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
George Fleming says
A thought-provoking article.
The synchronous speed is that which causes the generator to produce the grid frequency exactly. If the grid frequency is constant, so is the synchronous speed.
Asynchronous means that the machine cannot produce torque or power when turning at the synchronous speed. This is a characteristic of the doubly-fed induction generator. From Wikipedia:
“Doubly-fed electric machines are electric motors or electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system…
“…A practical wound-rotor doubly-fed electric machine system that does not rely exclusively on asynchronous (i.e., induction) principles…has never materialized from the electric machine establishment…
“…Consequently, the wound-rotor doubly-fed induction electric machine has been forced into antiquity, except in large installations where efficiency and cost are critical over a limited speed range, such as wind turbines.”
To emphasize, an asynchronous machine cannot operate at the synchronous speed. These statements may explain what asynchronous means, but the equation of “asynchronous” with “induction” is confusing. Synchronous machines also generate by induction. The generators used in central power stations are synchronous.
There is at least one wind turbine that uses a synchronous generator, the DeWind 8.2. It produces constant (synchronous) generator speed from variable rotor speed with a “superimposing gearbox.” This turbine was designed and tested by a German company. That company was bought by Korean company Daewoo, which is planning to install several 8.2 turbines in Texas.
The recently defunct Canadian company AAER claimed to have designed a turbine on the same principle as the DeWind 8.2. The Israeli company IQWind claims to have invented a variable diameter gear with which it plans to produce constant generator speed from variable rotor speed, but this is a different principle from that of the DeWind and AAER turbines.
The main purpose of producing constant generator speed from variable rotor speed is to eliminate the power electronics. All of the generators mentioned in the article above require power electronics to convert the wild AC to constant frequency for the grid.
A synchronous generator operating at synchronous speed does not need power electronics. It is more efficient and robust, and possibly less expensive, than the doubly-fed generator. It can provide fault ride-through, voltage support and reactive power with simple controls. These are the primary reasons that central power stations use synchronous generators.
Strong permanent magnets may allow for a more efficient generator than existing copper-wound machines, but the magnets require rare earth metals – on the order of 3,000 lbs for a typical wind turbine generator – and the generator could not produce grid frequency without power electronics. These are serious disadvantages.
It is often argued that the direct drive turbine will be more reliable than the geared turbine because it has fewer moving parts. Yet direct drive requires power electronics, and power electronics have an enormous number of moving parts on the molecular scale. They create heat and wear on the electronic switches and other elements of the device.
Eliminating the power electronics would drastically reduce the part count of the turbine, even though the turbine must be geared in that case. There would be no power inversion inefficiency, and the number of electronic circuits susceptible to damage from lightning strikes and other power surges would be greatly reduced. These are by no means the only advantages.
It is too early to conclude that the gearbox will always be a maintenance problem in wind turbines. We are discovering the reasons why they are failing too soon. New designs and arrangements for geared turbines will appear. Nacelle weight depends on several factors, and eliminating the gearbox does not necessarily reduce it. The trend to direct drive seems irrational to this observer.