Stability Analysis of Low-Voltage Distribution Feeders Operated as Islanded Microgrids

The increasing penetration of behind-the-meter distributed energy resources opens up opportunities to improve distribution grids' resilience and reliability. In this context, the paper studies islanded operation of radial low-voltage (LV) distribution feeders with a high penetration of prosumers equipped with grid-forming inverters. In contrast to the existing studies that focused on droop control as the main source of instability, we extend the analysis to the microgrid topology, in particular the number of grid-forming inverters. The eigenvalue analysis shows that with the increasing number of grid-forming inverters, the system becomes practically unstable with its critical eigenvalue approaching zero as progressively more inverters are added. Furthermore, we extend the analysis by explicitly modelling the DC side dynamics, including the DC/DC converter with its controller and inherent dynamic response of the DC energy source. Eigenvalue analysis shows that the DC voltage controller impacts the system stability by introducing new dominant eigenvalues. The results suggest that the DC energy source should have a fast response to ensure a stable operation. The small-signal stability results, benchmarked against time-domain simulations, indicate that careful parameter tuning is required to ensure stable islanded operation of LV feeders.