Coordinated frequency control of sending-end grids using over-speed deloading of wind turbines and DC transmission systems

To address the problems of limited power reserve and difficult high-frequency control in sending-end grids with a high penetration of renewable energy, a coordinated frequency control strategy based on wind turbine overspeed deloading and DC frequency limit controller (FLC) is proposed. Firstly, a single-machine equivalent model is established by considering the participation of wind turbines and the DC system in grid frequency control. Then, dynamic response indices of frequency stability are obtained through time-domain analytical calculation. Next, the virtual inertia coefficient and droop coefficient of the wind turbine overspeed deloading control, together with the control parameters of the DC FLC, are optimally tuned with the objective of improving the overall dynamic frequency response. After that, different coordinated frequency control strategies are designed for different disturbance power levels. Finally, the effectiveness of the proposed strategy is verified through case studies. It is shown that the proposed strategy can fully exploit the frequency regulation potential of wind turbines, effectively reduce frequency deviations in the sending-end grid under abnormal operating conditions, and improve the frequency stability of the sending-end grid.