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.