IEEE Electrification Magazine - September 2013 - 53

13,000

11,000

9,000

7,000

5,000

3,000

1,000

−1,000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 0
Time (h)
Utility Grid Supply
Battery Supply
Microgrid Supply

Islanding and Resynchronization

Electricity Price (cents/kWh)

Power (kW)

are regulated by the utility grid. In
island mode, the -natural-gas turbine
and battery storage would maintain
the microgrid frequency and voltage,
while solar PV and wind turbine units
serve portions of the campus load.
The short-term microgrid reliability is
enhanced by implementing three
major functions at IIT: islanding and
resynchronization, emergency
demand response, and self-healing,
which are discussed in this section.

Total Demand
Price of Electricity

Power (kW)

Electricity Price (cents/kWh)

The microgrid may increase its load
point reliability indices by setting up Figure 21. The day-ahead storage, microgrid, and utility grid supply on 17 July 2012.
its operation in island mode. Generally, there are two major reasons for
setting up a microgrid in island mode: 1) poor power
nization signals to the campus DER units through secquality at the utility grid, such as frequency or voltage
ondary control for mitigating any possible transients.
deviations, and 2) major faults at the utility grid. PMUs
The following criteria are to be satisfied for transition
and voltage/current meters at the point of common coufrom island to grid-connected mode:
pling (PCC) would report the utility grid malfunction to
1) The voltage magnitude difference at the PCC would be
the master controller, which will initiate the islanding
small.
process at the tertiary control level. The master controller
2) The frequency difference would be small to match the
will monitor building meters for supplying the local genvoltage phase angles at the switching instance.
eration dispatch. At islanding, the master controller may
3) The voltage angle with the lower frequency should lag
reduce the campus load through emergency demand
behind that of the higher frequency. Figure 25 shows
response to match the load with the local generation disthe voltage angle difference between the microgrid and
patch. The load reduction may entail shifting building
the utility grid at resynchronization instance. Assuming
loads and reducing curtailable building loads. Matching
that the microgrid frequency is slightly smaller than
the load with generation at islanding will reduce tranthat of the utility grid if VMG leads Vgrid , then the power
flow will be from the microgrid to the utility grid and in
sients and ensure a feasible microgrid operation considthe reverse direction at steady state. The flow from the
ering the ramping limits of DER unit -generation.
microgrid to the utility grid at resynchronization may
Figure 24 shows the campus load restoration in island
result in the overloading of the microgrid DER units.
mode at the Engineering 1 and Stuart Buildings located
The IIT microgrid resynchronization process is presentin Loop 1 on 19 July 2012. The load restoration started at
ed as follows. At first, the master controller will send the
6:19 a.m. on both buildings and was fully restored at 6:29
a.m. Figure 24(b) shows the inrush
current of a switched-on transformer located in the Stuart Building. In
13,000
30
island mode, any abrupt changes in
11,000
25
the local microgrid load are served
9,000
20
by the battery storage through pri7,000
15
mary and secondary controls. Once
5,000
the normal operation at the utility
10
3,000
grid is restored, the microgrid will be
5
1,000
resynchronized with the utility grid.
−1,000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 0
In island mode, the microgrid could
Time (h)
be operated at a frequency and a
voltage magnitude that are different
Utility Grid Supply
than those of the utility grid, which
Total Demand
could cause transients during the
Utility Grid Supply with Building Controllers
Price of Electricity
resynchronization process and damage the substation equipment. The
master controller will send synchro- Figure 22. The day-ahead economical demand response by building controllers on 17 July 2012.

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2013