Order Based Modal Analysis Versus Standard Techniques to Extract Modal Parameters of Operational Wind Turbine Gearboxes

Rotating machineries generally operate under very dynamic and complex conditions, during which structural nonlinearities, tonalities or other dynamic related phenomena may arise, affecting the assumptions made in the design phase. Techniques that allow accurately and confidently identifying the dynamic response are then of paramount importance in such complex scenarios. Wind turbine gearboxes are a typical example of such machines, as they operate under strong transient conditions caused by the turbulent and non-stationary wind speed as well as fluctuations in the electrical grid. If models can help predict these interactions, dedicated experiments need to be foreseen to characterize operational response and validate/improve the developed numerical models. For these reasons, a comprehensive test campaign has been performed at the ZF Wind Power test rig on a multi megawatt gearbox in the framework of a research project. This paper describes the different experiments performed on the gearbox in as many operating conditions as possible (standstill, stationary or transient at different torque levels) as well as the data analysis techniques used to extract a dynamic model from the measured responses. In particular, accelerations acquired in the different conditions are processed using Experimental, Operational or Order Based Modal Analysis. The objective of the paper is to understand how the structural response varies in different operating conditions, and how different methods behave in extracting modal parameters in the analyzed operating conditions.