IEEE Electrification - March 2022 - 15

drop or rise, takes place under various
load conditions. All conductors in
power systems have an intrinsic
impedance that results in a variable
voltage profile along the length of the
line when current flows. Active voltage
intervention (increasing or reducing
voltage to preferred operational limits)
might use electromechanical or electronic
components, from generators to
other devices. Such devices include
load tap changers, voltage regulators,
capacitor banks, synchronous condensers,
and others, along with earlystage
commercialization of solid-state
technology (power electronics), e.g., a
static synchronous compensator.
For synchronous generators, their
automatic voltage regulator adjusts
the output voltage either by adjusting power delivery via
the main field or real and reactive power output by modulating
the exciter field current. Voltage stability and reactive
power sharing among parallel-connected synchronous
generators is achieved via Q-V droop control such that each
machine follows a linear relationship between reactive
power and voltage.
Grid-forming inverters natively provide voltage regulation
via their Q-V droop laws, often called volt-volt
ampere reactive (volt-VAR) control, which closely matches
the behavior of synchronous machines. Mirroring terminology
from frequency control, this is generally called
primary voltage control to emphasize that these control
actions are done locally, without communication. Thus,
grid-forming inverters can be especially helpful in providing
voltage support in weak grids.
Recent advances in voltage control for inverter systems
are mostly concentrated in microgrid systems with droop
control. Virtual impedances have been used to improve
reactive power sharing and mitigate parameter sensitivity.
To further enhance reactive power sharing and reduce
steady-state errors, communications-based secondarylevel
controllers have been proposed. However, novel
methods should be devised for deployment in bulk power
systems to reduce communication dependency for scalability
and resilience.
Recent findings have also uncovered adverse interactions
between grid-forming inverters and synchronous
machine excitation systems that regulate voltages, and
similar issues have been observed on grid-following control
types. These interactions can destabilize hybrid systems
and appear to be common to both grid-following
and grid-forming inverter controls.
Interactions and voltage oscillations may occur in
systems of grid following with grid-support functions.
Here, the piecewise linear volt-VAR relations on grid-following
inverters and the time delays and filters used to
This must be
balanced by the
need for system
resilience because
islanded operation
is a key benefit of
grid-forming inverters
as a response
to widespread
catastrophic events.
tune volt-VAR control actions are
known to introduce undesired interactions
between grid-following inverters
and voltage-regulation equipment.
Methods to mitigate interactions
between all types of inverter controls
and other generation should be investigated
for inverter-dominated grids.
System Protection
The effect of grid-forming inverters on
power system protection is fundamentally
different than that of gridfollowing
inverters and has not been
extensively studied. In theory, the
fault current from grid-forming inverters,
though it may vary by the control
schemes, may have a subtransient
behavior that more closely mimics
synchronous machines and is significantly larger than
that supplied by grid-following inverters. The short circuit
currents from grid-forming inverters can be equivalent to
synchronous generation but are normally constrained to
4-6 p.u. for short time periods (<10 cycles) before steadystate
limits (<2 p.u.) are imposed. A larger short circuit
subtransient response will be limited primarily by the
short circuit response of componentry in the grid-forming
inverters, related to its internal impedance. The short-time
response is limited by semiconductor ratings, whereas the
steady-state response is limited by inverter hardware
parameters, e.g., thermal management.
By design, traditional three-phase grid-tied inverter
controls will not provide zero- or negative-sequence currents,
which can be used to identify unbalanced fault conditions
more easily; inverter controls are designed to
suppress negative-sequence currents. It is recommended
to program grid-forming inverters to source zero and
negative currents, mimicking a fault behavior of synchronous
machines, in an unbalanced fault condition. This
would yield an increase in the efficacy of traditional protection
mechanisms compared to the pure grid-following
control case and would significantly simplify the identification
of unbalanced faults.
A protection issue unique to grid-forming inverters is
operation in islanded/microgrid mode when a portion of
the power system is disconnected from the bulk grid. Traditional
grid-following inverters automatically shut off in
an islanded condition, with the absence of an external
voltage, but grid-forming inverters can continue to operate
islanded from the area grid (in many cases, such
resilient microgrid operation is a primary benefit of gridforming
inverters). To maximize the benefit, some form of
islanding protection still will be needed for grid-forming
inverters to safely operate in islanded mode while ensuring
the safety of electrical personnel and other bystanders.
This must be balanced by the need for system resilience
IEEE Electrification Magazine / MARCH 2022
15
IEEE Electrification - March 2022

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