Effect of oil aging states on the space charge behavior in oil-paper insulation under DC electric field

In order to deeply understand the effect of oil aging state on space charge behavior in oil-paper insulation under DC electric field, transformer oil samples with different aging states were prepared by accelerated thermally aging, and their physical and chemical properties were analyzed. Then new Kraft papers were dried in vacuum and immersed in oil above with different thermally aging states, and three kinds of oil-paper samples were achieved. The pulsed electro-acoustic (PEA) method was used for space charge measurement under both DC voltage-on and voltage-off conditions at room temperature. The impact of oil aging state on the characteristics of space charge injection, accumulation and decay was discussed. Experimental results show that homocharges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the aging state is worse, more charges are induced at both cathode and anode, as well as more space charges accumulate in the bulk. Besides the more the oil aging state is, the larger the area of negative charges is. For voltage-off condition, aging oil is beneficial for space charge decay. Moreover, trapped positive space charges decay faster than trapped negative charges.