In order to improve the grid-connected efficiency of the large scale offshore wind farm, HVDC was used widely. However, when the wind turbines and wind generators are two-mass or multi-mass shaft model, the control mode of power electronic equipment at rectifier may introduce negative damping to nearby generators and cause torsional vibration named subsynchronous torsional interaction. This phenomenon may lead to system instability. This paper gets electrical damping coefficient curve base on complex torque coefficient approach firstly, and works out the phase lag between the electromagnetic torque and the rotate speed at nature frequency of the shaft. Then a supplementary subsynchronous damping controller is designed to damping the oscillation between shaft masses. The effectiveness of the controller can be verified through time domain simulation and eigenvalue analysis. The simulation results showed that the oscillation amplitude could decreased quickly when the controller was added to the system, which provides a theoretical basis when the offshore wind farms are connected to the onshore grid through a HVDC transmission line.
