Effect of Moisture Content on Dynamic Characteristics of Space Charge in Oil-paper Insulation under AC Field

Oil-impregnated pressboard samples with water content of 0.34%, 0.98% and 1.81 % were used to investigate the effect of moisture content on dynamic characteristics of space charge under AC stress. Space charge contribution were observed by a fast space charge detecting system based on the automatic equipartition phase shift (AEPS) principle. It shows that space charges are injected from the top electrode for the sample with moisture content of 0.34%. The peak value of homo-charge in both electrodes significantly increase for the sample with moisture connect about 0.98%. It may be associated with the reduction of the charge injection barrier between oil-paper insulation and electrode. And the injection speed for ground electrode increases more than the injection speed for top electrode. This is because the material of two electrodes are different, and the energy level of charges in ground electrode is higher. With moisture content increases, the depth of traps within oil-paper may increase, lead to charges injected from ground electrode easier to be trapped. As moisture content rises from 0.98% to 1.81%, the space charge accumulation phenomena after charge injection and extraction in AC positive half-period is dominated by homo-charge injected from ground electrode, and may suppress the electric field distortion near ground electrode.