An Improved Start-Up Strategy for the DR-MMC Hybrid HVDC Topology Applied in Offshore Wind Power Transmission Systems

The hybrid HVDC topology based on a series connection of diode rectifiers and a modular multilevel converter (DR-MMC) has emerged as a promising solution for offshore wind power transmission (OWPT). Developing an effective startup strategy for this topology is crucial for its practical application in OWPT systems. However, existing strategies either require additional equipment such as power supply cables, full-bridge submodules, and dc circuit breakers, or expose significant safety and reliability concerns. This article develops an improved startup strategy for the DR-MMC applied in OWPT systems. Firstly, a zero dc-link current control is introduced after starting wind turbines, allowing the replacement of dc circuit breakers with more cost-effective bypass switches. Secondly, a coordinated control of submodule capacitor voltage is proposed to prevent both overvoltage and excessive discharge before DRs conduct. Thirdly, a smooth DR conducting approach, which involves first increasing and then gradually decreasing the dc-link voltage, is employed to avoid potential surge currents. The proposed startup strategy combines the advantages of low cost, high safety, and sufficient reliability, which cannot be achieved simultaneously with conventional ones. Both simulation and experimental results validate the effectiveness of the proposed strategy.