Yaw control for reduction of structural dynamic loads in wind turbines

The potential for active attenuation of structural dynamic load oscillations, by means of continuous control of the yaw servo, is investigated. The mathematical models are obtained from the equations of motion corresponding to a system of flexibly interconnected rigid bodies. The study involves two structural dynamic modes: the tower bending, particularly in the lateral direction, and the motion of a turbine with a teetered hub. In both cases the turbine has two blades. The results show the importance of the yaw stiffness for the tower bending. Furthermore, in the particular case studied here, a periodic LQ controller could achieve the same lateral damping as a suspension system with a spring and damper, or PD controller, with less than 10% of the control power. The results clearly indicate the importance of considering the systemʹs periodic dependence on time in the controller design. This is not equally important when actively reducing the teeter motion. However, in this case the aerodynamic damping must be relatively low in order to be able to accomplish a significant reduction.