Page 204 - Wind Energy Handbook

178 WIND-TURBINE PERFORMANCE

Power C P
K P ¼ ¼ (4:1)
1 3 º 3
r(ÙR) A d
2
The C P – º and K P –1=º curves for a typical fixed-pitch wind turbine are shown
in Figure 4.6. The K P –1=º curve, as stated above, has the same form as the power
– wind speed characteristic of the turbine. The efficiency of the turbine (given by
the C P – º curve) varies greatly with wind speed, a disadvantage of constant speed
operation, but it should be designed such that the maximum efficiencies are
achieved at wind speeds where there is the most energy available.

4.2.2 Stall regulation

An important feature of this K P –1=º curve is that the power, initially, falls off once
stall has occurred and then gradually increases with wind speed. This feature
provides an element of passive power output regulation, ensuring that the gen-
erator is not overloaded as the wind speed increases. Ideally, the power should rise
with wind speed to the maximum value and then remain constant regardless of the
increase in wind speed; this is called perfect stall regulation. However, stall
regulated turbines do not exhibit the ideal, passive stall behaviour.
Stall regulation provides the simplest means of controlling the maximum power
generated by a turbine to suit the sizes of the installed generator and gearbox and
until recently, at the time of writing, is the most commonly adopted control method.
The principal advantage of stall control is simplicity but there are significant
disadvantages. The power versus wind speed curve is fixed by the aerodynamic
characteristics of the blades, in particular the stalling behaviour. The post stall
power output of a turbine varies very unsteadily and in a manner which, so far,
defies prediction, see Figure 4.13, for example. The stalled blade also exhibits low
vibration damping because the flow about the blade is unattached to the low
pressure surface and blade vibration velocity has little effect on the aerodynamic
forces. The low damping can give rise to large vibration displacement amplitudes

0.6 0.06 0.6
0.5 0.05 0.5
K P
0.4 0.04 0.4
C P
0.3 C
K P
P 0.03 0.3
0.2
0.02 0.2
0.1 C
P
0.01 0.1
0
0 1 2 3 4 5 6 7 8 9
λ 0 0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
l/λ
Figure 4.6 Non-dimensional Performance Curves for Constant Speed Operation