IEEE Electrification - March 2022 - 36

mismatch is solved by actively modifying the active or
reactive power references accordingly. However, this active
IDM is only effective with a constant power load, and it
cannot deal with the case when both the active and reactive
power mismatches are equal to zero.
Another active IDM used with GFM inverters is to add a
positive feedback branch to the reactive power control
loop. By intentionally introducing a destabilizing effect in
the islanded mode, the PCC voltage magnitude can be
forced to deviate outside the normal operating range for
the islanding detection. Yet, with the decrease of the short
circuit ratio, such a positive feedback loop may also destabilize
the GFM inverters in the grid-connected mode. To
solve this issue, a real-time discrete wavelet transform is
applied to conditionally activate the positive feedback,
which increases the implementation complexity. Up to
now, most of the reported active IDMs for GFM inverters
can only guarantee their effectiveness in limited application
scenarios, and there is no " silver bullet " currently
available for effectively detecting all types of islanding scenarios.
Therefore, there is an important need to develop
more effective and robust active IDMs for GFM inverters.
Challenges in Islanding Detection
of GFM Inverters
The Unavailability of Grid Code Requirements
and Standards
The provisions of the current international standards for
islanding detection are generally in terms of the detection
time, quality factor of the tested load, and nominal voltage
and frequency ranges. Several commonly used international
standards are summarized in Table 2. However,
the current international standards stipulate only the
islanding detection requirements for the widely used GFL
inverters, while no specific rules are formulated for the
GFM inverters.
It is an open issue if the current standards on islanding
detection are still suitable or should be amended for GFM
inverters. For instance, according to IEEE Standard
1547-2018, the GFL inverters should detect the formation
of an unintentional island and cease to energize the island
within 2 s since these inverters cannot maintain the stable
operation of the island. By contrast, GFM inverters are able
to stabilize an island because of the voltage-source characteristic.
Hence, it may become reasonable that the GFM
inverters only need to detect an unintentional island in a
timely manner, while they can continue to energize the
island as long as the voltage and frequency of the isolated
microgrid are within the normal operating range. In this
way, the reliability of the power system is enhanced compared
to the case with GFL inverters. Some other provisions,
such as the frequency and voltage limits and
detection time, may also need to be reconsidered for the
islanding detection of GFM inverters.
Conflicts Between GFM Function and IDMs
Since the GFM inverter behaves as a voltage source like
the SG, it is capable of resisting frequency and voltage
deviations and trying to bring them back to the nominal
values. However, the basic principle of islanding detection
is to intentionally make sufficiently large deviations of
electrical quantities by the IDMs. Hence, the GFM function
and the IDM may cause an opposite adjustment of frequency
or voltage magnitude, which, thus, makes their
functions counteract each other. An example is given to
clarify this challenge. Based on the mechanism of islanding
detection for GFM inverters, one possible IDM is to
increase the active power mismatch by modifying the
active power reference when the frequency deviation is
detected. For instance, if the measured frequency is below
the nominal value, the decrease of the active power reference
will cause the system frequency to be further away
from the nominal value, leading to a successful islanding
detection. On the other hand, the primary frequency control,
as one of the typical GFM functions, will increase the
active power reference when the measured frequency is
below the nominal value. Consequently, the primary frequency
control may interact with such an IDM, causing
the failure of both control functions.
A similar challenge also exists in
TABLE 2. International standards for islanding detection.
Standard
Detection
Time
IEEE 1547-2018
IEEE 929-2000
International
Electrotechnical
Commission 62116
Korean standard
German VDE 01261-1
UL
1741
36
2 s
2 s
2 s
0.5 s
0.2 s
2 s
Quality
Factor
1
2.5
1
1
2
1
IEEE Electrification Magazine / MARCH 2022
Frequency
Range
59.3-60.5 Hz
59.3-60.5 Hz
59.3-60.5 Hz
59.3-60.5 Hz
47.5-50.2 Hz
59.3-60.5 Hz
Voltage Range
0.88-1.1 p.u.
0.88-1.1 p.u.
0.88-1.15 p.u.
0.88-1.1 p.u.
0.88-1.15 p.u.
0.88-1.1 p.u.
the islanding detection of GFL inverters
with grid support services, which
makes the GFL inverters actively participate
in the regulation of the power
grid during abnormal conditions. Two
commonly used grid support services
with GFL inverters are the frequencywatt
control for frequency support
and the voltage-volt ampere reactive
(VAR) control for voltage support,
which, similar to the GFM inverters,
can also resist the voltage magnitude
and frequency deviations. There have
been several studies on the compatibility
between grid support services
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