Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 60

The Tower Subsystem
A HAWT tower raises the rotor and power train to the speciied hub elevation , the dis-
tance from the ground to the center of the swept area. Minimum tower height is determined
by the required ground clearance (distance between the lowest point of the swept area and
the ground). An increase in tower height above this minimum depends on the trade-off
between the marginal increases in energy capture (because average wind speeds generally
increase with increasing elevation) and the marginal increase in system cost, including con-
struction and maintenance costs.
The principal component in the HAWT tower subsystem is the tower structure itself,
which can be a steel or reinforced-concrete shell, or a steel truss. Cylindrical shell towers
such as those shown in Figures 2-3 and 2-4 are now used almost exclusively to support large-
scale wind turbines. A type of lightweight tower construction is shown in Figure 2-10 in
which tension cables are used for truss diagonals in place of structural sections. This reduces
wake-induced or tower shadow loads on downwind rotors with rigid hubs. A typical HAWT
tower subsystem also includes a device such as a ladder and/or a powered lift for mainte-
nance, as well as cables for carrying power, control signals, and operational data between the
nacelle yaw slip ring (or cable-wrap device) and the ground.
HAWT towers are usually supported on massive spread foundations of reinforced con-
crete, although local site conditions may make a smaller foundation tied down by rock an-
chors the most economical design. Anchor bolts securing the tower to the foundation usually
extend down to the bottom of the concrete. Resistance to overturning and allowable soil
pressures are the key design requirements for HAWT foundations.
The dimensions of the tower structure ( e.g. height, diameter, wall thicknesses, and base
shape) can be adjusted within limits to obtain the desired fundamental system frequency , for
purposes of minimizing any structural-dynamic responses of the HAWT to unsteady rotor
loads. The fundamental system frequency is slightly lower than the frequency of a lexural
pendulum with the same length and stiffness as the tower and a lumped mass equal to the
masses of the rotor and the nacelle plus approximately one-third of the tower mass. The ratio
of the fundamental system frequency to the rotor speed is an indication of the relative rigid-
ity of the support provided by the tower. A stiff tower is one in which the frequency-to-rotor
speed ratio is substantially larger than the number of blades. In other words, a stiff tower's
natural vibration modes all have frequencies higher than the repetitive forcing caused by
blades passing the tower. A soft-soft tower is one in which the fundamental frequency ratio is
substantially less than one. This indicates that a soft-soft tower can attenuate vibratory loads
with frequencies of 1P and higher, at least to some degree. An intermediate ratio character-
izes a soft tower . Modern HAWTs usually have soft or soft-soft towers. Stiff towers are
generally not cost effective because of their relatively high weight, nor do they bend enough
to signiicantly attenuate transient thrust loads from wind gusts.
The Ground Equipment Station
Located in the ground equipment station are those components which are necessary for
properly interfacing the HAWT with the electric utility or other distribution system. Power
conditioning and control equipment ( e.g. transformers, circuit breakers, and the electronic
components of variable-speed generating systems), a ground control unit, and data recording
devices are typical components in the ground equipment station. Some or all of these may be
located within the base section of the tower.