Research on Control Strategy for DC Single-pole Grounding Fault in MMC-HVDC System

Aiming at the direct current (DC) single-pole grounding fault of the full-bridge modular multi-level converter - high voltage direct current (MMC-HVDC) transmission system, the detailed mathematical model is established. The results show that the positive- and negative-pole DC bus voltages of MMC-HVDC system mismatch the DC components of the MMC upper and lower arm voltages during the DC single-pole grounding fault, which leads to rapid discharge of the arm capacitance. Consequently, the system will suffer serious over current. Therefore, a control strategy is proposed by separately controlling the upper and lower arms of the MMC during the DC single-pole grounding fault. With the proposed method, the DC components of the upper and lower arm voltages will follow the change of the positive- and negative-pole DC bus voltages, which ensures the safe and stable operation of the MMC-HVDC system. In addition, in order to further improve the economic operation capability of the MMC-HVDC system under the DC single-pole grounding fault, an optimized power allocation strategy is proposed. Finally, the simulation results in PSCAD/EMTDC verify the correctness and feasibility of the proposed control strategy and optimized power allocation strategy for the MMC-HVDC system under DC single-pole grounding faults.