Environmental Engineering Reference
In-Depth Information
19.4.4 Experimental Results with R-inverters
Experiments were carried out with a laboratory set-up, which consists of two single-phase
inverters controlled by dSPACE kits and powered by separate 42 VDC power supplies. The
values of the inductors and capacitors are 2
.
35 mH and 22
μ
F, respectively. The switching
frequency is 7
5 kHz and the frequency of the system is 50 Hz. The nominal output voltage is
12 V RMS.
K
i
was chosen as 4 for both inverters and the droop coefficients are:
n
1
=
.
0
.
4 and
n
2
=
2
P
2
.
The first experiment was carried out for a linear load with a rheostat of about 9
0
.
8;
m
1
=
0
.
1 and
m
2
=
0
.
2. Hence, it is expected that
P
1
=
and the
results are shown in the left column of Figure 19.4. The second experiment was carried out
with a non-linear load, consisting of a rectifier loaded with an LC filter and the same rheostat
of about 9
, and the results are shown in the right column of Figure 19.4. In both experiments,
the load was connected to Inverter 2 initially and Inverter 1 was synchronised with Inverter 2.
Inverter 1 was then put in parallel operation with Inverter 2 before it was disconnected. In both
cases, the two inverters were not able to share the load in the ratio of 2 : 1 as designed. The
output voltage was significantly lower than the nominal voltage as well. An important feature
is that both
E
1
and
E
2
were lower than the rated voltage 12 V because there is no mechanism
to increase the voltage set-point in the conventional droop control scheme (note that what is
dropped is the voltage set-point). The THD of the output voltage when the load was non-linear
was high as well, although this is not inherent with the droop control.
19.5 Inherent Limitations of Conventional Droop Control
As shown before, the conventional droop control strategy failed to share the load in proportion
to the power rating. The voltage also dropped significantly. These are due to the inherent
limitations of the conventional droop control, which will be revealed in this section by taking
R-inverters as an example. Although the analysis in the sequel is done for the case with two
inverters connected in parallel, as shown in Figure 19.5, it can be applied to multiple inverters
connected in parallel as well.
In this case, the active and reactive power of each inverter injected into the bus (Guerrero
et al.
2004, 2005) are
V
o
E
i
V
o
cos
δ
i
−
P
i
=
,
(19.3)
R
oi
E
i
V
o
R
oi
Q
i
=−
sin
δ
i
.
(19.4)
The conventional droop controller
E
∗
−
E
i
=
n
i
P
i
,
(19.5)
ω
i
=
ω
∗
+
m
i
Q
i
(19.6)
takes the form shown in Figure 19.6 to generate the amplitude and frequency of the voltage
reference
ω
∗
v
ri
for Inverter
i
(Guerrero
et al.
2006a, 2007), where
is the rated frequency.
Search WWH ::
Custom Search