Large-Signal Stability Analysis for Islanded Microgrids Based on Inner Control Loops Approach

Due to microgrids low inertia and low capacity, recovering stable operating conditions of microgrids depends primarily on the dynamic performance of control systems. It also depends on the technological constraints of power electronic converters related to the low thermal inertia of semiconductors devices, which requires limiting the transient current under large-disturbances. This paper investigates the large-signal stability of an islanded microgrid fed by a controlled grid-forming inverter in voltage-frequency mode and a controlled grid-following inverter in current control mode. It analyzes the dynamic response models of each inverter control system connected to an islanded microgrid and the interaction between the inverter control systems under a three-phase ground fault. Time-domain simulations are carried out with Matlab/SimscapePowerSystems. The obtained results show that grid-forming inverter imposes the dynamic response of the islanded microgrid through dynamics of the inner current control loop. Whereas grid-following inverter acts as a constant current source whose dynamics is influenced by the grid-forming inverter.