IEEE Electrification - March 2022 - 13

Grid-Forming Controllers
From here forward, the term " grid forming " acts as an
umbrella for any inverter controller that 1) regulates terminal
voltages, 2) can coexist with other grid-following
and grid-forming inverters and synchronous generation
on the same system, and 3) does not require a PLL or
communications to operate together with multiple gridforming
assets. As shown in Figure 2, grid-forming controllers
can be broadly categorized as droop, virtual
synchronous machines, and virtual oscillator controllers.
Droop control is the most well-established grid-forming
method; it was conceived in the early 1990s. Its key feature
is that it exhibits linear tradeoffs, often called droop
laws, between real power versus frequency and reactive
power versus voltage. This mirrors how synchronous
machines operate in steady state. They give rise to the following
properties regardless of whether they are
machines or inverters:
x system-wide synchronization: all units reach the
same frequency
x power sharing: each unit provides power in proportion
to its capacity.
Virtual synchronous machine control replicates the
dynamic behavior of a synchronous machine with
an inverter. The complexity of the emulated virtual
machine can vary greatly, from detailed models to simplified
swing dynamics. Implementations that close -
ly match machine characteristics have both Q-V and
P-omega characteristics and are often called synchronverters.
Virtual inertia methods are simpler and capture
only the dynamics of an emulated rotor and its steadystate
P-omega droop.
Virtual oscillator control is another inverter control
method that emulates nonlinear oscillators. As illustrated
in Figure 2, the model takes the form of an oscillator
circuit with a natural frequency tuned to the nominal
ac grid frequency and its remaining parameters tuned
to adjust the nominal voltage and control bandwidth.
Despite its unconventional appearance, it exhibits the
Q-V and P-omega droop laws in steady state that the
other grid-forming methods also offer. However, its simple
time-domain implementation and dynamical properties
offer enhanced speed.
Inverter Control State of the Art
and Open Research Questions
In this section, we review relevant research and outline
research needs related to the following five topics: frequency
control, voltage control, system protection, FRT
and voltage recovery, and modeling and simulation.
Frequency Control
Frequency control refers to generation control actions
designed to maintain system frequency near the nominal
value. In machine-based grids, the system inertia
strongly influences the frequency dynamics and
physically originates from the rotating masses of machine-based
generators. Since inverter-based resources
do not contribute inertia to a power system, it follows
that the replacement of machines with inverters will
reduce the system inertia and may increase the risk of
large frequency swings. Figure 3 illustrates the relationship
between decreased machine capacity and increased
frequency deviations across time. To address
this concern, grid-forming inverters may be used to
counteract both the loss of inertia and primary frequency
control provided by retired synchronous generation.
Similar to the natural behavior of synchronous machines,
grid-forming inverter-based resources would autonomously
react to frequency swings and adjust their
power injections during a low-frequency event.
Reduced inertia may result in a larger rate of change
of frequency and increasingly volatile system dynamics,
and it also necessitates faster control actions to arrest
frequency swings. Because the magnitude of the frequency
swing after a disturbance is largely tied to the imbalance
between generation and load, enough untapped
capacity must be reserved as headroom for frequency
control. A drawback is that unused capacity could represent
an opportunity cost for both renewable and fossilfueled
generation because power output must be
throttled to less than the available amount.
Referring to the controllers shown in Figure 2, we provide
a brief survey of the existing frequency control strategies.
The P-omega droop offered by grid-forming units
would govern the steady-state frequency deviation after
the initial transients have subsided. Typically, these
TABLE 1. A comparison of grid-following
and grid-forming controls.
Grid-Following Control
Assumes grid already
formed under normal
operations
Control of current injected
into the grid
Decoupled control
of P and Q
Needs PLL
Needs voltage at the point
of common coupling to
deliver P and Q
Cannot operate at 100%
power electronics penetration;
instability thresholds
(tipping points) exist
Grid-Forming Control
Assumes converters must
actively form and regulate grid
voltages
Control of voltage magnitude
and frequency/phase
Slight coupling between
P and Q
It may use PLL control to
switch between modes
Can black-start a power system
Can theoretically operate at
100% power electronics penetration;
can coexist with grid
following
Not standardized, inadequate
operational experience at a
systems perspective
IEEE Electrification Magazine / MARCH 2022
13
IEEE Electrification - March 2022

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