Kerr electro-optic field mapping in transformer oil/PET film composite insulation system for DC voltage application

We measured dc electric field distribution in transformer oil/PET film composite insulation system for a parallel-plane electrode configuration using Kerr electro-optic effect with ac voltage modulation technique. Experimental results revealed that the electric field strength in the center of the oil gap declined in an exponential form with time up to 10 hr for the oil/PET system, while that for the oil alone remained almost constant. For the composite system, the electric field strength in oil by positive 10 kV dc voltage application proved to be smaller than that by the negative one. The reduction of the electric field in the oil was explained in terms of charge deposition on the film and space charge formation within the oil. To investigate the effect of electrode material, we also measured the field distribution in the oil alone with aluminum or brass electrode as a cathode while keeping the anode material brass. It was found that the electric field distribution was almost the same irrespective of the cathode material in the applied electric field range from -5 to -45 kV/cm. Finally, we studied a field distribution in the oil containing asphalt so as to simulate the degraded oil condition