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wind turbine with an induction generator. Some commercial wind turbines have induction
generators that include multiple sets of windings, allowing for multiple rotational operating
speeds. The
Vergnet GEV MP 250
(Fig. 4-4) is an example of a wind turbine with a two-
speed induction generator.
Doubly-fed induction generators,
pioneered by the large-scale prototype Mod-5B and
Growian wind turbines in the 980s (Figs. 2- and 3-35), are now some of the most widely
used generators for commercial large-scale wind turbines because of their ability to operate at
variable speed [Herrera
et al
. 985]. This feature increases energy capture by allowing the ro-
tor speed to increase and decrease as wind speed changes, maintaining a high aerodynamic ef-
ficiency. Doubly-fed induction generators are asynchronous induction generators with a mul-
tiphase wound rotor. A multiphase slipring assembly contains brushes to contact the windings
on the rotor. Using an electronic converter, both the rotor and grid currents are controlled.
In addition to being able to vary the wind turbine rotor speed by ±30 percent, it is also
possible to control reactive power to the grid with a doubly-fed induction generator. Overall
generator efficiency remains high as well. Initial costs and maintenance expenses are higher
than those for a conventional induction generator, because of the required power-electronics
converter and the slip ring assembly. Examples of manufacturers of large-scale commercial
wind turbines with doubly-fed induction generators are
Suzlon, GE,
and
Vestas
.
Synchronous AC Generators
Synchronous generators are also widely used on all sizes of wind turbines. When con-
nected to a utility grid, a synchronous generator rotates at a speed that corresponds directly to
the line frequency. For synchronous generators connected to a power inverter, the generator
rotor frequency is equal to the inverter frequency. A stationary magnetic field is provided
by the generator's stator. Turbine rotors connected to synchronous generators often operate
at constant rotational speed. Key advantages of a synchronous generator are its ability to
provide power without line excitation and its ability to control reactive power by adjustments
of the rotor field current. An example of a commercial wind turbine using a synchronous
generator is the
Windflow 500
.
Permanent magnet synchronous
generators are becoming more prevalent in all sizes of
wind turbines. An advantage of this type of generator, which uses permanent magnets to
generate a stationary magnetic field, is that it can either be connected to the utility grid or
used as a stand-alone power source. Back-to-back power converters can be used to enable
variable speed operation for increased energy capture. Furthermore, with increases in the
physical diameter of the generator, it is possible to design a
direct drive
power train with
permanent-magnet generator, which eliminates the need for a gearbox. The
Enercon E-126
is an example of a large-scale commercial wind turbine that uses a direct drive synchronous
generator. Examples of medium- and small-scale turbines with directly driven generators
include the
EWT Direct Wind 750
and the
Southwest Windpower Skystream
.
DC Generators and AC Alternators
DC generators have been used for years in small-scale commercial wind turbines, par-
ticularly at remote sites where battery charging is needed. During the 930s, wind turbines
charged batteries using shunt-wound DC generators, which were relatively massive. These
pass current directly from the generator rotor through brush contacts, requiring regular brush
replacement. Today, only a few very small turbines (ratings from 200 to 500 W) use DC
generators, remaining essentially unchanged since they were first designed 75 years ago.
Many small-scale wind turbines use
AC alternators
which utilize materials more ef-
fectively than DC generators and draw current off the stator. As a result, the slip rings in
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