Modelling and measurement of field dependent resistivity of transformer oil

This article studies the apparent resistivity of mineral oil as a function of electric DC stress using theoretical and experimental methods for three different oils. The experimental measurement of the apparent resistivity is done using a cell with two bare Rogowski like electrodes separated by a 2mm gap. The dc current is measured after one hour and the stress is varied in steps. For low DC stress the apparent resistivity rises with increasing voltage, leveling off and dropping slightly for higher voltages. The resistivity is modeled using the ion drift model with equilibrium resistivity measured using dielectric response as input. The charge dynamics and the corresponding resistivity is simulated and the results compared with the measured values. The main features seen in the measurements are captured in the simulation and there is a good overall agreement between the theoretical data and the experimental values.