ELECTRIC-FIELD IN TRANSFORMER OIL MEASURED WITH THE KERR-EFFECT TECHNIQUE

The electric field distribution profile in transformer insulating oil was measured as a function of time, both after application of a dc step voltage and after polarity change of dc voltage, using a highly sensitive advanced Kerr electro-optic 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. The mobility of ionic carriers, 2.1 x 10(-5) cm2/Vs, in paraffinic transformer oil was obtained from the relationship between the saturation time of charge density and the applied dc voltage. The relaxation time of accumulated charge near the electrodes after short-circuiting the electrodes was independent of the pre-applied dc voltage.