IEEE Power & Energy Magazine - May/June 2021 - 107

in my view (continued from p. 108)
and fault-current characteristics than rotating machines.
2) Distribution of resources across
the microgrid: Each combination
of sources has different fault response characteristics.
3) Bidirectional flows: -Distribution
system protection was designed
for only radial faults for powe r f lows in one direction, but
most microgrids will have their
sources in a different location,
which will present fault currents
going in the other direction in
both grid-connected and island
modes.
4) Design of protection systems:
The design of protection systems
for a microgrid in islanded operating mode is different from
the connected operating mode.
Moreover, it is necessary to consider protection during the transition between these two operational modes, which may require
two different sets of protection
schemes and quick switching between the two schemes.
The interconnection of DERs to
distribution networks, as individual
devices, presents one set of challenges to protection. DERs within microgrids, connected to distribution
networks as systems, present a different set of issues. Therefore, system
protection solutions may need to vary
under different operational configurations. For example, when inverters
are used in microgrids, they need to
be designed to be compatible with
all modes of operation and provide
quality power equal to or better than
that of the utility.

Protection for Microgrids
on Secondary (Meshed)
Networks
Most microgrids installed today have a
single point of interconnection, but the
electric power grid continues to become
more interconnected and meshed over
time. The fractal grid of the future provides reliability and resilience through
may/june 2021

reconfiguration and self-healing capabilities, but this also creates challenges
to protect a meshed system. For example, networking two or more microgrids
together provides additional economic
and reliability benefits, and new protection schemes will have to be developed
to coordinate these types of interconnections. Microgrids on downtown secondary networks with a meshed circuit,
such as those in some urban downtowns,
present different protection challenges.
Today, the protection of meshed networks is based in part on an assumption
of unidirectional current flow through
certain parts of the system, and few viable alternatives to this approach are
presently available. Protection challenges arise when microgrids operate on a
portion of a mashed network. The DERs
in the microgrid can interfere with a network protector auto-reclosing scheme to
the point that it may not reclose because
the network voltage or phase angle is
outside the permissive closing boundary. Further, DERs within the microgrid
can cause inadvertent islanding if network protectors trip on reverse power
from the DER. These are highly consequential considerations when developing protection schemes for microgrids
in secondary networks in major cities.
R&D into any or all of these issues could provide valuable advancements in the protection of microgrids
in secondary networks. Examples are
technologies for low-cost differential
protection of IBRs in microgrids.
The ability to use inverter capabilities to assist in fault detection and isolation has not been studied sufficiently
at this time, and research into that possibility would be of high value. Beyond
this, inverters and protection could be
co-designed in a more holistic systemlevel process to provide a cooptimized
generation and protection system. This
possibility and its potential cost-benefit
tradeoffs should be explored further.
A microgrid connected to a meshed
network still must deal with reverse
flow through network protectors. By
developing frameworks for the design

and functional requirements of microgrid architectures, the process of
determining DER short circuit current
requirements, protection schemes,
and interconnection types could be
better defined.

IEEE Power & Energy
Society Support for
Microgrids
The IEEE Power & Energy Society
(PES) has been working to bring new
microgrid protection technologies and
solutions to system protection overall.
Specifically, the Microgrid Protection
Working Group, under the PES Power
System Relaying and Control Committee, is bringing new insights to special
protection schemes, remedial actions
schemes, monitoring technologies, and
control systems. New technologies that
have a bearing on protection system
performance during abnormal power
system conditions are being studied
with respect to microgrids, both remote and grid connected, in radial and
secondary networks. Furthermore, a
working group has been formed in the
IEEE Standards Association specifically for microgrid protection, IEEE
p2030.12, Guide for the Design of Microgrid Protection Systems, sponsored
by the Power System Relaying and
Control Committee. This guide will
cover the design and selection of protective devices and the coordination
between their modes of operation.
My appreciation goes out to IEEE
Power & Energy Magazine for publishing an issue on the ever-important topic
of microgrid protection and bringing
the work of several valuable initiatives
to the attention of its readership.

For Further Reading
M. Ropp, M. J. Reno, W. Bower, J. Reilly, and S. S. (Mani) Venkata, Secondary
networks and protection: Implications
for DER and microgrid interconnection, Sandia National Lab., Albuquerque, NM, Tech. Rep. SAND2020-1120
9692985, Nov. 2020.
p&e

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IEEE Power & Energy Magazine - May/June 2021

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