Design of a Gain Scheduling Pitch Controller for Wind Turbines by Using the Bode Diagram

For variable speed wind turbines (WT) operating in the full load range, a collective pitch control is used to limit the speed to its nominal value. The concept of a nonlinear PI gain scheduling controller is particularly suitable for compensating the nonlinear rotor behavior and is widely used in the wind industry. However, a simple calculation method to calculate its controller parameters while considering the full dynamics of the control loop is not available yet. In this paper the linearization process as well as a simple calculation method to find suitable controller parameters based on methods of classical control engineering is introduced. In contrast to previous methods the full dynamics of the open loop system, in particular the dynamics of the controlled pitch servomotors and the dynamics of the drive train with its first eigenfrequency, can be considered within the controller design. This is done by analyzing the behavior of the gain scheduling control and deriving a simplified linearized model of the open loop system. In this way it is possible to optimize the controller parameters by using the bode diagram. The usability of the simple design method is shown on the basis of simulation results considering the influence of wind fluctuations as well as wind gusts.