Transient Stability of Grid-Forming Inverters in Microgrids: Nonlinear Analysis

A current limiting scheme is employed for Inverter-Interfaced Distributed Generation (IIDG) units, mostly in the control loops, to limit the current within the tolerable range and to facilitate the IIDG units being connected to the grid during the transient. However, current-limiting/saturation of the droop-controlled grid-forming inverters affects their transient stability and may make them unstable, which yet has not been explored in the literature in the context of autonomous microgrids (MGs). It is disclosed in this paper that the sharp phase angle variation and the arbitrary resistive output impedance of grid-forming inverters in the current limiting mode make the autonomous MGs unstable. Time-domain simulations prove the idea.