The influence on partial discharge in vacuum by chromium oxide coating on alumina surface

We have to elucidate the partial discharge phenomenon in a vacuum for the vacuum interrupter development, which occurs with regional light emission and electron charging on ceramic surfaces. They are caused by electric field electrons emitted from high electric field points, e.g., cathode triple junctions, and secondary electrons emitted from ceramics when the electric field electron hits on ceramic surfaces. Therefore, the suppression of these electron emissions leads to the reduction of partial discharge occurrence. There are some researches about the suppression of field electron and secondary electron emissions. We focused on chromium oxide coating on ceramics surface that suppress these electron emissions effectively. The secondary electron emission coefficient of chromium oxide is lower than that of alumina ceramics, so the secondary electrons are hard to be emitted from chromium oxide coating regions. Some researchers investigate the effects of coating chromium oxide on breakdown performance, but few researchers do the effects on partial discharge in a vacuum. In this article, we investigated the effects on partial discharge in a vacuum with chromium oxide coating and measured Partial Discharge Extinction Voltage (PDEV), which is one of the parameters to evaluate the characteristics of partial discharge in a vacuum. We used the evaluation samples that represent the internal structure of a vacuum interrupter and have different chromium oxide coating regions in order to investigate the effect. As a result, it was confirmed that PDEV in the sample with coating was about 1.5 times higher than that of without, and the larger the coating region was, the higher PDEV was. From the above results, it turned out that coating chromium oxide to ceramic surface was the effective way to suppress partial discharge in a vacuum.