Date of Completion

9-28-2018

Embargo Period

9-27-2020

Major Advisor

Jiong Tang

Associate Advisor

Chengyu Cao

Associate Advisor

Xu Chen

Associate Advisor

Richard Christenson

Associate Advisor

Horea Ilies

Field of Study

Mechanical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Wind energy is commonly recognized as a major environmentally friendly renewable energy source. Modern wind turbines are large, flexible structures that may suffer from reduced life owing to extreme loads and fatigue when operated under highly turbulent wind field. In this dissertation, advanced controllers are synthesized to reduce overall cost of wind energy production by regulating power capture and at the same time decreasing the structural loading to enhance the durability of turbine components.

First, collective pitch control (CPC) methods are investigated to regulate speed and power, and to mitigate symmetric loads in high wind speeds. A new adaptive control strategy is formulated for the pitch control of wind turbine, aiming at making a trade-off between the maximum energy captured and the load induced. The adaptive controller is designed to both regulate generator speed and mitigate component loads under turbulent wind field when blade stiffness uncertainties exist. It is shown that the blade root flapwise load can be reduced at a slight expense of optimal power output. In order to achieve better power regulation in above-rated wind speeds, a disturbance observer (DOB) structure that is added to a proportional-integral-derivative (PID) feedback controller is formulated, aiming at asymptotically rejecting disturbances to wind turbines. The augmented DOB pitch controller demonstrates enhanced power and speed regulations in the above-rated region. When large-scale wind turbines operate in turbulent wind fields, periodic loads on blades are induced by wind shear, tower shadow effects and centrifugal forces. While collective pitch control (CPC) is unable to deal with periodic loads, the advent of individual pitch control (IPC) provides opportunities to mitigate periodic loads. A multivariable robust individual pitch control framework to reject periodic loads under model uncertainties is then developed in this research. The robust structured singular values (μ)-synthesis approach can reduce response peaks at high harmonic frequencies and guarantee the robust stability and robust performance with respect to uncertainties. With the proposed IPC strategy, one can achieve significant periodic load mitigation as well as fatigue alleviation in speed-varying wind fields.

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