ENGINEERS AT NREL have completed tests on an unusual wind-turbine drivetrain that is the collaboration of several companies. The design sports a single-stage gearbox designed by Romax Technology, a medium-speed permanent-magnet generator, and a power converter developed by DNV Kema with high-efficiency modules developed by Cree (now Wolfspeed). The goal of the project’s first phase, which began in 2011, was to design an advanced drivetrain that could improve reliability and efficiency, reduce the cost of wind energy, and scale to larger power ratings.
NREL project lead Jon Keller says the 1.5-MW design can scale to ratings as high as 10 MW. In 2013, the team received follow-on funding to develop the prototype and demonstrate the technology’s commercialization potential.
The new gearbox consists of a single planetary stage that uses compliant flex-pins and journal bearings to support the planets, thereby eliminating the lower-reliability, higher-speed stages found in traditional gearboxes. Traditional three-stage, high-speed gearboxes have been plagued with reliability issues caused by large and unpredictable loads imparted to gears and bearings by high wind and turbulence, utility faults acting through the generator, and hard stops.
“Our goal was to produce the best power density in the smallest package,” says Romax Technology mechanical engineer Travis Histed. “The journal bearings with positive pressure lubrication allow for reduced planet size and a fourth planet.”
The flex pins are used to let the system find a natural load equilibrium. “If one pin is carrying too much load, it sort of flexes out of the way and the others pick up the load. It is a way to improve load sharing over a conventional three-planet design,” says Histed. “At the end of the test sequence, we added a test to simulate a full year of what’s called dithering, when the rotor is locked and the wind rocks the rotor back and forth. We dithered it quickly at 2 Hz and without oil. The coming teardown will tell how well the bearing held up.”
The planetary-stage design has seen a lot of evolution since early days from SRB (Spherical Roller Bearing) to CRB (Cylindrical Roller Bearing) to integral CRB or TRB (Taper Roller Bearing). “The innovative design of the journal bearing is a breakthrough for the industry to improve efficiency and system reliability,” added Dr. Zhiwei Zhang, VP Engineering from Romax Technology. “The medium speed co-axial drivetrain is also a developing trend for wind-turbine technology because it realizes the balance between the complexity of the mechanical drivetrain and generator cost. It also removes the high speed stage which is usually vulnerable to failures such as White Etch Cracks or WECs. The medium-speed drivetrain will demonstrate additional reliability and efficiency advantages over conventional high-speed designs when it is scaled to higher power rates for offshore applications. ”
In terms of reliability, NREL’s Keller estimates a significant increase in gearbox reliability (compared to a conventional three-stage gearbox) because it doesn’t have an intermediate or high-speed stage where most failures occur. “The use of a double-tapered roller main bearing to isolate rotor loads and journal bearings in the planetary stage should also improve reliability,” he says.
There are improvements on the electrical side as well. The team is also exploring medium-voltage, wide-band-gap, silicon-carbide power modules. These state-of-the-art power modules are expected to reduce losses within the power converter, leading to increased efficiency, energy capture, and revenue. “The power converter modules contain silicon-carbide diodes that reduce switching losses. Depending on a point of comparison — traditional low voltage or medium voltage modules — there can be a 0.5 to 1.5% gain in module efficiency,” says Keller.
“In addition, the power converter now has algorithms to compensate the inverter voltage and current levels to reduce torque transients during utility faults. This reduces mechanical loads due to electrical utility faults,” he says. What’s more, the improved efficiency of the power converter modules results in less heat generation, which is likely to improve module reliability.
The design also offers significant reductions in weight. “We find that a medium-speed configuration tends to have about a 20% decrease in weight compared to a three-stage, DFIG configuration and, at 10 MW, about half the weight of a comparable direct-drive configuration,” adds Keller.
The team began testing the drivetrain prototype in a 2.5-MW dynamometer with the controllable grid interface at the National Wind Technology Center at NREL last July and finished testing in April. Technology readiness levels have been advanced and combined with a commercialization plan will lead to deployment of the drivetrain technologies. Successful deployment of the more efficient, reliable drivetrain will further reduce the cost of wind energy and ensure that U.S. companies are at the forefront of technical innovation within the global wind-power industry.