A study on electric conduction of transformer oil

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<;0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohm´s law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm<;E<;1.33 kV/mm), ln (I/E²) appears to be proportional to E^-1, which meets the field emission equation. At high electric field stage (E>1.33 kV/mm), the obtained I-V² characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well.