Torque and pitch angle control for variable speed wind turbines in all operating regimes

In this paper, a multivariable control strategy is proposed for variable speed wind turbines operating in the two primary regimes, below-rated and above-rated wind speeds. A torque controller, based on the achievement of zero speed-tracking error, is developed to follow a trajectory that allows the wind turbine to operate with maximum power extraction in below-rated regime. The turbine torque is unknown to the controller, and its effect on it is compensated by estimation. In above-rated wind speed regime, a pitch angle controller is added to maintain a constant rated power, while the torque controller regulates the rotor speed to follow a constant nominal speed. The robustness of the control strategy is guaranteed through the incorporation of the turbine torque estimator. Simulation results demonstrate the effectiveness of the proposed control system for wind turbine operating in all wind speed regimes.