Surface flashover characteristics of oil-paper insulation under combined AC-DC voltage

The valve side winding of the converter transformers withstood several types of voltage. This paper investigated the effects of voltage component on the surface flashover characteristics of oil-paper insulation under combined AC-DC voltage in different ratio. The whole flashover process of surface flashover of oil-paper insulation under combined AC-DC voltage was studied by using two types of electrodes. The experimental conclusion was shown that the flashover voltage increased with the DC ratio in the combined AC-DC voltage component. Flashover voltage decreased significantly when the radial electric field increases in the electric field component. Furthermore, electric field distributions versus combined AC-DC voltage in different ratio were calculated by Finite Element Analysis (FEA). Due to the different distribution characteristics of DC electric field and AC electric field, when the DC ratio was higher, the surface flashover location tended to pressboard surface. Conversely, when the AC ratio was higher, it tended to the oil gap between the electrodes. Moreover, based on high-speed camera, it was used to capture emitted light produced by surface flashover, record flashover initiation and propagation process in small oil gap between the electrodes.