Oklahoma, United States [RenewableEnergyWorld.com] Wind turbines are like every other power generation technology: where there are moving parts, there’s wear and tear. Ongoing maintenance is crucial. But the source of stress on a wind turbine is unique — constant exposure to the elements — and utility-scale wind farm operational history is still being written.
“Many of the country’s wind farms are still relatively new and preventative maintenance will become more important as they age,” said Richard Esce, business development manager at HMT Inc., a New York-based company that provides high voltage maintenance and engineering services to wind farms.
Good wind farm maintenance includes keeping all turbine parts fine-tuned and protecting the critical integration point to the utility’s electric system. There are ways to keep maintenance costs in check and if, as some analysts say, 10 percent of the annual cost to generate wind power is directly attributable to maintenance and repair, that would be money well spent.
In the Field
Wind farm manufacturers like Clipper Wind, Vestas and GE Energy provide ongoing maintenance services for their turbines, while independent maintenance and repair companies service many makes of turbines. (See Sidebar, bottom of page.)
Rick Hammill, service manager N.Y. East, GE Energy-Renewables, said a regular schedule of maintenance for each wind farm is set up according to GE’s engineering specifications. Hammill’s region includes New York, Pennsylvania, Massachusetts and New Jersey, and the company is currently expanding into West Virginia. The typical size of a wind farm in Hammill’s territory is 50 units per farm.
“We have an installed base of 571 GE 1.5-MW turbines right now and that will probably grow to 754 by this time next year,” he said. “In the maintenance schedule, there is a comprehensive annual and semi-annual structure to inspect the entire turbine to make sure it’s maintained per our engineering specifications.”
GE schedules maintenance for blades and gear box lubrication at regular intervals based on the maintenance manual and how the wind farm is operated. Yaw and blade angles are checked to verify that they are functioning as designed. In parts of the country where blade icing can be an issue, Hammill said guidelines based on temperature and humidity are followed to ensure the technicians’ safety.
Technicians, who are trained at GE’s training facility, work exclusively on GE’s 1.5-MW turbines. Standardization of the fleet is beneficial, said Hammill.
Better Alignment Equals Lower Maintenance Costs
With most turbines, regular maintenance includes checking yaw angles. A turbine that is out of alignment with the wind — that has a “yaw” angle to the wind — is bearing assymetric loads on its blades, bearings and hub. Gust loads on the blade can cause flexing and fatigue stress.
“With better control of the turbines we should be able to produce lower stress loads on the rotating parts of the turbine,” said Phil Rogers, CEO of Catch the Wind, which provides laser-based wind sensor technology. “That could lead to lower maintenance costs and less maintenance activities altogether in addition to increasing turbine efficiency and output.”
For every degree a wind turbine is out of alignment, energy output drops by 1 percent. A turbine that is only 15 degrees out of alignment loses 15 percent of its efficiency — and the average wind turbine is out of alignment for most of its life, said Rogers.
“Data we are starting to see confirm that there is significant misalignment,” he said. “As the turbines get larger, there will be a load increase on the turbine.”
Wind sensors are typically mounted on the nacelle behind the rotor, where they measure wind that is already past and align the turbines based on that data. But if sensors measure the wind before it gets to the turbine, the turbine can be aligned and the pitch of the blade can be controlled to capture the wind gusts and counter the turbulence affect. Rogers said research at some of the national laboratories shows that configuring the sensors this way could greatly reduce the stress loads on the blades and the rotating parts of the turbine — the hub, the bearings and the transmission gearboxes.
The Critical Grid Connection
While much attention is paid to maintenance on the wind turbine itself, the connection to the utility is also critical.
“Typically you will have an interconnect substation to the local utility and a collector substation where the collector circuits come in from the wind turbine towers and transformers at each tower,” said HMT’s Richard Esce. HMT provides high voltage maintenance and engineering services to many kinds of generating facilities, from hydro power plants to wind farms. For wind farms, HMT designs substations and maintains the infrastructure that supports the farm’s connection to the electric grid. The company’s commissioning and start-up projects include wind farms in New York, Indiana, Pennsylvania and West Virginia.
The wind farm’s substation, which is designed based on utility requirements, consists of the collector substation and the interconnect substation.
