IEEE Power & Energy Magazine - May/June 2021 - 37

By Matthew J. Reno, Sukumar Brahma,
Ali Bidram, and Michael E. Ropp

UTT E

RSTO C
K.C OM/LE OWOL FERT

for IBRs (i.e., grid forming and grid following)
are discussed to present how their short current
signatures and dynamic response under faults
impact microgrid protection.

Challenges in Microgrid
Protection

Recently, microgrids have gained much attention in the electric power industry due to 1) their
capability for improving power system reliability
and resiliency, 2) their impact on increasing the
use of renewable resources, 3) the reduced cost of
distributed energy resource (DER) equipment, and
4) the continuing evolution of applicable codes and standards. As a result, it is expected that microgrid deployment
worldwide will grow significantly over the next several years.
Most microgrids today are created by reconfiguring systems
that started as radial distribution systems at the facility or
community level.
Small conventional generators (e.g., diesel and natural gas)
remain important in microgrids due to their performance
properties, relatively low cost, and availability. However,
most microgrids today obtain at least some fraction of their
energy from variable-generation resources like photovoltaics (PVs), and energy storage systems (ESSs) are becoming
nearly ubiquitous in microgrids, as they allow the optimal
operation of small generators as well as higher integration
of variable-generation resources, and they can be used to
improve the system's performance when transitioning from
on-grid to off-grid mode.
Nearly all renewable energy systems and ESSs are IBRs,
meaning that they are connected to the ac power system
via a dc-ac converter called an inverter (the term inverter
is commonly used even for the bidirectional dc-ac converters used in ESSs). The inverter's behavior is largely
software defined, providing a high level of flexibility as
well as unique performance characteristics and opportunities. Today, there are microgrids in operation that are
energized entirely by IBRs, and even those that include
rotating generators are often operated in IBR-only modes
under certain conditions.
Microgrids, like all power systems, require protection
that de-energizes and isolates faults before they can harm
health or property. The protection of traditional distribution systems is designed with the underlying assumption
that the system is fed by one source (a substation) and has
a radial topology. They are protected by overcurrent functions and devices. However, two key challenges arise in
may/june 2021

protecting circuit elements in IBR-based microgrids in
this way.
1) Microgrids can be fed by multiple distributed sources.
The protection of conductors, transformers, and other
circuit elements must thus be designed for any feasible
combination of sources, and fault current directions
and magnitudes may change accordingly.
2) The fault current produced by IBRs does not lend
itself easily to traditional overcurrent methods of fault
detection and isolation.
* The semiconductor switching devices of inverters are
intolerant of overcurrent, requiring IBRs to limit
their fault current contributions to, typically, on the
order of 1.1-1.5 per unit (pu) of the inverter's nominal current rating. IBRs do not produce the large
fault currents typical of utility sources or synchronous generators.
* IBRs often limit their fault current using means that
cause the output waveform to be nonsinusoidal. The
harmonic content in the waveform will generally
be IBR specific, as it depends on how the firmware
achieves the current limiting. Protection systems must
be tolerant of this harmonic content.
* The phase angle between the IBR's fundamental
output current and fundamental terminal voltage is
set by the IBR's programming, and, thus, there are
no generic means for predicting it.
This article addresses the challenges related to the protection of inverter-based microgrids and reviews some ongoing research and solutions in the area. Different types of IBR
topologies and controls are elaborated, and the challenges
of inverter-based microgrid protection are highlighted. The
solutions for the protection of microgrids with a high penetration of IBRs are discussed.

IBR Overview
The behavior of IBRs under faulted conditions is strongly
determined by the inverters that interface the primary power
source (i.e., renewable energy sources or ESSs) to the ac
system. In this section, some fundamentals of inverters are
briefly reviewed to provide background.

Topologies
In general, inverters are divided into single- and threephase categories. Single-phase inverters are mostly used
for lower-power applications (typically fewer than 15 kW)
and in single-phase microgrids. Three-phase inverters are
used to integrate larger generation sources to the primary
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IEEE Power & Energy Magazine - May/June 2021

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