Research Papers

Methods for Controlling a Wind Turbine System With a Continuously Variable Transmission in Region 2

[+] Author and Article Information
Andrew H. Rex, Kathryn E. Johnson

 Colorado School of Mines, Golden, CO 80305

J. Sol. Energy Eng 131(3), 031012 (Jul 14, 2009) (8 pages) doi:10.1115/1.3139145 History: Received May 06, 2008; Revised December 23, 2008; Published July 14, 2009

Variable speed operation enables wind turbine systems to increase their aerodynamic efficiency and reduce fatigue loads. An alternative to the current electrically based variable speed technologies is the continuously variable transmission (CVT). A CVT is a transmission whose gear ratio can be adjusted to take on an infinite number of settings within the range between its upper and lower limits. CVT research in wind turbine applications predicts an improvement in output power and torque loads compared with fixed-speed machines. Also, a reduction in the harmonic content of the currents is anticipated by eliminating the power electronics. This paper develops a model that combines a CVT model with the FAST wind turbine simulator for simulating the system’s performance in MATLAB/SIMULINK . This model is useful for control development for a variable-speed wind turbine using a CVT. The wind turbine with CVT is simulated using two controllers: a proportional-integral controller and a nonlinear torque controller of the type commonly used in the wind industry.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Frequency conversion points for different speed control technologies. The fixed-speed turbine (top) can use certain types of induction and synchronous generators and is directly connected to the utility grid. The three variable speed turbines have variable rotational speeds that can be decoupled from the fixed-frequency utility grid via electrical or mechanical conversion.

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Figure 2

Components of a wind turbine system with CVT. The wind turbine blades drive the low-speed shaft, and rotational speed is then stepped up via a gearbox. The output of the gearbox, the CVT shaft, connects to the CVT, which can step up or down the rotational speed by an infinite number of ratios within its limits. The output of the CVT is the generator shaft, which connects to the generator.

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Figure 3

Single-phase equivalent circuit of an induction machine. The figure shows resistances Ri and inductances Xi at various points, as well as the effect of the slip s on resistance R2. For more generator details, see the Appendix.

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Figure 4

Completed CVT wind turbine model. The green subsystem labeled “FAST Nonlinear Wind Turbine” is a SIMULINK block available from NREL (11). The four blocks in the upper left (“CVT Control,” “CVT Speed Input,” “Induction generator,” and “CVT Trq Output”) implement the CVT and induction generator that are the focus of this research.

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Figure 5

Block diagram of the CVT control system. This control scheme assumes knowledge of the wind speed input to compute a reference angular speed ωref. Since the wind speed will not be available in the field, the implementation of this controller can be considered a proof-of-concept for this research, with a realizable controller to be developed in Sec. 4.

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Figure 6

Comparison of PI CVT control with fixed speed operation. Time-series data for the variable-speed (“with CVT”) and fixed-speed (“without CVT”) operations are shown for wind velocity, turbine radial velocity, electric power, power coefficient Cp, TSR, and load torque.

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Figure 7

Load torque versus time for fixed-speed operation (“No CVT”), and two types of variable-speed control (PD and PI). Note that the nonzero derivative gain in the PD controller increases fatigue load standard deviation by 78.8% compared with the PI controller.

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Figure 8

Comparison of PI CVT control with fixed speed operation for a simulation using wind input file 10C. In this case, CVT-enabled variable-speed control allowed for a 27.5% improvement in energy capture compared with fixed-speed operation, but also led to an increase in load torque variations.

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Figure 9

Nonlinear torque control-based CVT control. This control scheme is implementable in the field because it does not require wind speed measurement. Like the controllers presented in Sec. 3, CVT-enabled variable speed operation results in an increase in energy capture (in this case, 4.8% more than fixed-speed operation for wind input file 8C).




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