Receiving-end AC faults in line-commutated-converter-based high-voltage direct-current (LCC-HVDC) systems may trigger commutation failures (CFs) and cause sending-end transient overvoltage (TOV) during recovery. Existing work emphasizes rectifier reactive power control and DC current recovery, but the DC current undershoot from constant current controller over-regulation and its effect on sending-end TOV are not well studied. This paper analyzes the coupling among DC current, rectifier reactive consumption, AC filter reactive output, wind farm reactive response, and sending-end bus voltage during CFs. It reveals that the current undershoot reduces rectifier reactive consumption, aggravating the sending-end reactive surplus and elevating the peak TOV. A TOV assessment method incorporating the dynamic reactive response of wind farms is developed, along with a voltage-threshold-constrained DC current undershoot suppression strategy (VTC-DUS). The VTC-DUS calculates a current lower limit from a preset voltage threshold to form a modified DC current order, and adjusts the voltage-dependent current order limiter characteristic by the deviation between the modified and actual currents to suppress the excessive current drop. PSCAD/EMTDC simulations indicate that the peak TOV errors of the proposed method are below 0.01 pu for three-phase-to-ground faults and 0.03 pu for single-phase-to-ground faults, and that VTC-DUS effectively reduces the sending-end peak TOV under various fault conditions.

Given the wide application of DC grids, the protection equipment of power grids must be improved during the fault period. This study proposes an integrated multiport flexible voltage clamp circuit breaker with a DC chopper acting on the receiving end converter to solve the DC short circuit fault and surplus power because of AC low-voltage fault; it has a modular design. As a DC circuit breaker, the device utilizes the voltage-clamping principle and thyristor semi-control to remove faults. As the fault current increases, the branch circuit outputs different voltage levels by selecting different gears, thereby controlling the voltage-clamping effect. This device can distinguish between different fault types to prevent secondary shocks in the system. As a DC chopper, the voltage at both ends of the energy dissipation resistor is varied by switching submodules, consuming surplus power to complete AC low-voltage faults and minimizing the impact of low-voltage faults on the system’s transmission capacity. Finally, the effectiveness and applicability of the equipment are verified using wind turbines connected to a flexible DC transmission three-terminal power grid model in PSCAD/EMTDC, and two fault simulation types are analyzed. A comparison of the electrical quantities (fault current, system voltage and branch voltage) of the proposed circuit breaker with other similar equipment shows that due to the efficiency of the proposed equipment, the peak fault current is reduced by at least 35.8%. The required voltage stress of key power electronic equipment is reduced by at least 71.5%. Therefore, the equipment ensures that the per-unit voltage of the DC system does not exceed 1.05 during AC fault crossing.