Effect of suspending bubbles on tracking failure of oil-impregnated pressboard

Moisture in oil-immersed transformer will become small bubbles suspending in insulation oil at high temperature. In order to investigate the influence of suspending bubbles on tracking failure of oil-impregnated pressboard in oil-paper insulation, a model based on needle-plate electrodes was established. This model was used to study effect of the number of suspending bubbles generated by dropped distilled water on the discharge voltage, how the discharge signals varied during the process of tracking failure on the oil-impregnated pressboard surface with the effect of suspending bubbles, and the surface carbonization process of oil-impregnated pressboard during tracking failure. The results show that when the number of suspending bubbles increases with the increase of dropped distilled water, the partial discharge inception voltage in oilpaper insulation descends slowly while the surface flashover inception voltage descends sharply. The discharge process can be divided into three periods orderly according to the discharge signals during tracking failure. The discharge signals in each period have some relationship with each other and the recurrence plot (RP) method is used to analyze characteristics of the discharge signals. The surface carbonization process can be also divided into three periods the same as the discharge process. The carbonization on the oil-pressboard surface is slow under partial discharge while it´s fast under surface flashover. Because suspending bubbles make surface flashover much easier, so they make the tracking failure of the oil-impregnated pressboard much easier too, although the carbonization in the insulation oil is more difficult than that in the air. This paper is helpful to understand the process of tracking failure of oil-impregnated pressboard in insulation oil with the effect of suspending bubbles, which will be beneficial for the insulation protection in oil-immersed transformer.