Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 82

5.6 CONTROL
Because the power in the wind increases so rapidly, all wind turbines must have a way to dump
power (not capture power) at high wind speeds. The methods of control are:
1. Change aerodynamic efficiency
a. Variable pitch, feather or stall
b. Operate at constant rpm
c. Spoilers
2. Change intercept area
a. Yaw rotor out of wind
b. Change rotor geometry
3. Brake
a. Mechanical, hydraulic
b. Air brake
c. Electrical (resistance, magnetic)
All of these methods have been used alone or in combination for control in high wind speeds and
for loss of load control. There were two vertical-axis wind turbines where they actually changed
the rotor geometry; one was a V shape that became flatter in high winds, and the other was a two-
bladed giromill where the rotor geometry changed from an H shape to a <-> shape. A blade was
designed where the length could be change as the outer part of the blade moved into the rest of
the blade.
For control in high winds, most small wind turbines and farm windmills have a tail to yaw the
wind turbine out of the wind, to furl the rotor. This operation is also called furling. There are some
wind turbines where the rotor is rotated about the horizontal axis for the high wind speed control,
rather than yawed about the vertical axis. The results are the same; the intercept area has been
decreased.
A pitch control system is one method to control rpm, start up (need high torque), and overspeed.
Blades are in the feather position (chord parallel to the wind) during shutdown, and when the
brake is released, the feather position provides starting torque, and then the pitch is changed to the
run position (pitch angle around 0°) as rpm increases. The blades are kept at the same pitch over
a range of wind speeds, the run position. For high wind speeds and overspeed control, the blades
are moved to the feather or stall position (blades perpendicular, negative pitch, to wind) to shut the
unit down. The pitch can be changed to maintain a constant rpm for synchronous generators. For
an induction generator, variable-speed generator, or alternator that operates over a range of rpm
in the run position, over this range the tip speed ratio is constant, and the unit operates at higher
efficiency.
For fixed-pitch blades, there are two possible operations, constant tip speed ratio (variable rpm),
which is the maximum efficiency, and constant rpm. The blade has to have enough twist to produce
torque for start-up, or the induction motor/generator starts the rotor at the cut-in wind speed. The
constant rpm operation with induction generators means that the maximum efficiency is reached
only at the design wind speed. Above rated power, the power output is controlled by the reduced
aerodynamic efficiency, called stall control.
Part of the control system can be electronic, generally a microprocessor or microcomputer
( Figure 5.14 ) . In constant-rpm operation, such as an induction generator, the unit is connected
to the utility line after the rpm is above the synchronous rpm of the generator. In reality, an
induction generator is not strictly constant rpm, as there is a small change in rpm (slip) with
power output. Doubly fed induction generators have a large rpm range, around 50%, and are