Environmental Engineering Reference
In-Depth Information
Reference sine wave
Carrier wave
t
Figure 4.33
PWM construction of a switching pattern
Comparison of Switching Methods
Quasi-sine wave inverters are widely used for small standalone applications. For larger appli-
cations, and for grid-connected inverters, PWM is normally employed.
The above discussion has focused on inverters providing AC power at 50 or 60 Hz, but
inverters are also used to provide power at much higher frequencies. This is very useful
internally in converter systems because it allows the use of high frequency transformers,
which, as mentioned earlier, are much smaller, lighter and cheaper than 50/60 Hz transformers
of the same power rating. This is a topic that will be visited later when converters for renew-
able energy sources will be reviewed.
Output Control in a Grid-Connected Inverter
In a grid-connected inverter, the reference sine wave shown in Figure 4.33 is created so that
it is of the same frequency as that of the grid but it can be phase-shifted with respect to the
grid voltage. Referring to Figure 4.29 and using phasors rather than instantaneous values, it
is clear that
VVV
i
=+
o
x
where
V
i
is the fundamental component of the pulse-width modulated voltage output from
the inverter bridge,
V
o
is the output voltage, which is the grid voltage once the inverter is
grid - connected, and
V
x
is the voltage across the inductor. Furthermore, the voltage across the
inductor must lead the output current
I
o
by 90 ° , ie:
VI
x
=
j
X
o
where
X
is the inductor reactance.
Phasor diagram (a) in Figure 4.34 illustrates the general case and shows that control of the
amplitude and phase of
V
i
with respect to the grid voltage
V
o
provides control of both ampli-
tude and phase of the output current
I
o
. This is essentially the mechanism for the control of
the active and reactive power fl ows as described in Equations (4.8a) and (4.8b) in the case
of the synchronous generator.
Phasor diagram (b) shows the particular case where the inverter is operating at unity power
factor (zero reactive power fl ow) where the output current
I
o
is in phase with the grid voltage
V
o
. The required amplitude of
V
i
and its phase with respect to the grid voltage can readily be
calculated for any required output current
I
o
.
Furthermore, neglecting losses in the circuit, we know that power-out must equal power-in,
i.e.:
