GFM Inverter Control-Loop Analysis of an EMT-Based Transmission- Distribution Testbed for High-IBR Protection Studies
Hsing Cheng Liu
Electronic Devices, Vol. 15, No. 2, 2026.
DOI: https://doi.org/10.6025/ed/2026/15/2/55-71
Link: https://www.dline.info/ed/fulltext/v15n2/edv15n2_1.pdf
Abstract:
The increasing penetration of inverter based resources (IBRs) is fundamentally changing the dynamic and
protection behavior of modern power systems. Unlike synchronous generators, IBRs exhibit low physical
inertia, control limited fault current contributions, and complex interactions among multiple power electronic
control loops. Grid forming (GFM) inverters are particularly important because they can internally establish
voltage and frequency references, support weak grids, and provide inertia-like and frequency support
behavior. However, the nonlinear control limits embedded within GFM inverters can significantly alter fault
current signatures and affect the performance of conventional protection schemes. This study presents a
control loop analysis of a GFM inverter model extracted from an electromagnetic transient (EMT)-based
transmission distribution testbed developed for high IBR protection studies. The analysis is based on the
open access PNNL PSCAD/EMTDC testbed, which includes WECC-compliant GFM and grid-following inverter
models and enforces IEEE 1547 and IEEE 2800 ride through requirements. The GFM control architecture is
characterized from the black box model files, revealing active power/frequency droop control, reactivepower/
voltage droop control, PI-based overload mitigation, reactive power limiting, voltage reference
limiting, phase angle integration, and explicit phase current limiting. Key parameters identified include a Pf
droop coefficient of 0.01 pu, a Qâ€“V droop coefficient of 0.02 pu, active power limits of 0.0â€“0.9 pu, reactivepower
limits of Â± 1.0 pu, a voltage reference limit of 1.15 pu, and a maximum transient current limit of 2.0 pu.
The same GFM control source file is retained across the Base, PL19, and PL39 cases, representing
approximately 0%, 19%, and 39% transmission system IBR penetration, respectively. This consistent control
implementation enables isolation of the system level effects of increasing IBR penetration, including reduced
inertia, altered short circuit capacity, and modified fault current behavior. The analysis demonstrates that
GFM hard limits, particularly the 2.0 pu transient current limit and 1.15 pu voltage ceiling, can suppress fault
current magnitude, distort early cycle voltage and current waveforms, and potentially compromise distance,
directional, and overcurrent protection functions. Because the dataset does not include time domain
simulation outputs, dynamic metrics such as rise time, settling time, overshoot, and damping ratio are not
numerically evaluated. The study establishes a structured foundation for future EMT simulation-based
assessment of protection performance under increasing IBR penetration.