this requires the CHPs to switch from power set point control to isochronous control while continuing to supply power. The CHP controls do this automatically based on the status of the interconnection breaker. Role of the Microgrid Controller in Seamless, Unplanned Islanding The microgrid controller decides whether the microgrid is transfer ready, and it communicates this status to the protective relays. As shown in Figure 7, three key quantities can be examined to determine the transfer readiness: 1) total load 2) online generation capacity (also known as spinning reserve) 3) power flow from the utility. Microgrid loading and online generation capacity are important factors in determining transfer readiness because the load must not exceed the available generation. To understand why the power flow from the utility is an important factor in determining transfer readiness, consider what will happen when the interconnection breaker trips. The power that the utility was providing now suddenly needs to be supplied by the DERs. The microgrid sources must be able to accept that load step without shutting down and without an objectionable change in voltage or frequency. Lithiumion battery systems excel at accepting load because they can typically change the power output rapidly across their full range within milliseconds. Natural gas CHPs take longer to accept load because changing the engine power output is more gradual, taking up to several seconds to ramp from no power output to rated power output. Therefore, if the battery system is not available for dispatch, then the microgrid controller must check whether the amount of power flowing from the utility is within the CHPs' load-acceptance capability before considering the microgrid to be transfer ready. Role of the Protective Relays in Seamless, Unplanned Islanding Protective relays are used to quickly detect problems in the utility system. The microgrid protection strategy must detect two types of utility problems: short circuit faults on the utility and a loss of source. When the relays detect a utility problem (either a fault or a loss of source), the relays respond with one of two actions Utility R 44 kV Legend: 12 kV 52 CB52-1 R 12 kV 12 kV 600 V 600 V CB1 (Point of Interconnection) CB2 R 50 kW R 200 kW Multifunction Relay (Microprocessor Based) K Key Interlock Microgrid R K CB3 R CB4 CB5 R R ac 200 kW 8 kW CB6 CHP 265 kW Battery System 250 kW R CB7 R CB8 R CB9 ac dc dc CHP 265 kW R Solar 8 kW Memorial Gardens YMCA Thomson Park Customer Loads figure 6. A typical power flow under normal operating conditions. 78 ieee power & energy magazine may/june 2021