Relationship between vacuum surface flashover and charging characteristics for alumina ceramics of lowered resistivity

Alumina ceramic is widely used for an insulator of vacuum interrupter. Occurrences of surface flashover along the insulator in vacuum have been a major obstacle to apply the vacuum interrupter for the higher rated voltage class, more than 100kV or further higher voltage. Therefore a superior alumina having high surface flashover voltage performance is required. Such a vacuum flashover occurrence is considered to be related to surface charging on the insulator. To investigate more superior alumina ceramics for high voltage application and clarify the mechanism of the surface flashover phenomena in vacuum, we carried out surface flashover test under negative impulse voltage application to four kinds of alumina in which dielectric constant and resistivity differ, and measured surface charge distribution before and after flashover occurrence. From the results, we revealed that the moderate resistivity could lead to the improvement in flashover voltage.