Frequency content of switching overvoltages

Switching overvoltages are usually discussed in terms of their peak values, but their frequency content is also important. The relevant frequencies have now been analysed for certain field-test waveforms as well as for calculated voltage transients that result from energising a single line. The power frequency voltage supplied by the source is predominant, but the frequency associated with surge reflections from each end of the line is quite evident. There are also significant high-frequency components introduced when a line is energised from a lowimpedance source if switching occurs near the peak of the source voltage waveform. These components define the rate of rise of voltage just before the peak and therefore affect the positive-polarity breakdown characteristics of the line insulation. In selecting parameters at a fixed frequency for use in a mathematical analysis or in a physical model, the frequencies present in the transient voltages should be determined.