Environmental Engineering Reference
In-Depth Information
turbine on soft soil is quite insignificant. This confirms also the importance of SSI
for the dynamic response of turbine towers. The acquired results of medium hard
and hard soil are similar to the previous results calculated with rigid ground
connection. The TMD is highly efficient at wind speeds, which cause resonant
tower vibrations.
12.5 Conclusion
Tower vibrations occur in fore-aft direction depending on the wind speed and
structural properties in the form of periodic and transient oscillations. These
vibrations threaten the structural stability and reduce the lifetime of wind turbines
significantly. To mitigate tower vibrations, several methods have been investi-
gated. In this chapter, the efficiency of tuned mass damper (TMD) is analyzed for
wind and earthquake-induced tower vibrations of onshore wind turbines. Fur-
thermore, tuned liquid column damper (TLCD), which belongs also to the group of
TMD, has been studied. The numerical verification of efficiency of the analyzed
damper is carried out by means of a 5 MW reference onshore wind turbine.
Hereby, four calculation campaigns are presented: Onshore Turbine with TMD
and TLCD, seismically excited onshore turbine with TMD and onshore turbine
considering soil-structure interaction (SSI) with TMD.
As known from other conventional slender structures, TMD can especially
mitigate resonant vibrations effectively. The acquired results of onshore with TMD
and TLCD enhance this phenomenon. During low wind speeds, the 3P frequency of
the reference wind turbine is quite near the fundamental frequency of the tower
bending mode in fore-aft direction. Therefore, near the cut-in wind speed 3 m/s, the
reference wind turbine responses is mainly characterized by resonant tower
vibrations. By using a TMD or TLCD these vibrations can be effectively reduced.
The vibration energy level reduced by TMD and TLCD amounts to a value hereby
between 20 and 80 % depending on the wind speed. At higher wind speeds, the
resonant nature of the vibrations disappears and tower oscillations become more
transient. The efficiency of a damping at transient vibrations mainly depends on the
auxiliary damping added to the main structure. At higher wind speeds, the vibration
energy reduction acquired by TMD and TLCD reaches up to 20 %.
Also, in seismic regions electricity generation using wind power is getting
remarkable. Therefore, in order to numerically verify the efficiency of TMD, the
onshore reference wind turbine is seismically excited. Hereby, five historic
earthquakes are used: El Centro, Hachinohe, Kobe, Kocaeli, and Northridge. With
the exception of El Centro each earthquake causes resonant tower vibrations. It is
observed that similar to the transient wind-induced vibrations, the efficiency of
TMD during seismic impact caused motion is nominal. On the other hand, cal-
culated results show that by using a TMD, resonant tower vibrations, which
usually occur during the post-earthquake phase, can be effectively reduced.
