Experimental identification of modal parameters of an industrial 2‐MW wind turbine

Abstract

This contribution presents modal testing of a 2-MW wind turbine on a 100-m tubular tower with a 93-m rotor developed by W2E Wind to Energy GmbH. This research is part of the DYNAWIND project of the University of Rostock and W2E. Beside classical modal analysis schemes, this contribution mainly focusses on the application of operational modal analysis techniques to a wind turbine. Specific problems are addressed, and hints for modal testing on wind turbines are given. Furthermore, an effective measurement setup is proposed for identification of the modal parameters of a wind turbine. The measurement campaign is divided in two parts. First, a measurement campaign using 8 sensor positions on a rotor blade was done while the rotor is lying on ground. Second, a detailed measurement campaign was done on the entire wind turbine with the rotor locked in Y position using 61 sensor positions on the tower, the mainframe, the gearbox, the generator, and the low-voltage unit. While the rotor blade was tested by classical and operational modal analysis techniques, the entire wind turbine was tested by operational modal analysis techniques only. The mode shapes and eigenfrequencies of the wind turbine identified within the measurement campaigns are within the expected range of the design values of the wind turbine. But in contrast, the damping ratios differ strongly from those given in guidelines and literature. Furthermore, a strong influence of aerodynamic damping compared to structural damping is observed for the first tower mode even for a parked wind turbine. KEYWORDS modal testing, operational modal analysis, 2-MW wind turbine INTRODUCTIONThe life cycle of a wind turbine is mainly influenced by its dynamics. To avoid resonances in the variable speed range of a wind turbine, resonant frequencies of the entire turbine including substructure resonant frequencies as well as harmonic excitations must be known accurately. Whereas the harmonic excitation frequencies are multiples of the rotational speed and well known, resonant frequencies have to be calculated using a proper simulation model or identified experimentally. A verification of calculated results by measurements is the preferable approach. So the extensive knowledge and the deep understanding of the dynamics of a wind turbine allows precise prediction of its behaviour. For the identification of the modal parameters the classical modal analysis (CMA) and the operational modal analysis (OMA) are chosen.The lowest resonant frequencies of the entire turbine and corresponding mode shapes, affected mainly by tower and blades, have to be considered with respect to the first and third-order excitations (rotational frequency and blade passing frequency, respectively) to avoid severe resonance problems during operation. Resonant frequencies of the main frame have to be considered with respect to higher excitation orders resulting mainly from the rotational frequency of the generator. Dynamic deflections of the main frame, potentially inducing undesirable loads on the drive tra...

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