Output-only modal analysis on operating wind turbines: Application to simulated data

Summary Output-only (Operational) Modal Analysis (OMA) is a modern branch of experimental modal analysis; the main advantage of OMA is its ability to extract modal model using only measured responses. This makes OMA extremely attractive for modal analysis of big structures such wind turbines. However, there are issues preventing straightforward application of OMA to operational turbines, e.g. structure invariance during the test. The effect of rotor rotation manifests itself in the equation of motion with time-dependent coefficients. Formulating and solving eigenvalue problem lead to time-dependent eigenvalues and eigenvectors which become meaningless as modal parameters. Fortunately, so-called Coleman coordinate transformation (also known as multi-blade coordinate transformation) allows one to eliminate time dependency of the system matrices, thus converting the original time-varying eigenvalue problem to a time-invariant one. This study extends this approach to experimental modal analysis. Forward Coleman transformation is applied to the data measured on the wind turbine blades, which is then combined with responses measured on the tower. The methods of Operational Modal Analysis are then applied to the transformed data, resulting in modal frequencies, damping and mode shapes. Backward Coleman transformation is finally employed for the mode shapes for their visualization. The study demonstrates the method using simulated vibrational responses of operational 3MW wind turbine. The responses of the tower and blades were obtained from the simulation of operational wind turbine dynamics under realistic wind load using commercial aeroelastic code.