WIND ENERGY SYSTEMS - Cd3wd

Chapter 6—Asynchronous Generators 6–15
increase. The basic circuit is shown in Fig. 7. The switches can be solid state (triacs)
which are easily controlled by microcomputer logic levels and which can withstand millions of
operating cycles. Costs and reliability of this load control system are within acceptable limits.
Unfortunately, this concept leads to a marginally unstable system for the Darrieus turbine
and possibly for the horizontal axis propeller turbine as well. The instability can be observed
by examining the electrical power output of the Sandia 17 m Darrieus as shown in Fig. 8. The
power output to an optimum load is seen to pass through the peak turbine power output for
any wind speed, as was discussed in Chapter 4. The load powers for the four different resistor
combinations are shown as linear functions of n around the operating points. These curves
are reasonable approximations for the actual P e curves, as was pointed out by the discussion
following Eq. 17. We do not need better or more precise curves for P e because the instability
will be present for any load that varies at a rate less than n 3 .
Figure 7: Load adjustment by switching resistors.
We assume that the load power is determined by the curve marked R a1 and that the wind
speed is 6 m/s. The turbine will be operating at point a. If the wind speed increases to 8
m/s, the turbine torque exceeds the load torque and the turbine accelerates toward point b.
If the second resistor is switched in, the load power will increase, causing the turbine to slow
down. The new operating point would then be point c. If the wind speed drops back to 6
m/s, the load power will exceed the available power from the turbine so the turbine has to
decelerate. If the load is not removed quickly enough, the operating point will pass through
point f and the turbine will stall aerodynamically. It could even stop completely and need
to be restarted. The additional load must be dropped as soon as the turbine starts to slow
down if this condition is to be prevented.
Another way of expressing the difficulty with this control system is to note that the speed
variation is excessive. Suppose the resistance is R a1 + R a2 + R a3 and we have had a steady
wind just over 10 m/s. If the wind speed would slowly decrease to 10 m/s, the turbine would
go to the operating point marked d, and then as it slowed down further, the load would be
switched to R a1 +R a2 . The turbine would then accelerate to point e. The speed would change
from approximately 50 to 85 r/min for this example. This is a very large speed variation and
may pose mechanical difficulties to the turbine. It also places the operating point well down
from the peak of the power curve, which violates one of the original reasons for considering an
asynchronous system, that of maintaining peak power over a range of wind speeds and turbine
rotational speeds. We therefore see that the PM generator with a switched or variable resistive
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001