Study On Torsional Vibration Damping Using Power Converters For The Variable Speed Wind Turbine System

The abnormal operation events of modern grid-connected wind farms are growing with their scale continuously increasing. In these events, the wind farm active power oscillation at a lower frequency than the rated one, which are commonly caused by the grid disturbances and faults, have caused a wide interest of academic and engineering. Recent research shows, these active power oscillations are considered be associated with the mechanical dynamics of the shaft system. It eventually appears in the form of mechanical torque acting on the shaft, i.e. torsional vibrations, and could potentially lead to damage of wind turbines shafting mechanical structure. Although the damping situation can be improved mechanically, e.g. through suitably designed couplings with rubber material, there is a cost problem related to it. Another method developed in this thesis, is to modify the control algorithm of the power converters within the variable speed wind turbine system, to provide an auxiliary damping for the system. It is cost effective and easy-designed. Therefore, the main work of this thesis is as followings.Firstly, the cases of wind turbine torsional vibrations as well as the current research of vibration damping method are illustrated in China and abroad. And then, the damping algorithm based on the rotor side converter is developed, taking doubly fed induction generator as an example in this thesis. To verify the damping strategy, small-signal model of the overall system is built and the modal analysis as well as the spectrum analysis are conducted. In addition, various time-domain simulation cases are considered to confirm the vibration damping method. Finally, the STATCOM and Energy Storage System are employed to improve the system damping. The mathematic model and damping control strategies of two FACTS devices are developed respectively. The simulation results indicate that the control methods can improve the damping property of the wind turbine system; at the meanwhile the torsional vibrations are effectively suppressed.