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Research Papers

Disturbance Observer-Based Pitch Control of Wind Turbines for Enhanced Speed Regulation

[+] Author and Article Information
Yuan Yuan

Department of Mechanical Engineering,
University of Connecticut,
Storrs, CT 06269

X. Chen

Assistant Professor
Department of Mechanical Engineering,
University of Connecticut,
Storrs, CT 06269

J. Tang

Professor
Department of Mechanical Engineering,
University of Connecticut,
Storrs, CT 06269
e-mail: jtang@engr.uconn.edu

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received May 5, 2016; final manuscript received December 29, 2016; published online May 9, 2017. Assoc. Editor: Ryozo Nagamune.

J. Dyn. Sys., Meas., Control 139(7), 071006 (May 09, 2017) (10 pages) Paper No: DS-16-1233; doi: 10.1115/1.4035741 History: Received May 05, 2016; Revised December 29, 2016

Time-varying unknown wind disturbances influence significantly the dynamics of wind turbines. In this research, we formulate a disturbance observer (DOB) structure that is added to a proportional-integral-derivative (PID) feedback controller, aiming at asymptotically rejecting disturbances to wind turbines at above-rated wind speeds. Specifically, our objective is to maintain a constant output power and achieve better generator speed regulation when a wind turbine is operated under time-varying and turbulent wind conditions. The fundamental idea of DOB control is to conduct internal model-based observation and cancelation of disturbances directly using an inner feedback control loop. While the outer-loop PID controller provides the basic capability of suppressing disturbance effects with guaranteed stability, the inner-loop disturbance observer is designed to yield further disturbance rejection in the low frequency region. The DOB controller can be built as an on–off loop, that is, independent of the original control loop, which makes it easy to be implemented and validated in existing wind turbines. The proposed algorithm is applied to both linearized and nonlinear National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine models. In order to deal with the mismatch between the linearized model and the nonlinear turbine, an extra compensator is proposed to enhance the robustness of augmented controller. The application of the augmented DOB pitch controller demonstrates enhanced power and speed regulations in the above-rated region for both linearized and nonlinear plant models.

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Figures

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Fig. 1

Structure of the DOB-based control

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Fig. 2

Loop shaping of DOB

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Fig. 3

Wind disturbance and generator speed responses (under 5DOF linearized model): (a) stepwise wind disturbance and (b) comparison of time-domain responses of DOB and PID

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Fig. 4

Frequency-domain generator speed performance comparison of DOB and PID under 5DOF-linearized model and random wind field

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Fig. 5

Bode diagram of Q filter under different β

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Fig. 6

Zoom-in view of generator speed responses. REF refers to the rated generator speed.

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Fig. 7

Comparison of magnitude responses of sensitivity functions

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Fig. 8

Time-domain performance comparison of DOB, DOB*, and GSPI: (a) wind speed (18 m/s turbulent field), (b) generator speed, (c) power, and (d) pitch angle. : GSPI, : DOB, : DOB*

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Fig. 9

Frequency-domain performance comparison of DOB, DOB*, and GSPI, under 18 m/s turbulent wind file: (a) overall performance and (b) zoom-in view at low-frequency region

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Fig. 10

Generator speed error performance comparison of DOB, DOB*, and GSPI. The generator speed errors of DOB and DOB* are normalized with respect to the error of GSPI.

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Fig. 11

Comparisons of RMS pitch rate (a), average power (b), low-speed shaft torque moment DEL (c), blade root edgewise moment DEL (d), blade root flapwise moment DEL (e), tower base side-to-side moment DEL (f), and tower base fore–aft moment DEL, and (g) of GSPI and DOB*. : GSPI, : DOB*

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