Sustained oscillations of grid-forming IBRs under unbalanced perturbation: Modal analysis and EMT studies

• A novel decoupled-sequence model for GFM-IBRs is developed and validated. • Negative-sequence components degrade system oscillatory mode performance. • Modal analysis revealed oscillations caused by 2ω negative-sequence components. • Negative-sequence control enhances damping and improves system stability. • EMT studies confirm model accuracy under small unbalanced perturbations. Grid-forming inverter-based resources (GFM-IBRs) are considered crucial for power systems with high penetration renewable energy, but their negative sequence behavior under small perturbations remains understudied. This paper proposes a novel decoupled sequence dynamic modeling approach for GFM-IBRs incorporating double-fundamental-frequency negative-sequence components. Using eigenvalue-based stability assessment (EBSA) and electromagnetic transient (EMT) simulation, we reveal that: 1. The proposed modeling approach accurately captures GFM-IBR dynamics under unbalanced small-scale perturbations. 2. Negative-sequence components significantly impact system oscillatory modes. 3. Implementing additional negative-sequence control improves system damping. These findings provide new insights for designing robust GFM-IBR controls and enhancing stability in systems hosting high-penetration renewable systems.