Wind turbine generators typically generate electricity at 575V or 690V, which is stepped-up to 34.5 kV. Collector circuits — which can be overhead or underground, depending on topography — run from every wind turbine back to the collector substation.
The collector system is unique to wind farms. It consists of the cabling system and either outdoor pad-mounted or indoor turbine-mounted transformers at each wind tower, said John Pertgen, HMT technical service manager. Maintenance on the pad mount transformer involves taking oil samples annually to check for dissolved gases or general quality, but the cable system’s another story.
“The cable system honestly comes down more to repairs than maintenance,” said Pertgen. “That’s probably the biggest nightmare for a wind farm operator, the cables. The equipment selected and the installation method can lead to problems and we’ve seen more than our share of cable failures and termination failures.”
Part of the problem is caused by the inherent nature of wind farms: they’re in rural areas, they cover a lot of ground and the soil can be rocky, making trenching difficult. Cable circuits are long with many splices and if they aren’t properly installed, there could be premature failures down the road, said Bruce Emerson, HMT senior electrical project engineer.
When pad mount troubles are caused by manufacturing issues, unfortunately all the units are likely to be affected by the common problem.
“Some of the designs include a secondary molded case circuit breaker in the transformer compartment and that’s been a problem in a couple of instances in terms of nuisance tripping,” said Emerson. “It’s not a great environment for a solid state trip unit so we’ve seen some failures.”
From the collector substation the power is stepped-up to the utility interconnect substation.
“That’s where it actually gets interconnected to the local utility lines,” said Esce. “That’s where the utility says, ‘Here are our requirements and here’s how we want you to set it up. You get it designed and then we’ll approve it.’ ”
In HMT’s experience, not as many problems crop up there because the installation is utility-grade, which is generally more rugged and includes more redundancy.
InterNational Electrical Testing Association (NETA) maintenance testing specifications have been approved by the American National Standards Institute (ANSI) for electrical wind power system maintenance. For a typical collector substation, for instance, there are annual inspection requirements and every three years a more thorough circuit breaker, transformer and relay testing.
The substation’s protective scheme is critical and must be designed correctly to protect the utility’s lines. The breaker closest to the fault must trip first.
“We’ve been called out on jobs where the upstream breaker tripped instead of the downstream breaker,” said Pertgen.
Emerson, who designs the protection scheme, said one of the challenges is getting the design information transferred correctly out into the field. Microprocessor protective relays are very powerful but to accomplish the necessary integration is not easy.
“There are the protective settings in a relay and then the more complicated logic in the microprocessor that orchestrates the operation of the whole substation,” he said. “Getting that logic correct can be quite a challenge. Glitches may not be uncovered during commissioning and can lead to confusion down the road.”
HMT shared some other suggestions for improving wind farm maintenance:
- Schedule oil sampling for the transformers. “A history of trends tells you a lot about what’s going on inside the transformer,” said Esce. “You can see slow degradation and that’s fine but once you see a spike, you need to investigate.”
- Review the events and histories that the protective relays are capturing and recording. “We’ve seen instances where if you’d been looking, you could have spotted the early signs of a problem,” said Pertgen.
- Make the collector system easier to maintain and provide simpler ways to isolate portions of it for maintenance.
- When building a wind farm, be more concerned with reliability than original installation cost.
- Start working sooner with the utility. Don’t wait to worry about the electrical side until the end.
- Block out enough time in the schedule for the final acts of commissioning. “At the eleventh hour, when everyone wants to see the turbines spinning and go home for Christmas, that’s when we need to spend the time to check through everything,” said Pertgen. “We’ll never have that opportunity again. Final functional check-out and proofing-out the control scheme is critical.”
HMT is already planning for the future.
“We want to be prepared when these facilities start aging and start having problems,” said Pertgen. “For some of the equipment, like SF6 breakers, there are different protocols. In 10 years, because of the number that have been installed in the area, there will be a call for those familiar with handling them.”
Technical talent with experience working on wind farms will be at a premium, as in the power industry in general.
Sidebar: A Sampling of Wind Farm Maintenance Companies
Good plant performance and availability require constant surveillance according to Clipper Wind, and once a wind power facility is commissioned, Clipper offers ongoing operation and maintenance services. Clipper Fleet Services provides site engineering, construction, installation supervision, operations and maintenance services and warranty management in support of Clipper’s Liberty Wind Turbine fleet.
