Frequency Regulation of VSC-MTDC System with Offshore Wind Farms

Frequency control of the voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) system integrated wind farms enhances frequency stability by compensating power for the disturbed AC system. However, it is hard to reasonably allocate frequency regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system. Moreover, it is difficult for the frequency control of the wind farms to cope with the change in wind speed, so the risk of wind turbine stalls is high. Therefore, based on the kinetic energy of the turbines and the power margin of the converters, the frequency regulation capability of wind turbines is evaluated, and a dynamic frequency support scheme considering the real-time frequency support capability of the wind turbines and the system frequency evolution is proposed to improve frequency support performance. Subsequently, a power control scheme, adapting to wind speed change is developed, and the active power during the frequency support stage and restoration stage is switched according to the wind speed. Finally, a hierarchical and zoning frequency regulation system is designed to utilize the frequency regulation resources of different links in the MTDC system integrated wind farms. The simulation results show that the novel frequency responsive strategy maintains frequency stability after the wind speed changes and avoids multiple frequency dips.