Tower design and analysis - Wind Power Generation and Wind Turbine Design - page 529

Towers are usually classifi ed based on the relative natural frequencies of the
tower and the rotor blades. Opposite to the stiff towers, soft towers are those whose
fundamental natural frequency is lower than the blade passing frequency. A further
subdivision differentiates a soft and a soft-soft tower. A soft tower's natural fre-
quency is above the rotor frequency but below the blade passing frequency while
a soft-soft tower has its natural frequency below both the rotor frequency and the
blade passing frequency. These kinds of towers (soft and soft-soft) are generally
less expensive than the stiffer ones, since they are lighter. However, they require
particular attention and need careful dynamic analysis of the entire system to
ensure that no resonances are excited by any motions in the rest of the turbine.
2 Analysis of towers
2.1 Tower blade coupling
Design engineers are interested in understanding and analyzing the coupled dynam-
ics of wind turbine towers with associated components, especially with proliferation
of such systems worldwide for renewable energy production. As wind turbines are
becoming larger in size and are being placed in varying global wind environments,
knowledge of the dynamic behaviour is important. The behaviour of the subcompo-
nents of the system (the tower and rotor blades) as well as the dynamic interaction of
those components with each other is vital to ensure the serviceability and survivabi-
lity of such expensive power generating infrastructure. Following a conventional and
simplifi ed design analysis, the mass of the components (nacelle and rotor blades)
can be simply lumped at the top of the tower, and as long as the fundamental fre-
quencies of the tower and blades are far apart, a stochastic forced vibration analysis
could be carried out. While the simplicity of this is attractive, the fl exibility of large
rotor systems may result in either economically ineffi cient design due to the conser-
vatism required to accommodate the uncertainties of component interaction or an
unsafe design due to ignoring the coupling effects.
Published literature available regarding the dynamic interaction of wind turbine
components, especially from the point of view of the structural design of the tower
with the interaction of the mechanical rotor blade system is growing. Harrison
et al. [1] state that the motion of the tower is strongly connected to the motion of
the blades, as the blades transfer an axial force onto the low speed drive shaft
which is ultimately transferred into the nacelle base plate at the top of the tower.
The dynamic characteristics of a multi-body system have traditionally been
determined by the substructure synthesis or component mode synthesis method
[2, 3]. In coupled analyses, it is fi rst necessary to obtain the free vibration charac-
teristics of all sub-entities, prior to dynamic coupling. The free vibration proper-
ties of a tower carrying a rigid nacelle mass at the top may be evaluated by
techniques such as the discrete parameter method, the fi nite element method or by
using closed form solutions. The discrete parameter method was used by Wu and
Yang [4] in a study on the control of transmission towers under the action of sto-
chastic wind loading. Lavassas et al. [5] also used this technique to assess the