Transients or surges on the power system may originate from switching and from other causes but the most important surges are those caused by lightning. The lightning surges may cause serious damage to the expensive equipment in the power system (e.g. generators, transformers etc.) either by direct stroke on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves. It is necessary to provide protection against both kinds of surges. The most commonly used devices for protection against lightning surges are: (i)Earthing screen (ii) Overhead ground wires (iii) Lightning arresters or surge diverters Earthing screen provides protection to power station and sub station against direct strokes whereas overhead ground wires protects the transmission read more

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1. Direct stroke 2. Indirect stroke 1. Direct stroke – In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the pole to the ground. The over voltages set up due to the stroke may be large enough to flash over this path directly to the ground. The direct strokes can be of two types: (i) Stroke A and (ii) stroke B. (i) In stroke A, the lightning discharge is from the cloud to the subject equipment i.e. an over- head line in this case as shown in Fig 4 (i).The cloud will induce a charge of opposite sign on the tall object (eg. an overhead line in this case). When the potential between the clouds exceeds the breakdown value of air, the read more

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An electric discharge between cloud and earth, between clouds or between the charge centers of the same cloud is known as lightning. Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or -ve) with respect to earth or a neighbouring cloud that the dielectric strength of neighbouring medium (air) is destroyed. There are several theories which exist to explain how the clouds acquire charge. The most accepted one is that during the uprush of warm moist air from earth, the friction between the air and the tiny particles of water causes the building up of charges. When drops of water are formed, the large drops become positively charged and the smaller drops become negatively charged. When the drops of water accumulate, they form clouds, and hence read more

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