HVDC Ground Return Current Modeling in AC Systems Considering Mutual Resistances

Ground return currents (GRCs) in ac systems generated by HVDC monopolar operations can result in half-cycle saturation of transformers. Considering the ground potential rises of substations, which is represented by mutual resistances in this paper, an improved model of GRC flow in aca systems is presented. This model is verified by the IEEE benchmark test case for geomagnetically induced current (GIC) and the measured neutral direct current and voltage on neutral blocking devices (NBDs). Based on this model, how the dc bias current in ac systems is affected by mutual resistance is analyzed using measured earth resistivities and representative ac systems. Furthermore, the impact of mutual resistances on GICs is discussed. In the end, an enhanced model for optimal configuration of GRC mitigation is also proposed. The actual application of the optimal NBD configuration has been proven to be cost-effective in mitigating the GRC flow.