|
The Steel Winds project launched in the middle of 2007 under a blaze of publicity. What made it so newsworthy was that it was among the first urban wind farms and the first built on a brownfield site on what was once the famous Bethlehem Steel Mill on the shores of Lake Erie near Buffalo, N.Y. And it was the first site in the world to deploy the Liberty wind turbine, one of the largest built in the U. S. at the time. The eight units installed at the project were the first products to come off the assembly line of wind turbine manufacturer Clipper Windpower of Carpinteria, Calif. This culminated in an award as a 2007 Project of the Year in Power Engineering magazine.
But then problems began to crop up with some components. To make matters worse, some aspects of remediation had to be addressed in the dead of winter in below-freezing temperatures. By late spring 2008 most of those issues were in hand and Steel Winds—renamed the Niagara Project—is now enjoying a long series of consecutive months of high production.
“As the first commercial project using the Clipper Liberty 2.5 turbine, we had some issues coming out of the starting gate with the gearing and blades,” said Patrick Caramante, vice president of engineering at First Wind Inc. of Newton, Mass., the developer and owner. “Those were for the most part retrofitted and remediated in early 2008. Since May of last year, we have had a very successful run.”
Caramante has been in the power industry for 30 years, so he has plenty of perspective on what can and does go wrong when any new technology is introduced. To him, initial problems are par for the course and Clipper has been responsive to these “teething issues.”
First Wind is an independent North American wind energy company focused exclusively on developing, owning and operating wind energy projects. It currently operates five wind farms with another project actively under construction.
When it came time to select the turbines for its first facility, First Wind chose the 2.5 MW Liberty machine. The company wanted the latest technology in the largest commercially viable form and felt that was best represented by Clipper.
Most modern gearboxes use three-stage planetary and two helical gears, but this necessitates massive gear casings that go beyond the capabilities of most manufacturers. Excessive loads on the bearings have been blamed for the gearbox failures suffered by just about all wind turbine suppliers over the past decade. Clipper designers ditched the planetary arrangement in favor of a two-stage helical design. Reduced loads are achieved in the gearbox via the use of four permanent magnet generators instead of the usual single-wound rotor-induction generator. This effectively takes torque down by a factor of 16, compared to a factor of four in traditional designs. Weighing in at 36 tons, the turbine is one of the lightest in the industry. (See Figure 1.) In addition, Clipper included low-speed tapered roller bearings on the main shaft. Low voltage ride-through and variable-speed technology are built in.
“The Clipper Liberty machine’s multiple drive path design radically decreases individual gearbox component loads, which reduces gearbox size and weight,” said Bob Thresher, director of the National Renewable Energy Laboratory’s (NREL) National Wind Technology Center in Golden, Colo. “The new generators significantly reduce component mass by eliminating much of the copper that would be required for windings in the rotor. The machine will also take advantage of advanced feedback controls to reduce load excursions in turbulent wind conditions and optimize pitch schedules to reduce drivetrain loads and improve energy capture.”
Construction at the Niagara site began at the end of 2006 and was finished by March 2007, delayed by heavy winter winds coming off Lake Erie. Further delays were caused by the newness of the machines. The Liberty consists of four major components that have to be assembled on site: the baseplate, gearing, genset and the nacelle roof, which is split in two. Early attempts to put all this together at hub height proved problematic. The decision was made to complete assembly on the ground and erect it in one piece rather than in four.
Confines of the site contributed to First Wind’s decision to move away from turbines in the 1.5 MW range. With a finite area available, Liberty machines gained an additional 8 MW of power. These machines were made at Clipper’s new manufacturing facility in Cedar Rapids, Iowa.
The first eight Liberty turbines rolled off the assembly line at the end of 2006, destined for Niagara. In 2007, another 137 turbines were produced, followed by just under 300 wind turbines in 2008. With a brand new assembly line and turbine, odds were that some problems would crop up.
An onsite technician heard a noise coming from one of the turbines. Upon investigation, damage to some gear teeth was apparent in the drivetrain’s secondary gear stage. Clipper informed First Wind of the problem and took the remaining seven turbines offline while it conducted a root cause analysis (RCA) utilizing the Six Sigma quality process. This was split into multiple phases: define, measure, analyze, improve and control (DMAIC). Clipper also brought in two drivetrains for teardown and component analysis. The analysis showed that improper timing in the drivetrain led to uneven stress between the gears. In some cases, this caused a premature failure of the gear teeth.
In short, the manufacturing tolerance issue throughout the gear set had induced stresses on the teeth, Caramante said.
The RCA revealed the cause of the timing issue to be deficiencies in supplier manufacturing and quality control processes. Inspection revealed that gear sets from gearing suppliers failed to meet specifications. Clipper discovered the quality deficiency after its own generator test stand became operational and was used to verify load distribution on the gear teeth. During the “improve” phase of the DMAIC process, therefore, the company designed and built a new timing measurement fixture and developed a drivetrain qualification test confirming proper gear meshing under load. Gear set and timing quality is now verified by its suppliers prior to shipment to Iowa.
“Clipper analyzed gearset manufacture and assembly, found the cause and then requalified their manufacturers,” said Caramante. “Since replacing the gearboxes in the early part of 2008 they have been running very well with no recurrence of the problem.”
At the same time as the gearing setback, maintenance personnel also observed that an internal structural reinforcement panel in the root area of one blade at Niagara had come loose. Known as the aft shear web, this panel is a longitudinal spar that connects the high pressure and low pressure skins to each other in the widest part of the blade. The connection lacked the strength to withstand the loading experienced during turbine operation. Therefore, Clipper developed an additional shear clip to strengthen the connection of the spar to the blade skins.
As the final stage of its RCA, Clipper inspected the repaired blades. An inspector noticed a small crack in the skin six meters from the root, on the trailing edge of one blade. Closer examination of other blades showed that while a few had cracks, many displayed small wrinkles at the same place on the blade. This was traced back to a flaw in the manufacturing process that brought about folds in the laminate. Appropriate repairs were conducted during early 2008.
“Clipper discovered a manufacturing issue with the way the skin was being laid onto the blade,” said Caramante. “They went to Brazil and revamped the manufacturing process to eliminate any wrinkling concerns.”
The company began the process of blade repair in below-freezing conditions. Heating elements were used as well as blankets around the blade to produce the 60 C temperatures required to set the blade patch. Almost all blades at Niagara have been remediated. According to Caramante, a few blades with minor issues were left running through the high-wind period of winter and will be addressed during scheduled maintenance this summer when winds are at their lowest.
In 2009, First Wind brought online the 125 MW Cohocton Wind Farm in New York, using 50 Liberty machines. The company has also placed orders for more turbines for operations in Utah, Hawaii, Vermont and Maine.
Power Engineering July, 2009
| 
Articles 
| View all PennWell sites | View all PennWell events
Add RSS Feed
