On the Modeling of Nonlinear Electrical Conductivity and Field Distribution of Mineral Oil Under HVDC-Load

Abstract:
Mineral oil is widely used as an insulation material in HVDC transmission systems. Electrical modeling of this insulation material is of great importance to optimize its usage in the grid apparatus, especially under DC load. In that regard, the ion-drift model is a useful modeling approach capable of reconstructing the nonlinear dielectric behavior of mineral oil and was consequently employed in this study. To verify the model, the electric field and conductivity measurements of an oil sample under DC-electric field stress are compared to the obtained simulation results. The transient behavior can be attributed to the intrinsic charge carriers, which, in the case of a relatively moist oil have a greater initial concentration compared to a dry oil sample. The pronounced field enhancement is determined by the intrinsic charge carriers, which are able to either fully neutralize at the electrodes or cease moving and ”stack up”. It seems appropriate to state that an asymmetrical field distribution in the transient case can be the result of the difference in mobilities of positive and negative charge carriers. A persistent asymmetry in the stationary case might be caused by different injection strengths of individual charge carriers accompanied by their mobilities. The polarization current is compared to the simulation results to interpret the charge transport, especially in the transient case, where a good agreement with the measurement results can be achieved. Finally, the stationary field distribution of the simulation is discussed.