Research on hybrid multi-terminal high-voltage DC technology for offshore wind farm integration

Multi-terminal high-voltage DC (MTDC) technology is a promising way to transmit large amounts of offshore wind power to the main grids. This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations. A hybrid four-terminal HVDC system comprising two onshore line commutated converters (LCCs) and two voltage source converters (VSCs) connecting an offshore wind farm is constructed in PSCAD/EMTDC. A coordination control scheme based on the VSCs' AC voltage control and the LCCs' DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids. The operational characteristics of the system are analyzed. In addition, a black start-up method without any auxiliary power supply for the VSCs is proposed. The transmission scheme is tested through simulations under various conditions, including start-up, wind speed variation, and the disconnection of one VSC or of one LCC.