Electric field distribution in transformer oil containing an additive using the Kerr-effect technique under various DC stress conditions

The electric field distribution profile in transformer insulating oil as a function of time after application of DC step voltage and change of the DC voltage polarity was measured using a highly sensitive advanced Kerr electrooptic technique. The lower limit of the applied electric field intensity was 100 V/cm in insulating oil with an electrode length of 8 cm. Charge distribution as a function of time was calculated from the measured electric field distribution. Mobilities of positive and negative ionic carriers, 0.025 and 0.030 cm²/Vs, respectively, were determined from the relationship between the saturation time of charge density and applied DC voltage. The relaxation time of accumulated charge near the electrodes after short-circuiting the electrodes was independent of the preapplied DC voltage