In our case, it is clear that the 3f 0 peak dominates the low frequency
vibration, which is associated with the tower dam fluctuation.
Its second (6f 0) and third (9f 0) harmonics are also present
at lower levels. All these peaks show a broadband aspect, which
might be caused by the turbulent nature of the wind. Moreover,
the fundamental frequency of the main rotor, f 0, is also present,
but it has a small amplitude. This peak is narrower than the harmonics
of the blade passing frequency, which indicates a possible
mechanical origin related to an imbalance.
The selected vibration measurements have been plotted versus
the wind speed for all the wind turbines. The resulting scatter plots
for the four wind turbines with exactly the same gearbox are shown
in Fig. 6. For the rest of the units, the plots result in very similar
aggregates of points, so they have been omitted to avoid data
superposition. It is clear that, in general, the amplitudes of the rms
acceleration values in the band up to 10Hz tend to increase with
the wind speed. More precisely, two different behaviors can be
identified in some units. One corresponds to the range of wind
speeds up to the rated speed of 12m/s, and the other from the rated
speed up to the cut-out speed of 25 m/s. The former presents a
maximum of vibration levels between 8 and 10 m/s, and the latter
shows a more linear behavior with a positive slope; therefore, the
maximum levels are found at the highest wind speeds.
To easily quantify and show this increasing tendency and to
make a comparison among all the wind turbines, linear fits to the
data sets calculated using the least-squares method are plotted in
Fig. 7. These results show that all the wind turbines present a convincing
similarity in terms of their predicted tower vibration
amplitudes.
The mean values and the standard deviations of the slopes,
intercepts, and correlation coefficients R2 for all the units are
listed in Table 1. From this final result, a good approximation of
the expected rms value of the vibration acceleration in the frequency
band from 0 to 10 Hz can be calculated as a function of
the wind speed for all the units.