WIND ENERGY SYSTEMS - Cd3wd

Chapter 6—Asynchronous Generators 6–14
ke 2 P e =
ω2 R a
(R s + R a ) 2 + ω 2 L 2 W/phase (17)
s
We see that at very low frequencies or for a very large load resistance that P e increases as the
square of the frequency. At very high frequencies, however, when ωL s is larger than R s + R a ,
the output power will be nearly constant as frequency increases. At rated speed and rated
power, X s will be similar in magnitude to R s + R a and the variation of P e will be nearly
proportional to the frequency.
We therefore see that a PM generator with a fixed resistive load is not an optimum load
for a wind turbine. If we insist on using such a system, it appears that we must use some sort
of blade pitching mechanism on the turbine. The blade pitching mechanism is a technically
good solution, but rather expensive. The costs of this system probably far surpass the cost
savings of the PM generator over other types of generators.
One alternative to a fixed resistance load is a variable resistance load. One way of varying
the load resistance seen by the generator is to insert a variable autotransformer between the
generator and the load resistors. The circuit for one phase of such a connection is shown in
Fig. 6. The basic equations for an autotransformer were given in the previous chapter. The
voltage seen by the load can be varied from zero to some value above the generator voltage in
this system. The power can therefore be adjusted from zero to rated in a smooth fashion. A
microcomputer is required to sense the wind speed, the turbine speed, and perhaps the rate of
change of turbine speed. It would then signal the electrical actuator on the autotransformer
to change the setting as necessary to properly load the turbine. A good control system could
anticipate changes in turbine power from changes in wind speed and keep the load near the
optimum value over a wide range of wind speeds.
Figure 6: Load adjustment with a variable autotransformer.
One problem with this concept is that the motor driven three-phase variable autotransformer
probably costs as much as the PM generator. Another problem would be mechanical
reliability of the autotransformer sliding contacts. These would certainly require regular maintenance.
We see that the advantages of the PM generator in the areas of cost and reliability
have been lost in using a variable autotransformer to control the load.
Another way of controlling the load, which eliminates the variable autotransformer, is to
use a microcomputer to switch in additional resistors as the wind speed and turbine speed
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001