Environmental Engineering Reference
In-Depth Information
Table 7.1
Steady-state performance of the inverter with
L
=
2
.
35 mH
Type of
Z
o
L
R
C
o
=
325
μ
F (3rd
+
5th)
C
o
=
479
μ
F (3rd)
THD of
v
o
39.4% 25.9%
19.8%
25.9%
V
o
11.92
11.02
11.42
11.63
and the load was a full-bridge rectifier loaded with an LC filter
L
=
150
μ
H,
C
=
1000
μ
F
and
R
L
=
9
.
7.5.1 The Case with L
35
mH
The parasitic resistance of the inductor is assumed to be 0
=
2
.
. According to the analysis
in the previous section, the optimal capacitance
C
o
can be chosen as 479
.
1
μ
F to minimise
the effect of the 3rd harmonics in
v
o
and 325
μ
F to minimise the effect of the 3rd and 5th
harmonics in
v
o
. The steady-state performance of the inverters with these controllers is shown
in Table 7.1. The results of the systemwhen the inverters were designed to have resistive output
impedances (with
K
i
=
0) using the robust
droop controller proposed in (Zhong 2012c) are also shown in Table 7.1 for comparison. The
C-inverters considerably improved the THD of the output voltage: from 39
4) and inductive output impedances (with
K
i
=
.
4% obtained by
the L-inverters and 25
.
9% obtained by the R-inverters to 19
.
8%. The lowest THD (19
.
8%)
was obtained when the capacitor was designed as
C
o
=
F to minimise the effect of the
3rd and 5th harmonics. The output voltage, the THD of the output voltage and the current
curves for the inverters with different output impedance is shown in Figure 7.7. Apparently,
the C-inverters offered the best THD and the L-inverters offered the worst THD. It is worth
noting that the purpose of these simulations is to demonstrate that C-inverters can improve
the THD of the output voltage because of the extra freedom introduced to optimise the THD
but not to reduce the overall THD to meet industrial regulations. Other techniques, e.g. the
ones proposed in (Shen
et al.
2010; Zhong
et al.
2011), can be applied to further decrease
the THD.
325
μ
7.5.2 The Case with L
25
mH
According to the above analysis, a small filter inductor helps reduce the cost and the THD of
the output voltage. In order to demonstrate this, the filter inductor
L
=
0
.
=
2
.
35 mH was replaced
with an inductor
L
25 mH and the simulations were repeated. The parasitic resistance of
the inductor is assumed to be 0
=
0
.
. The steady-state performance of the system is shown in
Table 7.2. As expected, the THD was reduced significantly with comparison to the case with
L
.
045
=
2
.
35 mH. Moreover, the C-inverters again improved the THD from 8
.
2% to 7
.
0% for
the R-inverters and from 9
0% for the L-inverters. The best THD was obtained when
the capacitor was designed to minimise the effect of the 3rd harmonics as
C
o
=
.
2% to 7
.
F. The
output voltage, the THD of the output voltage and the current curves for the inverters with
different output impedance is shown in Figure 7.8. Note that there are some variations in the
THD of the output voltage. The values in Table 7.2 are the mean values.
4500
μ
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