Study of the induced voltage on an overhead line due to lightning discharge

The paper presents a study of the voltage induced on an overhead line due to lightning discharge. Lightning discharge releases energy over a large frequency band, generally from few hundreds of hertz to megahertz. The bulk of the radiated electromagnetic energy within VLF and ELF band, which is horizontally polarized, is picked up by the electrical power lines, and the interfering signal appears in the form of a surge on these lines. This surge propagates along the transmission line, and affects all equipment and appliances connected to the mains supply. Any lightning strike induces voltage in an open wire overhead line, communication line and other similar structures. The current that flows from cloud to ground is of transient nature and is expressed as superposition of two decaying exponentials with rise and fall time constants as parameters and peak current amplitudes. The nature of the return stroke current waveform is used in accordance with the one considered in. From the expression of current in time domain, the frequency spectrum covered by the lightning discharge can be estimated using Fourier Integral transform. Several studies, both theoretical and practical have been carried out by many researchers to provide insight of the lightning induced voltages. The induced voltage on an overhead line is related to the field produced by the transient current resulting from the lightning discharge. The transient voltage induced on the overhead line as well as communication line produces over-voltage effect to the system connected to the line. The field induced on the overhead lines can be estimated from knowledge of per unit length (PUL) parameters of the line and its geometrical structure and alignment. The PUL parameters are considered for a two conductor line with dielectric coating and having separation between them and the parameters are obtained following the method in reference.