Decoupled Distributed Control of Offshore Wind Farms Connected to DR-HVDC Based on Novel Adaptive Virtual Impedance

In offshore wind farms connected via diode-rectifier-based high-voltage dc, conventional distributed control strategies have significant coupling among active and reactive power, leading to large oscillations in wind turbine (WT) converter power, voltage, and grid frequency during transients. In this work, we propose a decoupled distributed control strategy based on a novel adaptive virtual impedance (VI) method. In detail, we newly propose a simple estimator to enable each WT controller to locally estimate the global average reactive power, without the need for communication and line impedance knowledge. By comparing the estimated global average value with the actual reactive power, we achieve dynamically adjustable VI, thereby completely decoupling the power control. In addition, a proportional feedforward control is integrated into the estimator, ensuring precise reactive power-sharing among WT converters. We also develop a small-signal model of the proposed method to theoretically confirm the decoupling characteristics, and to facilitate system stability analysis and key control parameters design. Experimental results validate the superior performance of power and frequency regulation during both steady and transient states, in comparison with the existing decoupling control method.