Surface charging of aluminas excited by impulse voltage in vacuum

An investigation was made of the charging properties of alumina insulators and the relation between the surface flashover properties and the surface condition. An electrode configuration composed of a needle electrode and a backside electrode, which formed an extremely nonuniform field, was used. Experimental investigation of the charging properties revealed that surface charging with positive voltage applications started at the anode triple junction and that surface charging formed by voltage application attenuated little with time. Surface analysis of the insulators before and after heat treatment and repetitive flashovers revealed that heat treatment at 150°C in vacuum reduced the oxides but could not remove the organic contaminants. However, after repetitive flashovers, the organic contaminants were removed from the alumina surface. Therefore, conditioning the alumina surface improves the flashover voltage as the number of flashovers accumulates