(MW) 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 200 100 0 -100 -200 V_pcc 4.32 (kV) 8.0 6.0 4.0 2.0 0.0 (MW) 4.48 P_main grid Freq_microgrid (kW) (Hz) 61.50 61.00 60.50 60.00 59.50 59.00 58.50 4.16 4.00 3.84 x h = 13 5.0 min = 5 10.0 15.0 20.0 25.0 30.0 figure 5. Microgrid frequency and voltage at the PCC in Case 1. secondary and tertiary control signals in a restoration path is described using the following three cases as examples. h = 13 the iit microgrid is depicted in figure 4. at t = 13:05:10 s, a permanent fault caused by a major storm on the utility side of the Pcc triggers the islanding of the microgrid by the master controller. in this case, the microgrid infrastructure remains functional on the restoration path. figure 5 shows that the microgrid frequency will drop slightly at islanding because the available microgrid dEr is smaller than its load. also, there will be a short spike representing the rotating inertia of the natural gas turbine at islanding. figure 6 shows the period when primary and secondary controls in the natural gas turbine and battery storage will stabilize the microgrid voltage and frequency. the 4.80 15.0 20.0 25.0 30.0 P_Load 4.40 (MW) (Hz) 10.0 4.60 60.00 59.50 4.20 59.00 4.00 V_pcc 2.60 (MVar) 4.32 (kV) 5.0 min = 5 secondary control will restore the microgrid frequency to 60 hz once the primary control for load sharing results in a lower frequency. the battery will discharge shortly after islanding (within 10-11.5 s) to maintain the frequency. the wind turbine and solar Pv units (8 kW and 123 kW, respectively) do not participate in frequency and voltage regulation. from 11.5-20 s after islanding, there is a frequency overshoot representing the secondary control system of the natural gas turbine that prompts the battery to charge. the charging will stop when the 60-hz frequency is restored. Freq_microgrid 4.16 4.00 3.84 h = 13 P_batt figure 6. The dispatch of utility grid and DER units in Case 1. 60.50 x P_SG2 Periods of Primary and Secondary Control Case 1: Microgrid Islanding and Emergency Demand Response 4.48 P_SG1 Q_Load 2.40 2.20 2.00 20 60 min = 6 100 140 180 220 260 x h = 13 20 60 min = 6 (a) 100 140 180 220 260 (b) figure 7. (a) The voltage and frequency and (b) the load of the IIT microgrid in Case 2. january/february 2014 ieee power & energy magazine 77