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Operational Modal Analysis – A Powerful Experimental Tool for
Assessing the Dynamic Behaviour of Wind Turbines
a report by
Sven-Erik Rosenow and Frank Weber
W2E Wind to Energy GmbH
The rapid development of powerful simulation tools such as the finite assembly is near or at an exciting frequency does not occur. More
element (FE) method and multibody analysis, and recently also the testing during prototyping might be warranted, since static and
combination of the two, accompanied by a rapid increase in computing dynamic deformations of the carrier structure can induce a range of
power, has enabled engineers to compute the static and dynamic problems for the components bolted to it. These problems include
properties of highly complex structures. However, the high degree of undefined air gaps in ring generators, unaccounted-for bearing
complexity of these structures demands equally powerful experimental deformations and nacelle-mounted transformers that break under
tools to verify and update the simulation tools by incorporating unanticipated dynamic load levels.
measurement data. One interface that correlates the two tool types
and verifies the simulation results is a modal model of the structure, Wind turbine manufacturers have only recently begun to use
represented by a set of modal parameters (see dynamic testing equipment to obtain a better understanding of Figure 1). The method
used to obtain the modal model is called modal analysis. the dynamic properties of their carrier assemblies and the way in which
they transfer primary and secondary loads. Due to the large dimensions
The computed modal analysis requires a physical model of the structure of the carrier structure, the test set-up, which is designed to excite the
described by the following parameters: mass, damping and stiffness. structure using real-world load levels, is expensive. Therefore, it is
The result is the modal model of the structure, which is described by the necessary to make compromises on how much of the complete assembly
following parameters: resonant frequency, modal damping and mode can be included in the test set-up. Unavoidably, some boundary
shape. The equivalent modal model can also be obtained by measuring conditions will be missing. The only test set-up that ensures a complete
the structural responses of the real structure (response model) and wind turbine representation is testing the wind turbine in the field.
analysing the measurement data via experimental modal analysis.
A New Level of Field Testing
It is rather difficult to artificially excite the entire wind turbine in theApplication to Wind Turbines
Since wind turbines are very flexible and highly dynamic structures, field. This is because the exciting force requires not only a minimum
verifying models by actual measurement is necessary for simulation tool amount of energy but also sufficient frequency content. Artificial
application. In this article we present an innovative experimental excitation using release lines is a proven method, but is difficult to
technique, available data processing software and our main realise in practice. The application of mechanical exciters, as an
measurement results. alternative, is ineffective because of its restricted induced energy
content at the lower frequency range and its high cost.
In order to avoid resonances in the variable speed range of a wind
turbine, accurate measurements of its resonant frequencies, The key advantage of the technique described in this article is that
substructure resonant frequencies and harmonic excitations are artificial excitation is not required. Transducers and analogue-to-digital
required. Whereas harmonic excitation frequencies are multiples of the (A/D) converters have advanced greatly in terms of sensitivity and
rotor rotational speed and are well-known, resonant frequencies have resolution. Very low dynamic response levels induced by wind or waves
to be calculated using a proper simulation model or identified can be measured with a high degree of accuracy. This facilitates
experimentally. When using simulations, verification of the calculated techniques that determine the physical properties of large structures
results via measurement is highly desirable. based on responses to the nearly white noise characteristic of natural or
ambient excitations. Simultaneously, the development of suitable
It is standard procedure to check resonant frequencies and stochastic identification methods has advanced. As a result, applying
corresponding mode shapes of the rotor blade(s) and tower against the output-only modal analysis, also termed operational modal analysis
first- and third-order excitation (1p and 3p) to avoid severe resonance (OMA), has become an alternative to classic input-requiring analysis
problems. Decoupled resonant frequencies of the rotor blade(s) and techniques. OMA is a low-cost and convenient, yet very powerful and
the tower can be obtained from the supplier design models with good accurate, tool to experimentally assess the physical properties of large
accuracy. They are verified via relatively straightforward measurements: engineering structures.
whereas the rotor blades are tested in a test rig at the supplier’s
facilities, the tower’s resonant frequencies can be measured after the When OMA is performed, the structural responses can be measured
wind turbine is erected in the field. under normal operating conditions as an option. For example, when
a bridge is tested for wind excitation it is not necessary to halt the
By contrast, testing of the resonant frequencies of the carrier assembly traffic flow; rather, traffic is considered and used as a typical
(carrier frame or carrier structure, including the equipment bolted to it) operational excitation source, in addition to the ambient wind.
is largely neglected. Consequently, verifying whether the carrier Likewise, if OMA is applied to a wind turbine, it is possible and
© TOUCH BRIEFINGS 2010
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