A wide variety of gearbox and generator options is in use in the wind industry. Of 21 wind turbines examined, the transmission/generator/conversion options number about 15. Permanent magnet generators are used in eight designs, including the Multibrid 5 MW machine; double-fed induction generators are used in five designs.
The report excludes consideration of what used to be an extremely popular concept -- fixed-pitch, stall-regulated wind turbines. These, it notes, "do not allow any control of the power produced by the wind turbine, and consequently cannot meet several of the most basic grid code requirements."
Grid code demands considered in the report are: tolerance to frequency and voltage variations (within prescribed limits); fault ride-through, reactive power; voltage control capability; the ability to provide frequency response service; and the ability to control the severity of power excursions.
The fixed speed induction generator which was once, alongside stall regulated machines, one of the most common features, does not score well in meeting these requirements (table). Double fed induction generators, on their own, do not score particularly well either: each of the designs that uses these does so in conjunction with a partial or full electronic converter. Variable ratio transmissions, where the variations in speed are implemented within the gearbox, are comparatively rare, but the concept scores well for frequency regulation and power control, less well for the other grid code requirements.
UpWind is only one of two wind energy projects chosen for funding under the EU's sixth framework program for research and development spending. Part of its remit is to study design aspects of very large wind turbines, for which it has been granted a EUR 14.3 budget over a five-year period (Windpower Monthly, May 2006). The total budget is EUR 23 million. UpWind has 39 participants from 11 European countries making up a consortium of 11 universities, ten research institutes, seven wind turbine and component manufacturers, six consultants, two wind farm developers and two certification bodies. The aim is to develop and verify improved models of wind turbine components which the industry needs for the design and manufacture of large wind turbines with the market for large offshore power plant in mind. The scope of the work is wide, covering aerodynamics, rotor structure and materials, foundations, control systems, remote sensing, condition monitoring, wake effects, economics and the interface with the electrical network.