Environmental Engineering Reference
In-Depth Information
This simply means that the per-unit output impedances of all inverters operated in parallel
should be the same in order to achieve accurate proportional real power sharing for the
conventional droop control scheme. This is the basis for the virtual output impedance approach
(Guerrero
et al.
2006b) to work properly. If this is not met, then the voltage set-points
E
i
are
not the same and errors appear in real power sharing.
According to (19.5), the real power deviation
P
i
due to the voltage set-point deviation
E
i
is
1
n
i
P
i
=−
E
i
.
For two inverters operated in parallel with real power consumption of
P
1
and
P
2
, the relative
sharing error is defined (Zhong 2012c) as
P
1
n
2
−
n
1
+
P
2
n
1
n
2
e
P
%
=
P
2
.
P
1
+
P
1
P
2
, the relative real power sharing error due to the voltage set-point
When
P
1
+
P
2
=
+
deviation
E
=
E
2
−
E
1
=
E
2
−
E
1
is
P
2
=
−
E
∗
n
i
P
i
P
1
P
1
−
P
2
P
1
P
1
P
2
P
2
E
E
∗
,
=
=
e
P
%
E
∗
where
n
i
P
i
is the inverse of the voltage drop ratio at the rated power for Inverter
i
. The smaller
the droop coefficient (or the voltage drop ratio), the bigger the sharing error; the bigger the
voltage set-point deviation
E
, the bigger the sharing error. For example, for a voltage drop
n
i
P
i
E
∗
10% and a voltage set-point deviation of
E
E
∗
ratio of
10%, which is very possible
for the reasons mentioned before, the error in real power sharing is 100%. The accuracy is
very low.
=
=
19.5.2 Reactive Power Sharing
When the system is in the steady state, the two inverters work under the same frequency,
i.e.
ω
1
=
ω
2
. It is well known that this guarantees the accuracy of reactive power sharing for
R-inverters (or the accuracy of real power sharing for L-inverters); see e.g. (Li and Kao 2009).
Indeed, from (19.6), there is
m
1
Q
1
=
m
2
Q
2
.
Since the coefficients
m
i
are chosen to satisfy (19.8), reactive power sharing proportional to
their power ratings is (always) achieved, i.e.,
Q
1
S
1
=
Q
2
S
2
.
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