Cross-wind modal properties of offshore wind turbines identified by full scale testing

This work presents the results of in-ﬁeld tests performed on a 2.5 MW, 80 m diameter wind turbine. During the two testcampaigns, dynamic response of the structure was monitored by using three different measurement systems, namely con-ventional strain gauges, photogrammetry and laser interferometry, whereas the turbine was both at parked condition androtating. The recorded data were analysed by using an operational modal analysis algorithm based on the least squarecomplex exponential method and several turbine parameters (eigenfrequencies and damping ratios) were extracted. Theobtained system parameters were then qualitatively compared with the results presented in a study from literature, whichincludes both aeroelastic simulations and in-ﬁeld measurements performed on a similar size and capacity wind turbine.Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS wind turbine; dynamic testing; operational modal analysis; optical measurement techniques; photogrammetry; laser interferometry

Operational modal analysis of a 2.5 MW wind turbine using optical measurement techniques and strain gauges

Output-only modal analysis of linear time-periodic systems with application to wind turbine simulation data

Operational Modal Analysis (OMA) is one of the branches of experimental modal analysis which allows extracting modal parameters based on measuring only the responses of a structure under ambient or operational excitation which is not needed to be measured. This makes OMA extremely attractive to modal analysis of big structures such as wind turbines where providing measured excitation force is an extremely difficult task. One of the main OMA assumption concerning the excitation is that it is distributed randomly both temporally and spatially. Obviously, closer the real excitation is to the assumed one, better modal parameter estimation one can expect. Traditionally, wind excitation is considered as a perfect excitation obeying the OMA assumptions. However, the present study shows that the aeroelastic phenomena due to rotor rotation dramatically changes the character of aerodynamic excitation and sets limitations on the applicability of OMA to operational wind turbines. The main purpose of the study is to warn the experimentalists about these limitations and discuss possible ways of dealing with them.

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Applicability Limits of Operational Modal Analysis to Operational Wind Turbines

Challenges in testing and monitoring the in-operation vibration characteristics of wind turbines

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.

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