Start-up Strategy for DR-MMC Paralleled Hybrid HVDC Integrated with Offshore Wind Power

Due to the advantages of small size, low cost and simple operation control, diode rectifier (DR) based HVDC scheme is one of most competitive options for grid integration and transmission of large offshore wind farms. However, DR scheme faces the challenges of requiring the islanded operation capability of offshore AC wind farms. To synthesis the advantages of modular multilevel converter (MMC) and diode rectifier (DR), this paper proposes a hybrid HVDC system composed of paralleled MMC and DR in power sending end integrating with offshore wind farms. The proposed system can realize islanded system start-up and operation without storage equipment installation and wind turbines (WTs) control strategy modification. The hybrid system establishes the AC voltage at sending end through offshore MMC, and realizes the gridconnected operation of part WTs at first. Then, with the growth of sending end AC bus voltage, DR is activated for the rest wind power transmission. The start-up strategy for each type-IV WTs is also shown to provide detailed process of WTs integration. The PSCAD/EMTDC simulation of a type-IV wind farm connected to a ±200 kV/800 MW hybrid HVDC system is performed, and the results verify the effectiveness of the startup control strategy.