A Damping Method for Torsional Vibrations in a DFIG Wind Turbine System Based on Small-Signal Analysis

Torsional vibrations present in the drivetrain excited by turbulent winds or grid disturbances (often manifesting as voltage sags) can produce severe stresses on the components of wind turbines. This study presents a damping method for the rotor side converter of a doubly fed induction generator (DFIG)-based wind turbine to improve system damping during small grid disturbances. Within the damping strategy, torsional damper is included into the power loop of the rotor side converter to provide damping for the system. During vibrations, a well-designed low-pass filter (LPF) is employed by the torsional damper to extract the dynamics of the generator speed and to generate a damping signal. To demonstrate the proposed damping approach, the small-signal stability model of a wind turbine connected to an infinite bus system is presented, and the eigenvalue analysis is conducted to verify the damping effect introduced by the torsional damper. In the case study, grid voltage sag at the point of common coupling (PCC) is applied to excite the torsional vibration of the wind turbine shaft. The validity and effectiveness of the proposed damping approach are testified by using the simulation results against theory analysis.