Stability analysis for isolated AC microgrids based on PV-active generators

The current trend in isolated microgrids is oriented to distributed renewable energy generators, such as photovoltaic (PV) generators and their corresponding distributed energy storage systems (ESS) as an unit denoted as active generator (PV+ESS). In an isolated microgrid, every distributed generator (ESS or PV) may operate in two operation modes (grid-forming or grid-following) in accordance to specific operational conditions. Nevertheless, at least one distributed generator should assume the regulation of the common bus, and seamless transition between operation modes should be ensured for all the distributed generators. In particular, ESS's based on batteries require at least two different mode of charge. As consequence, the topological operation mode of the microgrid is affected by the changes of the operation mode of each distributed generator. Typically, droop control loops are used for interconnecting several different distributed generators in parallel to a common bus, whose parameters determine the stability and damping of the microgrid operation. In this paper, a small-signal stability analysis is applied to an isolated AC microgrid composed of (PV+ESS) active generators, regarding three different operation modes of the microgrid. The proposed analysis allows the designers an adequate selection of the droop coefficient in order to ensure the stability of the microgrid under different topological operation modes.