The Breakdown Voltage of Oil Gaps with High DC Voltage

The electrical strength of technically clean insulating oil in a direct voltage field is lower when the high-voltage electrode is sharper and of positive polarity. In a uniform field, with distances up to 100 mm, approximately 30 percent lower withstand values are obtained with direct voltages than with alternating voltages (rms values); this is also valid when the oil is heated up to transformer operating temperature. For longer applications of the test voltage (up to 30 minutes) a reduction in the electrical strength of approximately 15 percent, compared with 1 minute values, is to be expected. With direct voltages having a superimposed alternating component, the same results apply if the peak value of the combined voltage is used.