For a grid-connected AC microgrid (MG), when a fault occurs at the microgrid, adaptive overcurrent (OC) or directional overcurrent (DOC) relays must be activated as fast as possible to protect all distributed generation (DG) units and loads. Fault currents inside the grid-connected AC microgrid can be significantly varied because fault current contributions from the main grid and DG units are different depending on various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs) in the microgrid. A traditional fault analysis method cannot be applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. Therefore, this paper proposes a simplified and automated fault current estimation approach for grid-connected AC microgrids, which is effective to quickly and accurately calculate fault current contributions from IBDGs and RBDGs and the grid fault current contribution to any faulted sections in the microgrid. This simplified and automated fault current calculation approach is mainly focused on grid-connected and small-sized low-voltage (LV) AC microgrids with the support of communication system. Under the grid-connected operation mode of the MG, fault tripping current thresholds of adaptive OC/DOC relays can be properly adjusted by the proposed fault analysis method. In particular, relying on fault current distribution coefficients of IBDGs, RBDGs, and the utility grid, pick-up currents of the adaptive OC/DOC relays in the LV AC microgrid are instantly and accurately self-adjusted according to different MG configurations as well as the operation status of DG units during the grid-connected mode.