GE has installed more than 10,000 wind turbines worldwide, comprising more than 15,000 MW of capacity. The company offers installation, operation and maintenance services for its wind turbines. (Rick Hammill, service manager N.Y. East, GE Energy-Renewables, discusses GE’s program in this article.)
Turbine maintenance plays a vital role in maintaining the operational life of a Vestas turbine, which averages about 20 years, said Andrew Longeteig, Vestas spokesperson. Without regularly scheduled professional maintenance, turbine lifetime can be shortened. Vestas focuses on three key service areas: preventative maintenance, scheduled and unscheduled services and supplying Vestas-certified spare parts and components. Vestas has 38,000 wind turbines installed worldwide.
Encore Power Services
Texas-based Encore Power Services (EPS) provides field services, parts and component repair services for wind turbine generators, including GE, Siemens, Gamesa, MHI, NEG, Micon, Clipper, Vestas, Kenetech and Zond. EPS also offers unscheduled maintenance for wind turbines, such as base bolt tensioning, blade replacement and gearbox replacement.
Gemini Energy Services
Gemini Energy Services is a full-service wind industry operations and maintenance provider. James Haley, director of operations, said the company’s strength lies in its unique workforce of turbine technicians, which is composed primarily of former military professionals who have received wind-specific training to excel up-tower. Based in San Diego, Gemini deploys its technicians to customer sites for permanent O&M roles or sends “quick-action traveling reliability teams” for corrective action. Haley says Gemini’s technicians are “battle-proven and wind-powered.”
Shermco Industries’ field services include on-site generator maintenance and repair, electrical system maintenance, vibration analysis and laser alignment. Headquartered in Irving, Texas, the company’s generator remanufacturing facility is authorized for work on every major brand and its engineering services provide balance-of-plant (BOP) electrical problem solving, including 15, 25 and 35 kV cable evaluation, testing and repair.
Upwind Solutions is an authorized OEM service provider that offers full-service operations and maintenance for utility-scale wind projects. Specialized services include gearbox bore scope inspections, generator bearing inspections and replacements, 24/7 monitoring, BOP services and root-cause analysis. Working from Medford, Ore., Upwind’s experience includes the construction and operation of more than 3,000 MW of onshore wind energy projects in North America as well as two large offshore wind installations.
In 2007, Wazee Wind, a division of 88-year-old Wazee Electric, was created to meet the growing demand for on-site wind generator repairs and maintenance in the Rocky Mountain region. Wazee offers complete wind turbine generator repairs, rewinds and maintenance, with service in both its motor repair shop and up-tower at wind farm sites nationwide. Wazee is factory-certified by Hitachi and also provides after-warranty service for ABB, Siemens, Gamesa, Vestas, GE Wind, VEM, Winergy and other manufacturers.
WennSoft offers a suite of business software for renewable energy organizations to help them manage projects and subcontractors, maintain and build facilities, integrate financial data and connect field employees and customers.
Wood Group Renewable Energy Services (RES) provides O&M for industrial wind farms, including day-to-day operations, turbine optimization and gear box repair. The company also offers commissioning services, crane services and control system installation and service. RES is part of the Wood Group, an international energy services company for turbines and other high-speed rotating equipment used in the global oil and gas, power and industrial markets.
TGM Wind Services
Abilene, Texas-based TGM inspects wind turbine blades and towers using aerial utility platforms coupled with on-board phased array ultrasonic technology. (See photo, below.) The Bronto Skylift machines can withstand winds speeds of 35 mph and lift up to 1,000 pounds of men and materials in a fully enclosed platform to a maximum height of 295 feet. TGM assesses weld integrity on tower sections and locates bonding or laminating defects in the turbine blade and reports on manufacturer defects or structural damage that occurred during transport or erection.
Below: TGM technicians inspecting wind turbines from an aerial utility platform. The Bronto Skylift can withstand winds speeds up to 35 mph. Photo, TGM.
Nancy Spring is senior editor at Power Energineering.
This article was reprinted with permission from Power Engineering as part of the PennWell Corporation Renewable Energy World Network and may not be reproduced without express written permission from the publisher.