IEEE Electrification - March 2022 - 31

new challenges on the control, stability,
and protection of these systems.
One major concern is islanding, which
can be categorized into unintentional
and intentional islanding. Compared
to intentional islanding, which is used
as a feasible solution to enhance the
grid resiliency, unintentional islanding
may cause undesirable consequences,
such as the following:
1) Without planned and coordinated
protection and control, the
voltage magnitude and frequency
would differ greatly from the
nominal values, which may cause the damage to the
end-user equipment.
2) An out-of-phase reclosure results in possible transient
overcurrent or overvoltage, which may break down
the power generation.
3) There are possible hazards to utility workers.
4) There can be confusion between the utility and independent
power producer on the responsibility for
degraded power quality.
Therefore, it is critical to detect unintentional islanding
in a timely manner.
Extensive research efforts have been made to address
the unintentional islanding of IBRs; however, they are
focused primarily on grid-following (GFL) inverters. Driven
by the grid code requirements on islanding detection, various
islanding detection methods (IDMs) have been developed
for the islanding detection of GFL inverters. However,
with the increasing penetration of GFL inverters, the safe
and stable operation of a power grid is challenged by several
limitations of these GFL inverters, such as the instability
issue in weak or faulty grids, the inability to operate in the
islanded mode, and the reduction of the system inertia.
To overcome these obstacles, grid-forming (GFM) inverters
are emerging as a promising solution. Because they are
controlled as a flexible voltage source, GFM inverters are
expected to operate more flexibly than legacy SGs in the
future inverter-dominated power systems. Nevertheless,
the voltage-source behavior of GFM inverters may become
a double-edged sword when unintentional islanding
occurs. On the one hand, GFM inverters have the capability
to sustain a stable islanding operation, thanks to their voltage
controllability. On the other hand, GFM inverters continue
to energize the islanded system without an
awareness of the islanding situation, which may cause
undesirable consequences due to the reclosure or maloperation
of utility workers. Hence, the unintentional islanding
of GFM inverters is emerging as a new risk to the
reliability of future inverter-based power systems.
This article begins by discussing the necessity of
islanding detection for GFM inverters. Then, the mechanism
of islanding detection for GFM inverters is revisited,
and the main differences from GFL inverters are
The unintentional
islanding of GFM
inverters is emerging
as a new risk to the
reliability of future
inverter-based power
systems.
highlighted. State-of-the-art solutions
for the islanding detection of GFM
inverters are briefly reviewed, and
four critical challenges faced in the
effective islanding detection of GFM
inverters are shared.
The Necessity of Islanding
Detection for GFM Inverters
The concept of GFM inverters was initially
introduced for microgrids and
later extended to large interconnected
power transmission grids. Unlike a
GFL inverter, which has a currentsource
characteristic, a GFM inverter behaves as a controllable
voltage source behind an impedance. Figure 1
illustrates the typical control diagrams of a GFM inverter
and a GFL inverter together with their equivalent circuit
representations.
Thanks to its voltage-source characteristic, a GFM
inverter can flexibly operate in both grid-connected and
islanded modes without significant reconfiguration of the
control system. GFM inverters can actively participate in
the regulation of grid voltage and frequency in the gridconnected
mode while preserving the ability of active and
reactive power controls. Further, GFM inverters are able to
naturally operate in the islanded mode. Hence, even if
unintentional islanding occurs, GFM inverters can autonomously
maintain the voltage and frequency of the resultant
island system in the allowed operating range.
Consequently, even without the knowledge of unintentional
islanding, it is possible for GFM inverters to achieve
a smooth transition from grid-connected to islanded
mode. However, it does not mean that the islanding detection
of the GFM inverters can be ignored. Considering the
possible overvoltage or overcurrent caused by the reclosure,
the GFM inverters should know the islanding state
and initiate the relevant control actions for a smooth transition
from islanded to grid-connected mode.
The possible hazard to utility workers is another concern
for the necessity of islanding detection for GFM inverters.
Moreover, since GFM inverters generally have different
control objectives in grid-connected and islanded modes,
i.e., the precise control of output power in response to grid
commands in the grid-connected mode and the tight regulation
of voltage and frequency in the islanded mode, an
effective islanding detection can help the GFM inverters to
timely modify the control references and parameters to
accommodate the corresponding control objectives.
The Mechanism of Islanding
Detection for GFM Inverters
One solution for the effective islanding detection of IBRs is
based on the communication between the inverters and
the utility, which is known as a remote detection scheme.
Alternatively, the islanding detection can also be realized
IEEE Electrification Magazine / MARCH 2022
31
IEEE Electrification - March 2022

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