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Keeping an eye on fatigue in wind turbines

September 18, 2003

Wind farms are springing up everywhere like mushrooms. Business in this alternative sector of the energy industry is booming, thanks in part to federal and former state subsidies. The total output of the wind turbines installed in Germany today is theoretically enough to supply the whole capital of Berlin with electricity. But the durability of the power plant and hence its economic viability is dependent on the capricious moods of the energy source. The wind blows at variable strengths, shaking the rotor blades and causing frequent variations in the mechanical load. The change in the torque acting on the drive shaft leads to vibrations which can eventually damage the shaft, the shaft couplings and the gear. For this reason, manufacturers and operators of wind turbines need sensor systems to monitor the load and provide advance warning of signs of impending failure.

A suitable "watchdog" has been developed by the Fraunhofer Institute for Industrial Mathematics ITWM in collaboration with a sensor manufacturer, assisted by the Institute for Electrical Machines, Drives and Power Electronics at the University of Dortmund. "The chief advantage of this sensor is that it is a non-contact device, and does not interfere with the mechanical structure of the turbine," stresses its head Professor Stefan Kulig. "As we have learnt from earlier project work, manufacturers of turbo generators and operators of conventional power stations prefer not to have to make major modifications to their existing plant." The sensor makes use of what is known in technical parlance as the magnetostrictive effect: The magnetic properties of the steel components vary according to the applied mechanical forces, such as mechanical torque. The sensor induces a magnetic field, for instance in the rotating shaft. Four coils measure the resulting changes. These data are processed to obtain a measurement of the torsion.

"Modern wind turbines need online monitoring systems," affirms Dr. Andreas Wirsen of the ITWM. "Thus, the status reports output by our data analysis programs can be immediately transmitted to the operator." The software provides near-real-time analysis of the high-resolution data. The applied mathematical technique can even be used to calculate torsion in components which - for structural reasons - cannot be fitted directly with sensors. To acquaint manufacturers of wind energy systems with the benefits of this monitoring system, the scientists will be present at the world's leading wind energy exhibition, HUSUMwind, in Hall 3, Stand A 306, from September 23-27.

Fraunhofer-Gesellschaft