In thyristor equipment installed near 66kV GIS-cable system, mal-firing of thyristor was generated by surge induced in earthing system during closing operation of GCB. Though earthing impedance was lowered by laying a floor with copper plate, mal-firing could not be suppressed. So investigation using full-scale experimental equipment was made. As the results, the mechanism and method for suppression of this mal-firing are clarified. The resume of results is as follows.
(1) High frequency surge induced in earthing system contains two frequency conponents, those are, conponent in the frequency range of several hundred kHz and conponent over 10MHz.
(2) The frequency conponent which generates mal-firing is in high frequency range over 10MHz.
(3) Condensers attached at joint between GIS tank and power cable sheath and at joint between transformer and power cable sheath are effective for suppressing mal-firing.
(4) Earthing mesh is not effective to suppress surge over 10MHz induced in earthing system.
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