Frequency dependency of the partial discharge signal propagation characteristics in gas-insulated switchgear (GIS) was investigated in a laboratory and an underground substation. The laboratory test indicates that signal damping can be attributed to two different mechanisms. One is reflection due to irregularity of characteristic impedance such as spacers, and the other is mode shift from TEM to TE or TM. Field tests show that signal damping phenomena are independent of propagation direction, and the mode shift gives rise to a large signal damping. The major higher mode seems to be TE11. Amplitudes of observed waveforms using narrow-band filters, that agree well with the corresponding frequency components observed with a spectrum analyzer, show random behaviour depending upon the measuring point caused by the effects of standing waves. This indicates that narrow-band filters do not seem to be suitable for a partial discharge location system from a frequency components comparison. Thus, the realizability of the system using broad-band filters instead is discussed.
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