Small-Signal Stability Analysis for Grid Forming Converters Connected to Resistive Grids

Grid forming converters (GFMs) are expected to enhance the stability of power systems with high penetrations of inverter-based resources (IBRs). While their advantages over grid-following inverters are well recognized, the risk of subsynchronous oscillation (SSO) associated with small-signal stability of GFMs, especially when connected to resistive grids, remains insufficiently explored. This paper presents a comprehensive yet simplified method to evaluate SSO risk induced by the lack of small-signal stability. To address the complexity of multiinput multi-output (MIMO) systems, the eigenvalue loci method is introduced. The study demonstrates that the GFMs tend to become unstable, particularly when grid resistance is high and the coupling between power synchronization and reactive power controllers cannot be neglected. The analytical results are validated through controller hardware-in-the-loop (C-HIL) experiments, confirming the increased SSO risk in resistive grids and the effectiveness of the proposed evaluation method. These findings highlight the importance of considering grid resistance and controller coupling in GFM design and provide practical guidelines for proper investigation of small-signal stability of the GFM in future power systems with high IBR penetration.