Optical measurement of electric field in transformer oil/pressboard composite insulation system and discussion on charge dynamics

We measured an electric field in transformer oil/pressboard composite insulation system under dc voltage application using Kerr effect in the oil. 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. We discussed this phenomenon by a charge accumulation and potential distribution model considering the difference between the charge accumulation rate on the pressboard of positive and negative charges. Discussion results were also described based on experiment with pressboards changing its thickness and location in the oil. Charge dynamics was also discussed when the polarity of dc voltage was reversed.