Environmental Engineering Reference
In-Depth Information
Towers for small wind turbines are generally taller in relation to their rotor diameters
than those of large turbines, in order to place the rotor at heights where the winds are more
energetic. Unlike the predominance of free-standing towers with larger commercial turbines,
guyed towers
have proven popular for small turbines (see Fig. 2-0). Small turbines are often
mounted on tubular towers made from a variety of materials including steel and fiberglass as
well as solid wood poles. For turbines rated from to 5 kW, wood poles offer an inexpensive
tower option, particularly where concrete and fiberglass towers have limited availability such
as in remote areas of developing countries. Wood poles are graded for different loads and can
be sized to the loads produced by the turbine.
Truss
or
lattice
towers are less costly and can sometimes be easier to install than free-
standing tubular or pole towers of the same height. Installation of a lattice tower usually re-
quires only simple hand tools. However, use of truss towers is declining because of problems
associated with the fasteners (which can become loose) and with fatigue of one or more of
the tower legs. Fatigue problems have been caused by improperly transferring the loads from
the tower top plates to the tower legs. Tower-to-foundation interface problems have gener-
ally been associated with poor foundation installations. Tower problems are accentuated by
excessive yaw motion and rotor vibration in highly turbulent wind regimes
Although some machines employ guyed towers to achieve additional height and there-
fore energy capture, the majority of wind turbine towers are not guyed. Guyed systems
require additional maintenance, primarily because thermal cycling causes periodic loosening
of guys and turnbuckles at most sites. Some manufacturers employ
tilt-down
towers, which
generally permit the use of a winch rather than a crane to lower and raise the tower. There
is no consensus within the industry regarding the “best” approach, though tilt-down towers
have clear advantages in remote or hurricane-prone areas, or for stand-alone applications
where crane availability is a problem.
Generator, Electrical System, and Controls
Generators and associated electrical control systems convert the mechanical power of
the rotor into electrical power. The generator type is chosen on the basis of the turbine's rated
power and the use of the electrical energy. The generator choice is also highly dependent on
the method of controlling rotor aerodynamic power and speed, as well as on the choice of the
speed increaser in the drive train. Today, both
synchronous
and
asynchronous
generators are
used in all sizes of wind turbines, but the majority of generators are asynchronous.
Asynchronous AC Generators
Single-speed asynchronous generators, typically
induction generators
, have been very
common in the wind turbine industry around the world and comprise much of the installed
base. They are often used with fixed-pitch stall-regulated wind turbines. In large- and
medium-scale turbines, most induction generators are
three-phase
with 690 VAC output. In
small-scale turbines,
single-phase
20/240 or 400 VAC outputs are most common. Induc-
tion generators are mechanically and electrically identical to induction motors. They produce
power when their rotational speed exceeds the synchronous speed of equivalent induction
motors.
Key advantages of induction generators result from their simplicity and include char-
acteristics of good reliability, low cost, and reduced electrical components and control. In-
duction generators, however, require that the magnetizing flux be provided by the utility
grid or in the case of a stand-alone system, an electrical energy storage device. Induction
generators also contribute to inductive reactance on the power line, often to the detriment of
the utility system operations. The
Nordic Windpower N1000
(Fig. 4-2) is an example of a
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