Current-Limiting Control Design for Grid-Forming Capability Enhancement of IBRs Under Asymmetric Grid Disturbances

Recent technical specifications for grid-forming (GFM) inverter-based resources (IBRs) require the terminal voltage to remain as balanced as possible during asymmetric disturbances to enhance GFM capability. However, under severe unbalanced fault conditions, GFM IBRs are forced into current-limiting operation, which substantially impairs their ability to support voltage balance. This presents a critical challenge for advanced current-limiting control (CLC) in GFM IBRs. To address this challenge, a general CLC design guideline is proposed. The core principle is to align the phase angle of the impedance emulated by the CLC with that of the external AC grid impedance in both positive and negative sequences, thereby improving voltage balance and enhancing GFM capability. Following this guideline, two CLC implementations are developed using virtual impedance and current reference saturation frameworks. The proposed CLC schemes undergo rigorous grid code compliance evaluation, transient stability analysis, and robustness assessment. Extensive simulation studies, including comparisons with state-of-the-art CLC methods, as well as controller hardware-in-the-loop experiments, validate the improved transient stability and enhanced GFM capability achieved by the proposed method.