IEEE Electrification - March 2022 - 11

M
ANAGING THE STABILITY OF TODAY'S
electric power systems is based on decades
of experience with the physical properties
and control responses of large synchronous
generators. Today's electric power systems
are rapidly transitioning toward having an increasing proportion
of generation from nontraditional sources, such as
wind and solar (among others), as well as energy storage
devices, such as batteries. In addition to the variable nature
of many renewable generation sources (because of the
weather-driven nature of their fuel supply), these newer
sources vary in size-from residential-scale rooftop systems
to utility-scale power plants-and they are interconnected
throughout the electric grid, both from within the
distribution system and directly to the high-voltage transmission
system. Most important for our purposes, many of
these new resources are connected to the power system
through power electronic inverters. Collectively, we refer to
these sources as inverter-based resources.
The operation of future power systems must be based
on a combination of the physical properties and control
responses of traditional, large synchronous turbine
generators as well as those of inverter-based resources
(see Figure 1). The major challenges stem from the recognition
that there is no established body of experience
for operating hybrid power systems with significant
amounts of inverter-based resources at the scale of
today's large interconnections.
To operate such large hybrid power systems, the
assumptions that underlie current generation design and
control approaches must be reexamined and, where
appropriate, modified or even redefined to take explicit
account of the new challenges and opportunities presented
by these inverter-based forms of generation. We
should expect that new control approaches, operational
procedures, protection, and planning tools and processes
will be required.
Present
Synchronous generators regulate their terminal voltages
and respond to changes in grid frequency through
changes in their power output. We refer to these generation
sources as grid forming. Today's inverter-based generation
sources generally use phase-locked loops (PLLs), which
rely on externally generated voltages from synchronous
machines to operate. We refer to these types of inverter-based
generation sources as grid-following inverters. In
case of unintended separation of the power system or after
a blackout, islanded systems comprising only these types of
inverters cannot operate autonomously. This limitation of
the grid-following inverters has inspired an investigation
into grid-forming control methods for power electronic
inverters, which provide functionalities that are traditionally
provided by synchronous machinery. Early work on this
topic started in the 1990s, focusing on power systems with
small footprints (e.g., microgrids) and on small islands (such
as Kauai, Hawaii). Today, grid-forming controls are being
considered for deployment in bulk power systems because
of their ability to enhance the stability of these grids when
loads are largely being served by inverter-based resources.
This article reviews the challenges involved in integrating
inverter-based resources into the electric power system
and offers recommendations on technology pathways to
inform the academic community, industry, and research
organizations. We will 1) discuss the difference between
grid-following and grid-forming control approaches for
inverter-based resources; 2) review relevant research and
outline research needs related to five grid-forming inverter
topics: frequency control, voltage control, system protection,
fault ride through (FRT) and voltage recovery, and
modeling and simulation; and 3) introduce a road map
that outlines an evolutionary vision in which grid-forming
inverters play a growing role in power systems that, in
turn, leads to the identification of nearest-term priorities
for research. This article builds upon the Research Roadmap
on Grid-Forming Inverters (Lin et al. 2020). Interested
Future
Generator
Inverter
Generator
Inverter
(a)
(b)
Figure 1. (a) The present power system has historically been dominated by synchronous generators having a large rotational inertia with a relatively
modest amount of inverter-based resources, such as photovoltaics, wind, and batteries. (b) Future systems will have a significant fraction
of generation interfaced with power electronics and might be dominated by inverters. This implies a need for next-generation grid-forming controllers
that ensure grid stability at any level of penetration with inverter-based resources.
IEEE Electrification Magazine / MARCH 2022
11

IEEE Electrification - March 2022

Table of Contents for the Digital Edition of IEEE Electrification - March 2022