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Some preliminary results from the NWTC direct-drive, variable-speed test bed

Description: With the remarkable rise in interest in variable-speed operation of larger wind turbines, it has become important for the National Wind Technology Center (NWTC) to have access to a variable-speed test bed that can be specially instrumented for research. Accordingly, a three-bladed, 10-meter, downwind, Grumman Windstream machine has been equipped with a set of composite blades and a direct-coupled, permanent-magnet, 20 kilowatt generator. This machine and its associated control system and data collection system are discussed. Several variations of a maximum power control algorithm have been installed on the control computer. To provide a baseline for comparison, several constant speed algorithms have also been installed. The present major effort is devoted to daytime, semi-autonomous data collection.
Date: October 1, 1996
Creator: Carlin, P.W. & Fingersh, L.J.
Partner: UNT Libraries Government Documents Department

Analytical expressions for maximum wind turbine average power in a Rayleigh wind regime

Description: Average or expectation values for annual power of a wind turbine in a Rayleigh wind regime are calculated and plotted as a function of cut-out wind speed. This wind speed is expressed in multiples of the annual average wind speed at the turbine installation site. To provide a common basis for comparison of all real and imagined turbines, the Rayleigh-Betz wind machine is postulated. This machine is an ideal wind machine operating with the ideal Betz power coefficient of 0.593 in a Rayleigh probability wind regime. All other average annual powers are expressed in fractions of that power. Cases considered include: (1) an ideal machine with finite power and finite cutout speed, (2) real machines operating in variable speed mode at their maximum power coefficient, and (3) real machines operating at constant speed.
Date: December 1, 1996
Creator: Carlin, P.W.
Partner: UNT Libraries Government Documents Department

Damage measurements on the NWTC direct-drive, variable-speed test bed

Description: The NWTC (National Wind Technology Center) Variable-Speed Test Bed turbine is a three-bladed, 10-meter, downwind machine that can be run in either fixed-speed or variable-speed mode. In the variable-speed mode, the generator torque is regulated, using a discrete-stepped load bank to maximize the turbine`s power coefficient. At rated power, a second control loop that uses blade pitch to maintain rotor speed essentially as before, i.e., using the load bank to maintain either generator power or (optionally) generator torque. In this paper, the authors will use this turbine to study the effect of variable-speed operation on blade damage. Using time-series data obtained from blade flap and edge strain gauges, the load spectrum for the turbine is developed using rainflow counting techniques. Miner`s rule is then used to determine the damage rates for variable-speed and fixed-speed operation. The results illustrate that the controller algorithm used with this turbine introduces relatively large load cycles into the blade that significantly reduce its service lifetime, while power production is only marginally increased.
Date: December 31, 1998
Creator: Sutherland, H.J. & Carlin, P.W.
Partner: UNT Libraries Government Documents Department

Results from the NREL Variable-Speed Test bed

Description: The NREL Variable-Speed Test bed turbine has been used to examine the performance and controllability of a variable-speed, variable-pitch turbine. Control strategies that eliminate drive-train torque fluctuations in high winds have been published before and example data are given here. The energy capture of a variable-speed wind turbine depends in part on its ability to successfully operate at the peak of the C{sub p}-{lambda} curve. The losses associated with the inability of the rotor to stay exactly on top of the curve at all have been found and quantified. New control strategies for improving energy capture in moderate winds are also proposed. The potential exists to improve overall energy capture by 5% or more.
Date: November 1, 1997
Creator: Fingersh, L.J. & Carlin, P.W.
Partner: UNT Libraries Government Documents Department

The History and State of the Art of Variable-Speed Wind Turbine Technology

Description: The National Renewable Energy Laboratory's (NREL's) National Wind Technology Center (NWTC), directed by the Department of Energy (DOE) is pursuing several research projects in variable speed. In the near future the laboratory will be reevaluating its ongoing experiments and plans. The starting point for this reevaluation will be a sound understanding of the current state of the art in design and application of variable-speed technology. This report, which outlines current technologies and historical applications of variable-speed, will provide a baseline in deciding the course of research in the upcoming years.
Date: March 9, 2001
Creator: Carlin, P.W.; Laxson, A.S. & Muljadi, E.B.
Partner: UNT Libraries Government Documents Department

Circle diagram approach for self excited induction generators

Description: When an induction generator is connected to a utility line supply, the voltage and frequency at the terminal output are the same as the voltage and frequency of the utility line supply to which the generator is connected. The reactive power needed by the induction generator is supplied by the utility and the real power is returned to the utility. With a fixed frequency dictated by the utility, the induction machine starts generating above the synchronous speed. The range of speed is also limited by the slip. At a very high slip, the copper losses increases as the current increases. On the other hand, in an isolated operation, the induction generator operates in self-excitation mode. It determines its own voltage and frequency. These two quantities depend on the size of the AC capacitor, the induction machine parameters, the electrical load, and the speed of the generator. The operating speed of the induction generator is extended without generating excessive loss. This paper presents an analytical study by utilizing a circle diagram to illustrate the operation of the induction generator in isolated operation. The steady-state calculations are presented to support the analysis. Possible applications for the system in variable-speed generation are currently under investigation. The output can be directly connected to equipment that is non-sensitive to the frequency (a heater, battery charger, etc.) or can be connected to a converter to get a fixed-frequency AC output.
Date: May 1, 1993
Creator: Muljadi, E.; Carlin, P. W. & Osgood, R. M.
Partner: UNT Libraries Government Documents Department