Computational study of very fast transients in GIS with special reference is effects of trapped charge and risetime on overvoltage amplitude

Very fast transient phenomena in gas insulated substations (GIS) have been studied by computer simulation. The influence an overvoltage amplitude of trapped charge, risetime and section length of GIS is described. Under identical conditions, the amplitudes of overvoltages caused by very fast transients at the same point in a circuit, but having different trapped charges, are connected by a simple relation due to the linear nature of the circuit. In the case of high predicted overvoltages, the assumption of 1 pu trapped charge causes a large overestimation of the overvoltage which can be in excess of 20%, in comparison with the overvoltage due to 0.4 pu trapped charge, a value which, in practice, is rarely exceeded. For an open ended circuit, the assumption of zero risetime can cause an overestimation of more than 30% in the overvoltage in comparison with the case of 5 ns or more risetime. Variation of the trunk section length has a greater effect on the overvoltages than that of a branch section. The difference in the overvoltages at an open end for various section lengths can be in excess of 50%. In some cases even a small variation of section length causes a very large change in overvoltages