Ultimate bounds and regions of attraction for two-inverter microgrids with primary and secondary frequency control loops

In this paper we study frequency stabilisation in inverter-based microgrid systems with primary droop controller and three types of secondary control strategies. Particularising the microgrid to the case of two inverter buses, we propose structured models for which frequency regulation, ultimate bounds and an estimate of the region of attraction associated with the ultimate bound set are obtained in closed-form analytical expressions.