The formation characteristics of bubble in oil-paper insulation and its effect on the breakdown

The presence of bubbles will reduce the electric strength of the transformer insulation, and even lead to insulation breakdown. In this paper, the characteristics of bubble formation in oil-paper insulation with different moisture content are studied. Through the preparation of insulating pressboard with different moisture content, the bubble phonomenon is observed in the experiment of rapid heating. The test results show that the higher the moisture content of the insulating pressboard is, the lower the initial temperature of the bubble formation becomes. The initial temperatures of the bubble formation are 142 degrees C, 120 degrees C and 94 degrees C respectively for the three moisture contents 2%, 4% and 6% of pressboard. Then the effect of bubble on oil-paper insulation is verified through AC breakdown test. With the bubble appearing, the breakdown voltage of the oil-paper insulation decreases. The failure pattern changes from bulk breakdown to surface creep breakdown. The flashover voltage becomes the critical voltages of the oil-paper insulation when the bubble appears. In the gap of 1mm, the breakdown voltage decreases by 28.6% compared with the non-bubble condition.