Research on hierarchical coordinated variable-gain frequency regulation strategy for wind farm MMC-LCC grid-connected system

• A hierarchical synergetic-MPC based variable-gain control strategy is designed for wind farm MMC-LCC grid-connected system to realize coordinated frequency regulation under communication-free mapping of frequency disturbance. • The DC voltage variable-gain regulation strategy is proposed for receiving-end converter station to achieve the inertia support and coordinated frequency regulation of the receiving-end grid based on the synergetic control. • The synergetic-MPC based variable-gain frequency regulation strategy is proposed for wind farm utilizing pitch angle reserve capacity to coordinate with the receiving-end grid frequency regulation and DC voltage restore. The MMC-LCC hybrid HVDC transmission system, composed of Line Commutated Converters (LCC) and Modular Multilevel Converters (MMC), has found applications in HVDC grid-connection of wind farm due to its characteristics such as economic efficiency in long-distance and large-capacity power transmission and the advantage of being suitable for wind power external transmission in weak AC systems. To achieve wind farm DC transmission providing active frequency regulation support, based on the frequency droop control for the constant voltage of the receiving-end LCC converter station, synergetic control is adopted to design a droop gain control strategy that utilizes DC capacitor energy storage for frequency support, and an active power command control strategy for frequency regulation coordinated by wind farm operating under limited power. Furthermore, considering the operation state differences of wind turbines, with the optimization objectives of reducing the pitch adjustment amplitude of wind turbines and improving the accuracy of the frequency regulation active power of wind farm in tracking the active power adjustment command, a variable-gain strategy for coordinated frequency regulation of wind turbines in different operation states is designed using model predictive control. Simulations verify the effectiveness of the proposed hierarchical variable-gain coordinated frequency regulation strategy. The simulation results also indicate that this strategy can improve the frequency regulation dynamic characteristics of the receiving-end power grid and has the advantage of optimizing the dynamic recovery process of the DC capacitor voltage in coordination with the wind farm.