Environmental Engineering Reference
In-Depth Information
been demonstrated in large scale wind turbines. A wound rotor has the advantage of control-
lability of the generated voltage through fi eld current control. This reduces the DC link duties
compared to a rotor with permanent magnets in which the generator voltage is just propor-
tional to the speed. However, with permanent magnets the generator is more effi cient due to
the absence of the excitation loss. In either case the variable frequency AC in the machine
stator need not be sinusoidal and the freedom to use a trapezoidal waveform, for example,
can be exploited in the electromagnetic design of the machine. Additionally, as the AC output
is rectifi ed, it is not required to generate a frequency as high as 50 Hz at the highest rotational
speed.
Gearless Wind Turbines
Use of a synchronous generator through a full converter also reopens the possibility of elimi-
nating the gearbox. This approach has been successfully followed by the company Enercon
since 1993. The fi rst consideration in designing a gearless wind turbine is that the rated torque
of any electrical machine is roughly proportional to the volume of its rotor. The gearbox in
a typical 1 MW wind turbine has a ratio of about 70. Eliminating the gearbox increases the
torque and hence the volume of the required generator rotor by roughly the same factor. To
achieve the volume without the weight and to achieve a suffi cient rate of fl ux cutting, a very
large-diameter ring-shaped generator is used. It is still considerably heavier than the usual
1500 rev/min generator found in most wind turbines, but Enercon consider that this is justifi ed
by the elimination of the gearbox.
The Enercon ring generators have a large number of poles: 72 in the case of the 1.5 MW
E-66. The rotational speed of between 10 and 22 rev/min thus produces a frequency of
between 6 and 13.2 Hz. Also, their generators are six-phase, which helps to reduce the ripple
on the DC coming out of the diode bridge rectifi er. The gearless confi guration has been used
in all Enercon models from the 300 kW E-30 in 1993 to the new 4.5 MW E-112.
As shown in Figure 4.49, the machine-side converter could be a simple diode rectifi er,
which is typical in small wind turbines. In larger wind turbines, a controlled rectifi er is useful.
Depending on the choices above, the DC bus may or may not keep a constant voltage as the
speed varies. The grid-side converter is likely to be an IGBT based inverter that is required
Large-diameter
synchronous
generator
DC-DC
converter
Grid-
frequency
AC
Slip-rings
Transforme
r
Diode bridge
rectifier
IGBT
converter
Grid
6-phase
variable-frequency AC
DC link
Figure 4.49
Gearless wind turbine using a synchronous generator
