Influence of Deep Earth Resistivity on HVDC Ground-Return Currents Distribution

During monopolar operation or commissioning of high-voltage direct-current (HVdc) links, ground-return current (GRC) leads to power transformer dc bias issues. A GRC model is proposed in this paper, and the penetration depth, penetration ratio, and earth surface potential of various earth models are simulated. The result indicates that the influence of deep earth resistivity increases rapidly by the increasing distance from substations to HVdc electrodes. The simulation results are compared with measured data of GRCs distribution in the Hubei power grid of China. The result demonstrates that the simulation considering deep earth resistivity is credible and the shallow earth model could lead to poor accuracy. Furthermore, earth resistivity measurements are carried out at different geological regions and the deep earth characteristics and the GRCs distribution are quite different. Thus, it can be concluded that to calculate GRCs distribution using uniform earth structure without measurement is not appropriate. Finally, the relationship between accurately measured earth depth and the accuracy ratio of GRCs calculation is analyzed. The result of this paper contributes to determine the depth of earth model in GRCs simulation modeling and earth resistivity measurements.