The use of induction generators in wind power applications has been common since the early development of the wind industry. Most of these generators operate at fixed frequency and are connected directly to the utility grid. Unfortunately, this mode of operation limits the rotor speed to a specific rpm. Variable speed operation is preferred in order to facilitate maximum energy capture over a wide range of wind speeds. This paper explores variable speed operating strategies for wind turbine applications. The objectives are to maximize energy production, provide controlled start-up, and reduce torque loading. This paper focuses on optimizing the energy captured by operating at maximum aerodynamic efficiency at any wind speed. The control strategy we analyze uses rotor speed and generator power as the feedback signals. In the normal operating region, rotor speed is used to compute a target power that corresponds to optimum operation. With power as the control objective, the power converter and generator are controlled to track the target power at any rpm. Thus, the torque-speed characteristic of the generator is shaped to optimize the energy capture. The target power is continuously updated at any rpm. In extreme areas of the operating envelope, during start-up, shutdown, generator overload, or overspeed, different strategies driven by other system considerations must be used.

Caselitz, P., Grebe, R., Kruger, T., Pischel, K., and Schoo. A., 1993, “Control of Variable Speed Wind Turbines With Superposition Gearbox,” 1993 European Community Wind Energy Conference Proceedings, Mar. 8–12, Lubeck-Travemunde, Germany, pp. 526–529.
Drouilhet, S., Muljadi, E., Holz, R., and Gevorgian, V., 1995, “Optimizing Small Wind Turbine Performance in Battery Charging Applications,” NREL/TP- 441-7806, presented at Windpower ’95, Washington, DC, Mar. 27–30.
Eggleston, D. S., and Stoddard, F. S., 1987, Wind Turbine Energy Design, Van Nostrand Reinhold, New York.
Fardoun, A. A., Fuchs, E. F., and Carlin, P. W., “A variable speed, direct drive transmission wind power plant,” Windpower ’93 Proceedings, San Francisco, CA, pp. 134–141.
Holley, E., 1995, “Speed Control System for a Variable Speed Wind Turbine,” U.S. Patent 5,155,375, Oct. 13.
Lipo, T. A., 1984, “Variable speed generator technology options for wind turbine generators,” DOE/NASA Workshop on Horizontal-Axis Wind Turbine Technology, Cleveland, OH, May 8–10.
Muljadi, E., Flowers, L., Green, J., and Bergey, M., 1995, “Electric Design of Water Pumping With Wind Power,” 14th ASME-ETCE Wind Energy Symposium, Houston, TX Jan. 29-Feb. 1.
Quatron, D. C., 1990, “A Theoretical Investigation of the Impact of Pitch Control on the Fatigue of Wind Turbines,” 1990 European Community Wind Energy Conference Proceedings, Madrid, Sept. 10–14, pp. 533–537.
Schluter, L. L., and Vachon, W. A., 1992, “Advanced Control Stragegies for Wind Turbines,” Windpower ’92 Proceedings, Seattle, WA, pp. 267–275.
Torrey, D. A., and Child, S. E., 1992, “Development of variable-reluctance wind generators,” Windpower ’93 Proceedings, San Francisco, CA, pp. 258–265.
Xu, L., 1993, “A High Efficient Variable Speed Wind-Power Generating System Using Doubly Excited Brushless Reluctance Machine,” Windpower ’94 Proceedings, San Francisco, CA, pp. 266–271.
This content is only available via PDF.
You do not currently have access to this content.