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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)Electronics 2021, 10, x FOR PEER REVIEW
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are known as grid-feeding converters and also require DC-link voltage as one of the sig-
nals in its constant current control loop [49]. The control loops concept for grid-forming
and grid-feeding converters are shown in Figure 2.
Microgrids require primary and secondary voltage control when operating in the is-
landed mode, while tertiary voltage control is also effective during their operation in the
grid-connected mode.
Figure 2. Control loops for grid-forming and grid-feeding converters [49].
In grid-connected mode, the grid normally absorbs all the power generated by IC in
a microgrid. However, in the cases of grid loss, each inverter should receive, from a su-
pervisory controller, new settings of the output power suitable to the microgrid load.
However, slow-acting supervisory control may lead to significant DC link voltage rise due
to the excess of circulating power among the ICs during low load [50]. A voltage support
strategy for grid feeding converter with new coordination between the active and reactive
current injection considering the voltage level as well as the margin residue is proposed
in [26] to improve the dynamic voltage stability of the islanded microgrid. Margin residue
is introduced in this strategy to avoid any damage in the inverter due to the over current
flow more than the maximum current. Voltage stability within acceptable limits can also
be assured through proper design and implementation of ICs in an islanded hybrid MG.
Microgrid loadability during contingency can be balanced by interfacing with an adjacent
microgrid. In islanded AC–DC hybrid microgrids (HMG), the amount and direction of
power transfer is realized through a droop characteristic implemented by the IC. The IC
interfacing autonomous AC and DC subgrids always consider the loadability of both sub-
grids by measuring the frequency of AC grid and voltage of DC grid. The normalized
frequency and normalized DC voltage are then utilized to determine the droop character-
istic of HMG [41,51]. In an islanded HMG, IC changes the operating role based on power
transfer direction. It is seen as a current source by heavily loaded subgrid or as a load for
lightly loaded subgrid. When operating in grid-forming mode or voltage-controlled
mode, ICs share the responsibility of maintaining the voltage with other DGs while if IC
cannot control the voltage at its AC terminal, it switches to the current-control mode to
operate in the grid supporting mode.
2.3. The Effect of Size and Duration of Disturbance
Another line of study for the microgrid voltage stability can be classified in terms of
size and duration of disturbances, and the physical nature of voltage instability [52]. There
may be different factors leading to various forms of disturbances resulting in voltage in-
stability problems. Large disturbance voltage stability is concerned with the system’s ca-
pability to regulate bus/feeder voltages following large and long-term contingencies be-
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