Hybrid Flexible HVDC System and Its Control Strategy for Isolated Renewable Energy External Delivery

High-capacity long-distance HVDC is indispensable for the centralized integration of deep-sea offshore wind farms and sandy-region renewable energy bases. Hybrid flexible HVDC, which adopts voltage-source converters at the sending end and current-source converters at the receiving end, meets the technical needs and potentially offers better economic efficiency in terms of converter station capital cost. However, technical challenges remain, including passive sending-end charging and start-up, joint grid formation of multiple converters, commutation failure immunity at the receiving end, and minimum continuous current maintenance. This paper proposes a hybrid flexible HVDC system scheme along with its control strategy. The scheme employs modular multilevel converters (MMCs) at the sending end and hybrid commutated converters (HCCs) at the receiving end, and a small-capacity uncontrolled diode rectifier (UDR) is configured in parallel with the HCCs. Operating characteristics of the three types of converters are analyzed, and the overall system operating strategy is presented. Electromagnetic transient simulation results show that the UDR provides a system charging circuit and can automatically supply continuous current when renewable generation power fluctuates; for bipolar HVDC or hierarchical HVDC, virtual synchronous generator control of the MMCs enables the joint grid formation and realizes the power balance between multiple converters; for faults in the receiving grid, the HCCs are able to actively turn off the valves at risk of commutation failure, ensuring a smooth ride-through.