The elements of a power system (e.g. generators, transformers, transmission lines, insulators etc.) are subjected to over voltages i.e. voltages greater than the normal value. These over voltages on the power system may be caused due to many reasons such as lightning, opening of circuit breaker, the grounding of conductor etc. Most of the over voltages are of large magnitude but may still be important because of their effect on the performance of circuit interrupting equipment and protective devices. An appreciable number of these over voltages are of sufficient magnitude to cause insulation break down of the equipment in the power system. Therefore, power system engineers always device ways and means to limit the magnitude of the over voltages produced and to control their effects on the operating equipment.
VOLTAGE SURGE
A sudden rise in voltage for a very short duration on the power system is known as a voltage surge or transient voltage.
Transients or surges are of temporary nature and exist for a very short duration but they cause over voltage on the power system. They originate from switching and from other causes but by far the most important transients are those caused by lightning striking a transmission line.
When lightning strikes a line, the surge rushes along the line, just as a flood of water rushes along a narrow valley when the retaining wall of a reservoir at its head suddenly gives way. In most of the cases, such surges may cause the line insulators to flash over and may also damage the nearby transformers, generators or other equipment connected to the line if the equipment is not suitably protected.
Fig 1 shows the wave-form of a typical lightning surge. The voltage build-up is taken along y-axis and the time along x-axis. It may be seen that lightning introduces a steep fronted wave. The steeper the wave front, the more rapid is the build-up of voltage at any point in the network. Voltage surges are specified in terms of rise time t1 and the time t2 to decay to half of the peak value.
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