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

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 grid‐connected 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 new generation of high‐voltage, high‐speed power electronic components allows a wide range of variable‐speed operation for very‐large‐scale machines. Over the past 30 years a number of designs have been tested, a few of which have entered commercial operation. A number of these designs and their histories are described. A detailed description of a wide range of electrical methods for allowing variable‐speed operation is provided.