Coordinated Control Strategy for Sending-End Converters in Wind-Integrated Real Bipolar MMC-HVDC Systems

[Objective] Modular multilevel converter based high voltage direct current （MMC-HVDC） technology with a real bipolar connection is one of the mainstream technical solutions for efficient grid connection and cross-regional consumption of large-scale islanded wind farms. It has been widely adopted in HVDC projects such as Zhangbei and Rudong in Jiangsu. However，the control strategy for the sending-end positive and negative pole converters faces the dual challenge of balancing bipolar transmission power and providing stable voltage support for the wind farm. To address this，a grid-forming coordinated control strategy for bipolar converters is proposed，which combines an active power-frequency boost scheme with a reactive power-voltage droop scheme. [Methods] The coupling mechanism among active power，phase，and frequency at the sending-end bipolar converters is analyzed. An active power-frequency boost control strategy is established，clarifying the power balance and frequency coordination of bipolar converter under wind power fluctuation and the adaptive regulation mechanism under capacity-limited conditions. The coupling relationship between the voltage and reactive power of the sending end converters and wind farm is revealed. A reactive power-voltage droop control strategy with capacity and active power constraints is established. The coordinated stability mechanism of AC bus voltage under reactive power fluctuation is investigated. [Results] Simulation results demonstrate that the proposed strategy can effectively maintain dynamic power balance between the bipolar converters and provide stable voltage and frequency support for the AC system under various conditions，including system power fluctuations and single-pole capacity limitations. [Conclusions] The grid-forming coordinated control strategy proposed can effectively address the core challenges of power balance and voltage and frequency support for the sending-end bipolar converters in islanded wind farm MMC-HVDC systems under complex operating conditions，thereby improving the operation stability of the system.