Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 327

7.2.4 Power control
Wind energy converters require appropriate control mechanisms to limit power
extraction at higher wind speeds (Fig. 7.17). Power controls prevent mechanical
deterioration of the rotor and are also required due to the capacity (thermal) limi-
tation of the generator (i.e. matching the installed electric capacity).
On principle, power and speed controls need to be distinguished /7-3/. If the
number of revolutions must be kept constant, or almost constant, power has to be
controlled accordingly. It must not exceed the installed generator capacity, as the
latter would become thermally overloaded and deteriorated in the end. If, by con-
trast, the number of revolutions is variable within certain limits (Table 7.1) the
maximum number of revolutions must not be exceeded to prevent mechanical de-
terioration of the rotor and other movable components. Moreover, the capacity
must be monitored.
Currently, two different control methods are applied for commercially available
wind energy converters to limit extractable wind power. They are referred to as
stall and pitch controls. Both methods are suitable to limit the power absorbed by
the rotor.
Stall control. Power absorption from wind can be limited by the so-called stall
effect (intentional flow separation) (Section 7.1). For this purpose wind energy
converters must be connected to a sufficiently strong grid and have to be operated
at a constant number of rotor revolutions - regardless of the actual wind speed.
Due to the described operational changes, inflow conditions at the rotor - at a
constant number of rotor revolutions and its individual blades - change to an ex-
tent that airflow breaks down at certain (elevated) wind speeds (Fig. 7.19); be-
cause of the resulting eddy currents, the rotor slows itself down or maintains the
effective torque at a constant level.
At wind velocities above the cut-in wind speed and below the nominal wind
speed, the lifting force at the rotor blade required for rotor drive is obtained by the
airflow effective on the profile. The aerodynamic angle of attack α between effec-
tive approach (inflow) velocity vI and the rotor profile chord increases with in-
creasing wind speeds and a constant, or almost constant, number of rotor revolu-
tions.
When the nominal wind speed range is reached, the angle of attack becomes so
high that, due to the strong deviation, the airflow can no longer follow the surface
contour. The flow stalls from the upper profile side (suction side; refer to Fig.
7.19). Because of the airflow stall the rotor lift is reduced; thus power extraction
from the wind can ideally be kept at a constant level.
Flow stall at a rotor blade does not always occur at the angle of attack meas-
ured at stationary airflow for a profile (so-called static stall). The phenomenon of
flow stall rather depends on the progress of the angle of attack (e.g. during gusts)
and on the three-dimensional flow incident on the rotor blade (radial flow induced
by centrifugal force) (so-called dynamic stall). Both may delay the stall, i.e. air-flow