Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 81

Fig. 5.3 Shares of type groups installed per year in 2004 and 2007 in Germany
5.3.2 Induction Generators for Direct Grid Coupling
Induction generators directly feeding into the grid are normally cage induction ma-
chines. The grid imposes the frequency and supplies the magnetizing reactive power.
In normal operation the speed deviates only by slip values of 1% and below from
synchronism. Different from variable speed systems, this concept is often called
constant speed. Together with a stall controlled wind turbine the system can obvi-
ously operate with optimal tip-speed ratio at one single speed only.
Often a second speed is provided in the generator, generally by means of a pole
changing winding. Thus the lower occuring wind speeds can be utilized by a second
stage of higher pole number at lower rotational speed (Danish Concept). The pole
numbers are usually 4 and 6, giving a speed ratio of 3:2. In principle the same effect
can be accomplished when a smaller generator coupled used, coupled to the main
machine.
The capability of such a wind energy system is characterized by the power curve
and the annual energy yield. The latter is defined in dependence of the average
wind velocity at hub height. For calculation it is usual to assume a Weibull wind
distribution with form factor 2, i.e. a Rayleigh distribution, and constant air density.
Figure 5.5 shows an example power curve, based on measurements made in a test
field, with uncertainties indicated [IEC61400].
A sketch of such a typical system is depicted in Fig. 5.6. It shows the data acqui-
sition stage where operational and weather data are scanned, and the data processing
stage located in a computer which is at the same time the control computer. Indi-
cated is the soft switching device in form of a phase-controlled thyristor inverter,
serving to limit the rush-in currents.