New challenges met by gearboxes for larger wind turbines

With the latest wind turbines now rated at more than 6MW, and plans to erect such monsters offshore, the challenge of ensuring that their gearboxes as totally reliable has grown even greater than it was before. It is possible to build wind turbines without gearboxes, but there is a cost penalty because the generators then have to be of sufficient diameter to accommodate a large number of poles. Close to each other at the Hannover Fair were the stands of RE Power, which has installed three 6.15MW, 126m rotor diameter turbines close to the German-Danish border, and Hansen Transmissions, which supplies the gearboxes. These units have two planetary stages and one helical stage. Both companies are presently majority owned by the Indian Suzlon Energy, which is headquartered in Pune. Since the plan is to install numbers of these monster machines offshore in the next few months, with all the costs associated with hiring crane barges, should they malfunction, we asked Mario Desmit, Hansen Transmission's marketing communications manager, if they were truly reliable. He insisted that they were, thanks to accelerated life testing in specially constructed test rigs at the company's production facility in Lommel, Belgium. A dynamic bearing system test rig has five hydraulic cylinders to apply axial and radial loads of up to 400kN axial force and 1500kN radial force per cylinder. It cost €2.6 million. In addition, the company also has a gearbox text rig which has a nominal power of 13.2MW, a peak power 16.8MW and cost more than €10 million. This incorporates 1,000 tonnes of steel and more than 1,000m3 of concrete. Hansen has developed what it calls a 'Design Operational Robustness Test' or DORoTe which translates a set of simulated wind loads, as supplied by customers, into a test programme which delivers corresponding rotor torques and speeds to the gearboxes. Two years of operational wind conditions can, the company says, be translated into three months of continuous testing by "Increasing loads within acceptable levels". Start, stop, normal operation with variable loading, run-pause and emergency stops are all included in the test plan. Multi body computer simulation (MBS) also plays an important part allowing an unlimited number of virtual sensors to be applied to models, as opposed to the limited number that can be applied in reality. Nonetheless, in a validation campaign on gearboxes on the test rig, they are fitted with more than 200 real world sensors in order to measure torque, acceleration, displacement, speed and rotational speeds. This enables validation of the MBS models and their gearbox transfer functions, which can be incorporated into customer MBS models of overall wind turbines. After the mechanical tests, the gearboxes are completely disassembled and all components inspected. It should be noted at this point that Hansen Industrial Gearboxes, which is based in Antwerp in Belgium and was also exhibiting at the Hannover Fair was acquired by Sumitomo Heavy Industries on March 4th 2011 and is now a separate company. It is currently in the process of being rebranded: Hansen Industrial Transmissions as opposed to Hansen Transmissions. Applications for Hansen Industrial gearboxes include: aerators, mixers and agitators, cooling towers, air cooled condensers, conveyors, hoists, mills, kilns, pumps, toasters, pulpers, cable cars, scrapers and screw pumps. Design Pointers • 'New wind turbines and gearboxes are rated at 6.15MW • Several are installed onshore and it is planned to install units off shore within the next few months • The bearings and gearboxes have been subjected to very sophisticated modelling and mechanical testing, and so will hopefully not malfunction