Nonlinear Behavior and Reduced-Order Models of Islanded Microgrid

An islanded microgrid consisting of grid-forming converters, being a high-order nonlinear system, exhibits rich nonlinear dynamical phenomena. The use of appropriate reduced-order models offers useful physical insights into the behavior of the system without the need for excessive computational resources. In this article, we derive a number of reduced-order models capable of describing the slow-scale dynamics of an islanded microgrid comprising a number of grid-forming converters. It is shown that slow-scale Hopf and homoclinic bifurcation behaviors arise from the stability of the voltage loops of grid-forming converters and are unrelated to the transmission network dynamics. Therefore, omitting the network dynamics does not affect the accuracy of reduced-order models in representing the slow-scale dynamics of the system. This is especially beneficial for modeling the microgrid with a complex transmission network. Furthermore, on this basis, all inner loops can be omitted when studying saddle-node bifurcation, leading to the development of power-flow-based reduced-order models. Finally, the stability of an islanded microgrid with a complex transmission network is evaluated.