IEEE Electrification - March 2022 - 33

by monitoring the electrical quantities at the point of common
coupling (PCC), which belongs to the local detection
schemes. The islanding condition is confirmed if the monitored
electrical quantities deviate outside the normal operating
range. Compared to remote detection schemes, local
ones are more preferable in field applications because of
their lower cost and higher reliability; thus, they are the
main focus of this article. For local detection schemes, the
power mismatches between the inverter output and load
consumption, which refers to the active and reactive
power exchanged with the grid in the grid-connected
mode, determine the variations of certain electrical quantities
at the PCC after an islanding occurrence. Such dependencies
between the electrical quantities and power
mismatch reveal the mechanism of islanding detection,
which is highly related with the control types of the IBRs.
The mechanism of islanding detection for GFL inverters
has been well acknowledged. Because of their different
behaviors from GFL inverters, it is important to revisit the
mechanism of islanding detection for GFM inverters. To
that end, a generic test system recommended by IEEE Standard
929-2000 and IEEE Standard 1547-2018 is considered in
this work, as shown in Figure 2, where the local load is
selected as a paralleled RLC load, imposing a great challenge
to the islanding detection. ∆P and ∆Q denote the
active and reactive power mismatch, respectively.
Figure 3 illustrates the GFM control scheme used in the
following analysis, which is composed of the outer P-f and
Q-V droop control and the inner single-loop
voltage-magnitude control.
Practical applications of this GFM
control scheme have been found in
the Consortium for Electric Reliability
Technology Solutions Microgrid
and in high-voltage dc transmission
systems. The P-f and Q-V droop
coefficients used for the test are Kp =
0.05 p.u. and Kq = 0.1 p.u., respectively.
It is known that the resistance R
of the RLC load determines the
active power mismatch, while the
natural resonant frequency f0 of the
RLC load is mainly related with the
reactive power mismatch. Hence, by
varying the parameters of the RLC
load, different degrees of power mismatch
can be emulated for the
islanding tests of the GFM inverters.
In the following tests, the occurrence
of unintentional islanding is
emulated by switching off the utility
breaker S1 at t = 3.5 s. As the main
focus of this part is to reexamine the
mechanism of islanding detection
for GFM inverters rather than the
effectiveness verification of IDMs,
Inverter
Breaker
IBR
L R
C
Local Load
Q
Qref
Pref
+-
P
Figure 3. The GFM control scheme implemented by the outer P-f and Q-V droop controls with the
inner single-loop voltage-magnitude control. VCO: voltage-controlled oscillator.
IEEE Electrification Magazine / MARCH 2022
33
+-
Kq
Kp
the inverter breaker S2, which is used to cease the operation
of inverters once the unintentional islanding is detected, is
always kept in the closed state.
The first simulated scenario is the case with a large
active power mismatch and a nearly zero reactive power
mismatch. The corresponding simulation results are
shown in Figure 4(a), where the grid fundamental frequency
is 60 Hz. According to IEEE Standard 929-2000, the
frequency and voltage limits for islanding detection are
(59.3 Hz, 60.5 Hz) and (0.88 p.u., 1.1 p.u.), respectively. From
Figure 4(a), it is observed that the system frequency after
islanding drifts outside the frequency limit, while the PCC
voltage magnitude exhibits a relatively small deviation.
Recalling the mechanism of islanding detection for GFL
inverters (i.e., the frequency deviation after islanding is
mainly caused by the reactive power mismatch and the
voltage magnitude deviation attributes to the active power
mismatch), it is found that, for GFM inverters, the frequency
deviation after islanding is the result of the active
power mismatch, which is completely opposite to the situation
with GFL inverters.
The second simulated scenario is the case with a large
reactive power mismatch and a nearly zero active power
mismatch. The corresponding simulation results are
shown in Figure 4(b). It is interesting to note that, even
with a large reactive power mismatch, both the frequency
and PCC voltage magnitude have only relatively small
variations after islanding and are still within the
P + jQ
S2
PCC
PL + jQL
∆P + j∆Q
S1
Utility Breaker
Grid
Figure 2. A generic system recommended by IEEE Standards 929-2000 and 1547-2018 for
islanding detection tests.
V0
++
ω1
++ ω
1/s
ϑref
Vpcc
+-
kiv/s
Vref
VCO
vref
Gate
Signal
PWM

IEEE Electrification - March 2022

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