Environmental Engineering Reference
In-Depth Information
Internal ring gear
Fan
Bearing
Electric
Motor
Gearbox
Brake
Pinion gear
Figure 12: Electric pitch control system .
The electric pitch control systems have been developed alternatively with the
hydraulic systems. This type of control system has a higher effi ciency than that of
hydraulic controlled systems (which is usually less than 55%) and avoids the risk
of environmental pollution due to hydraulic fl uid being split or leaked.
In an electric pitch control system as shown in Fig. 12, the motor connects to a
gearbox to lower the motor speed to a desired control speed. A drive pinion gear
engages with an internal ring gear, which is rigidly attached to the roof of the rotor
blade. Alternatively, some wind turbine manufacturers use the belt-drive structure
adjusting the pitch angle. The use of electric motors can raise the responsiveness
rate and sensitivity of blade pitch control. To enhance operation reliability, the use
of redundant pitch control systems was proposed to be equipped in large wind
turbines [ 53 ].
5.4.2 Stall control
Besides pitch control, stall control is another approach for controlling and pro-
tecting wind turbines. The concept of stall control is that the power is regulated
through stalling the blades after rated speed is achieved.
Stall control can be further divided into passive and active control approaches.
Passive stall control is basically used in wind turbines in which the blades are
bolted to the hub at a fi xed installing angle. In a passive stall-regulated wind tur-
bine, the power regulation relies on the aerodynamic features of blades. In low and
moderate wind speeds, the turbine operates near maximum effi ciency. At high
wind speeds, the turbine is automatically controlled by means of stalled blades to
limit the rotational speed and power output, protecting the turbine from excessive
wind speeds.
Compared with pitch control, a passive stall control system has a simple struc-
ture and avoids using a complex control system, leading to high reliability of the
control system. In addition, the power fl uctuations are lower for stall-regulated
turbines. However, this control method has some disadvantages, such as lower
effi ciency, the requirement of external equipment at the turbine start, larger
dynamic loads acting on the blades, nacelle, and tower, dependence on reliable
brakes for the operation safety. Therefore, this control technique has been primarily
 
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