IEEE Electrification - March 2022 - 16

because islanded operation is a key benefit of grid-forming
inverters as a response to widespread catastrophic
events. A robust set of standards is necessary to balance
autonomous grid-forming operation in grid-connected
mode and islanded/microgrid operation as well as during
line maintenance by electrical personnel.
Although synchronous generation has well-defined
and predictable currents and voltages during transient
events (or well-understood models/experimental testing)
that allow for protection engineers to ensure subtransient
and transient reactance are within system
specifications, no well-defined sets of models and tests
are provided from inverter manufacturers. Detailed
analytic modeling and simulation efforts, similar to
those already underway for grid-following controls, are
needed to examine the effects of grid-forming implementations
on power system protection and provide a
consistent framework for protection design for inverter
installations. A robust standards ecosystem that can
mandate the consistent behavior of grid-forming inverters
from different manufacturers to the same contingency
scenarios is needed to obtain reliable protection of
grids. Without such a framework, protection engineering
must carry out extensive studies on inverter behavior or
extensive redesign of the protection system, which
increases the risk, complexity, and cost of inverter
installations. In addition, we must explore whether
today's protection schemes are appropriate and effective
long-term solutions for a grid with grid-forming and
grid-following inverters or whether a paradigm shift is
needed to fully benefit from the fast dynamics of power
electronics inverters.
FRT Capability and Power System
Voltage Recovery
Transmission faults can cause deleterious electromagnetic
transients that propagate throughout a geographic
area. During and after such events, it is desired that generating
resources are capable of 1) withstanding such deleterious
transients and 2) driving the grid to a new
operating point by regulating terminal voltage magnitudes
and frequency. This ability is referred to as voltage
ride-through, disturbance ride-through, or FRT capability.
The North American Electric Reliability Corporation
(NERC) standard PRC-024-3 mandates all generating
resources to remain connected during defined voltage
and frequency excursions to support the Bulk Electric
System. Figures 4 and 5 illustrate classes of PRC-024-3
time-duration envelopes that enclose a set of positivesequence
voltage magnitudes and frequency that shall be
tolerated by generation resources in a variety of interconnections.
Notably, voltage and frequency requirements in
the Quebec interconnection can be more challenging to
satisfy than those for the Eastern, Western, and Electric
Reliability Council of Texas (ERCOT) interconnections
because the Quebec envelopes are more permissible.
16
IEEE Electrification Magazine / MARCH 2022
A limitation of the present grid codes is that they are
conceptualized from observations of modern power
systems that are dominated by synchronous machines.
If a significant amount of inverter-based generation
displaces synchronous machinery, such voltage recovery
ability might be challenged. In contrast to synchronous
generators, which can supply relatively large
off-nominal currents for short periods of time, power
inverters have hard current limits that could greatly
restrict the current dynamic voltage recovery capability
of future power grids. For example, inverter-based generation
might be limited to support the voltage recovery
of grids with high penetrations of motor loads,
which might slow down the voltage recovery because
of high inrush currents. Hence, inverter-based generation
with grid-forming control may need to operate
under low-voltage/high-current conditions for longer
times than they do today.
To timely tackle the aforementioned problems, it is
critical to investigate a suitable set of FRT envelopes that
inverter-based systems might have to tolerate in the
future. For example, it could be beneficial to determine a
current ride-through envelope that serves to engineer
inverters that tolerate motor-stalling events. Other problems
to address pertain to the development of analysis
tools to ascertain the compliance of grid-forming controls
in the context of FRT codes, determining disturbances
that drive a set of grid-forming inverters outside
a nontip zone, coordination with protective relays, and
feasibility of communication-less protection systems for
fast response.
Modeling and Simulation Approaches
A common assumption applied to a wide range of modern
simulation tools is that a power system has a hypothetical
synchronous reference speed-i.e., a center-of-inertia
speed-that remains relatively close to nominal (e.g., rad/s)
during and after a transient. Consequently, the power
transmission network has been classically represented by
an abstract algebraic system in which electrical variables
are sinusoids cycling at this reference speed.
Such an assumption is justifiable in classic systems
because synchronous machinery with relatively large
rotor inertia constants can maintain close to nominal
rotor speed during and after faults. Increased inverterbased
generation might invalidate the constant-frequency
assumption because of the lack of rotating inertia.
Specifically, the cycling speed of generated voltages by
inverters with controls, such as droop and virtual oscillator
control, might change abruptly during faults. This can
occur because the cycling speed of these controllers
depends on the instantaneous power/current. For example,
the ac power provided by an inverter could be as low
as zero during faults.
Because of the high computational burden of largescale
electromagnetic transient simulations, synchronous
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