Study of the charge dynamics in mineral oil under a non-homogeneous field

The presence of mobile charge carriers can affect the frequency response of complex permittivity of insulating liquid when the frequency is low. It has been found that the experimental results of the complex permittivity of mineral oils are in a good agreement with Coelho's frequency response theory when an extra contribution to the complex permittivity arising from DC conduction is taken into account. It has been reported that small quantity of high mobility charge carriers are responsible for this DC conduction. In our previous work, we have found that if the charge carriers are fast enough so that they can reach the opposite electrode in a cycle, the motion of these fast charge carriers can only contribute to the imaginary part of the complex permittivity under a homogeneous field. In this paper, the polarization induced by the motion of those fast charge carriers under a non- homogeneous field has been studied. It has been found when the field is not high, the total current caused by the drift and diffusion of those fast charge carriers is proportional to the external voltage. A modified space charge polarization theory has been proposed. The frequency dependence of the complex permittivity obtained experimentally has been fitted using this modified Coelho model and compared with our previous simulation. This modified model enables one to gain a better understanding of the frequency response in mineral oil in low frequency regions.