IEEE Power & Energy Magazine - May/June 2021 - 41

To protect semiconductor switches, most inverters include
both hardware and software switch current limiters
that keep the IBRs' fault current level low.
modes when the microgrid transitions between grid-connected
and islanded operation.
For example, in grid-connected mode, both a PV inverter
and an energy storage inverter may use current-controlled
grid-following inverter controls with no zero- or negative-sequence injections. During the microgrid transition to
islanded operation, the PV inverter will likely stay in gridfollowing control mode, but the energy storage inverter may
be designed to transition to grid-forming mode and, thus,
able to inject negative- and zero-sequence current.

Impacts of IBR on Conventional
Protection Schemes

used for detecting nonsymmetrical faults is highly
compromised under the high penetration of IBRs.
✔✔ Protection system coordination: High penetrations of
IBRs distributed throughout the system can impact
the coordination of fuses, reclosers, and overcurrent
-protection relays due to the fault current injections from
multiple locations. Furthermore, because IBRs will
produce, essentially, a fixed current into a fault, there is
no decrease in available fault current as a fault becomes
more electrically distant from the IBR, and coordination must rely on varying time delays only. Depending
on the utility practice, a minimum coordination time
interval should be met between primary and backup
overcurrent devices, independent of the fault locations,
state of the microgrid, or dispatch of the energy sources in the microgrid. The fault currents from the high
penetration of IBRs may change the tripping times of
primary and backup protection relays and violate the
minimum required coordination time interval. IBRs
can also adversely impact fuse-saving schemes.

Sequence Currents (pu)

Sequence Currents (pu)

The unique fault current characteristics of IBRs can ad--
versely impact the performance of conventional protection schemes.
✔ ✔ Impact of IBRs on overcurrent protection: The
low-fault-current contribution from IBRs can adversely impact the operation of overcurrent protection
devices. This impact is more salient in an islanded
microgrid, where no fault
current is supplied from the
upstream grid, and the mi1.5
crogrid IBRs are the sole
Fault
1.25
source of fault currents. Un1
der this scenario, the con0.75
ventional overcurrent devicNo Negative- or
Zero-Sequence
es fail to detect and isolate
0.5
Current
microgrid faults if the fault
0.25
current that they sense is
0
0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1
0.11 0.12
below their current pickup
Time (s)
settings, and it is difficult to
(a)
choose overcurrent settings
1.5
that provide sufficient sen1.25
sitivity and selectivity.
1
✔✔ Impact of IBRs on directional
0.75
protection: The potentially
0.5
unpredictable negative-se I0 = I1 = I2
0.25
quence cu r rent of I BRs
0
significantly impacts the
0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1
0.11 0.12
performance of protection
Time (s)
schemes that highly rely on
(b)
this quantity, for example, in
Positive
Negative
Zero
determining the direction of
faults. Moreover, the performance of negative-sequence figure 4. The experiment results for a single-line-to-ground fault applied to a threeovercurrent relays that are phase (a) grid-following and (b) grid-forming inverter.
may/june 2021	

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IEEE Power & Energy Magazine - May/June 2021

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