Small-Signal Stability of Grid-Forming Converters Under Fault Conditions

The threshold virtual impedance (TVI) method is gaining significant research interest for limiting current in grid-forming (GFM) converters under fault conditions. This method is valued for its ability to maintain voltage source behavior during such faults. However, sequence extraction and negative-sequence control (NSC) are often overlooked in small-signal stability assessments under these conditions. This paper develops a small-signal impedance model for GFM converters based on the TVI current limitation method during fault conditions. Using developed small-signal impedance model, the impact of NSC on system stability during faults is analyzed. Additionally, key TVI parameters, such as the low-pass filter (LPF) cutoff frequency and the impedance ratio X/R are examined. The findings suggest that NSC, a lower LPF cutoff frequency, and a lower X/R ratio enhance system stability under fault conditions. However, a lower cutoff frequency may not effectively limit fault currents, and a lower X/R ratio can lead to reduced voltage magnitude during faults. Time-domain simulations validate these analytical results.