Modeling and Control of Large and Flexible Wind Turbines in Variable Speed Mode

As wind turbines are increased in size and power, flexible structure of wind turbines is becoming increasingly important and cannot be neglected. In this paper a four-mass drive-train model is presented by considering flexible structure modes of wind turbines. Due to the complexity of the four-mass model of the drive train, the focus of this work is on the design of nonlinear variable speed controllers for a reduced-order, two-mass model of the drive train. Both the state feedback controller and the measurement feedback controller (i.e., using the information of rotor speed only but not the torsional angle) are proposed to achieve asymptotic tracking control for prescribed rotor speed reference signals, thus capturing maximum wind power. Simulation results are provided to demonstrate the effectiveness of the proposed control schemes. Generalizations of our control system design methods to larger and flexible wind turbines with high-order drive train models are currently under investigation and will be reported elsewhere.