Generators - Wind Energy Systems for Electric Power Generation - page 50

Other Concepts

Other types of machines can be considered for application in wind energy systems.
All of them are of the synchronous type, where the induced frequency at given rota-
tional speed is considerably larger than in conventional alternating-pole machines.
For ratings above some kilowatts however, these unconventional concepts have only
appeared as special products or prototypes. The following types are notable.

Modular magnet machines

In these machines the stator is composed of single modules carrying a coil winding.
The rotor carries permanent magnets, surface mounted or in flux concentration ar-
rangement. For vehicle propulsion the machine features preferably an outside rotor.

Variable reluctance machines

The principle design is known from stepping motors and finds application as
switched reluctance motors. Evidently this machine can also be used as generator.

3.4 Generator Comparison

From the performance of the machine types discussed in the previous chapters the
following conclusions can be drawn:

• The asynchronous machine, especially in the form of cage induction machine,

is a robust and low cost generator. In the conventional solution directly coupled
to the mains, the required reactive power is drawn from the grid. This constant
speed technology may be improved by arranging for a second speed in the pole
changing concept (Danish concept), preferably in the ratio 3:2.
When using the wound rotor asynchronous machine the slip power can be
recovered. This was done in the static Kramer system. The modern solution is
the so-called doubly-fed asynchronous machine which allows, by means of a
converter, to extract or feed power in the rotor circuit. Operation with variable
speeds in a ratio of typically 2:1 requires a converter designed for approximately
1/3 of the rated power.
To use the induction machine as a stand-alone generating system, a controlled
supply of the magnetizing reactive power is necessary. This can be done by a self-
controled (active front-end) machine-side inverter.
Principal mechanical limitations prohibit very high pole numbers, say above
8. Hence asynchronous generators in wind systems are normally driven via a
gear box.

• In the synchronous machine the required magnet flux is provided by permanent

magnets or by excitation current fed into a field winding. In the latter case re-
active power and terminal voltage, respectively, are adjustable independent from