Operational Modal Analysis of a Wind Turbine Mainframe using Crystal Clear SSI

Experimental investigations of the dynamic behavior of wind turbine components as well as of the turbine’s overall dynamics are important for the evaluation of applied prediction models. Operational Modal Analysis (OMA) was applied to identify resonant frequencies, corresponding mode shapes and damping values. Experimental investigations were carried out at the main frame of a Fuhrlander AG 2.5 MW wind turbine with a rotor diameter and a hub height of each ca.100m. Structural responses were recorded at a stopped turbine excited by the stochastic wind only. The complex structure of the mainframe as well as its interaction with the tower and the turbine blades makes it non-trivial to perform the modal analysis. In the paper the new Crystal Clear Stochastic Subspace Identification (CC-SSI) technique is applied. This technique produces very clear stabilization and easy automatic mode extraction even for complicated systems with a large number of modes as considered here. At a wind turbine, there is usually a restriction in easily applicable measurement locations. Since time series were recorded only at the main frame of the turbine, overall mode shapes of the entire turbine were identified applying a combined identification using Finite Element Analysis (FEA).