Vulnerability analysis of VSG-controlled grid-forming inverters under asymmetrical and symmetrical fault conditions

Abstract

As inverter-based resources increasingly replace synchronous generation, grid-forming (GFM) inverters are becoming essential for maintaining stability in low-inertia power systems. However, the dynamic behavior of GFM control under severe fault conditions remains a critical vulnerability, particularly regarding the inherent technical conflict between voltage restoration objectives and hard converter current limits. This paper presents a comprehensive dynamic analysis of a virtual synchronous generator (VSG)-controlled inverter subjected to a full spectrum of grid disturbances, ranging from asymmetrical Line-to-Ground and Line-to-Line faults to severe symmetrical Three-Phase faults. Using detailed time-domain simulations, the study systematically quantifies the failure mechanisms of standard VSG architectures. The results demonstrate that without dedicated fault ride-through (FRT) coordination, the controller’s pursuit of voltage regulation drives the inverter into dangerous operating states. Specifically, asymmetrical faults are shown to induce sustained power oscillations and negative-sequence currents that standard control loops fail to mitigate, while symmetrical faults precipitate extreme synchronous current surges that immediately exceed thermal safety margins. Furthermore, the analysis reveals that unmitigated inertia emulation can exacerbate post-fault frequency instability during recovery. These findings establish a critical benchmark for GFM vulnerability and underscore the urgent necessity for developing enhanced FRT control strategies that can dynamically prioritize converter protection over voltage support during transient events.