Structural health monitoring techniques applied to operating wind turbines

Modal parameter estimation, to obtain natural frequencies, modal damping and mode shapes is a key step to characterize the dynamical behavior of a structure. Operational Modal Analysis (OMA), also known as output-only modal analysis, is a technique for estimating the modal parameters only on the basis of the measured vibration data without any information on the excitation forces. This information can then be used to improve numerical models in order to monitor the operating and structural conditions of the system. While application of OMA on parked wind turbines is straightforward, the same is not true in case of operational turbines, i.e. in power production configuration. This is due to the fact that most of the key OMA assumptions are not respected by an operational wind turbine. Coleman transformation, also known as Multi Blade Coordinate (MBC) transformation, is used for having information on the dynamic interaction between the nonrotating body (tower-nacelle) and the spinning rotor. It converts the time periodic system into a time invariant one with the assumption that the system is isotropic. Standard OMA techniques can then be applied to the time invariant model in order to get the modal parameters. This paper shows the application of MBC transformation and OMA techniques to wind turbines in operational conditions. Results are compared to the ones obtained applying OMA to the corresponding time-variant systems in order to show the improvement in understanding the dynamic behavior of operating wind turbines after using MBC transformation.