James Lawson, Contributor
January 10, 2013 | 6 Comments
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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