0195136047.pdf

13.6 PRACTICAL APPLICATION: A CASE STUDY 611

Optimum

operating

point

Cp

Cp max

λ

λopt

Figure 13.6.1Typical power coefficient versus tip-speed

ratio characteristic.

P/

Pmax

shaft power

w = 10 rod/s

v, m/s

−0.2

0

0.5

1.0

Maximum

shaft

power

20 30

14 18 22 26

10

Figure 13.6.2Typical mechan-

ical power versus wind speed

characteristics of a wind turbine

for various values of shaft speed.

• Constant-speed, constant-frequency (CSCF) systems


• Variable-speed, constant-frequency (VSCF) systems


• Variable-speed, variable-frequency (VSVF) systems

The generating units in these WECSs are commonly the induction and synchronous generators.
Constant shaft-speed operation requires more complex and expensive mechanical and/or
hydraulic control systems for accurate control of the shaft speed. This is usually accomplished by
controlling the turbine blades. Since the turbine operates with a low efficiency for wind speeds
other than the rated speed, only a small portion of the available wind energy is extracted. Hence
variable-shaft-speed systems have been developed.
In variable-shaft-speed operation, the turbine is allowed to rotate at different speeds with
the varying wind speed. Optimum power transfer is possible, while the actual speed of rotation
is determined by the torque–speed characteristics of both the turbine and the generator. In such
an operating mode, major control means are inevitably placed on the electrical side, since the
control of electric systems is easy to implement, more reliable, and less costly than the control of
mechanical systems.
Figure 13.6.3 illustrates a typical double-output induction generator (DOIG) scheme, in which
the DOIG is equipped with two controlled converters to allow power flow in both directions. Power