Electronic Devices Hsing Cheng Liu 15 2 2026 https://doi.org/10.6025/ed/2026/15/2/55-71 https://www.dline.info/ed/fulltext/v15n2/edv15n2_1.pdf 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 Q–V droop coefficient of 0.02 pu, active power limits of 0.0–0.9 pu, reactivepower limits of ± 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.