Analogous average vibration signatures with similar amplitudes are found in 14 wind turbines of the same model and under similar wind loads. The low-frequency spectrum of the nacelle–tower system vibrations shows a maximum amplitude peak at the blade passing frequency and lower amplitude peaks at the second and third harmonics. These frequencies are mainly excited by the
tower dam effect.The instantaneous vibration amplitudes show different evolutions depending on whether the wind speed is lower or higher than the rated speed. Nevertheless, a linear fit has been calculated for each unit as an approximate model to describe the vibration behavior over the entire operating range . The average linear trend can be used for setting a more reliable
and precise threshold level to protect the turbines against excessive vibrations.