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Determining Wind Turbine Reliability: Test, Test, then Test Again

Reliability is the key attribute of a turbine today, particularly offshore. There are many other important measures of wind turbine performance, notably the cost per unit of electricity generated, but they count for little if a turbine breaks down. And how do you investigate reliability? Test, test some more – and then test again.

Besides reliability, wind turbine testing proves a new design’s validity and shows how well a sub-system — or the complete turbine — actually performs in practice. Testing component-level performance and reliability is also vital: accurately predicting the lifespan of critical elements like main bearings or generators allows operators to plan servicing and replacement throughout a turbine’s lifetime.

Manufacturers have long had their own test facilities, but there are significant innovations at independent third-party sites. The borders can often blur: centres such as the Fraunhofer Institute frequently work on long-term research projects with businesses and their investors, while Sandia’s new test site at Lubbock, Texas features both academic and commercial partners.

Independent centres can offer very large or very specialised equipment. And they can also be seen as impartial — which is crucial for project developers, increasingly focused on the reliability of the equipment that underpins years-long, multi-million-dollar investments. ‘There is a huge expansion of wind power and a recognition that to put the next generation of turbines offshore, you need to prove them first,’ says Jim Tuten, project manager for the Wind Turbine Drivetrain Test Facility at North Carolina’s Restoration Institute. ‘The capital backers of these projects also want some assurances. Testing allows you to exercise the unit to its capacity in controlled conditions and without being at the mercy of the wind.’

To complement laboratory work, Spain’s CENER hosts many other testing, design and analysis services, and also runs a test wind farm offering high-speed winds that can take turbines up to 6 MW (CENER)

As well as services ranging from design assistance to electrical testing, these facilities have three main offerings: blade and drivetrain testing — and longer-term turbine trials at outdoor sites that offer grid connections — monitoring equipment and meteorological masts.

Blades are delicate composite structures but face extreme cyclic loads over 25 years or more. Their failure can have severe repercussions in safety, downtime and public image. If many turbines need to be retrofitted, significant costs arise. So validating a blade’s design, its manufacturing process and reliability over time are essential to the success of manufacturers, developers and the industry as a whole.

Standards such as IEC-61400-23 or the UK’s ISO-17025 accreditation govern the full-scale structural testing of rotor blades. To measure specific stresses and strains, and to map blade properties such as static deformation, ultimate strength, fatigue performance or deformation at resonant (Eigen) frequencies, testers employ various methods such as static, single-axis loading of blades at different points along its length or multi-axial, dynamic loadings that better mimic the complex forces found in real life applications.

A full endurance (fatigue) test applies 20 years or more of cyclic loads and might take three to four months, while a static (ultimate strength and resonance) test takes one to two weeks. Applying huge forces and measuring the results demands investment in expensive apparatus such as vibration actuators and linear variable differential transformer (LVDT) equipment that centres must constantly upgrade. In its literature, the Fraunhofer Institute quotes €300,000 – €400,000 for a four- to six-month comprehensive blade test. But as Dr Arno van Wingerde, head of Fraunhofer IWES Competence Centre Rotor Blades, points out, ‘compared with a failed wind turbine, that’s a bargain’.

Higher power and more comprehensive drivetrain testing is the other main area of development at independent testers. HALT (Highly Accelerated Life Tests) subjects drivetrains to accelerated wear environments to swiftly confirm design and component integrity, and there’s a shift underway to test whole nacelles rather than individual components. Drivetrain testers can also be applied to other types of generator or gearbox, such as those used in tidal or wave turbines.

‘The wind energy sector is still striving to get a full understanding of integrated testing and the drivetrain has to be investigated particularly carefully,’ says Dr Jan Wenske of Fraunhofer IWES. ‘Gearbox, generator/converter system and pitch system failures are currently the main cause for downtime. Increased reliability of the drivetrain leads to a higher energy output, lower O&M costs and thus to an increased profitability of expensive offshore wind parks.’

Today, Clemson University’s Restoration Institute in South Carolina is spending nearly $100 million to build one of the world’s largest drivetrain research facilities. A unit capable of testing at up to 7.5 MW will be ready by early next year, while a second, even larger, tester will follow, capable of testing turbines that can generate up to 15 MW. ‘The facility will implement new, advanced equipment to simulate blade forces at force and power levels unavailable anywhere else,’ says Project Manager Jim Tuten. ‘The centre will provide for electrical system testing as well so that combined mechanical and electrical issues can be addressed.’

