Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 27

used for small and medium wind turbines. Since the capacity of wind turbines has
entered the multi-megawatt power range in recent years, pitch control has become
dominant in the wind power market.
The active stall control technique has been developed for large wind turbines. An
active stall wind turbine has stalling blades together with a blade pitch system. Since
the blades at high wind speeds are turned towards stall, in the opposite direction as
with pitch-control systems, this control method is also referred to as negative pitch
control. Compared with passive stall control, active control provides more accurate
control on the power output and maintains the rated power at high wind speeds.
However, with the addition of the pitch-control mechanism, the active stall control
mode increases the turbine cost and decreases operation reliability.
With megawatt wind turbines becoming the mainstream in the wind power indus-
try from the late 1990s, pitch control is more favorable than stall control. It has been
reported that the number of pitch-regulated turbines is four times higher than that of
stall-regulated turbines and the trend is going to continue in coming decades [41].
5.4.3 Yaw control
In order to maximize the wind power output and minimize the asymmetric loads
acting on the rotor blades and the tower, a horizontal-axis wind turbine must be
oriented with rotor against the wind by using an active yaw control system. Like
wind pitch systems, yaw systems can be driven either electrically or hydraulically.
Generally, hydraulic yaw systems were used in the earlier time of the wind tur-
bine development [54]. In modern wind turbines, yaw control is done by electric
motors. The yaw control system usually consists of an electrical motor with a
speed reducing gearbox, a bull gear which is fi xed to the tower, a wind vane to gain
the information about wind direction, a yaw deck, and a brake to lock the turbine
securely in yaw when the required position is reached. For a large wind turbine
with high driving loads, the yaw control system may use two or more yaw motors
to work together for driving a heavy nacelle (see Figure 7).
In practice, the yaw error signals obtained from the wind vane are used to cal-
culate the average yaw angle in a short interval. When this average yaw angle
exceeds the preset threshold, the yaw motor is activated to align the turbine with
the wind direction. Thus, with heavily fi ltered wind direction measurements, the
actions of yaw control are rather limited and slow.
5.4.4 Other control approaches
In the early time of wind turbine design, ailerons were once used to control the
power output. This method involves placing moveable fl aps on the trailing edge
of rotor blades [55]. The ailerons change the lift and drag characteristics of the
blades and eventually change the rotor torque, which enable to regulate rotor speed
and rotor power output. However, this method was less successful and was soon
abandoned.
Another possibility is to yaw the rotor partly out of the wind to decrease power.
This technique of yaw control is in practice used only for tiny wind turbines
(>1 kW) [ 56 ].