Frequency Support Method for Offshore Wind Power VSC-HVDC System Based on Dual-Terminal Symmetric Coordinated Inertial Synchronization Control

Large-scale offshore wind power voltage source converter based high voltage direct current systems (VSC-HVDC) can regulate the power of wind turbines through the coordinated operation of dual-terminal converter stations, and improve the frequency regulation capability of the onshore power grid. In order to solve the problem of poor coupling between the output power of offshore wind farms (WFs) and the frequency of the onshore power grid while ensuring the stability of the DC bus voltage of the dual-terminal converter station, this paper proposes a dual-terminal symmetric coordinated inertial synchronization control method, which transmits the frequency of the grid with a dual-terminal symmetric control structure in the frequency-voltage correspondence. A non-communication DC voltage estimator is designed in the offshore converter station to estimate the change in the onshore DC voltage, with which the frequency of the onshore grid to the AC grid of the offshore WFs can be accurately restored. Then the offshore WFs can provide active frequency support by responding to this change. Simulation results conducted by PSCAD/EMTDC software verify the effectiveness of the proposed control method.