Study of the depolarization current in mineral oils

Polarization and depolarization current measurement is a non-destructive testing method to study the dielectric properties of insulating materials. Two kinds of mineral oils with different aging time have been measured and compared. When the charge carriers drift to the electrodes, they may be blocked and accumulated in the vicinity of the electrode to form the charge layers. When the oil is discharged, the charge carriers in these charge layers will start to move backwards to the bulk and result in a depolarization current. The depolarization process can be affected by both diffusion and electric drift. In this paper, the diffusion theory has been used to study the depolarization in mineral oil and the experimental results have been fitted using exponential equations. It seems the depolarization current is mainly contributed by the diffusion of the charge carriers in the mineral oil. The assumption that there are two kinds of charge carriers in the mineral oil that made in frequency domain measurement has been verified in this paper.