DC field measurement in oil/pressboard composite insulation system by electro-optic Kerr effect

In oil-immersed power transformers, an insulating oil/pressboard (PB) composite insulation system is generally applied. In the system, charges accumulate at the oil/PB interface and electric field distribution changes depending on time and oil condition. In this paper, the authors directly measured the DC electric field in oil/PB composite insulation system using Kerr electro-optic field measurement. From the results, they proposed the charge behavior model. They calculated the electric field distribution using the finite element method (FEM), taking account of measured charge behavior. Finally, they confirmed that the charge model explains the experimental results well, including in the case of the polarity reversals.