Environmental Engineering Reference
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Figure 13: Fatigue loads spectrum for a 500 kW stall wind turbine operating in
complex terrain at 13 m/s mean wind speed [32].
considered in Fig. 10. Note that the material fatigue data are given similarly and
therefore it is possible to assess the fatigue strength of the design. Of course fatigue
assessment requires the complete load spectrum so that the targeted life time of the
machine is reproduced. The procedure is detailed in the IEC standard [35] and involves
a long list of 10-min simulations covering the entire range of mean wind speeds.
8.3 Stability assessment
Stability appeared as a problem when wind turbines were fi rst up-scaled to 600 kW.
At that time most wind turbines were stall regulated and research identifi ed
dynamic stall as the main driver of aeroelastic instability [21, 36]. In order to ana-
lyze aeroelastic stability, specifi c tools were developed and validated against full-
scale tests measurements; see for example [37-40]. The majority of these models
combine the BEM aerodynamic theory with multi-body system dynamics using
beam theory, similarly to the previously described analysis.
For stall regulated machines, the damping of the tower lateral bending mode as
well as the blade lag modes is marginal at high wind speeds where stall is expected
to have its major effect as shown in Fig. 14. Two sets of results are considered,
both obtained for zero structural damping which in log scale is estimated to add
6-10%. The fi rst set, denoted as “fi xed RPM” corresponds to the modelling of the
drive train as a spring. In this case, the damping of the tower lateral bending mode
and the blade lag modes is negative at all wind speeds. Since there is no other
source of damping except what unsteady aerodynamics contributes, it follows that
this contribution is negative. Fixing the RPM is not realistic and the generator
should be also included. A simple generator dynamics consists of assuming that
the generator torque is proportional to the rotor speed which will add extra damp-
ing. The corresponding results are denoted as “generator”. There is a substantial
increase in the damping of the tower lateral bending mode and the symmetric lag
bending mode of the blade. If structural damping is added the situation will become
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