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Small Wind Energy Systems 159

12
c=3 m/s
10
c=5 m/s
Rayleigh wind distribution (%) 8 6 4 c=11 m/s
c=7 m/s
c=9 m/s
c=13 m/s

0 2
0 5 10 15 20 25
Wind speed (m/s)

FIGURE 7.5 Annual wind speed study fit with a Rayleigh probability density function.

1 year to the next, it is natural that a cycle of 1 year should be the minimum when used for evaluat-
ing the wind resources.
For optimally designing an electrical generator, the random nature of the wind distribution at a
particular site is considered, defining the machine frequency and voltage ratings. The majority of
the losses are due to the necessity of a gearbox to match the generator speed with the turbine speed.
Therefore, it is interesting to consider eliminating the gearbox in order to design an overall optimal
system.

7.4 SELF-EXCITED INDUCTION GENERATORS FOR
SMALL WIND ENERGY APPLICATIONS
A small wind power plant may use an SCIG as an IG, and it is common to have capacitors for pro-
viding the generator the required magnetizing current for self-starting conditions from black start.
An SEIG usually has simplified controllers for speed, voltage, and frequency. It is also common
to use IGs for small wind energy systems, because they are cheaper, more rugged, and robust than
other electrical machines. However, they have larger losses and also need a gearbox. Nevertheless,
IGs do not need sophisticated synchronization devices and have natural, or intrinsic, protection
against short circuits. In stand-alone operation, an SCIG can be used; sometimes, a small PMSG
is preferred together with a power converter. DFIGs are used for larger power applications, with a
power converter connected on the rotor of the machine. SCIGs are used for many industrial applica-
tions, so their widespread use facilitates the application as a generator in developing countries. An
IG requires either reactive power from the utility grid to operate or will need an inverter with a bat-
tery or capacitors for self-excitation.
A correct design of an SEIG involves many factors, such as

1. Instantaneous power and amount of energy in the primary source
2. Machine construction characteristics
3. Self-excitation process
4. Load starting procedures
5. Speed ratio of a multiple-stage gearbox