Dynamic Performance of Parallel Heterogeneous Grid-Forming Inverters During Islanding and Grid Resynchronization

Modern power systems with high inverter penetration face reduced inertia and increased frequency instability during disturbances, necessitating grid-forming inverters (GFMs) that emulate synchronous-machine dynamics. This study examines the parallel operation of a Virtual Synchronous Machine (VSM) controlled and a synchronverter-controlled GFM in a low-inertia microgrid across the full operational cycle of grid-connected, islanded, and resynchronization phases. A unified simulation framework was developed to systematically evaluate the impact of coordinated variations in virtual inertia (H) and damping coefficient (Dp) on frequency dynamics, power sharing, and stability under power imbalance and grid-code-compliant reconnection conditions. The results demonstrate that a higher virtual inertia limits the rate of change of frequency and nadir after islanding, while increased damping enhances oscillation suppression and settling performance. During resynchronization, both GFM satisfy IEEE Std 1547-2018 limits on voltage, frequency, and phase-angle differences, confirming stable interaction and seamless transition between the operating modes. These findings reveal the complementary strengths of heterogeneous VSM-synchronverter operation and the critical role of unified parameter tuning for robust microgrid performance in low-inertia environments