Direct measurement of electric field using Kerr electro-optic method in transformer oil/pressboard composite system at DC polarity reversal

We measured an electric field in transformer oil/pressboard composite insulation system under DC voltage application using Kerr electro-optic measurement system. When a pressboard was inserted between two parallel-plane electrodes, the electric field between the pressboard and the cathode decreased monotonously with time. On the other hand, the electric field between the pressboard and the anode increased with time, reached a maximum value and then fell down. In order to clarify the mechanism, we measured an electric field at DC polarity reversal and at different temperatures in transformer oil. For understanding of these phenomena, we proposed a charge accumulation model considering the difference in charge accumulation rate on the pressboard surface between positive and negative charge.