Multi-Degree-of-Freedom Synergistic Control Method for VSC-HVDC Integrated Offshore Wind Farm Under Severe Onshore Disturbance

Severe onshore disturbances, such as rejection action of main protection under the onshore grid fault, lead to massive power imbalances in the voltage source converter-based high voltage direct current (VSC-HVDC) integrated offshore wind farm for a long duration. Existing methods are conditioned on conventional AC faults, which cannot handle the conflict between DC voltage security and collector bus voltage limitation caused by the severe onshore disturbance, leading to integrated system outage or excessive wind turbine withdrawal. A novel theory of multi-degree-of-freedom synergistic control based on the quantity adjustment of offshore-connected wind turbines and the AC bus voltage reduction of the offshore-end converter is proposed, and the multi-degree-of-freedom controllable range (MCR) for the integrated system is established. The attenuation characteristic of MCR is found, and the calculation methods of the minimum tolerable active power of the grid-end converter and the maximum tolerable disturbance duration, which avoid DC overvoltage and the forced tripping of wind turbines, are proposed. Finally, the multi-degree-of-freedom synergistic control method is proposed. As verified by simulations, the proposed method can significantly enhance the uninterrupted operation capability of the integrated system and maximize the quantity of offshore-connected wind turbines after disturbance to facilitate onshore grid restoration.