Selective Phase Tripping for Microgrids Powered by Synchronverter-Interfaced Renewable Energy Sources

Synchronverters are inverters that imitate the behavior of synchronous generators to enhance the dynamics of renewable energy sources (RESs) powering microgrids. However, the synchronverter's inrush currents during faults could reach intolerable levels. Limiting the synchronverter's fault currents could obstruct protection relays from identifying faulted phase(s) during unbalanced faults, and hence, jeopardizing selective phase tripping (SPT). This paper unveils the root causes behind the deviation in phase selection that hinder SPT in microgrids powered by synchronverters. Virtual-impedance fault current limiters (VI-FCLs) are proposed for synchronverters to ensure accurate SPT by commercial relays and limit their sequence and DC inrush currents. Based on a short-circuit analysis, the positive- and negative-sequence VI-FCLs, as well as the active-to-reactive power ratio of synchronverters, are regulated to enable SPT and protect synchronverters from inrush currents. Simulation results using PSCAD/EMTDC ensure the effectiveness of the proposed control scheme in enabling reliable SPT in microgrids with synchronverters. The efficacy of the proposed scheme is assured by examining various fault types, a wide range of fault resistances, and the grid-connected and islanded modes.