Lightning protection practices followed in overhead lines. Overhead lines are exposed to open air and therefore, they are subject to over-voltages due to both external and internal causes. Lightning is the main cause of the occurrence of overvoltage in overhead lines. Different types of transients are experienced by overhead lines. Transients faced by lines may be divided into two following broad categories: 1. DC transients 2. AC transients. Lightning is unidirectional voltage waveform of very high peak. Both direct and indirect lightning strokes have severe adverse effect on transmission lines. It can cause breaking of insulators, lines and other members hampering the service of the electric power system. Thyrite-type lightning arrester is used for diverting very high voltages. Multiple numbers of disc separated by gap are placed in stack and enclosed in porcelain container. Gap reduces flow of current at normal condition. Surge absorber absorbs energy from travelling waves and reduces their high ampli-tude. The capacitor-based voltage transformer is used for measurement of high voltage. Dissipation factor is the measure of dielectric strength of insulating medium. Dissipation factor is measured using bridge circuit. Schering bridge is commonly used for measurement of dissipation factor. Lightning has been protected by earth wires while other overvoltages are protected by surge arresters, surge absorber and other protective devices.
Lightning and surge protection, Page 1 of 2
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