Revisiting the Definition of Grid-Forming Inverter-Based Resources
Ambuj Gupta, Balarko Chaudhuri, Mark J O'Malley · 2025
Regulators, system operators, and consortia worldwide have published definitions and technical specifications for grid-forming (GFM) capabilities and functional requirements. The most widely accepted definition requires a GFM inverterbased resource (IBR) control to maintain a (nearly) constant internal voltage phasor in the sub-transient to transient time frame, where the internal voltage phasor is typically considered to be at the inverter terminals. However, the literature lacks conclusive evidence to explicitly substantiate this claim. This letter questions this definition by comparing the transient response of an ideal voltage source and a GFM to a voltage magnitude and phase change in the network. It is shown that a GFM exhibits (nearly) constant voltage characteristics across the filter capacitor rather than at the inverter terminals. We suggest that system operators and regulators revisit the definition and, thus, characterization of GFM IBRs.