Nonlinear Dynamic Modeling and Large-Signal Stability Analysis of Isolated Microgrids

The distributed generation of microgrids is mainly from renewable energy sources, where the flexible energy conversion is achieved through power electronic converters. However, microgrids suffer from a lack of rotational inertia and poor immunity to disturbance, making them more susceptible to transient instability that cannot be assessed by small-signal analysis. To analyze the effects of large disturbances on the transient stability of the system, nonlinearities in the microgrid consisting of grid-forming (GFM) and grid-following (GFL) converters are described mathematically. Then, a generic nonlinear dynamic model of the isolated AC microgrid is established in this paper. Moreover, the iterative algorithm based on Takagi-Sugeno (TS) multi-modeling is employed to estimate the domain of attraction (DOA). The main novelty of this paper is to quantify and analyze the transient stability of the microgrid consisting of multiple types of converters and nonlinear loads. Furthermore, the role of complex nonlinear loads on the DOA is fully considered to make it close to the actual operating scenario. The validity of the transient stability margin is also verified by time-domain simulations.