IEEE Power & Energy Magazine - January/February 2014 - 80

(MW)

5.0

(MVar)

P_main grid

Load Restoration by Tertiary
Control
P_SG1

P_SG2
Response of Natural Gas
Generator to Tertiary Control

4.0
3.0
2.0
1.0
300

(kVar)

(kW)

Q_SG1

Q_SG2

2.00
1.50
1.00

100

100
0
-100
h = 13

Q_main gridGrid

0.50
P_batt

200

5.0
4.0
3.0
2.0
1.0
0.0
-1.0
2.50

(MW)

8.0
6.0
4.0
2.0
0.0
-2.0

Q_batt

50
0
-50
-100

20
min = 55

100

x
h = 13

(a)

20
min = 55

100

(b)

figure 13. The dispatch of utility grid and DER units in Case 3: (a) real power and (b) reactive power.

to the natural gas turbine and battery storage during the load
restoration to maintain the Pcc-rated frequency and voltage
(4.16 kv and 60 hz). figure 7(b) shows that at t = 13:06:30
s, the campus load will begin to increase progressively from
4.13 MW and 2.07 Mvar to 8.13 MW and 4.07 Mvar. the primary controls of the natural gas turbine and battery storage
will respond to load increments by adjusting the frequency
with each step. figure 7(a) shows that the secondary control
stabilizes the microgrid voltage and frequency before increasing the load in the next step. the natural gas turbine supplied
4 MW and 2 Mvar before load restoration; this will increase to
8 MW and 4 Mvar after load restoration.
in figure 8, the highlighted period in red shows primary and secondary control signals for regulating the
microgrid voltage and frequency. the primary control
will increase real and reactive power dispatch of both the
battery storage unit and the natural gas turbine (sG1 and
sG2 are the synchronous generators of the natural gas
turbine) according to their droop characteristics. the secondary control signal of the master controller will restore
the microgrid frequency and voltage. figure 9 shows that
at t = 13:33:05 s, the master controller will procure a
real-time optimal dispatch to adjust the set points of battery storage units and building controllers through tertiary control. figure 10(b) shows that the total served
real and reactive load on campus reaches 8.38 MW and
4.19 Mvar, respectively. figure 10(a) shows the periods in
which the microgrid voltage and frequency are adjusted by
master controller signals, which are submitted to primary
controls of the natural gas turbine and battery storage unit.
80

ieee power & energy magazine

Case 3: Resynchronization of the Microgrid
with the Utility Grid
once the normal operation of the utility grid is restored, it
will communicate with the master controller through tertiary
control to resynchronize the microgrid load with the distribution network. accordingly, the microgrid master controller will send secondary control signals to both the natural gas
turbine and the battery storage unit to minimize the voltage
magnitude and phase differences between the microgrid and
the utility grid prior to resynchronization. When the Pcc
voltage is at the rated value and the microgrid frequency is
slightly lower than its rated value, its voltage angle should
also lag that of the utility grid for resynchronization.
figure 11(a) shows that the resynchronization process
starts at t = 13:55:20 s with a secondary control signal sent by
the master controller to the natural gas turbine to adjust the
microgrid frequency to 59.9 hz for phase angle synchronization. figure 11(a) shows that the voltage magnitude is set at
its rated value by the secondary control signal that is sent to
both the natural gas turbine and the battery storage unit. the
master controller also sets the reference frequency of the battery storage unit to be equal to the microgrid frequency so that
the battery maintains its dispatch during resynchronization.
figure 12(a) shows the phase angle difference between
the microgrid and the utility grid bus voltages at the Pcc;
a positive difference indicates that the utility grid voltage
is leading that of the microgrid. the frequency difference
(0.1 hz) between the iit microgrid and the utility grid
will cause the voltage phase difference to ramp up. at
t = 13:55:26.75 s, all resynchronization conditions are satisfied,
january/february 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2014