Civil Engineering Reference
In-Depth Information
FIGURE 7-5
Generator output power variation with wind speed in the blade pitch-regulated and stall-
regulated turbines.
important, as it determines all major components and their ratings. The
alternative strategies and the corresponding speed control methods fall in
the following categories.
7.2.1
One Fixed-Speed Drive
The fixed-speed operation of the generator naturally fits well with the induc-
tion generator, which is inherently a fixed-speed machine. However, the
turbine speed is generally low, whereas the electrical generator works more
efficiently at high speed. The speed match between the two is accomplished
by the mechanical gear. The gearbox reduces the speed and increases the
torque, thus improving the rotor power coefficient C
. Under varying wind
speed, the increase and decrease in electromagnetically converted torque
and power are accompanied by the corresponding increase or decrease in
the rotor slip with respect to the stator. The wind generator generally works
at 1 to 2 percent slip. The higher value benefits the drive gear, but increases
the electrical loss in the rotor, which leads to cooling difficulty.
The annual energy yield for a fixed-speed wind turbine must be analyzed
with the given wind speed distribution at the site of interest. Since the speed
is held constant, the turbine running above the rated speed is not a design
concern. However, it is possible to generate electrical power above the rated
capacity of the generator. When this happens, the generator is shut off by
p
