Frequency Regulation of VSC-MTDC System with Offshore Wind Farms

Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stabili-ty by compensating the power for a disturbed AC system.How-ever,it is difficult 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 manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind 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 system frequency evolution is pro-posed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the fre-quency-regulation resources of different links in the MTDC sys-tem with wind farms.The simulation results show that the nov-el frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.