Impact of Grid Topology on Pole-to-ground Fault Current in Bipolar DC Grids: Mechanism and Evaluation

The fault current level analysis is important for bipolar direct current (DC) grids, which determines the operation and protection requirements. The DC grid topology significantly impacts the current path and then the fault current level of the grid, which makes it possible to limit the fault current by optimizing the grid topology. However, the corresponding discussion in the literature is indigent. Aiming at this point, the impact of grid topology, i.e., the connecting scheme of converters, on the pole-to-ground fault current in bipolar DC grids, is investigated in this paper, and the ground-return-based and metallic-return-based grounding schemes are considered, respectively. Firstly, the decoupled equivalent model in frequency domain for fault current analysis is obtained. Then, the impacts of converters with different distances to the fault point on the fault current can be analyzed according to the high-frequency impedance characteristics. Based on the analysis results, a simplified fault current index (SFCI) is proposed to realize the fast evaluation of impact of grid topology on the fault current level. The SFCI is then applied to evaluate the relative fault current level. Finally, the simulation results validate the model, the analysis method, and the SFCI, which can effectively evaluate the relative fault current level in a direct and fast manner.