IEEE Power Electronics Magazine - June 2023 - 21

inverters are described. A focus on the comparison of
GFL and grid-forming (GFM) inverters based on a more
comprehensive white paper developed by the SCC-21
Task Force on Advanced Inverters supporting industry
standards is needed in the next few years to reduce system-wide
IBR events on the electric system.
Introduction
Large inverter-based system events have been experienced
by power utilities world-wide. The Odessa events [1]
resulted in an instantaneous loss of 1,116 MW of IBRs associated
with photovoltaic (PV) and the ERCOT system frequency
went down from 60 to 59.2 Hz in a matter of cycles
(Figure 1) on 4 June 2022 during midday with high solar PV
power generation.
These events may limit the wide-scale adoption of IBRs
and thus the integration of renewable energy generation. It
is therefore important for the power community to address
the concerns with IBRs.
This article describes the concepts of advanced inverters
as they apply to various applications that in turn determine
the inverter(s) functional characteristics.
Inverter
applications include the following:
■ Inverters in distribution grids: inverters used as distributed
energy resource (DER) interfaces, including wind,
solar PV, and battery energy storage systems (BESS)
(IEEE Std 1547-2018).
■ Inverters in grid-connected and islanded microgrids
(IEEE Std 2030.7-2021).
■ Inverters in standalone distribution grids: uninterruptible
power supplies (UPS), islanded systems (islanded
microgrids), and isolated/remote grids (IEEE 1562-2019,
IEEE Std 2030.7-2017).
■ Inverters in larger transmission grids: IBR used in wind
and solar farms, and static synchronous compensators
(STATCOMs) (IEEE Std 2800-2022, IEEE Std 1052-2018).
■ Aggregated DER/IBR for grid services (IEEE Std
2030.11-2021).
Inverter Operating Modes
Apart from limitations on the inverter's short-circuit and
over-current capability, the operation of the inverter is
defined by control loops implemented to realize the
required grid functions. In its simplest formulation, these
are the pulse-width modulator, current limiters and reference
voltage amplitude and frequency control.
The resulting standard control loops include the following,
as shown in Figure 2:
a) Current or Power Control-This mode of operation
allows independent control of active and reactive current/power,
or P-Q control, as found in GFL inverters.
This mode requires synchronization [typically using a
phase-locked loop (PLL)] with the electric grid to
which the inverter is connected so that power can be
exchanged between the two synchronous sources, the
inverter and the grid. This operation is like that of a
synchronous machine synchronized to the grid and
feeding active and reactive power into the grid.
b) Voltage Amplitude and Angle/Frequency Control-
This mode allows V-f control, as found in GFM inverters.
This mode of operation requires an internal oscillator to
set the frequency of inverter voltage and control of the
amplitude of the inverter voltage. This operation is similar
to a standalone synchronous generator (SG).
Inverter Control-Historical Note
The concepts of P-Q, i.e., GFL, and V-f, i.e., GFM, control
have been developed since inverters were first developed.
From force-commutated thyristors to controllable
devices, including the gate turn-off thyristor (GTO) and
integrated gate-commutated thyristor (IGCT), the IGBT,
and the newer silicon carbide (SiC) based MOSFETs and
IGBTs. The terms grid-following and grid-forming inverter
have been coined recently but are not new functions [2].
Several groups are developing roadmaps and design
improvements to make IBRs more robust for integrating
renewables into the grid [3].
FIG 1 ERCOT system frequency during Odessa-2022 fault [1].
June 2023 z IEEE POWER ELECTRONICS MAGAZINE 21
IEEE Power Electronics Magazine - June 2023

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