No abstract

No abstract

We have investigated charging and flashover characteristics of a polymeric or glass insulator exposed to ac voltage in vacuum in order to develop compact and reliable high voltage VCBs (Vacuum Circuit Breakers). This paper focuses on charging characteristics of a cylindrical model insulator. The charging of an insulator is investigated using an electrostatic probe that measures the electric field near the triple junction on the grounded electrode. This method allows a time-resolved measurement of the charging process. The insulator was made of borosilicate, fused quartz or polymetyl methacrylate, and was in the shape of a right cylinder with 10 mm in thickness. It has been clarified that the charging is characterized by three sequential states; initiation, quasi-stable and stable states, and that the polarity of the charge is positive for these states irrespective of the voltage phase. The charging characteristics with ac voltage are compared to our previous results with dc voltage excitation. We find that the charge magnitude at the stable state coincides with that obtained by dc. The electric field on the grounded electrode, and therefore the charge magnitude, decreases with the surface roughness, and decreases as the insulation strength is increased. A computer simulation has been conducted to investigate the quasi-stable state, which clarifies that the transition in surface charge distribution being synchronous to the voltage phase is responsible for causing the quasi-stable state.

No abstract

This paper describes experimental results on surface charging of partially mechanically processed insulators in vacuum. The charging phenomena were observed when insulators were exposed to a ramped dc voltage, by using a capacitive probe embedded in the cathode. First, we examined a partially roughened cylindrical insulator. The height of the roughened surface measured from the cathode, was varied while keeping the total length of the insulator constant. Second, in the same way, we examined a cylindrical insulator having a truncated conical frustum or a chamfer at its cathode-side end. Based on these experiments we have proved that the partial mechanical processing is effective for suppressing insulator charging in vacuum.