Control strategy for offshore wind power HVDC transmission system based on series-connected DRU-MMC topology

This paper investigates control strategies for large-scale offshore wind power high-voltage direct current (OWP-HVDC) transmission systems employing a series-connected diode rectifier unit and modular multilevel converter (DRU-MMC) topology. First, a comprehensive onshore-offshore coordinated reduced-voltage startup strategy is proposed. By leveraging the bidirectional conduction capability of the MMC, this method enables smooth system energization and safe grid connection of WTGs. Furthermore, to enhance steady-state performance under wind power variations, an enhanced DC voltage control strategy is developed for the offshore MMC, supported by a detailed small-signal stability analysis and rigorous validations under DC and AC grid disturbances. A comparative analysis is then conducted among three distinct grid-forming control schemes: fixed AC voltage control, active power control, and the proposed DC voltage control. Simulation studies on the large-scale OWP-HVDC system based on the hybrid converter reveal that fixed AC voltage control fail to properly regulate DC voltages of the hybrid converter, resulting in severe MMC DC voltage ripples of up to 14.4 %. In contrast, the proposed DC voltage control strategy precisely stabilizes the MMC DC voltage, limiting the ripple to a mere 2.9 % while maintaining bus voltage fluctuations within acceptable 1.3 %. These results indicate that DC voltage control is the most robust grid-forming strategy for series-connected DRU-MMC systems, offering superior power allocation and enhanced protection against over-modulation.