Kerr electro-optic field measurement in transformer oil/solid dielectrics composite insulation system

We measured the electric field in transformer oil/solid dielectrics 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 at cathode side of pressboard decreased with time. On the other hand, the electric field at anode side increased initially with time, reached a maximum value and then fell down. Next, we measured the electric field after polarity reversal of dc voltage. The results revealed that the electric field reached a magnitude of 2.7 times as much as the average dc applied field. We interpreted the above results in terms of a charge dynamic model considering the difference in charge accumulation speed on pressboard between positive and negative charges.