The frequency fluctuation of the offshore wind power generation system based on MMC-HVDC will affect the normal operation of the healthy wind plants when fault occurs or in other transient conditions. In serious cases

large-scale off-grid wind turbines may take place. For this reason

we analyze the relationship between frequency and d-axis and q-axis of the AC voltage in offshore wind power system

and propose a frequency control scheme. A frequency loop is introduced into the offshore converter controller to stabilize the frequency of the bus voltage. Considering the distinctions of frequency in different locations

a scheme of frequency cooperative optimization control is proposed to further optimize the frequency fluctuation at the wind turbine collector substations. Finally

a simulation model of offshore wind power plants based on MMC-HVDC was built on the MATLAB/Simulink. The research results show that

with cooperative optimization control

the frequency fluctuation in wind power system bus voltage and wind turbine collector substations can be reduced to less than 30% of that without control in conditions of system fault and abrupt change of operation. The proposed method can be adopted to effectively suppress the frequency fluctuation of wind power generation system based on MMC-HVDC in transient conditions.