Furthermore, the presented results are shown to be consistent with recent experimental observations of current in lightning strokes to the high stack in Japan and Toronto CN Tower and of the associated electromagnetic fields. Finally, to show the validity of the model, some results calculated by NEC-2 are compared with the results calculated by Virtual Surge Test Lab (VSTL), which is based on Finite Difference Time Domain (FD'l'D) method. In such case, reduced scale model is considered because of large computation time of FI)TD method.