Influence of Inverter-Based Resources on Microgrid Protection: Part 2: Secondary Networks and Microgrid Protection

Secondary networks are deployed when exceptionally high-reliability electric service is required for specific loads. Secondary network protection makes extensive use of the fact that the available fault currents in such systems are typically very high. Also, most faults in secondary networks must be isolated from both sides, which requires some special considerations. Today, microgrids are also being deployed as a means of increasing power system resilience on radial circuits. Many microgrids are energized exclusively by inverter-based resources (IBRs), either all the time or under certain operational conditions. The protection of IBR-sourced microgrids is entirely different from that of secondary networks. IBRs are often distributed throughout a system, their fault currents are generally low, and the current flow is not unidirectional. Thus, if it were ever contemplated to deploy a microgrid on a secondary network, significant system protection challenges would have to be overcome.