AC Fault Ride-through Control Strategy for Sending End of LCC-MMC Hybrid DC Transmission System

When the grid commutation type converter is adopted on the rectifier side, and the modular multi-level converter adopted on the inverter side in the hybrid DC transmission system, the commutation failure problem will be solved on the traditional DC transmission inverter side. Aiming at the interruption of DC power transmission caused by the AC fault at the sending end of the hybrid DC transmission system, a fault ride-through control strategy based on reducing the AC and DC component of the inverter side bridge voltage is proposed. Based on the analysis of the AC fault characteristics at the sending end, a method is designed to determine the number of the sub-modules on the inverter side according to the degree of voltage drop on the rectifier side AC bus. Relying on the quantitative analysis of the reduced value of the inverter side DC voltage, the sending power transmission capacity of the DC system after a terminal AC failure is maintained. Taking into account the active and reactive power constraints of the converter, a method for setting the DC voltage regulation limit of the inverter station is designed. The method of synchronously regulating the AC and DC components of the converter bridge arm voltage is used to reduce the inverter side DC voltage, which can meet the system's requirements for the modulation ratio, so that the inverter side AC outlet voltage will not be distorted and is suitable for the large voltage dropping on the sending end AC bus. Finally, the effectiveness of the proposed control strategy is verified by the simulation and comparative analysis of the sending-end communication faults with different severity. © 2021, Power System Technology Press. All right reserved.