Flashover and Charging Characteristics of a Long Solid Insulator Exposed to ac Voltage in Vacuum

In our previous studies, with a comparatively short cylindrical insulator of 10 mm length, we have found that the charging process due to ac voltage excitation in vacuum is characterized by three different states; Initiation, Quasi-stable state and Stable state. In all of the states, the insulator surface acquires positive charge irrespective of applied voltage amplitude or phase. Also, we have reported that the magnitude of the charge decreases with the roughness of insulator, and that this results in the improvement of surface insulation strength. To apply the above results for practical insulators used for vacuum valves of VCBs, we have investigated the characteristics for a longer insulator up to 50 mm made of borosilicate (Pyrex®) . As a result, we have confirmed that the charging process is basically the same as that observed in the previous studies. We have also confirmed the effectiveness of roughening the surface on reducing the charge magnitude and increasing the flashover voltage.