Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 91

5.3.4 Synchronous Generators
5.3.4.1 General
In wind energy application synchronous generators are used in variable speed sys-
tems only. The converter to decouple the frequency between machine and grid has to
be designed for full load, different from the doubly-fed induction generator concept.
Generators of larger ratings are generally equipped with an excitation winding,
fed via slip-rings from a separate exciter. Machines with permanent magnet (PM)
excitation have been the domaine of smaller ratings, but are gaining their share for
ratings also in the Megawatt class.
Synchronous machines are more suitable for designs with large pole numbers
than induction machines are. Hence they are the option for direct driven generators,
sparing the gear box in the system. Generators of considerable diameter and pole
number values are found in the gearless Magawatt systems.
A possible fully-fed system configuration is shown in Fig. 5.15. The synchronous
generator SM driven by a wind turbine supplies an uncontrolled rectifier RI to feed
a d.c. intermediate circuit DC1 of variable voltage. Connection to a constant voltage
intermediate circuit DC2 is made by a step-up inverter SI. The self-controled grid-
side inverter GSI is suitable to adjust the power factor. A filter F is used to comply
with power quality requirements.
The generators are normally constructed conventionally with interior rotor and
radial magnetic flux direction. It is observed that synchronous machines with PM
excitation gain more and more interest.
In recent years several unconventional concepts were developed, allegedly of-
fering advantages because of possible integration of generator rotor and wind rotor
hub. Solutions avoiding both the heavy gearbox with speed ratios around 100:1 (as
in the standard doubly-fed induction generator systems) and the large diameter gen-
erator with pole numbers of more than 70 (as in the gearless synchronous machine
system) have been introduced. Notable are
- VENSYS concept [Vensys GmbH] using a direct driven permanent magnet exte-
rior rotor synchronous generator,
Fig. 5.15 Synchronous generator with full-load converter (legend see text)