The voltage-source-converter based high-voltage direct current(VSC-HVDC)transmission technology has become an ideal solution for grid-connection of remote offshore wind farms. As the offshore wind generation unit using large-scale wind turbines

the elastic coefficient of each mass of the shaft is larger than that of conventional inland wind generation unit. When the offshore wind farms integrated to the power grid through VSC-HVDC

two subsynchronous oscillations are triggered

i. e.

the torsional vibration of the turbine shaft and the subsynchronous oscillation caused by the interaction between the wind turbine and the VSC-HVDC converter. This paper developes a small signal model for grid-connection of offshore wind farm through VSC-HVDC. Then the two oscillation modes and the corresponding strong correlation state variables are analyzed by the participant factor analysis. On this basis

the root locus curve of the strong correlation state parameter is plotted by the eigenvalue analysis method

and its influence on the electrical damping characteristics of the system is analyzed. Based on the signal test method

this paper proposes a co-suppression method of supplementary excitation damping controllers for DFIG-based wind turbines and VSC-HVDC system with supplementary damping control. The co-suppression method is verified by the simulation on DIgSILENT/PowerFactory.