IEEE Power & Energy Magazine - May/June 2021 - 46

Instead of protecting every feeder section, subsystems can
be protected. An island can be subdivided into multiple subareas, each of which can be treated as a zone for differential protection. This will significantly reduce the expense,
although with decreased selectivity. Most of these issues
may also be relevant to islands at the subtransmission level,
but the topology and existing transmission infrastructure
may better absorb the cost of upgrades compared to distribution systems. For example, such systems would have
breakers at both ends of lines already available. This is also
true for urban distribution feeders that operate in a ringmain configuration.
Since the local protection options are either limited
or unreliable, communication-enabled protection options
can be considered. Since microgrids would typically have
a communication infrastructure and a controller, this
approach could be feasible. For example, the actual differential function can be integrated with the microgrid
controller to reduce the expense of relays. However, protection-related data must get priority in the communication network. This option may adversely affect the speed
of differential protection, but due to low fault currents, this
may not pose a problem.
Adaptive protection using directional overcurrent relays
exploiting the communication platform of microgrids can
modify protective actions according to system condition
changes. Since the fault profile is different in grid-connected and islanded modes, the focus in this approach is to
provide two different sets of settings for each relay to work
under these modes. Adaptive protection using only directional overcurrent relays can work as long as the fault contribution from inverters is large enough to be distinguishable from other events like peak load and in-rush current,
but the adaptive protection can also change the protection
scheme of the microgrid when in islanded mode to other
schemes, such as distance protection, voltage-restrained
overcurrent, or communication-assisted approaches, that
will work with low fault currents.

Conclusions
System protection has been developed and refined through
decades of experience to protect system components and
maintain stability against the fault response of synchronous
generators, which is typified by dangerously high magnitudes of fault currents and rapidly changing angles. Fault
currents from IBRs are in the range of overloads, and they do
not have rotating parts, which obviates the issue with angular instability. However, legacy protection systems designed
to tackle traditional fault responses become unreliable in the
presence of IBRs. Inside inverter-based microgrids, more
complications arise due to the change in the very topology
that is assumed to underpin its protection.
This article provides insights into the stable operation and
fault response of inverters at up to 100% penetration of IBRs
in a distribution system microgrid, both for grid-connected
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ieee power & energy magazine

and islanded modes of operation, including mode transition.
Many traditional protection schemes have drawbacks, but
communication-based options, including differential protection and adaptive protection, can emerge as potential solutions to protect such microgrids.

Acknowledgments
We thank Jacob Mueller and Nicholas S. Gurule of Sandia
National Laboratories for their contributions to the figures
used in this article.

For Further Reading
J. Rodriguez, S. Bernet, P. K. Steimer, and I. E. Lizama, " A
survey on neutral-point-clamped inverters, " IEEE Trans. Ind.
Electron., vol. 57, no. 7, pp. 2219-2230, 2010. doi: 10.1109/
TIE.2009.2032430.
S. S. Venkata, M. J. Reno, W. Bower, S. Manson, J. Reilly, and G. W. Sey Jr., " Microgrid protection: Advancing the
state of the art, " Sandia National Lab., Alberquerque, NM,
SAND2019-3167, 2019.
A. Haddadi, M. Zhao, I. Kocar, U. Karaagac, K. W. Chan,
and E. Farantatos, " Impact of inverter-based resources on
negative sequence quantities-based protection elements, "
IEEE Trans. Power Del., vol. 36, no. 1, 2020. doi: 10.1109/
TPWRD.2020.2978075.
T. Kauffmann et al., " Short-circuit model for type-IV
wind turbine generators with decoupled sequence control, "
IEEE Trans. Power Del., vol. 34, no. 5, pp. 1998-2007, 2019.
doi: 10.1109/TPWRD.2019.2908686.
L. Che, M. E. Khodayar, and M. Shahidehpour, " Adaptive
protection system for microgrids: Protection practices of a
functional microgrid system, " IEEE Electrific. Mag., vol. 2,
no. 1, pp. 66-80, 2014. doi: 10.1109/MELE.2013.2297031.
M. Ropp and M. J. Reno, " Influence of inverter-based
resources on microgrid protection: Part 2, " IEEE Power &
Energy Mag., vol. 19, no. 3, pp. 47-57, 2021.
S. Brahma, " Protection of distribution system islands fed
by inverter-interfaced sources, " in Proc. IEEE PES PowerTech 2019, Milan, Italy.
T. Patel, P. Gadde, S. Brahma, J. Hernandez-Alvidrez,
and M. Reno, " Real-time microgrid test bed for protection
and resiliency studies " , in Proc. North American Power
Symp. 2020, Tempe, AZ, April 2021.

Biographies
Matthew J. Reno is with Sandia National Laboratories, Albuquerque, New Mexico, 87185, USA.
Sukumar Brahma is with Clemson University, Clemson,
South Carolina, 29634, USA.
Ali Bidram is with the University of New Mexico, Albuquerque, New Mexico, 87131, USA.
Michael E. Ropp is with Sandia National Laboratories, Albuquerque, New Mexico, 87185, USA.

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