IEEE Power & Energy Magazine - May/June 2021 - 81

Seamless, unplanned islanding is achieved
through a combination of control and
protection systems.
Figure 9 shows how the current supplied by the DERs
increased substantially when the fault occurred. After
approximately seven power system cycles, the interconnection breaker opened to disconnect the microgrid from
the utility system. During the fault, the DERs' voltage was
depressed. Following the disconnection of the microgrid
from the utility system, the microgrid voltage recovered
while the utility voltage collapsed.
After this successful operation, the microgrid operated in
island mode for approximately 2 h. The microgrid was then
successfully resynchronized with the utility to return it to
grid-tied operation.

Microgrid Control Interactions
With Protection
As previously described, the protection design of the 44-kV
utility system and the additional 600-V backup protection
for this microgrid provides a comprehensive solution for
keeping DERs and other equipment protected in the event
of a fault. However, protective relays only react to faults or
other undesirable system conditions. When combined with
a secondary-level control architecture, the entire microgrid
can act proactively to avoid potential faults or other undesirable conditions. Seamless, unplanned islanding is achieved
through a combination of control and protection systems.
Power-export protection and asset-availability functions also
use a combination of both control and protection systems.

after tens of seconds, and significant power export causes
a trip within 1 s. If these limits are exceeded, all three of
the DERs (the CHPs, PV, and battery) are tripped offline.
This approach maintains the utility power to the loads while
removing DER contributions.
To avoid this protective trip event, the microgrid controller must adjust the power output of the DERs to maintain a
positive power flow from the utility. This is achieved through
the following control actions:
✔✔ For gradually changing loads, the microgrid controller
has user-defined set points that allow microgrid operators to set a minimum and maximum net power-import
target. The microgrid controller measures the total
net power input every few seconds. Any time the net
power input is below the defined minimum or above
the defined maximum, the microgrid controller issues
new set points to the CHP generators. This method assumes that the CHP reaction time to a new set point is
sufficient for maintaining a positive net power import.
✔✔ For dramatic load changes, the battery energy storage system is used. One of the primary loads in the
microgrid is approximately 150 kW and runs in a repeated, but unpredictable, cycle throughout the day.
This 150-kW load step and corresponding 150-kW
load rejection occur approximately every 2-3 h. The
microgrid controller must command the battery energy storage system to quickly charge during the load

Power-Export Protection
While Grid Connected

may/june 2021

Amps (RMS)

2,000

Output Current of One CHP Generator

1,500
1,000
500
0
0.4

kV (RMS)

When the microgrid is connected
to the utility source, it is prohibited from continuously exporting
power. The available DER assets
have more capacity than requir-
-ed to supply the loads and could
export as much as 300-400 kW
back into the 44-kV utility system.
The microgrid controller must constantly adjust the set points of the
DERs to avoid pushing power back
into the utility system.
Power export is measured at
the point of interconnection (CB1
or CB2) and defined in two levels in the protective relays: any
active power export causes a trip

0.3
0.2

Microgrid Voltage
Utility Supply Voltage

0.1
0
49.2

49.4

49.6
Time (s)

49.8

figure 9. The voltage and current plots showing the transition from the grid-connected
to the microgrid islanded mode.
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IEEE Power & Energy Magazine - May/June 2021

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