Forming Process of Cellulose Impurity Bridge in Oil-Paper Insulation under DC Voltage

Cellulose impurities are easy to accumulate and form bridges in oil-paper insulation, resulting in partial discharge and even insulation failure. In this paper, formation conditions and bridge-forming process of cellulose impurity in oil-paper insulation under DC voltage are studied. The experimental system of oil-paper insulation under DC voltage is built, and the optical diagnosis platform is used to explore whether the cellulose impurities generate and form bridges under different DC voltages. Based on the experimental conditions, the dynamic model of cellulose impurity movement in oil-paper insulation is established, and the bridging process of cellulose impurities in oil is analyzed. The results show that the generation of cellulose impurities under DC voltage is related to the average discharge and accumulated discharge. When the average partial discharge reaches 42 nC, the probability of cellulose bridging obeys the two-parameter Weibull probability distribution with accumulated discharge as the independent variable. In the moving process, the cellulose impurities may collide with each other in the oil, then continue to accumulate towards a stronger electric field region after the charge transfer, and finally form an impurity bridge with thick ends and fine middle. This study quantitatively analyzes the formation conditions of cellulose impurities and explains the whole process of cellulose impurities from movement to bridge, which can provide a theoretical reference for the evaluation of oil-paper insulation performance.