Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 99

5.3.5 Examples of Commercial Systems
Table 5.1 presents a compilation of characteristics of different sizes and concepts
of commercially available wind energy systems, selected at random from a market
survey as examples [Source: Windenergie 2006 -Market survey-, BWE]. Several
classes of rated power are covered by the three technical concepts currently in use:
- induction generators, with two speeds by pole-changing (5.3.2),
- induction generators, doubly-fed, with rotor-side converter (5.3.3.3),
- synchronous generators, fully-fed, with converter (5.3.4),
partly with permanent magnet excitation (5.3.4.3)
5.4 Systems for Island Operation
5.4.1 Systems in Combined Generation
5.4.1.1 General
The main drawback of wind power converters is the unsteady performance depen-
dent on wind velocity. Especially below cut-in speed no power is available. There-
fore in island grid application other power sources and/or power storage devices are
necessary to satisfy consumer requests.
A well-known combination consists of Diesel generation and wind systems.
There are several concepts aimed to serve different load types.
In fuel-saving operation one or more Diesels are continually running; the wind
system is connected when there is sufficient wind. The Diesel generator is preferably
a self-controled synchronous machine. The wind generator may be a direct coupled
induction machine, its magnetizing current being supplied by the Diesel generator.
When there is enough wind to serve the consumer needs, the Diesel can be
stopped. A free-wheel clutch allows operating the synchronous machine as a ro-
tating condenser parallel to the induction generator.
Short-time storage devices in stand-alone systems are mostly lead-acid batteries.
Cost aspects normally prevent their expansion to long-time storage equipment.
Note that a switching concept depending on consumer priorities can also be con-
sidered, where lighting and communication equipment have first priority, and house-
hold appliances and heating have to stand back in times of low wind velocity.
5.4.1.2 Combination with Diesel Generation
Figure 5.20 is the sketch of an island grid configuration, containing two Diesels with
synchronus generators, and rotating phase shifter for controlled supply of reactive
power. Two small and one medium sized wind systems with induction generators