Figure 3.
Effect of reactive power compensation on the P-V characteristic.
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The penetration of DG units in a microgrid can increase or decrease the voltage
stability margin depending on their power factors as well as their locations. For the long-
term voltage stability, it is more beneficial to have DGs operating at generating lagging
power factors to improve the voltage security margin, i.e., increasing the distance to voltage
collapse. When a DG operates at a leading power factor, the short-term voltage stability is
generally improved as the voltage dips are reduced.
2.5. Coordination of Voltage Control Loops
Reactive power is generally supplied for improving the bus/feeder voltage profile dur-
ing short-term faults. In islanded microgrid clusters, where generating units are nearby and
feeder lengths are relatively short, it is easy to achieve desired reactive power-sharing by
manipulating voltage control associated with DGs. Any variations in the terminal voltages
of DGs are almost closely reflected in the rest of the MG buses or feeders [
57
]. In practice,
proper reactive power-sharing among multiple DERs in a microgrid is most commonly
done through the DG interfaced converter’s Q-V droop control loop. Voltage stability
can be improved by adding DC links among radial feeders. But, AC/DC loops change
the network topology from radial to mesh, thus making its operation and control more
difficult [
58
]. Appropriate coordination of Q-V droops is crucial to avoid high circulating
reactive power flows, which may result in large voltage oscillations [
59
]. Conventional Q-V
droop control in islanded microgrid suffers from limitations such as poor voltage regulation
due to the inappropriate reactive power sharing due to line impedance mismatch and
non-identical bus voltages. There is a linear reduction in the magnitude of the reference
output voltage of a DG with the increase in the injection of its reactive power [
60
]. Thus,
DGs with steeper Q-V droop slopes may present poor dynamic performance especially in
the presence of non-linear loads. Higher droop gains in the primary control can reduce the
effect of line resistances on the current sharing accuracy, but also can cause large voltage
drops in the output terminal of the converters [
49
]. Furthermore, during overloading and
islanding conditions, DGs with voltage support capability may have to operate near their
limits [
58
]. Some of these DGs could reach their limits due to a subsequent contingency. In
such a situation, they normally switch to the current-control mode. As a consequence, the
entire microgrid could lose voltage control and eventually collapse.
Considering current mismatches and voltage drops caused by primary droop control,
a secondary controller is needed for maintaining voltage stability in the microgrid. Cen-
tralized approaches such as hierarchical control and distributed control approaches have
been proposed as an alternative to improve the load sharing accuracy (e.g., [
12
,
32
]). The
secondary controller measures the DC-link voltage, calculates the voltage correction factor,
and provides it to all converters in the microgrid to increase their output voltages.
A centralized hierarchical primary-secondary voltage control scheme for maintaining
the DC-link voltage at only one bus/feeder/PCC of a microgrid is presented in Figure
4
,
where R
d
represents the droop setting, R
1
is the line resistance, V
*
is the rated DC-link
voltage, V
MG
is the measured DC-link voltage, and δV is the calculated voltage correction
factor. In the hierarchical control approach, an additional control layer is added (secondary
control) to communicate with the primary droop control. The secondary control sets
the parameters for the primary control. In the distributed control approach, instead
of using single secondary control, distributed controllers are implemented in each of
the participating converters. These controllers communicate among each other using a
common bus [
61
]. This approach normally ensures accurate load sharing and better voltage
regulation as compared to that of hierarchical control.
Various decentralized secondary controllers are presented in the literature for main-
taining voltage stability at different buses/feeders in a microgrid network. A consensus
global average voltage estimator-based distributed secondary controller is discussed for
average voltage stability maintenance of the grid-supporting buses; however, this scheme
is not applicable for load buses in a microgrid network [
62
].
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the parameters for the primary control. In the distributed control approach, instead of
using single secondary control, distributed controllers are implemented in each of the par-
ticipating converters. These controllers communicate among each other using a common
bus [61]. This approach normally ensures accurate load sharing and better voltage regu-
lation as compared to that of hierarchical control.
Various decentralized secondary controllers are presented in the literature for main-
taining voltage stability at different buses/feeders in a microgrid network. A consensus
global average voltage estimator-based distributed secondary controller is discussed for
average voltage stability maintenance of the grid-supporting buses; however, this scheme
is not applicable for load buses in a microgrid network [62].
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