Transient Stability Enhancement of Multi-Inverter Microgrids

Large-scale distributed renewable energy sources (RESs) are entering the power system through grid-forming and grid-following inverters (GFMIs and GFLIs). These inverters can cluster together to create an islanded microgrid that optimizes the use of RESs. However, the microgrid may lose synchronization during a temporary short-circuit fault. It has been observed that the critical fault clearing time can increase if GFLIs inject a significant amount of reactive power or if GFMIs operate at slightly higher cutoff frequencies in their low-pass filters. This phenomenon is related to a specific hidden periodic orbit and the relative region of the stable equilibrium point's (SEP's) basin of attraction. Due to the presence of the aforementioned hidden periodic orbit, the phase-locked loop is able to operate normally during the fault, which effectively ensures the synchronization stability for the post-fault system. The sustainability of this hidden periodic orbit depends on the appropriate injection of reactive power by GFLIs. Moreover, increasing the cutoff frequencies of GFMIs leads to a homoclinic bifurcation, which enlarges the SEP's basin of attraction and guarantees successful resynchronization of the inverters after the fault is cleared. Simulations and experiments have confirmed these findings.