IEEE Power & Energy Magazine - November/December 2021 - 20

services. Other features are intrinsically different between
technologies. GFL inverters cannot black-start a grid or reliably
operate in a grid without a voltage reference to follow.
In contrast, GFM inverters present themselves similarly,
yet not identically, to synchronous machines. A GFM inverter
What Is 100%?
Companies, countries, and cities around the world have
declared their intention to go " 100% something " at some
point in the future. That something might be clean, renewable,
or carbon-free energy, to name a few. To dispel any
confusion, consider Figure S1. Almost all variations on 100%
reside within the 100% carbon-free set. In that space, many
creates a tightly controlled internal voltage phasor. This is
analogous to a synchronous machine, where the inertia of
the rotating mass is inherently coupled to the grid. This voltage
persistence and the accompanying virtual inertia with
a GFM inverter is a designed control action. It can be
devices use synchronous machines. However, their dwindling
presence raises concerns. We already know we can
live comfortably with a synchronous machine-dominated
mix of everything in the noncarbon set, so the question for
today is: what happens when we move into the inverterdominated
space?
* Synchronous Versus
Inverter Based
* Renewable
Generation Versus
" Dynamic Enabler " Versus
Fossil Versus Nuclear
Batteries
Fuel Cells
Flywheels
SVC/STATCOM
HVDC
Inverter
Based
Synchronous
Machines
Variable-Speed
Pumped Hydro
Solar
Photovoltaics
Wind
Pumped
Hydro
Hydro
Geothermal
Biomass
SolarThermal
Nuclear
Compressed
Air
Storage
Synchronous
Condensers
Fossil
All Candidates for
Grid Forming
100%
Renewable
Generation
Carbon Free
figure S1. Power generation and grid support technologies. SVC: static var compensator; STATCOM: static synchronous compensator.
Grid Following Versus Grid Forming
Grid Following
Most IBRs currently in service operate in GFL mode. They
rely on fast synchronization with the external grid to tightly
control their active and reactive current outputs. If these inverters
cannot remain synchronized during grid events and
challenging network conditions, they are unable to maintain
controlled, stable outputs.
Grid Forming
The primary objective of GFM control for IBRs is to maintain
an internal voltage phasor. When GFM controls are applied
in bulk power system (BPS)-connected IBRs, the voltage
phasor is held constant in the subtransient-to-transient time
frames. This enables IBRs to immediately respond to changes
20
ieee power & energy magazine
in the external system and maintain stability and control during
challenging network conditions. The phasor must be controlled
to maintain synchronism with other devices and control
active and reactive currents to support the grid. When
GFM controls are applied in non-BPS-connected IBRs (for
example, black-start and microgrids), this synchronization
functionality is removed or limited, and the voltage phasor
may be held relatively constant through time. This enables a
plant to operate in an electrical island and define its frequency.
There are many variations of GFM and GFL controls. Both
are subject to physical equipment constraints, including voltage,
current, and energy limits; mechanical equipment constraints
(in wind turbines); and external power system limits.
november/december 2021

IEEE Power & Energy Magazine - November/December 2021

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