Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 64

Recently there has been significant interest in using assisted stall control
(sometimes known as active stall). In this technique a slow blade pitch actuator is
used to position the blades, but the main control mechanism remains aerodynamic
stall. One reason for using assisted stall is that the braking requirements for very
large wind turbines require blade actuators, and so these can be used for assisted
stall without additional equipment.
Variable-speed wind turbines generally employ pitch regulation to limit the
power into their rotors, but stall regulation has also been used (Burton et al ., 2000).
However, for stall regulation to be effective, the rotational speed, w R , must be kept
constant. This can be done using the power electronic converters, but a key
advantage of variable-speed operation of large wind turbines is that the rotor may
accelerate when hit by a gust of wind and so reduce mechanical loads. Stall reg-
ulation, with the requirement to maintain constant speed, reduces this advantage.

3.5 Fixed-speed wind turbines

Fixed-speed wind turbines are electrically quite simple devices consisting of an
aerodynamic rotor driving a low-speed shaft, a gear box, a high-speed shaft and an
induction or asynchronous generator. From the electrical system viewpoint they are
perhaps best considered as large fan drives with torque applied to the shaft from the
wind flow. Before describing the main components and characteristics of these
turbines, the basic characteristics of an induction machine are reviewed.

3.5.1 Review of the induction (asynchronous) machine
The induction machine consists of the stator and rotor windings. When balanced
three-phase currents flow through the stator winding a field rotating at synchro-
nous speed, n s , is generated. The synchronous speed, n s , in revolutions/minute is
expressed as

120 f s
p f

n s ¼ ð 3 : 8 Þ

where f s (Hz) is the frequency of the stator currents, and p f is the number of poles. If
there is relative motion between the stator field and the rotor, voltages of frequency
f r (Hz) are induced in the rotor windings. The frequency f r is equal to the slip
frequency sf s , where the slip, s , is given by

n s n r
n s

s ¼ ð 3 : 9 Þ

where n r is the rotor speed in revolutions/minute. The slip is positive if the rotor
runs below the synchronous speed and negative if it runs above the synchronous
speed (Krauss, 2002; Kundur, 1994).
Figure 3.10 shows the schematic of the cross-section of a three-phase induction
machine with one pair of field poles, and Figure 3.11 illustrates the stator and rotor