Charge Distribution and Charging Characteristics of Cylindrical Alumina Insulator with Shield Rings in Vacuum

This paper deals with charging characteristics on the insulator surface in vacuum after the AC voltage application. In designing Vacuum Interrupters (VIs) which is current interruption parts of Vacuum Circuit Breakers (VCBs), one of the consideration points is the control of surface flashover events in vacuum. It is believed that such a flashover occurs via charging on the insulator surface. In this study, we measured the charge distributions of cylindrical alumina insulators in vacuum after the AC voltage application. Those insulator samples are equipped with electric field relaxing shield rings (shields) at both ends of the insulator to relax electric field around the triple junction (TJ), and we prepared three kind of shield rings which have different gap length from the alumina insulator surface. As a result, positive charges were formed on the insulator surface of all samples by voltage application. The amount of charge was increased almost linearly to the applied voltage. In addition, when repeating high voltage application test under same condition, the amount of charge was gradually decreased by conditioning effect of shields, and finally reached a stable value by repeating voltage application tests. After that, when a higher voltage rather than above conditioning voltage was applied to the experimental sample, the charge amount was slightly increased.