Wind Energy

20 W. Bierbooms and D. Veldkamp
Table 3.1. Relative mean equivalent flap moment range (calculated divided by
measured; 100 = perfect correspondence) and relative standard deviation σ
Wind speed (ms −1 ) 11.4 14.7 19.5
0 constraints 87 (σ=4) 108 (σ=5) 107 (σ=4)
1 constraint 92 (σ=4) 103 (σ=4) 109 (σ=3)
3 constraints 92 (σ=3) 104 (σ=5) 112 (σ=3)
higher frequency phenomena. Even so, these comparisons are useful, for example
it can be seen from Fig. 3.5 that apparently the pitch controller in the
calculations is more “nervous” than the actual pitch system.
How good the load prediction is, is measured with the fatigue damage
equivalent load. In Table 3.1 some results are given. Each selected measured
10 min. period was reproduced 20 times, respectively without constraint, and
with wind constrained in 1 and 3 points. Mean and standard deviation of
calculated load ranges are normalized by the measured load range. There is
some improvement here and there, but on the whole it is not convincing. From
the lack of improvement it must be concluded that the fatigue loads are mainly
determined by high frequency wind variation, while the constraints only fix
the low frequency wind (as can be seen in Figs. 3.3 and 3.5). We stress that at
present it cannot be established whether this conclusion has general validity,
because the limited number of load cases that were investigated.
3.6 Conclusion
If the method of constrained wind is used in load verification, the low frequency
part of the wind and turbine loading can be reproduced well, which
makes it possible to compare time traces directly. From the three load cases
investigated here, it appeared that there was no improvement in predicted fatigue
damage equivalent load ranges. However, more work is needed to verify
this conclusion.
References
1. Wim Bierbooms, Simulation of stochastic wind fields which encompass measured
wind speed series – enabling time domain comparison of simulated and measured
wind turbine loads, EWEC, London, 2004