By Teresa Hansen, Associate Editor
Over the past decade, land-based wind turbines have grown from about 700 kW to well over 2 MW. This up-scaling in size, however, has subjected gearboxes and other components to even greater loads and stresses. Traditional gearing and bearing manufacturers, in particular, are struggling to scale existing technology to power these juggernauts while maintaining tolerances.
“Wind turbines are one of the most demanding applications for gearboxes due to variable loads that are extremely difficult to predict,” says Charles D. Schultz, chief engineer at Brad Foote Gear Works in Chicago, and author of the book, An Introduction to Gear Design. “As wind turbines get bigger, the design challenges get bigger too.”
Massive torque, for example, is transmitted through the three-stage planetary gearboxes typically used in multi-megawatt turbines. In response, manufacturers developed huge, expensive ring gears and bearings, stretching the limits of current technology and causing some designers to investigate new configurations.
“Planetary gearboxes suffer too many failures due to bearing issues and excessive loads,” says Amir Mikhail, senior vice-president of Clipper Windpower Inc., Carpinteria, Calif. “This is one of the reasons we decided to go in a different direction-we wanted to create a more reliable machine. In particular, we wanted to design a machine with a highly reliable, low maintenance gearbox.”
Over the past four years Clipper developed a 2.5 MW wind turbine with a compact two-stage helical gearbox. Known as the “Liberty,” Clipper’s patented design reduces loads, minimizes the likelihood of damage and increases gearbox lifespan by using multiple generators and a multiple path, distributed gearbox. Four high-speed output shafts split the load by a factor of 16-four times greater than in commercially available gearboxes. To ease maintenance, high-speed gear sets can be replaced without removing the gearbox.
Clipper Windpower’s Liberty turbine offers breakthroughs in wind generation.
While gearboxes in comparable turbines weigh 50 to 70 tons, the Clipper model weighs 36 tons, including the gearbox, brakes and housing. Low-speed tapered roller bearings take thrust loads on the main shaft. Clipper’s patented variable speed technology, which includes tower damping techniques to eliminate harmful vibrations, results in fewer parts and higher reliability.
The turbine control system incorporates high-speed embedded processors needed for algorithm computations, which are repeated every 50 milliseconds. Unity power factor is produced down to a low rated power percentage, reducing the need for volt-ampere-reactive (VAR) correction. Its low voltage ride-through capability extends up to three seconds. The control system also reduces loads by anticipating resonant conditions within the drive train structure and generators.
The Liberty design incorporates lightning protection equipment that has already proved its value. Lightning struck the Liberty I turbine three times over the past year, yet it continued to operate without downtime. Two lightning receptors that run down a conductor are installed on each blade tip. Brushes on the main bearing carry the current down the tower via ground conductors. A steel mesh integrated Faraday cage prevents damage to parts inside the nacelle. A second Faraday cage houses the controls. These design elements enable the machine to ride out lightning storms without shutting down.
In addition, a two-metric ton service crane installed in the Liberty reduces both repair time and the costs normally caused by depending on a third-party crane.
Mikhail says these changes may bring about a shift in turbine lifespan. Most turbine manufacturers advertise a 20-year life cycle; Clipper intends to push the envelope by 10 more years.
“The Liberty turbine power train meets the 30-year design life requirements,” says Mikail.
Permanent Magnet Generators
The Liberty machine uses four Clipper MegaFlux permanent magnet (PM) generators. This PM generator configuration was selected over the wind industry’s most commonly-used doubly-fed and wound field synchronous generator due to its high efficiency, low maintenance requirements and lower overall service cost.
In doubly-fed generators, current excites the rotor, creating a magnetic field. This process pulls current directly from the unit, diminishing efficiencies and subjecting the rotor and rotor bearings to magnetic fields that result in stray currents, which can cause high stresses and result in pitting and bearing failures. PM generators, on the other hand, are already magnetized, so it isn’t necessary to pull power from the unit. As a result, the wind turbine is able to operate at higher efficiency over a wider range of loads.
“The MegaFlux generator offers high efficiency in the area of power factor performance, staying near unity down to 5 percent to 10 percent of load,” says Mikhail.
At only 3.5 x 3 feet, the MegaFlux generator uses no brushes and does not require coupling or a switch clutch between the gearbox and the generator because the low short-circuit current results in lower short circuit torque. This allows fault torques to be transmitted without harming the gearbox. The generator’s form-wound Class H stator insulation is rated for medium voltage while the standard random-wound insulation is suitable only for lower voltage.
The MegaFlux Clipper generator weighs less than 4,000 pounds and can be handled by the on-board crane. In the event of a generator or converter fault, the drive train system can operate with two or three generators, delivering half or three-fourths power output until service can be performed.
Clipper offers two generator configurations: the Tewac, a fully enclosed water- to air-cooled model offering a contamination-free enclosure for harsh environmental applications; and the IP54 air-cooled system that can be used for less demanding applications.
Clipper’s Liberty I prototype has been running in Medicine Bow, Wyo., since March 2005. The site has offered a wide range of weather conditions including extreme temperatures, high turbulence wind squalls, lightning, ice and snow. The site is remote, with little maintenance infrastructure. These harsh conditions have both challenged the machine and provided the testing team and maintenance crew with lessons learned, which they are applying to the Liberty.
Over the course of its latest testing period, Liberty performed well within its design expectation. The power curve, turbine architecture and system design were confirmed by the DOE’s National Renewable Energy Laboratory (NREL). Further tests verified that the power balance between generators was within the 7 percent design specification.
The Clipper machine and blades were certified for 93 and 96 meter rotors against international industry standards by Germanischer Lloyd WindEnergie GmbH. Independent engineering reviews were conducted by wind consultancy Garrad Hassan. Extensive testing and evaluation of the 2.5 MW turbine demonstrated that the new drive train design mitigates gearbox stresses. First-year operating data indicates substantially higher overall reliability compared to standard gearbox designs.
Further, the Liberty’s PM generator attained higher efficiencies at low wind speeds compared to today’s standard design. The controller offers simple and effective grid integration, and its power factor regulation technology with ride-through capability exceeds current and planned standards for electric grid operation.
“We’re very pleased with the Liberty prototype’s first 12 months of operation” says Mikhail. “Testing has confirmed its design specifications and demonstrated high performance. We’re already incorporating the lessons we’ve learned over the past months into our first production units.”
Clipper’s 54,000 square-foot assembly facility recently opened in Cedar Rapids, Iowa. The plant has started producing Liberty wind turbines, which are scheduled for deliveries starting in mid-2006.