Environmental Engineering Reference
In-Depth Information
0.5
0.4
0.3
C
p
0.2
0.1
0
0
5
10
15
λ
Figure 2.8
C
p
-
λ
curve for a three- bladed rotor
result in a
C
p
-
curve that sits below the one shown in Figure 2.8. The more the blades are
pitched away from the optimal position, the lower the output at any given combination of
wind and rotor speed. Wind turbines that can control their output by pitching the blades in
this manner are called
pitch - controlled
. An alternative is to allow the process of stall to limit
the wind turbine output. Such
stall regulated
machines have a fi xed blade pitch and run at a
nominally fi xed rotor speed so that wind speeds above rated result in stalling of the blades,
thus limiting power output around the rated value. Stall regulation was widely used for the
smaller turbines (50-500 kW), but this passive approach to power control lacks the potential
for operators to adjust the turbine output in response to external conditions. In part as a con-
sequence of this, practically all MW sized wind turbines are now pitch-controlled in some
way. An approach known as
active stall control
combines pitch and stall regulation. Dynamic
power control is by stall but the blades are slowly adjusted to make sure stall occurs at the
correct power level.
The mechanical shaft power created by the wind turbine rotor is converted to electricity
by an electrical generator with a conversion effi ciency that may reach 98% for large genera-
tors. Variable speed operation requires a frequency conversion through a power electronic
converter, a process that reduces somewhat the overall effi ciency. The generator and fre-
quency conversion aspects are discussed in Chapter 4.
λ
2.4.4 Power Variability
The short, medium and long term variations in wind speed shown in Figure 2.3 affect power
system operation in different ways. This is suffi ciently important, given the signifi cant and
growing proportion of wind capacity in a number of power systems, to merit more detailed
discussion.
Variability from Second to Second
At low/medium wind speeds the electrical output from a single wind turbine could vary sub-
stantially. In Chapter 5 it will be shown that this may have a detrimental effect on the power
