Page 275 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

262 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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Lines of constant Locus of maximum
power delivery
Turbine torque Turbine torque/speed curves
power

for increasing wind speed

Turbine rotational speed

FIGURE 10.7 Family of a typical wind turbine torque versus their rotational speed for variable wind velocities.

wind turbine generators are usually based on inexpensive permanent magnet generators. Chapter 7
discusses a PMSG wind turbine controller with maximum power optimization and a back-to-back
PWM grid-connected inverter in Figure 7.13.
Figure 10.7 shows a family of a typical wind turbine torque versus their rotational speed for vari-
able wind velocity, where a change in the maximum power point can be observed when the wind
velocity changes.
Large-scale wind turbines have built-in optimized speed and/or pitch control. However, the market
for small-scale wind turbines does not accommodate expensive solutions. Gearboxes are often used
in high-power wind turbines, but they significantly impact low-power wind turbines for two reasons:
(1) they have low efficiency, since the viscous loss (1% of rated power per gearbox stage, which is
proportional to the mechanical speed), and (2) they are the major factor (at least 19%) in the down-
time in wind turbine generation due to their maintenance needs. Therefore, an optimized electric
generator should be designed in order to have the best efficiency for a low wind velocity operating
range, without a gearbox and a direct-drive system would be preferable. Small wind turbines have
considerable cost constraints in order to be competitive in rural systems and applications for farms
and villages. Typically, small turbine generators are made of permanent magnet machines, which are
not optimized for capturing wind energy at the low wind speed range (up to about 7 m/s).

10.6.1 Mechanical Control

A pitch controller is used in the wind turbine to control the mechanical power extraction from the
turbine. As wind is intermittent and stochastic, there will be cases when the wind speed is higher
than the rated wind speed for nominal power, and the wind generator will generate higher than
power the rated conditions. This may lead to damages to the generator winding and/or may cause
system instability. Therefore, the mechanical power extraction is limited from the turbine side using
a pitch controller. A conventional pitch controller is shown in Figure 10.8, first the error signal

e 1 x s 0 90
P gen Controller β
+ – 1+T d s 0
P ref Pitch servo Rate limiter Hard limiter

FIGURE 10.8 Block diagram of a pitch controller.