The Influence of Thermal Aging on Space Charge Distribution in Oil-impregnated Paper under AC Field

The winding insulation in transformer usually bears the combined action of DC and AC components during the operation. The positive and negative charges are injected from the electrodes respectively, while under the AC field, the injection of positive and negative charges alternately occurs on the single electrode due to the periodic change of the electric field polarity, and the direction of carrier migration changes with the applied field. Thus, the charge behavior under AC field is more complicated. In this paper, the space charge distribution in oil-impregnated paper with different aging times was measured under different frequency sinusoidal electric fields. According to the phase characteristics of charge distribution, the effect of thermal aging on the charge behavior under AC field was analyzed from the perspective of charge source. It shows that there is charge hysteresis in the insulation, that is, the residual charges injected in the previous cycle form a heterocharge accumulation after the voltage is reversed, resulting in serious field distortion. According to the preliminary analysis of the results from early aging period, the more severe the aging, the less obvious the charge hysteresis is, which may be related to the decrease of the trap depth caused by thermal degradation. It is speculated that the charge hysteresis under AC field is an important cause of accelerating the electrical aging of the insulation.