Tower design and analysis - Wind Power Generation and Wind Turbine Design - page 542

G DISP-CF yielded a difference of 6% from G DISP-NI . The difference in the value
dropped to less than 1% when the modes move further apart at = 3.14 rad/s. It

was also observed from Tables 1 and 2 that the displacement and bending moment
GRFs obtained showed some disagreement, with the values of G BM-NI being higher
than those of G DISP-NI . The largest disagreements were observed at the single DOF
model and the two DOF model case of

Ω

Ω = 0 rad/s, where differences of 7 and 5%

were observed.

4 Vibration control of tower

As the wind turbines grow bigger in size and become fl exible with the increase in
rotor diameter, it is not only enough to estimate the design forces and ensure the safety
of the wind turbine. Additionally, it is necessary to control the vibration response of
the fl exible wind turbine tower. It has been observed that wind-induced accelerations
may be the reason for the unavailability of wind turbine with increased downtime and
may cause damage to the acceleration sensitive subcomponents and devices in a wind
turbine [46]. Hence, it is important to consider structural vibration control strategies
for wind turbine towers for operational reliability of wind turbines.
Vibration control strategies for fl exible and tall structures susceptible to large
wind-induced oscillations in general are becoming increasingly important, partic-
ularly with the current tendency to build higher and lighter. HAWTs are no excep-
tion, having experienced a dramatic increase in scale in the past decade. This is
particularly evident in offshore wind turbines, with rotor diameter measuring over
120 m. As the design approach is based on strength considerations, stiffness does
not increase proportionally with increase in height and these fl exible turbines may
experience large-scale blade and tower deformations having non-linear character-
istics, which may prove detrimental to the functioning of the turbine. Thus, there
is distinct merit in investigating the vibratory control of both wind turbine blades,
e.g. using blade pitch [47, 48] and towers [49], using an external energy damper.
Among the several structural vibration controllers available, tuned mass damper
(TMD) as a passive vibration control device has become popular. It suppresses
vibration by acting as an energy dissipator. Considerable amount of literature now
exists on the use of TMDs for fl exible structures [ 50- 52 ]. Use of a TMD for sup-
pression of vibration in a wind turbine tower including blade-tower interaction has
been studied by Murtagh et al. [49]. They provided a simple analytical framework
in order to qualitatively investigate the effect of a TMD on the fore-aft response of
a wind turbine tower.

4.1 Response of tower with a TMD

The displacement response of a wind turbine tower including blade-tower interac-
tion and rotationally sampled turbulence acting on the rotor blades, and with an
attached TMD may be expressed as [49]:

[ Mxt ]{ ( )} + [ Cxt ]{ ( )} + [ Kxt ]{ ( )} = { Ft ( )} + { Vt V ( )} + { F t ( )}

(20 )

T T T T B

DAMP