A Multi-level Grid-forming Control Strategy for Offshore Wind Farm Integration via HVDC for System Frequency Support

The High Voltage Direct Current (HVDC) system is one of the important ways to achieve deep sea wind power collection and grid connection. However, offshore wind farms which connected to the grid through HVDC are difficult to perceive grid frequency fluctuations and often rely on frequency information transmission to participate in grid frequency regulation, which puts high demands on the timeliness and accuracy of signal transmission. Therefore, this article proposes a multi-level grid-forming control strategy for offshore wind farm integration via HVDC for system frequency support, in which both offshore wind turbines and receiving end converter stations adopt grid-forming control. The sending end converter station controls the DC voltage of the HVDC system, allowing the offshore wind farm to directly participate in maintaining the energy balance of the HVDC system and improving its response speed to changes in grid frequency. Under this strategy, the receiving converter station behaves as a synchronous machine with inertia to the grid, and can spontaneously provide frequency support to the power grid. A simulation model of an offshore wind farm connected to the grid through HVDC was built based on MATLAB/Simulink software. The stability and inertia response characteristics of the wind farm connected system under this control strategy were analyzed to verify the effectiveness of the proposed method.