Surface trap and carrier transport of aged and pristine oil-paper under harmonic voltage by surface potential decay

Harmonic voltage can accelerate the aging of insulating materials. In this paper, an ac voltage with low-order harmonic components was applied on oil-paper insulated bushing with a duration of one month. Oil-paper samples extracted from bushing were tested by Surface Potential Decay (SPD) after positive and negative corona charging. Experimental results of SPD illustrate that the surface potential of aged oil-paper sample drops faster than the pristine one. Surface trap distribution is obtained for both pristine and aged samples. Besides, the hole and electron motilities are calculated based on transit time and initial surface potential. The conclusion can be reached that during the harmonics aging experiment, the scission of cellulose occurred under the thermal, electrical and mechanical stress, inducing surface traps and leading to higher nobilities of electron and hole.