Washing withstand voltage tests on station insulators

An investigation was undertaken to determine the cause of flashovers on semi-conducting glaze (also called resistive glaze) station post insulators produced by two different manufacturers, that were experienced by Pacific Gas and Electric Company (PG&E) during pressure washing in the Moss Landing Substation. The insulators that flashed were being struck by over spray water caused during washing of adjacent insulators. Comparative tests were performed at NGK's High Voltage Laboratory in Komaki, Japan with semi-conducting glaze station posts, ordinary glaze station posts, and pin type insulators energized at 70 kV (phase to ground) under different conditions of wetting and artificial contamination. The tests confirmed that semi-conducting glaze insulators had excellent performance while contaminated and in heavy clean mist, or while clean but subjected to heavy salt mist. However, when an extreme volume of water from overspray cascaded across the sheds of contaminated semi-conducting insulators they could flash as the mechanisms that give semi-conducting glaze insulator their excellent performance are defeated. Under these conditions the washing withstand voltage of ordinary and semiconducting glaze insulators is the same. Two countermeasures, modifying the wash pattern and reducing the volume of water by using a nozzle with a small orifice diameter, were found to be effective in reducing flashover probability when washing insulators in a substation.