| Modern wind turbines are experiencing rapid growth in both physical size and rated capacity. As designs grow larger with lighter construction materials, dampening flexible modes with active control becomes more critical for avoiding structural fatigue issues in order to extend the turbine service life. Prior to the year 2000, with few exceptions, industrial wind turbine controllers were designed using single-loop proportional-integral-derivative (PID) techniques with trial-and-error tuning of the loop gains. Structural oscillation was mitigated through stiff turbine designs and controllers were intentionally limited in response bandwidth to avoid excitation of natural frequencies. It is now known that complex wind interactions with the whole structure cause nearly all flexible modes to be excited to some extent during normal operation and flexible turbines tend to exacerbate structural fatigue issues.;This dissertation documents the first known application of the theory of disturbance utilization control (DUC) to the problem of active pitch control in large, flexible wind turbines. Controllers are designed for a mathematical model of a flexible utility-grade wind turbine operating in above-rated winds, and it is shown through simulation that positive "utility" (useful energy) exists in turbulent wind inflow. It is further demonstrated that the optimal DUC controller harnesses this energy to dampen several flexible modes leading to decreased pitch actuator demand and improved structural fatigue life, all while also regulating generator speed.;Comparing results to previously-published disturbance accommodation controllers, it is shown that DUC reduces blade damage from flapping by 22%, drive-train fatigue damage by 14%, total blade actuator motion by 63%, and results in a 60% reduction in maximum blade actuator pitch rate in turbulent environments. |
