Environmental Engineering Reference
In-Depth Information
8
Bypassing Harmonic
Current Components
As discussed in the previous chapter, the filter capacitor C can be regarded as part of the load
instead of part of the inverter and the output impedance of an inverter can be changed to resistive
or capacitive with a simple proportional or integral controller. In general, C-inverters offer
better voltage quality than R-inverters and L-inverters. Simulation and experimental results
have indicated that the voltage THD is high for R-inverters and L-inverters when a non-linear
load is connected. It can be improved via reducing the filter inductor but the inductor cannot
be too small. In this chapter, a voltage controller is introduced to improve the voltage quality.
Moreover, it is found that the physical interpretation of the strategy is to bypass harmonic
components of the load current.
8.1 Controller Design
For the single-phase inverter shown in Figure 8.1, the inductor current
i
is measured to
construct a proportional controller so that the output impedance of the inverter is forced to be
resistive and so that it dominates the impedance between the inverter and the AC bus, as shown
in Chapter 7 to design R-inverters. This is also widely referred to as a virtual resistor (Dahono
2003, 2004; Dahono
et al.
2001; Li 2009) to dampen the resonance of the LC filter. Here, it is
used to force the output impedance of the inverter to be predominantly resistive with a large
enough
K
i
. However, a large
K
i
results in high THD in the output voltage if non-linear loads
are present. In order to improve the voltage quality, the output voltage
v
o
can be measured
and compared with the voltage reference
v
r
to form a voltage loop, as shown in Figure 8.2.
The following two equations hold for the closed-loop system consisting of Figure 8.1 and
Figure 8.2:
u
=
v
r
−
K
i
i
+
K
R
(
s
)(
v
r
−
v
o
)
,
u
f
=
sLi
+
v
o
.
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