Small-Signal Synchronization Stability of Sequence-Decomposed Grid-Forming IBRs

To facilitate relay operation and mitigate unbalanced voltage conditions, recent specifications mandate negative sequence control in grid-forming inverter-based resources (GFM IBRs). Adhering to this requirement, sequence-decomposed modules (SDMs) must be integrated into the control system to detect and extract sequence components. However, the introduction of SDMs induces cross-coupled effects in the dq-frame, potentially making GFM IBRs susceptible to subsynchronous oscillations (SSOs)—an aspect not thoroughly investigated previously. This study addresses this gap by initially exploring the stability issue using single-input single-output small-signal models of GFM IBRs with SDMs. To mitigate SSO instability risks, a damping enhancement control scheme is then proposed, based on the fundamental hybrid synchronization control principle. By leveraging the q-axis positive sequence voltage from the SDM as a feedforward loop, system stability is significantly enhanced with promising damping capabilities. Simulation and controller hardware-in-the-loop tests, conducted across varying grid strengths, validate both the theoretical analysis and the effectiveness of the proposed control.