Trends in wind-power generators

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.

Permanent magnet
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.

Direct-drive designs
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 Duty
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.

WPE

A 3-MW drivetrain all in one package

A recent wind-turbine drive combines a two-stage gearbox, coupling, and permanent-magnet generator into one compact package. One design goal for the HybridDrive from manufacturer Winergy (winergy-ag.com) is to reduce drivetrain size. A direct connection between gearbox and generator allows shortening the drivetrain by about 35% and nearly eleminating alignment tasks. The company says it demonstrates a peak efficiency of over 94%.

Its compact dimensions present several advantages when designing a wind turbine. For instance, when the HybridDrive replaces existing machinery, it is possible to use the reclaimed space for the converter and transformer instead of mounting those in the tower. Relocating transformers to the nacelle reduces low-voltage cable losses, which further improves efficiency. Depending on design, tower costs can be reduced due to the lower nacelle weight. Transportation costs also drop because of the smaller nacelle.

Another plus is that the PM generator uses only 20% of the rare-earth materials used in similarly sized direct-drive units. Dependence on a particular and costly raw material, often neodymium, is considerably lower making long term costs easier to calculate.

The drivetrain is available with just one or a dual-bearing rotor shaft. Winergy adds that engineers can order the drive with either journal or conventional roller bearings for the planetary gears.

Despite its compact size, the drive is sufficiently modular to allow for disassembly and replacing parts when necessary. For instance, a service crane in the nacelle can lift individual drive modules. So if major service becomes necessary, there will be no need for a crane callout, which considerably reduces service costs. Furthermore, says Winergy, reliability and quality of the drive is optimized because a single supplier is responsible for the design and quality assurance of the drivetrain.

The first design of the drive has an output of 3 MW and is intended for use in offshore turbines. The developer says the concept is easily increased to the 6 to 7 MW range.

 

Three generator designs serve many turbines

ABB says it has developed permanent magnet (PM) generators for applications that call for direct drives, and medium and high-speed. The electrical performance of individual generators can be tweaked for best performance in cooperation with the wind-turbine OEM. For a closer look:

A low-speed gearless design works best in direct-drive applications. The turbine and generator form a compact and structurally integrated unit. The design is said to give free access to all parts for easy installation and maintenance. The simple and reliable low-speed rotor without separate excitation or cooling system results in minimum wear, reduced maintenance requirements, lower life-cycle costs, and a long lifetime.

A medium speed PM generator provides a compact low-speed system. The turbine main bearing and PM generator combined with a single-stage gear box to work at high efficiency and require little maintenance. This design features a simple and reliable low-speed generator rotor without separate excitation or cooling systems. The designs result in less wear, reduced maintenance requirements, lower life cycle costs, and a long lifetime.

And the third design arrangement is a high-speed PM generator similar to a doubly-fed version with even smaller space requirements. It is said to package high power in a small size. Its typical speed range is from 1,000 to 2,000 rpm using a 6 or 8-pole generator. Other pluses are a mechanically identical design to mainstream equipment but without slip rings.

ABB also offers wind turbine generators in conventional synchronous versions.

Efficient generator stays cool and does not cog

Clean Energy Technologies, Calgary, Alberta, Canada  is said to have developed a generator for wind turbines that is more efficient than existing designs. The company’s Axial Flux Permanent Magnet (AFPM) generators work at high efficiencies and produce more power at lower wind speeds due to a coreless design. The generators eliminate traditional cogging issues which makes them well suited for wind-turbine applications.

Traditional or radial-flux permanent magnets (RFPM) orient the magnetic flux outward or radially from the turbine shaft while the AFPM design works by orienting the magnetic flux along the axis of the turbine shaft. A coil wraps around a disc at the center axis. Magnetic discs then rotate on the sides of the coil disc and generate electricity. This kind of power generation is said to be ideal for wind power applications because its initial operating torque (cut-in speed) is lower than existing RFPM methods.

AFPM power generation divides into inner and outer classifications. With an inner configuration, only the magnetic disk rotates while the generator housing remains fixed. In the outer configuration, the whole generator body rotates by fixing the magnetic disk to the body. Application requirements determine the configuration.

There is another consideration. A generator producing electricity also produces heat. As the heat increases, efficiency decreases. To solve this problem, Clean Energy generators are liquid cooled. This significantly reduces, says the company, the wear that high temperatures cause, and thus improves the generator’s life-span. The table compares a few details of the competing designs.

Generators and services for wind turbines

Electrical design firm Potencia developed a 5 kW wind turbine in 1975 but has evolved as a designer and supplier of generators for the wind-turbine industry. The company has since developed 500 kW induction generators, and 750 kW doubly-fed wound-rotor generators in the 90′s. More recently, the Mexico-based company has developed permanent-magnet generators for multi-megawatt turbines for Clipper Windpower, Carpinteria, Calif.
Potencia says it has manufactured generators classified as induction, wound rotor, salient pole, and permanent magnet, and that its custom generators can meet any specification. The firm adds it is aware of the need for high-reliability in equipment and so guarantees a lowest temperature-rise and highest efficiency for its units. The generators are manufactured with U.S. materials and dollar-based competitive prices with shipping to any place in North American.

The company also refurbishes several generator designs. The tables list a few specs for a new Leroy Somer, wound-rotor generator in a 660kW unit, and then performance figures for before and after it was refurbished.

A few specs for a Leroy Somer generator

Performance figures for before and after rebuilding the generator

Gearbox, generator, and controls in one package

GE Drivetrain Technologies, a unit of GE Transportation, Erie, Pa, says the company has launched a wind generator and control systems division to serve wind turbine owners around the globe.

The new company will immediately offer doubly-fed induction and permanent magnet generators in the 2 to 6 MW power range. The company produces more than 3,000, 2 MW and greater generators annually and has sold more than one million generators and motors globally. The company says these generators have been proven reliable in extreme environments in applications as diverse as railway locomotives, mining trucks, and off shore drilling rigs.

“Our objective is to apply GE Transportation’s capability to advance the state of the art in wind energy generation technology,” says GE Drivetrain Business Leader Prescott Logan. “GE Transportation’s global supply chain, including existing generator-manufacturing facilities in Erie, Pa. and Monterrey, Mexico creates near-term production capability.”