Electrical Apparatus magazine, December 2013

Electrical Apparatus magazine, December 2013

This is a summary of the Electrical Apparatus December 2013 featured technical article,  “The wind turbine’s weak link,” by Richard L. Nailen, P.E.    Order a copy of this issue

Most large wind turbines around the world include a complex multi-stage gearbox coupling the typical 10 to 15 RPM of the rotor blades to the 1,500 or 1,800 RPM of the electric generator. Designers expected gearboxes to give at least 20 years of trouble-free service. However, they often fail within 3 to 11 years. Repairs and replacements are costly (up to $200,000 U.S.) and time-consuming (as long as eight months). Gearbox failure is the largest contributor to wind turbine downtime.

The wind energy industry has recognized that the stresses imposed on gears and bearings have exceeded expectations. A new IEC design standard 81400-4 is a step forward, but tests are continuing in the U.S. and Europe to improve understanding of the failure modes involved. Chief among them are Hertzian fatigue; combined bending and torsional stresses in the main rotor shaft; transient torque loads caused by unpredictable wind gusts; capacitor switching; and generator shutdown or disconnection.

Contributing to failure are impurities in gear and bearing steels, and contamination in lubricating oil. Sophisticated filtration systems are being devised to monitor particle size and density in the oil, with filtration down to 3 micron particle size. Synthetic oils with numerous additives are most useful, especially in the lower-speed gearing where high torque and low speed combine to hinder film formation. Oil viscosity and traction coefficient must be closely controlled. Unfortunately, no ideal oil composition suits the entire gearing assembly.

Extensive testing by the U.S. Department of Energy, gearbox manufacturers, and European agencies such as the FVA in Germany have pointed the way to several design improvements. They include specifications for cleaner steel, superfinishing of gear teeth, replacement of some cylindrical roller bearings by preloaded tapered roller bearings, and application to bearing and gear tooth surfaces of coatings such as black oxide and magnesium phosphate.

Maintenance remains an issue. Mounted on high towers in often remote land areas or offshore, turbines are not subject to easy surveillance. Remote condition monitoring systems–especially for vibration tracking–are a promising solution. Frequent oil analysis and prompt oil changes, however, will remain challenging.

To order a back issue with the full article, “The wind turbine’s weak link,” call 312-321-9440 or visit our online webstore.

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