Space Charge Behaviors of Thermally Aged Kraft Paper impregnated with Synthetic Ester Liquid

Due to DC bias, space charges are formed in the insulation system of certain HVDC converter transformers, which can cause electric field distortion and eventually insulation failure. Studying the space charge behaviour is important in designing and operating HVDC transformers. This paper investigates the impact of paper-ageing on the space charge behaviour of insulation systems with synthetic ester and Kraft paper insulation as a potential candidate for HVDC transformers. The maximum charge density at the electrodes and the total accumulated charges in the sample increased at the early stage of ageing but later decreased almost to the same level as the new paper samples. A fast charge dissipation was observed during the volt-off period, indicating a possible shallow trap generation in aged paper samples.