What is Voltage surge or Transient voltage ?
Definition: 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 (a few hundred μs) but they cause overvoltages on the power system. They originate from switching and from other causes but by far the most important transients are those cause 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 give way.
In most of the cases, such surges may cause the line insulators (near the point where lightning has struck) 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.
The waveform of a typical lightning surge shown in fig below :
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| Waveform 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.
In most of the cases, this build-up is comparatively rapid, being of the order of 1—5μs. Voltage surges are generally specified in terms of rise time t1 and t2 to decay to half of the peak value. For example, a 1/50 μs surge is one which reaches its maximum value in 1μ and decays to half of its peak value is 50 μs.
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