Environmental Engineering Reference
In-Depth Information
Synchronous Generator Supplying an Autonomous Network
A synchronous generator driven by a turbine through a gearbox is exclusively used to supply
an autonomous network. In such networks the wind turbine may be the only source of power
or more often may be supported by a storage system, e.g. a battery plus converter, or another
source such as a diesel driven generator. At present, the relatively few examples of this
scheme are restricted to small islands or to remote communities. If the wind turbine is the
main source of energy, it is also in charge of system frequency and voltage. For a fi xed fre-
quency grid, the wind turbine has to be controlled to operate at fi xed speed in spite of varia-
tions in wind speed and in system demand. This can be achieved through (a) a stall regulated
wind turbine with frequency sensitive prioritized load switching or (b) a combination of vari-
able pitch wind turbine control plus a controllable dump load and/or a storage system such
as batteries with a bidirectional converter. Such autonomous systems are at present unusual
and will not be referred to further.
Fixed Speed Wind Turbines
Figure 4.43 shows the classical turbine - gearbox - asynchronous generator - grid system used
extensively in the 1980s. It is simple, reliable and very well proven in practice. Many of the
wind turbines installed during the 1980s and 1990s were of this type and are still in service
today. In this scheme the induction machine is of cage confi guration and, due to the inevitable
slip, the speed varies marginally with power so the speed is not strictly constant. The small
mechanical compliance due to the slip provides some degree of electrical power smoothing
during gusty conditions and is of benefi t to reducing loads in the mechanical drivetrain.
Where a large induction machine is to be directly connected to the network (without speed
control), some form of
soft starter
is generally required in order to limit the currents and
torque during start-up. A soft starter is not a converter as such but simply limits the surge of
current necessary for an asynchronous generator to absorb in order to establish the rotating
magnetic fi eld when fi rst connected to the mains. As it uses thyristors it is much cheaper than
the transistor based interfaces.
Figure 4.44 shows a power-electronic soft starter comprising three pairs of antiparallel
thyristors. Initially, the thyristors are fi red very late in the cycle so that little current fl ows.
Gradually, the fi ring is advanced until the thyristors are effectively 'on' continuously, at which
Squirrel-cage
induction
generator
Gearbox
Transformer
PFC
Soft
starter
Grid
Figure 4.43
Directly connected squirrel -cage induction generator
