Full Envelope Wind Turbine Control Systems Design

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Wind turbines are a means of providing clean, safe and abundant energy. There is limited***environmental impact and the operating costs are low. On the other hand, initial capital costs for***wind turbines are high. As such, long life and predictable operating patterns are desirable. This***proposal aims to improve the life and predictability of operation of wind turbines through full***envelope control. Wind turbines operate in two regimes: moderate wind speeds where the goal is to***extract maximum power and higher wind speeds where power is regulated near capacity. In the***transition between moderate and higher wind speeds the system should efficiently and safely move***between regimes. Most large wind turbines are of a variable speed, variable pitch configuration***where two actuations are used: field excitation of the generator stator, governing rotation as a***braking force; and blade pitch, which affects the aerodynamic lift and drag and generated forces.***For lower operating costs, pitching of the blades is avoided as much as possible as this involves***mechanical components requiring lubrication and bearings with finite lifespans. Thus, at moderate***wind speeds only field excitation is used, while in power regulation mode, both blade pitching and***field excitation are employed. Transitioning from one regime to the other involves a change in the***number of actuators, along with changes in control system goals and in dynamic characteristics. In***the transition region, overspeeding of the turbine can result in exceeding rated conditions for***bearings and torques and forces in the blades and tower structure. All of these issues can greatly***reduce the life expectancy and increase maintenance costs.***Promising methods for feedback control of wind turbines include linear parameter varying concepts,***optimized disturbance rejection methods and model predictive control. Further, switched systems***will allow more flexibility in dealing with varying objectives, number of actuators, and varying***dynamics. Work here will consider the extrapolation of these methods to handle transition region***control and full envelope operation of the turbine. Transition between regimes can induce***oscillations in speed and torques and also increase wear on the pitch actuators. Intelligent choices of***control strategy to avoid nonlinear oscillations would be of benefit to practical implementation.***Finding peak power is fraught with difficulty. Varying dynamics due to blade wear and damage,***uncertain power curve data, and complex interaction between turbines and with the environment***can cause errors in estimating parameters for maximum power extraction. Employment of advanced***feedback control techniques as described in this proposal can improve the efficiency, maximum***power extraction and improve life of the turbine.