Environmental Engineering Reference
In-Depth Information
7
Control of Inverter
Output Impedance
The filter capacitor of an inverter can be regarded as a part of the load and, as a result, the
output impedance of the inverter is inductive. Such inverters are referred to as L-inverters. As
mentioned in Chapter 2, the output impedance of an inverter plays an important role in the
THD of the output voltage. In this chapter, control strategies are presented to design the output
impedance to be resistive and capacitive, respectively. The corresponding inverters are referred
to as R-inverters and C-inverters. Moreover, it is shown that C-inverters can be designed to
significantly improve the THD of the output voltage. Both simulation and experimental results
are presented.
7.1 Inverters with Inductive Output Impedances (L-inverters)
The circuit of a single-phase inverter under consideration is shown in Figure 7.1(a). It consists
of a single-phase H-bridge inverter powered by a DC source, and an LC filter. The inverter
is connected to the AC bus via a circuit breaker CB and the load is assumed to be connected
to the AC bus. The control signal
u
is converted to a PWM signal to drive the H-bridge so
that the average of
u
f
over a switching period is the same as
u
, i.e.
u
u
f
. Hence, the PWM
block and the H-bridge can be ignored in the controller design (Zhong 2012a).
Although the inverter consists of an LC filter, the capacitor C can be regarded as a part of
the load instead of a part of the inverter (Zhong 2012c). This reduces the control plant to an
H-bridge and an inductor, as shown in Figure 7.1(b). The advantages of this are: (1) it reduces
the order of the control plant to be 1; and (2) it considerably simplifies the design and analysis
of the controller, which facilitates the understanding of the nature of inverter control.
Since the average of
u
f
over a switching period is the same as
u
, which is the same as the
reference voltage
≈
v
r
, there is approximately
u
=
v
r
≈
u
f
=
v
o
+
sLi
,
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