Environmental Engineering Reference
In-Depth Information
Transformer
Grid-frequency AC
Wound-rotor
induction
generator
Grid
Gearbox
Slip-rings
Low-
frequency
AC
DC link
Speed setpoint
IGBT
Converter
IGBT
Converter
Figure 4.47
DFIG (doubly fed induction generator)
ting the induction generator to follow commands on torque change very rapidly. This enhances
the energy extraction capability from gusts and relieves drivetrain stresses. For this computa-
tion to be executed, it is necessary to feed into the converter controller information on the
rotor speed and its angular position, the magnitude and phase of the rotor and stator currents
and information on the torque-speed curve that must be followed to maximize energy
extraction.
Reactive power, or power factor, could also be externally specifi ed. Although either con-
verter can be arranged to inject reactive power into the mains (the former through the
rotor-stator path, the latter directly into the mains), it was found that the task can be more
economically performed through the rotor-connected converter.
The function of the mains-connected converter is, in most schemes, to transfer the required
active power from the DC link to the grid or vice versa. This is carried out through a voltage
control loop which ensures that the DC voltage is maintained fi xed. For example, if the rotor-
connected converter decides that more energy should be transferred from the wind turbine to
the DC link (as a response to an increase in wind speed), the DC link capacitor will be over-
charged and its voltage will increase. This will prompt the mains-connected converter to
transfer more energy from the DC link to the mains, restoring voltage equilibrium. Because
of the substantial variation in the magnitude of the voltage in the rotor of the asynchronous
generator, the DC rail voltage is kept low. For an economic design this necessitates the con-
nection of the mains-connected converter to the grid through a transformer.
Both converters are controlled through PWM techniques which ensure minimum harmonic
injection into the mains. To protect the power electronics, an overvoltage protection is
mounted on the rotor terminals to short circuit the rotor in case of equipment malfunction.
The converters in Figure 4.47 are rated to carry only the rotor power, typically 30% of the
wind turbine rating.
Technical details of the various wind turbine generator concepts presented above can be
found in reference [8].
Wind Turbines with Full Converters
The confi guration shown in Figure 4.48 uses two full converters. Full, in this context, means
that all the power goes through the converters. The converters allow the standard squirrel-cage
 
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