operates with PE-interfaced units (wind turbine and battery storage) at a penetration level of more than 60%, while the diesel generator covers the rest of the energy demand. When the fault occurs, all of the generating units experience a high voltage sag [Figure 5(a)], which is below the fault ride-through requirements imposed by the Greek noninterconnected before the fault ride-through limit is exceeded. Improper coordination can lead to unit disconnections, thus jeopardizing the overall system security. Such a case is presented in Figure 5. A symmetrical fault occurs in the island microgrid operating with high PE-interfaced unit penetration. Before the fault inception, the system Circuit Breaker Closed L WT B Circuit Breaker Open PV Load Bus 5 ICB4 Photovoltaic Wind Turbine Diesel Generator Batteries Energy Storage PEs Fault Current Fault Bus 6 CB5 CB4 F1 CB7 F2 F3 CB6 Fuses L L ICB2 PV CB2 CB1 CB8 Diesel Generator Bus 4 Bus 3 CB9 F5 CB11 F6 F7 Bus 1 CB10 L WT ICB3 Off-Grid Microgrid L CB3 Bus 2 B (a) Time to Trip (s) 100 Battery Energy Storage 10 1 0.1 Diesel Feeder Relay (CB4) Fuse 1 (F1) Fuse 3 (F3) 50 75 100 Current (A) (b) 200 300 figure 4. (a) An off-grid island microgrid and (b) the impact of topology changes in the protection system performance. may/june 2021 ieee power & energy magazine 25