Factors affecting breakdown phenomenon in transformer oil with uniform and nonuniform fields

The conduction current in oil under AC and positive DC voltages increases in general with increasing field strength or oil temperature. When the oil is subject to positive DC voltage the leakage current is too much smaller than that with AC voltage. The breakdown voltage of oil decreases as the oil quality decreases. The breakdown voltage is lower for positive polarity then for negative polarity of DC voltage. The breakdown voltage increases to a maximum at a temperature of 80 °C and then decreases with further increase of temperature. The breakdown voltage values under uniform field are lower than those under nonuniform.