Space Charge Characteristics in Oil and Oil-impregnated Pressboard and Electric Field Distortion after Polarity Reversal

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.