 

Renk Labeco’s 7 MW and 15 MW test rigs break new ground in reproducing the forces exerted on the drivetrain by the rotor (Clemson University Restoration Institute)

Renk Labeco’s 7 MW and 15 MW test rigs break new ground in how they mimic the forces exerted on the drivetrain by the rotor. Power from the gearbox output is fed into a hydraulic load application system — a load disk mounted on the test stand’s central shaft. Radial and axial loads are applied to the disc by 72 hydraulic actuators to simulate real-life, three-dimensional forces and bending moments that gearboxes and generators must withstand. ‘The biggest challenge for the project is that we have equipment under design to test equipment that hasn’t been designed yet,’ quips Tuten. Capable of low voltage ride through (LVRT) and zero voltage ride through (ZVRT) testing, the lab will also examine generator grid compatibility. Future plans include a 15 MW hardware-in-the-Loop (HIL) capability that will aid testing of other types of electrical equipment as well as ever-larger generators.

The Wind Technology Testing Center (WTTC) in Charlestown, Massachusetts is the other sizeable stateside development. Overseen by the Massachusetts Clean Energy Center (MassCEC) and its partners, the WTTC opened in May 2011 and offers three test stands, each of which can handle blades up to 90 metres. ‘This is the world’s largest structural testing lab for blades, and it’s the only one in the USA,’ says Executive Director Rahul Yarala. ‘We’re extremely busy with blade testing and are working closely with manufacturers and developers.’

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Capable of exerting forces of up to 84 MNm, WTTC offers the full suite of certification tests along with the latest blade prototype development methodologies, R&D partnerships, blade repair capabilities and hands-on workforce training. Fatigue tests employ the National Renewable Energy Lab’s (NREL) patented resonant test system technology. As well as the WTTC, NREL’s smaller-scale National Wind Technology Center still offers blade, drivetrain and many other types of testing, research and development for turbines up to 5 MW. The WTTC also proves that test centres can act as hubs for further investment: TPI Composites has opened a blade R&D centre and prototyping factory 74 km south of Charlestown. TPI will be a WTTC customer, and cited it as key to its decision to set up shop in Massachusetts.

Another new wind turbine blade-testing facility is planned for Clarkson University, based in Potsdam, New York, this time focusing on smaller blades in the 12-15 metre range. The Center for Evaluation of Clean Energy Technology will be built and run in conjunction with other partners by London-based Intertek plc, using a $4.2 million grant from the New York State Energy Research and Development Authority.

Still on the East Coast, Poseidon Atlantic in Northampton, Virginia, will offer sites for turbine prototype testing, infrastructure, wind measurements and wind turbine testing and certification services. The site is planned to hold between five and ten turbines, with a maximum size of around 5 MW. The owners expect to start construction in late 2013, with the first turbines online before the end of that year.

Asia’s only independent test centre is SGS Wind Energy Technology Center (WETC) in Tianjin, China. Opened in June last year, this focuses on full-scale blade testing for blades up to 70 metres long. The centre offers natural frequency, static, fatigue and ultimate static tests as well as a full range of other consultancy and solutions such as blade-specific non-destructive testing and composite technology training.

As befits the current world leader in both installed wind capacity and turbine manufacturing, Europe has the largest spread of test facilities both in operation and under development. Germany’s Fraunhofer Institute opened its new Competence Centre Rotor Blades testing facility in Bremerhaven last year. As well as several beam test rigs and material climate chambers, the centre’s two test rigs can handle blades up to 70 metres and 90 metres respectively. The 90 metre rig – a giant 1000 tonne block of steel – can load blades along their length to a maximum of 1800 kN and bend them by up to 30 metres. Uniquely, it allows the blade to be tilted too. To cope with demand, another test rig for 50 metre blades will go into operation this year.

The latest development here is the DyNaLab (Dynamic Nacelle Laboratory). When active in 2014, it will have a drive output of 10 MW and will be used to test complete nacelles ranging from 2 MW to 7.5 MW. A 40MVA artificial grid will allow complete electrical certification of test turbines, and the institute is currently working to scope the test stand’s final specification. There are also plans for a new lab for foundations and support structures at Fraunhofer’s Hannover location. ‘This facility gives us the possibility to test the dynamic behaviour of the complete structure with near-realistic simulation of soil/seabed-support structure interactions,’ says Dr Jan Wenske. A medium voltage lab, a pitch-system test bench and a mainshaft lifetime test bench are also under development, he adds.

