Environmental Engineering Reference
In-Depth Information
E
*
Fundamental
droop controller
-
K
RMS
e
-
E
1
P
n
1
v
o
s
v
r
v
r
1
Q
ω
t
1
m
1
i
s
*
ω
ω
t
v
o
i
3
rd
-harmonic
droop controller
v
r
3
ω
t
v
o
i
v
rh
h
th
-harmonic
droop controller
Figure 21.5
Droop controller consisting of a robust droop controller at the fundamental frequency for
R-inverters and several harmonic droop controllers at individual harmonic frequencies
21.4 Simulation Results
Simulations were carried out with two inverters connected in parallel to verify the strategy. The
values of the inductors and capacitors are 2
F, respectively. The fundamental
frequency of the system was 50 Hz and the rated output voltage was 12 VRMS. The inverter
load was a rectifier bridge connected to a 9
.
35 mH and 22
μ
resistor after an LC filter with a 0
.
15 mH inductor
and a 1000
F capacitor. The inverters were designed, according to Chapter 7, to have resistive
output impedances with a proportional current feedback of
K
i
. The harmonic droop controller
shown in Figure 21.5 provides the voltage reference
μ
v
r
to the inner-loop current controller.
The droop controllers at the fundamental frequency were also designed according to (Zhong
2012c) with the droop coefficients
n
1
=
2
.
2 and
m
1
=
0
.
14 for Inverter 1 and
n
1
=
1
.
1 and
m
1
=
0
.
07 for Inverter 2. The current feedback gains were chosen as
K
i
1
=
4 and
K
i
2
=
2
and the parameter
K
e
was chosen as
K
e
=
20. The 3rd and 5th harmonic droop controllers
were adopted with coefficients chosen as
n
h
=
50.
The switching/sampling frequency needs to be high enough in order to handle the 5th
harmonics. Table 21.1 shows the voltage THD obtained with three switching frequencies. It
shows that 4 kHz is not high enough, which resulted in a high THD. The results when the
strategy was not used were also shown in Table 21.1 for comparison. The harmonic droop
control strategy considerably improved the voltage THD, by 45%. The output voltage and the
5 and
m
h
=
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