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Wind turbine gearbox concept for high wind speed complex terrain applications

Little ingenuity has been shown in the design of geared generator drives in wind turbines. The common approach is to increase the size of the gearbox to match the high dynamic torque, which occurs. This however, results in large, expensive, and not very reliable gearboxes not appropriate for turbines subjected to very turbulent wind flows as for the case of operation on high wind speed complex terrain sites. Statistics on major wind turbine failures reveal gearbox malfunction as the second main cause of wind turbines break down. In addition, available gearbox design standards do not account for the highly variable amplitude dynamic loading typical in wind turbine applications.

The novelty of the work performed within the MEGAWIND project is that an integrated design methodology was used for the mechanical drive systems, to minimise dynamic overload torque. The gearbox was specifically designed for high wind speed and high turbulent conditions aiming at developing a highly reliable, low cost, low weight geared drive system for large-scale wind turbines. Special attention was given to the gearbox to achieve very low noise emission, minimizing gear noise at all power levels.

Combining innovative and careful design of the whole system with very advanced gear design and stress analysis techniques of gears resulted in an improved gearbox concept for this demanding wind turbine application. The work that was carried out focused on the development of the conceptual design of novel mechanical drive system investigating alternative gearbox configurations. Evaluation of the performance of these drives followed, by analysing their dynamic performance with an appropriate dynamic model. The final selection of the most appropriate involved also the optimisation of the parameters of the mechanical drive to give minimum dynamic overload. Detailed design of the mechanical drive e.g. shafts, bearings, gears and mounting concluded the development of the gearbox.

At the same time effort was put on developing a comprehensive instrumentation system to monitor dynamic shaft torques, axial rotor bearing loads, gear-case vibrations etc. The advanced instrumentation system was for the first time installed in an operating in complex terrain site 1.3MW wind turbine to closely monitor loads of the drive train developed inside the gearbox. Extensive field-testing was performed by experienced testing organisations according to the guidelines of IEC 61400-21 standard, including all necessary measurements for the evaluation of the developed concept, emphasising on the measurements taken by the intensively instrumented gearbox.

Refinement of the mechanical drive system will be conducted based on these measurements. From the work conducted within the MEGAWIND project it is expected that it will have a positive effect on the development of gearboxes for the next generation wind turbines. The developed design methodology to achieve an optimal gearbox for a dedicated use in a specific wind turbine is expected to improve the cost effectiveness and the reliability considerably, not only on on-shore conditions (as in the case of complex terrain applications), but also on offshore.

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