On the polarity effect of the breakdown voltage for dielectric liquids in an inhomogeneous field

The polarity difference of the breakdown voltage of some pure dielectric liquids and transformer oil is qualitatively explained by analysing the electric field distribution of a point-plane test cell under the condition of unipolar injection of charge carriers. Although the space charge reduces the electric field at the point, it leads to an increase in the stressed volume thus increasing the probability of breakdown. The magnitude of the space-charge contribution is inversely proportional to the mobility of the injected charge carriers. Since in some of these liquids electrons exhibit a much higher mobility than positive ions, the influence of the space charge is reduced for the point negative compared with the point positive.