Most indoor polymeric insulators are used for a long term over 30 years. Those insulators rarely fail due to partial discharge depending on the used material and installation environment. Therefore, we have been developed a new risk analysis method of partial discharge occurrence on indoor polymeric insulators. We conducted experiments simulated dry-band discharge and uncontaminated-band discharge, and defined the decision making criteria for occurring partial discharge. The criteria for dry-band discharge are insulation resistance under maximum humidity and surface resistance ratio before and after drying. The criteria for uncontaminated-band discharge is surface resistance ratio of before and after cleaning. In order to on-site measuring the surface resistance in the on-site switchgear, we developed portable measuring device. This device which consists of high humidity generator, insulation resistance tester and surface contact sensor can measure the surface resistance between pin electrodes under local high-humidity space. We analyzed the partial discharge risk on different switchgears used for over 25 years using this device. As a result, the risk due to partial discharge of long-term used insulators are kept extremely low by periodically maintenance and environmental control.
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