Fraunhofer is looking to build further turbine test sites, he says. Germany already has the DEWI test site near Wilhelmshaven, currently holding ten prototypes totalling over 19 MW, while a new site in Janneby, Schleswig-Holstein, will open at the end of this year. Developed by GL Garrad Hassan and two partners, Janneby will have eight test locations suitable for turbines up to 150 metres in height. Users will be able to access services that include wind, acoustic, power performance, load, power quality and LVRT measurements. A LiDAR test site is also planned.

The Netherlands hosts Europe’s largest testing site in Lelystad, where the first turbine started operations in June last year. Operated by Ecofys, Lelystad can service ten turbines with tip heights up to 200 metres. The main Dutch testing facility is Knowledge Centre WMC. Able to test blades up to 60 metres long, it has operated since 2003.

Spain’s CENER was the world’s largest independent turbine testing facility when it opened in 2008. Under one roof, it hosts a full range of static and fatigue tests for blades up to 100 metres, and can test drivetrains of 2-6 MW. To complement lab work, CENER hosts many other electrical testing, design and analysis services, and also runs a test wind farm offering high-speed and high-density (Class 1) winds that can take turbines up to 6 MW. ‘The drivetrain lab offers a six-degrees-of-freedom drivetrain testing bench, a torque-only nacelle test bed and a generator test bench,’ says Pablo Ayesa Pascual, director of CENER’s Wind Energy Department. ‘The first two are capable of applying a torque of 6 MNm and all have been dimensioned for 8 MW of main actuation. We see a 12 MW power test bench in the near future, however we have no formalised plan as yet.’

Over in the UK, an investment in the region of £150 million ($240 million) means the National Renewable Energy Centre (Narec) will soon rank among the top global independent testers. Joining the existing 50 metre test facility, the new 100 metre blade-testing centre will be the world’s largest and should be ready for commissioning by September 2012. The 130 metre-long building will house a single Moog hydraulic blade-testing rig. There will also be 3 MW and 15 MW drivetrain testers; the former is intended primarily for tidal turbines, but could be used for wind too. The 3 MW centre is close to completion while the 15 MW facility is slated for completion by August 2013. The centre already offers a broad spread of services including a dry dock, subsea and electrical testing, and a cold chamber that can drop down to -20C. Narec is also planning an offshore turbine test site at Blyth, Northumberland, for 2014. ‘Our USP will be the ability to test on- and offshore at the same time,’ says Steve Abbott, Narec’s marketing and communications manager. ‘We’re geared to large offshore turbines and marine facilities, and will have the ability to replay extreme events onshore. Our customers – manufacturers and project developers – want to demonstrate turbines in UK sea and seabed conditions, with learnings that translate to other North Sea locations.’

Turbine testing development in Denmark is focused on the Lindoe Offshore Renewables Center (LORC) in Odense. Still at the planning stage, it will include facilities for welding research, foundation testing and other mechanical testing for lubricants, gears, bearings and oil filters. Innovative ‘helicopter testing’ will evaluate how blade coatings cope with mechanical strain, UV exposure and chemical resistance, as well as tip and leading edge erosion.

The other big development in Denmark is Risø DTU’s new turbine testing field at Østerild in northern Jutland. With space for seven turbines up to 20 MW and 250 metres high, Østerild augments the Høvsøre test centre, which can hold five turbines with a maximum height of 165 metres. Turbulence at Østerild gives manufacturers a wider range of test wind conditions, measured more accurately by 250 metre-high met masts. Advanced grid connection technology will allow greater experimentation, such as checking turbine tolerance to varying grid frequency. Østerild’s infrastructure is nearing completion, with two test sites already available and it will host the second prototype of Siemens’ 6 MW offshore turbine later this year.

With this rapid expansion of test facilities, manufacturers and developers have more choice in where to develop prototypes and prove new designs. Though centres often form long-term partnerships with manufacturers, a mixture of knowledge sharing and competition between independent providers can only help to raise test standards, and so improve turbine reliability and performance.

‘Companies need effective testing to prevent silly mistakes that cost money and affect the industry’s reputation,’ says MassCEC’s Rahul Yarala. ‘Of course, there’s only so much business out there and every lab needs revenue. That’s a good thing: it improves test timing, methods and cost effectiveness.’

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