Operational Modal Analysis of Wind Turbine Drivetrain with Focus on Damping Extraction

Due to changes in generator topology, wind turbines are operating in much wider speed ranges and thus at more varying operating conditions. This has a positive influence on the energy production, but results in much wider gear mesh excitation ranges that can lead to tonalities. As such, a high-quality characterization of the modal model of the gearbox housing and its interaction with the turbine nacelle is essential to pinpoint the specific resonances responsible for these tonalities. Not only the knowledge of the resonance frequencies is essential, also the damping should be well characterized. This allows to rank the modes with regard to importance for design modifications. This paper investigates the performance of different modal estimation methods to assess these damping values. An SSI, p-LSCF and Polymax Plus estimator are benchmarked with one another. A sensitivity study is done to optimize the configuration parameters of the different modal estimators specifically for damping extraction. A simulation example is first used to perform this optimization. Acceleration signals embedding the modal content of a rotating machine are simulated to serve as inputs for the modal estimators. Once detailed insights are gained, data of a wind turbine is used. Acceleration measurements along the gearbox housing together with encoder measurements serve as inputs. Damping estimates from rotating data sets are compared. The impact on the accuracy of the damping estimates is illustrated for the different modal estimators.