60.40 Freq_microgrid Period for Secondary Control (MW) 60.20 (Hz) 13.0 12.0 11.0 10.0 9.0 8.0 7.0 60.00 59.80 59.60 4.48 V_pcc 7.0 4.16 4.00 5.0 4.0 3.84 h = 13 Q_Load 6.0 (MVar) (kV) 4.32 P_Load 3.0 20 min = 55 40 60 80 x 100 h = 13 20 min = 55 (a) 40 60 80 100 (b) 180 120 60 0 -60 -120 -180 8.0 phase diff 5.0 Period for Secondary Control Resynchronization Instant -5.0 -10.0 x P_switch 26.50 26.60 h = 13 min = 55 6.0 (MW) phase diff 0.0 (°) (°) figure 11. (a) The frequency and voltage and (b) load of the IIT microgrid in Case 3. 26.70 26.80 26.90 27.00 4.0 2.0 0.0 8.0 V_switch (kV) 4.0 0.0 -4.0 -8.0 2.0 I_grid_A (kA) 1.0 0.0 -1.0 -2.0 x h = 13 20 min = 55 40 60 80 100 (a) (b) figure 12. Resynchronization conditions at the PCC in Case 3. (a) The phase angle difference between the microgrid and the utility grid bus voltages at the PCC and (b) the crucial instants before and after the coupling switch at the PCC is closed. Note that the switch is closed when the angle difference is 2.5°. Case 2: Load Restoration in Island Mode once the rated microgrid frequency and voltage in the restoration path are stabilized as described above, the master january/february 2014 controller will send signals to building controllers to perform load restoration through tertiary control. figure 7(a) shows that the master controller will send secondary control signals ieee power & energy magazine 79