Delay-dependent small-signal stability analysis of inverter-based islanded microgrids

The stability of an inverter-based hierarchical microgrid is challenged by inverter controls, network dynamics, load models and communication delays among the interacting agents. Moreover, the adoption of non-dispatchable grid-following (GFL) inverters further lowers the system inertia in islanded Microgrids (MGs). Therefore, both grid-forming (GFM) and GFL inverters shall coordinate to perform frequency/voltage regulation through their grid-support function. This paper investigates the role of GFM-GFL coordination on the operational stability of real-world Microgrids (MGs) with communication delays. A complete non-linear state-space model is derived for the MG. The time-delayed system model is discretised in a system of ordinary differential equations to determine delay margins using spectral analysis. Later, the same model is linearised around an operating point to determine delay margins using time-domain analysis. The equilibrium point is disturbed by load perturbation and the system response is observed for different values of communication delays and control topology. The delay margins estimated theoretically are validated using the time-domain simulations in MATLAB/SIMULINK. Simulation results are compared to determine the efficacy of grid-support function in improving operational stability of islanded MGs. The developed evaluation scheme is tested on a multi-inverter islanded power system.