Influence of Field Relaxation Ring Length on Vacuum Flashover Voltage in Model Sample of Vacuum Interrupter

In a vacuum interrupter (VI), surface flashover events are one of the obstacles for improving its higher voltage performance. To suppress the vacuum flashover along the insulator surface, field relaxation rings apply to it around the connection point between the metal and alumina insulator, which is called as Triple Junction (TJ) in order to reduce the high field around the TJ point. To obtain higher voltage performance in vacuum interrupter, it is necessary to optimize the shape of field relaxation rings for example, its length, tip radius and gap length etc. In this study, we investigated the vacuum flashover voltage in model vacuum interrupter samples which were equipped with the field relaxation rings inside cylindrical alumina insulator. We carried out the flashover tests for the model samples changing the length of field relaxation rings. As a result, it was confirmed that the install of field relaxation ring increased the vacuum flashover voltage in vacuum interrupter. But in case of the short ring length, the flashover performance was inferior to that of sample having a certain length ring. On the other hand, in case of applying a long ring length, the flashover voltage also decreased rather than the certain appropriate ring length. Therefore, the design of ring length is necessary to select the appropriate length considering the field relaxation for TJ point and the tip of shield rings.