Frequency dependency of the partial discharge signal propagationcharacteristics in gas-insulated switchgear (GIS) was investigated in alaboratory and an underground substation. The laboratory test indicatesthat signal damping can be attributed to two different mechanisms. Oneis reflection due to irregularity of characteristic impedance such asspacers, and the other is mode shift from TEM to TE or TM. Field testsshow that signal damping phenomena are independent of propagationdirection, and the mode shift gives rise to a large signal damping. Themajor higher mode seems to be TE11. Amplitudes of observed waveformsusing narrow-band filters, that agree well with the correspondingfrequency components observed with a spectrum analyzer, show randombehaviour depending upon the measuring point caused by the effects ofstanding waves. This indicates that narrow-band filters do not seem tobe suitable for a partial discharge location system from a frequencycomponents comparison. Thus, the realizability of the system usingbroad-band filters instead is discussed
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