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Voltage Stability of Power Systems with Renewable-Energy Inverter-Based Generators: a reviewBog'liq electronics-10-00115-v3 3012107720, notification-file, application-file, 1669973412 (3), 1669120852, 1671794695, 1671786083, 1671606844, 1671627717, 6-Hhg2maExef6D4dssx4y3oBHURCKfsq, AgioGbFzDYdNWpPFYeiuNAhafTAYCWxy, 1, Axborot texnologiyalari va kommunikatsiyalarini rivojlantirish v-www.hozir.org, - Raspberry Pi for Beginners Revised Edition 2014 (2011)Figure 3. Effect of reactive power compensation on the P-V characteristic.
2.5. Coordination of Voltage Control Loops
Reactive power is generally supplied for improving the bus/feeder voltage profile
during 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 ter-
minal 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 im-
pedance mismatch and non-identical bus voltages. There is a linear reduction in the mag-
nitude 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 pri-
mary 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]. Further-
more, 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-con-
trol mode. As a consequence, the entire microgrid could lose voltage control and eventu-
ally collapse.
Considering current mismatches and voltage drops caused by primary droop con-
trol, a secondary controller is needed for maintaining voltage stability in the microgrid.
Centralized 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 maintain-
ing 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 (second-
ary control) to communicate with the primary droop control. The secondary control sets
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