According to the World Wind Energy Association, the world market for wind turbine installations set a new record during 2011 - reaching a total size of 42 GW (37.6 GW in 2010). And according to preliminary data gathered by WWEA, total capacity worldwide stands at around 239 GW, enough to cover 3% of the world's electricity demand.

To meet this growing demand, more wind turbines – and ever larger units – are needed quickly. But speeding up development is a daunting task given the increasing complexity of the designs and the need for machines to operate reliably for decades in adverse weather conditions. This translates into more testing to be performed on each of the custom-designed units.

One company active at the sharp end of supplying components for wind turbine manufacturers is Moventas, a tier-one supplier of gearboxes for wind turbines. And according to Jari Toikkanen, Manager of the Research and Test Group, the number of noise and vibration tests has quadrupled in the last five years, with many projects requiring same-day turnaround.

Toikkanen explains that tests are done primarily to improve product reliability and meet strict demands from regulatory agencies such as the American Gear Manufacturers Association (AGMA) and European ISO standards: “In addition to greater product development efforts for these units, wind turbine OEMs are demanding more vibration tests that measure behaviour in greater detail than ever before,” says Toikkanen.

Gearbox resonances

Particular attention is focused on studying vibrations of the wind turbine’s massive gearbox, which uses a combination of planetary and helical gearing to step up rotor speed 100-fold to drive the electrical generator. Another major component of interest is the torque arm connecting the gearbox to the turbine framework. By way of example, for a large 3 MW model (made by Moventas) the gearbox weighs in the region of 30 tonnes, measures two metres in diameter, and is two-and-a-half metres in length. The torque arm alone is four metres wide from bushing to bushing, half a metre thick, and weighs another five tonnes.

Engineers perform extensive modal impact testing to test the resonances of these components. Why? To check they don’t match the “excitation frequencies” of the surrounding structure or gear mesh frequencies, whose vibrations could damage the framework, rotor blades, drive shafts and the huge tower (the tallest of which is over 120 metres).

Generally, says Toikkanen, the goal is to avoid the modal frequency range of 80 to 250 Hz for the torque arm, and 400 to 800 Hz in the rest of the housing structure. When resonances are identified within or near these ranges, engineers shift the modal frequencies by modifying the geometry of the gearbox components and torque arm – typically optimising stiffness properties by changing part thicknesses and shapes.

Toikkanen notes that the process is complicated by the variable gearing frequencies that cause gearbox and torque arm vibration modes at different rotor blade speeds – from an input rotation of a few rpm for a light breeze, to a maximum of ten times that for gale-force winds.

Boosting test productivity through technology

To overcome the various testing challenges needed to cope with such a high demand for testing, Moventas has now implemented its LMS Test.Lab software (with an LMS SCADAS 8 channel mobile data-acquisition system that can take all modal analysis measurements in a short time). The system contains integrated tools that Moventas engineers need for modal analysis:

• test set-up;
• control;
• measurement;
• signal conditioning;
• result analysis;
• data management;
• and report generation.

According to Toikkanen, built-in workbooks and step-by-step prompts show engineers where to enter parameters and how to proceed through the process, and this ease of use means measurements can be taken much quicker than previously.

The company's system also improves testing productivity in on-line monitoring, says Toikkanen: “We can see results immediately as measurements are being taken…with real-time visualisation, we can verify the test on the spot, see first-hand how the structure deforms with every hammer impact, and readily identify the root cause of any unexpected resonances.”

Such visualisation is important to Moventas engineers, and animated mode shapes are displayed together on the same screen with plots such as frequency response functions (FRFs) - showing vibration amplitude versus frequency at key locations on the gearbox. This enables engineers to see immediately how the gearbox housing bends and twists at various frequencies, so they can readily identify which bearings are transmitting vibrations - and determine critical gear-mesh harmonics.

Fast-response engineering projects

The difference the package has made, says Toikkanen, means that routine tests can be run “in a few days instead of weeks”. And when faster turnaround is needed, a battery of modal tests can be run in the morning with results analysed and documented the same day.

In addition to implementing LMS Test.Lab, the company has worked closely with LMS Engineering Services on projects requiring additional calculation resources – projects where fast response was needed to meet requirements for key wind-turbine manufacturer customers. Defining the scope and specifications of durability analysis calculations for these projects was coordinated in conjunction with Petri Lahtinen, chief structural analyst at Moventas.

In one project, LMS Engineering Services provided critical fatigue life analysis needed by a wind turbine manufacturer to certify a turbine. The analysis needed to check that two critical wind-turbine gearbox cylindrical components – a torque arm and gear planet carrier – would withstand loads expected over a 20-year operational lifetime. LMS engineers created finite element models of the components and applied unit load cases to determine the stress-time series on each part. This stress-time series together with the complete load time histories for the components were then used with durability simulation software (LMS Virtual.Lab) to determine fatigue life prediction for the base material.

In a further project, LMS measured gearbox rotational vibration on the low-speed input and high-speed output shafts. Signals from accelerometers mounted directly on the low-speed shaft were analysed using the system above. Signals for the high-speed shaft were obtained from a laser vibrometer system measuring rotational velocity. A series of operating response colour maps identified rotational vibration and related resonances for both shafts.

As turbine manufacturers continue to demand more rigour from their components suppliers, such streamlining of testing (demonstrated in this article) will be crucial to drive innovation in turbine development - and reduce the cost of energy generated from a project.

About the author: Jari Toikkanen is manager of the research and test group at Moventas, a tier-one supplier of gearboxes for wind turbines.