Voltage Stability of Power Systems with Renewable-Energy Inverter-Based Generators: a review

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2. Voltage Stability of Microgrids
Microgrids are managed autonomously, and act as either consumers or generators
from both grid and market perspectives [
14
]. Microgrids can isolate themselves from the
main power network in case of a fault or large disturbance [
15
,
16
]. The disconnection
or reconnection is done through a single point of connection to the utility known as the
point of common coupling (PCC). A microgrid must meet the established interface and
interoperation requirements at the PCC, such as the one defined in the IEEE Standard 1547
for connecting inverters [
17
]. A microgrid integrated with the main grid presents several
challenges in terms of electric grid operation and system reliability maintenance. Voltage
stability in a microgrid can be defined as its ability to retain the buses/feeders’ voltage
level within an acceptable range during normal operating conditions as well as after any
contingency event. Voltage instability is initially a local phenomenon and starts with an
imbalance of reactive power. A sudden change in loads or a sudden change in load flow
capacity like tripping of a transmission line are the two major causes of voltage instability.
Keeping the voltage stable is one of the crucial aspects of microgrid operation and control,
as the relatively low voltage levels, uncompensated loads, and current-limited inverter
operation in microgrids put the network at risk for voltage instability and collapse [
2
]. In a
system with voltage instability, there is at least one bus or feeder in the system for which
the voltage magnitude (V) decreases with the increment of the reactive power injection (Q)
at the same bus or feeder, i.e., dV/dQ is negative. In other words, a microgrid integrated
system is voltage unstable if V-Q sensitivity is negative for at least one bus/feeder. Mi-
crogrid voltage stability phenomenon is broadly classified based on disturbances in the
system; however, it can be further classified based on various other factors: whether the
microgrid operates grid-connected or islanded; whether the study is about the DC-link
voltage stability or it is about the system voltage stability; whether the system response to
a small disturbance is examined or its response to large disturbance; and all other factors
that affect the microgrid voltage stability such as the Q-V droop sensitivity, the inverter dy-
namics, load dynamics, and dynamics of other components, e.g., under load transformers
tap changers. Figure
1
shows some of these classifications and the corresponding factors.

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Islanded MGs with the domination of IBGs in their generator mix can be more vul-
nerable to disturbances than conventional power grids. Static voltage stability analysis is
usually not sufficient for MGs [18]. It is obvious that to be effective in enhancing voltage
stability, IBGs should have active and reactive power control to support the system. Many
voltage support strategies for grid-connected IBGs have been reported in the literature,
such as [19–22]. Enhancing voltage stability of islanded MGs with voltage support is re-
ported in [23,24]. To mitigate the voltage instability of islanded MGs, prioritized reactive
current injection from the inverter has been presented as a means for voltage support.
Since islanded MGs contain considerable resistive line parameters [25,26], a sufficient ac-
tive current component is also required in conjunction with the reactive current to en-
hance the dynamic voltage stability. Based on this requirement, effective coordination be-
tween the active and reactive current components for an IBG has been proposed in [27].

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