Weber_Outsourcing_book_temp.qxd 26/02/2010 11:52 Page 59
Operational Modal Analysis for Assessing the Dynamic Behaviour of Wind Turbines
Figure 1: Options for Mechanical System Identification and Verification Figure 2: Physical Carrier Structure and Model (insert) with
Measurement Locations
Computed prediction of system behaviour
Computed modal analysis Forced vibration calculation
Physical model Modal model Response model
Matrices of mass, Resonant frequencies, Matrices of
damping and stiffness damping and mode shapes frequency responses
Experimental identification of Experimental modal analysis
parameter matrices
Experimental identification of system behaviour
desirable to perform the test during its most important and complex amount of data collected depend on the frequency range being
operational state – power production. investigated. Third, the transducers have to be positioned in a manner
in which all mode shapes of interest are clearly identifiable. Preferably,
In output-only testing there are three main rules to follow. First, the recorded data will cover all relevant vibration information for the
multiple input loads acting stochastically on the structure are required, structure. If the number of transducers that can reasonably be handled
which means that excitation is random not only in time but also in is less than the number of desired measurement locations, one set of
space. The space criterion is met for megawatt-class wind turbines with transducers can be moved to additional locations. The remaining set
their extra-large rotor diameters. Second, measured data require has to remain in place as a reference.
sufficient signal-to-noise ratios and signal lengths. The latter is a
minimum of one hour, as it is the function of the lowest frequency of If the data are measured as described, the rest of the process is ‘easy’,
interest and the damping ratio. The sampling frequency and thus the since user-friendly software tools with a lot of processing ‘brains’ are
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