Cold-wet-switch-on characteristics of semi-conducting glaze insulators

Semi-conducting glaze insulators show higher flashover voltages under contaminated conditions when they are continuously energized with the operating voltage. Such a superior performance is owing mainly to the drying effect by the Joule´s heating caused by the leakage current flowing in the semi-conducting glaze layer. So, under cold-wet-switch-on conditions, in which drying effect does not exist, such a superior performance cannot be expected. Based on the test data obtained under heavily wet conditions simulating the worst wet conditions in fields, present design withstand voltage is proposed. However, considering shorter interruption duration in most of the outages in electric power systems, the present design criterion may not be adequate for rationalized actual applications. Now we are investigating the time variation of surface resistance after interrupting the operating voltage on artificially contaminated insulators under natural wet conditions. Based on the analysis of the data obtained so far, a higher design withstand voltage can be practically proposed even for cold-wet-switch-on conditions.