Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 314

for wind energy converter operation as it creates high strain inside the power
train, especially during gusty wind. By means of a direct current intermediate cir-
cuit or by isolated operation, synchronous generators may be operated at variable
numbers of revolutions and frequencies.
Synchronous generators also provide idle power which is required for the op-
eration of various consumers (e.g. motors). When compared to asynchronous gen-
erators, synchronous generators are characterised by slightly higher efficiencies.
Asynchronous generator. Asynchronous generators are also provided with a fixed
stator and a pivoting rotor. However, excitation (creation of a magnetic field) of
the rotor is performed differently. Rotors of asynchronous generators are provided
with windings that have direct or shunt short-circuits. When an idle asynchronous
generator is connected to an alternating current grid, voltage is induced into the
rotor winding, similar to a transformer. The applied frequency is equal to the fre-
quency of the applied voltage. As this winding is short-circuited, there is heavy
current flow, so that a magnetic field is created inside the rotor. Since the rotor
magnetic field tends to follow the stator magnetic field the rotor is accelerated.
The faster the rotor turns, the lower is the resulting relative speed of the rotor
winding and the rotating field and thus the voltage induced into its winding. Dur-
ing motor operation, the synchronous number of revolutions will be approached
until the weakening rotor magnetic field is still sufficient to compensate for the
friction losses of the rotor in idle mode. However, the synchronous number of
revolutions cannot be reached as there would be no current induced into the rotor
windings, no magnetic field and thus no torque. The specific difference between
both numbers of revolutions of the rotor and the rotating field in relation to the
rotating field is referred to as slip. Machine operation is thus asynchronous. The
more weight is put on an asynchronous generator, the higher is the resulting slip,
as higher capacities require stronger magnetic fields. More slip is associated with
more induced voltage, more current and a stronger magnetic field. During motor
operation, the operating speed is always below, and during generator operation
always above the synchronous number of revolutions. Due to these excitation
conditions, voltage and current are not in phase, so that reactive power is required.
Depending on the respective power, appropriate condensers need to be connected
or disconnected. This disadvantage is even more severe for isolated systems. In
countries such as Germany or the Netherlands, the respective reactive power re-
quired for public grid operation may be supplied by the available power stations
equipped with synchronous generators.
This "flexible" operational behaviour (Fig. 7.12) is desirable for asynchronous
generators operated in conjunction with inflexible grids (e.g. European 50 Hz
grid), to reduce the strain of the wind energy converter, and particularly the power
train, during gusty wind. However, without respective adaptations, only very
small asynchronous generators present a slip of up to approximately 10 %. Slip
decreases with an increasing machine size. Common generators with capacities