Wind-power generators fall into three primary types—induction, permanent magnet, and superconducting. Induction generators, sometimes referred to as asynchronous generators, have dominated the market for years due to their low manufacturing costs and extensive experience in the power-generation industry. However, in recent years generators implementing rare-earth, permanent magnets made from neodymium have gained some market share. The third category is filled by just one company so far, but if successful, others will join.
Wind-turbine manufacturers are tasked with providing state-of-the-art technology and effective maintenance standards. They are moving toward greater overall system efficiency, higher reliability, and fault ride-through capabilities that permanent-magnet designs provide. Additionally, permanent magnet generators include other valuable features such as low-speed electricity generation, and decreased size and weight.
According to generator manufacturer, The Switch, “Several independent studies by industry specialists have concluded that permanent-magnet generators and full power converters represent the preferred future drive-train technology.” Reasons for this trend include lower costs across the entire system and reduced maintenance requirements thanks to the elimination of the gearbox.
In 2010, the wind industry’s number one maintenance concern was the gearbox. As much as 25% of overall turbine downtime was due to gearbox failures of some fashion. As a result, some turbine manufacturers implemented variable-speed permanent magnet generators and solid-state electronic converters, thereby eliminating the need for a gearbox. This direct-drive design has two primary advantages. The first and most obvious is the increased expected availability provided by the elimination of a maintenance headache. The second is decreased weight in the nacelle. Weight is a characteristic considered early in design phases. Eleminating a gearbox results in a substantial weight decrease.
Combined drive trains
There are, however, drawbacks to direct-drive designs. For one, large generators may be several meters in diameter. To remedy this problem one wind-turbine drive takes permanent magnet generator integration to the next level. Manufacturer Winergy has designed a new HybridDrive system that integrates a two-stage gearbox and generator into one housing, resulting in a 35% decreased driveshaft length and a peak efficiency over 94%.
The HybridDrive’s compact dimensions present several advantages. For one, lesser space requirements make it possible to place a transformer up-tower, thereby reducing low-voltage cable losses. Another advantage is that this design requires only 20% of the rare-earth materials used in a similarly rated direct-drive generator. Finally, a service crane in the nacelle can lift individual drive modules. So if major service is necessary, a crane callout is not.
Offshore wind development brings a whole new set of challenges to the industry. Since subsea structures and installation services represent the majority of wind farm costs, project owners are looking for ways to maximize their return per tower. The problem is that 5 to 6 MW is about the peak capacity for current generator designs. One main limiting factor is the heat generated by such high production combined with neodymium’s low heat tolerance. There are however, a few firms with ideas for increasing generator capacity. American Superconductor, for example, will licensing its 10-MW SeaTitan technology. Amperium wire, a superconducting material used in this turbine’s generator, replaces traditional copper windings, and a representative poleset was completed and tested in conditions as high as 30° Kelvin. To keep the generator functioning at these temperatures, a new rotor-cooling scheme was developed that uses a rotating cryogenic rig.
Filed Under: Generators, Turbines