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

ABSTRACT
Early wind turbines used for performing mechanical work (pumping, grinding and cutting) optimized aerodynamics by being allowed to run at variable speed. Some of the earliest DC electric wind turbines were allowed to run at variable speed. With the advent of gridconnected AC turbines, rotational speeds were limited in order to control the wind turbine AC frequency output to equal the grid frequency. With the advent of semiconductor devices, attempts began as early as the 1970s to allow variable-speed operation of large-scale turbines. The introduction of a newgeneration of high-voltage, high-speed power electronic components allows awide range of variable-speed operation for very-large-scalemachines. Over the past 30 years a number of designs have been tested, a few of which have entered commercialoperation.Anumberof these designsandtheirhistories are described.Adetailed description of a wide range of electrical methods for allowing variable-speed operation is provided.

Introduction
The earliest horizontal-axis windmill to use the principles of aerodynamic lift instead of drag may have been introduced in the 12th century. These horizontal-axis sail turbines were allowed to run at varying speeds, limited only by braking or furling to control their speed during storms. This behaviour occurred naturally, and for most uses a particular speed was unimportant. These designs operated throughout Europe and in the Americas into the present century. In the 700 or so years since the first sail wing turbine, craftsmen discovered many of the practical structural and operational rules without understanding the physics behind them. It was not until the 19th century that these principles began to be clearly understood. In the early 19th century the classic American water pumper was introduced. The need for this machine was driven by the phenomenal growth of agriculture in the American Midwest, beginning with the opening of the northwestern prairie states in the early 1800s. More than a million of these machines dotted the Midwest and West starting in the early 1850s. Even now these multibladed farm windmills can be seen throughout the western United States and Canada, where the energy and storage requirements for providing drinking water for cattle are well matched to the wind water pumper’s power, the storage capacity of the associated stock tank, and the wind statistics of the Great Plains. These machines use the most rudimentary aerofoils (often flat plates or slats of wood) and are allowed to rotate proportionally to wind velocity. For the purposes of direct mechanical water pumping, this variable-speed operation works effectively. Even though the American water-pumping design gives up something by its dependence on a flat-plate aerofoil, its simplicity, ease of construction, and reliability still make it ideal for its intended purpose.

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http://nrel.gov/docs/fy01osti/28607.pdf