Environmental Engineering Reference
In-Depth Information
Type of Electrical Generation
Early wind power plants produced DC electricity, as do many small systems today
which are designed for remote stand-alone applications. Batteries are used for energy
storage, and generator speed is allowed to vary or is controlled only within modest limits.
Interfacing with conventional utility grids or diesel-electric systems, however, requires an
AC output and more stringent controls on
power quality
and
synchronization
. Until recently,
most medium- and large-scale wind turbines utilized
synchronous generators
. Larger
turbines can afford the controls necessary for the more-dificult synchronization
requirements. Utilities generally favor synchronous generators, because they provide their
own
reactive power
or
VARS
and can usually deliver excess VARS to the line when needed.
Most designers of wind turbines in the 1- kW to 100-kW range have selected
induction
generators
, because they are relatively inexpensive and easy to synchronize with the grid.
They also provide some valuable
power-train damping
. Their principal disadvantage is that
they consume considerable reactive power. When induction generators were used in limited
numbers, this was not signiicant and was usually ignored by the utility. The problem can
be solved through the use of
capacitors
in the system at a small additional cost. However,
capacitors may introduce a potential safety hazard, which is the possibility that
self-
excitation
after a
fault
opens the utility line could cause the turbine to keep generating.
Appropriate additional controls can eliminate this type of hazard.
The eficiency of an electrical generator usually falls off rapidly below its rated output.
Since the power in the wind luctuates widely, this becomes a major consideration in the
selection of rated wind speed and rated power. Some small- and medium-scale systems
have utilized two generators of different sizes. The smaller generator operates near its rated
power at low wind speed, switching over to the larger generator during higher winds. The
costs of the additional smaller generator and controls must be balanced against the losses
associated with operating the larger generator at a low percentage of its rated power.
Of necessity, both induction and synchronous generators must be operated at constant
speed in order to maintain the
grid frequency
. It has long been recognized, however, that
variable speed operation has two major advantages over synchronous operation. First, the
aerodynamic eficiency of the rotor is improved at low wind speeds if the rotor speed is
also reduced. Second, system dynamic loads are attenuated by the “lywheel” action of the
rotor, as it speeds up and slows down in response to wind gusts. There are several methods
to produce constant frequency power from a variable speed generator, at some cost.
However, the primary deterrent to the incorporation of variable speed in all but the smallest
wind turbines has been the dificulty in predicting and preventing harmonic resonances.
By the mid-1980s, advances in structural dynamic analysis,
variable-speed constant-
frequency (VSCF)
generators, and
power electronics
combined to make variable speed
operation practical in larger sizes. The Mod-5B HAWT, the Sandia 34-meter and the Eolé
VAWTs, and most new systems under development in Europe have VSCF generators.
HAWT Tower Stiffness
The towers for most early HAWTs were made of heavy
steel trusses
designed more
for stiffness than strength. High stiffness was needed to keep the
fundamental (lowest)
natural frequency
of the system higher than the blade passage frequency, in order to
minimize the possibilities of resonant vibrations and associated structural dynamic problems.
Some smaller wind turbines also used
guy cables
for stiffening. The development of better
analytical design tools allowed a change to
steel shells
for towers, which is now the
predominant coniguration. These are so-called
soft tower
designs, in which the
fundamental system frequency is less than the blade passage frequency. Care must be taken
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