Effect of sub-atmospheric pressure on the conduction current in transformer oil

The effect of subatmospheric pressure on the quasi-steady-state and burst currents in transformer oil is investigated. Nonuniform high DC field is applied to aged and fresh transformer oil samples. The applied pressure is reduced from 10³ torr to 50 torr and then increased correspondingly for each applied voltage inside a controlled vacuum chamber. Generally, it is found that at a fixed-field voltage the quasi-steady-state current remains almost unchanged with pressure variation. At a certain critical low subatmospheric pressure, the current increases sharply. The value of this critical pressure increases as the applied voltage is increased. Variation of pressure has similar effects on burst current. The results reveal that the gas bubble mechanism is the dominant factor affecting the conduction current and breakdown in insulating liquid at low pressure. A critical pressure initiates the formation of a stream of gas bubbles after which the current increases sharply, leading to a breakdown