As a solution for optimized, reliable and flexible operation of distribution systems, multiple AC microgrids can be interconnected. In this paper, a low-order low-frequency small-signal model is proposed for large-scale interconnected AC microgrids in order to analyze stability and dynamics as well as synthesize high-level controllers, e.g. grid-tie power controller. A sensitivity analysis-based technique is introduced for finding effective modules of fully inverter-based AC microgrids on the dominant low-frequency modes. The low-order model of AC microgrids including both droop-based and PQ-controlled distributed generation units is obtained by removing non-effective modules and reconfiguring the effective modules. The concept of virtual swing equation and the aggregation modeling method are employed to achieve a single-order model for each AC microgrid with any number of sources. The analysis and synthesis of the large-scale interconnected microgrids can easily be done using the proposed single-order model. The frequency analysis and control of three interconnected AC microgrids are presented as a case study, which leads to introducing the inter-microgrid oscillatory modes.
