Output-only Modal Analysis on Operating Wind Turbines: Application to Simulated Data

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.