Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 236

Fig. 11.7 never reaches the 48 V required for most battery systems. As mentioned
in the previous section, the key feature of modern power electronics is the ability
to use PWM to match generator and load characteristics. In fact PWM is even
more powerful as shown by the following example of a boost converter comprising
an insulated-gate bipolar transistor (IGBT) or similar device capable of switching
very rapidly with little power loss.
Figure 11.8 shows a simple boost converter where the input voltage V in comes
from the rectifier and the output voltage V out is the DC bus voltage in Fig. 11.3 .
L, D, and C are the inductance, diode, and capacitor, respectively, and S is the
IGBT whose switching can be controlled to provide PWM. V out is related to V in by
the remarkable equation

V out ¼ V dc = 1 a

ð Þ ð 11 : 4 Þ

which is independent of L, D, and C whose values are fixed by the need to
maintain ''continuous conduction'' and keep within the maximum allowable AC
ripple in V out . a is the duty cycle, the fraction of time that the switch is shut, e.g.
Tafticht et al. [ 12 ]. V out is greater than V dc and greater than the peak AC voltage
from the inverter if one follows the boost converter. However, if V out is too large
with respect to V dc then a must be made large and diode D must transfer a lot of
energy to the capacitor C in a short period of time. This stress on the diode and
capacitor reduces boost converter efficiency: the maximum practical boost factor
1/(1 - a) is about 3. Note on the other hand that if V out is too close to V dc then
a must be made small with the consequence that switch S must quickly transfer
significant energy to the inductor L which may again reduce boost converter
efficiency. A buck-boost converter, having the ability to step-up or step-down the
DC bus voltage, may be used in place of the boost converter to overcome
switching stresses, [ 12 ].
If V out is not fixed, varying a can provide MPPT. There are two basic ways of
doing this e.g. Munteanu et al. [ 11 ]. The first is through some form of ''perturb and
observe'' scheme by which a is altered to see if the power output increases. This
involves measuring the boost converter output (the DC bus) current and V out . If the
power is increased then a is varied further in the same direction. If the power
decreases, then the variation is reversed. In practice, however, it is often safer to

Fig. 11.8 Schematic of a
simple IGBT boost converter

D

L

V dc V out

S C