Environmental Engineering Reference
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Fig. 5.2 Common concepts
of systems feeding into the
grid (Legend see text)
converter; the machine may be electrically excited, via slip-rings or brushless (d),
or by permanent magnets (e).
A closer look into the concepts which are realized in the vast majority of wind
parks gives. Figure 5.2. Part (a) depicts a conventional system, with an induction
generator directly connected, driven by the wind turbine via a gear box, where speed
ratios of around 100 are common for ratings of 1500 kW and above. To avoid high
rush-in currents after switching, it is usual to have a soft-starting device, consisting
of a phase-controlled power electronic circuit. The figure may easily be amended
for a generator with two winding systems of different pole pair number, according
to the Danish Concept.
Figure part (b) is typical for systems with a synchronous generator, preferably di-
rectly driven which implies a design with a large number of pole pairs. Variants are
known where a gear box of only moderate speed ratio of around 10 is used (Multi-
brid), allowing a smaller generator size. The power is fed to the grid via a converter
with intermediate d.c. circuit which must be designed for full load (fully fed).
Figure part (c) is the circuit common for a system with doubly fed slip-ring
induction generator. In contrast to part figure (b) the converter rating is typically
only 35% of full load to allow for a speed range of 1:2. Power electronic adap-
tive devices are shown as intermediate circuit converters, but other configurations
are also possible. Modern equipment uses active front end inverters on the machine
side as well as on the grid side, to allow reactive power supply and power factor
adjustment.
5.2.3 Systems for Island Supply
Wind energy systems for feeding power into an island grid in absence of a util-
ity may operate in concepts combining different types of generation, such as
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