Low-Frequency Small-Signal Modeling of Interconnected AC Microgrids

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. inter-microgrid power flow controller. A sensitivity analysis-based technique is introduced to find significant modules of fully inverter-based AC microgrids for preserving 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 insignificant modules of the detailed model and reconfiguring the significant modules. The concept of virtual swing equation and the aggregation modeling method are employed to achieve a single-order model from the low-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.