Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 25

5.4.1 Pitch control
The pitch control system is a vital part of the modern wind turbine. This is because
the pitch control system not only continually regulates the wind turbine's blade
pitch angle to enhance the effi ciency of wind energy conversion and power gen-
eration stability, but also serves as the security system in case of high wind speeds
or emergency situations. It requires that even in the event of grid power failure,
the rotor blades can be still driven into their feathered positions by using either
the power of backup batteries or capacitors [51] or mechanical energy storage
devices [ 52 ].
Early techniques of active blade pitch control applied hydraulic actuators to
control all blades together. However, these collective pitch control techniques
could not completely satisfy all requirements of blade pitch angle regulation, espe-
cially for MW wind turbines with the increase in blade length and hub height. This
is because wind is highly turbulent fl ow and the wind speed is proportional to the
height from the ground. Therefore, each blade experiences different loads at dif-
ferent rotation positions. As a result, more superior individual blade pitch control
techniques have been developed and implemented, allowing control of asymmet-
ric aerodynamic loads on the blades, as well as structural loads in the non-rotating
frame such as tower side-side bending. In such a control system, each blade is
equipped with its own pitch actuator, sensors and controller.
In today's wind power industry, there are primarily two types of blade pitch
control systems: hydraulic controlled and electric controlled systems. As shown in
Fig. 11, the hydraulic pitch control system uses a hydraulic actuator to drive the
blade rotating with respect to its axial centreline. The most signifi cant advantages
of hydraulic pitch control system include its large driving power, lack of a gear-
box, and robust backup power. Due to these advantages, hydraulic pitch control
systems historically dominate wind turbine control in Europe and North America
for many years.

Blade

Hydraulic Actuator

Oil
Tank

Control
System

Pump

Figure 11: Hydraulic pitch control system .