IEEE Power & Energy Magazine - May/June 2021 - 73
to export power when the load suddenly decreases, as
shown in Figure 4.
Protection-Speed Requirements
Microgrid protection must also operate very quickly in
response to both local and external utility system faults.
Microgrid DERs have low inertia, which can result in rapid
DER frequency changes when faults are present. To ensure
a stable response to a fault in a low-inertia microgrid, the
mi-crogrid protection must operate very quickly to separate
all the fault-current sources (including the utility system and
microgrid DERs) from the faulted components. For example,
if there was a fault on the circuit serving North Bay's Thomson
Park load while islanded, the microgrid frequency is expected
to rise, and the voltage will be depressed. The CHP generators will accelerate while the voltage depression will cause
the grid-following inverters, in this case, the PV inverters, to trip offline. Therefore, the fault must be interrupted
promptly by the local microgrid protection on CB9 to limit
the generators' acceleration, allowing a stable recovery
while continuing to serve the unfaulted microgrid load circuits without interruption.
Fast microgrid protection operation is also critical in response to faults while grid tied. For example,
if a close-in bolted fault occurred on one of the three
microgrid load circuit circuits (i.e., downstream of CB7,
CB8, or CB9) while grid tied with the CHP generators
running, the local microgrid protective relay and circuit
breaker must clear this fault promptly so that the CHP
generators' frequency and phase angle do not significantly shift from prefault conditions, which would result
in generator damage.
Fast and communication-based microgrid protection
operation is also preferred for protection coordination while
grid tied. The microgrid's 600-V switchgear introduced multiple series-connected protective devices (e.g., CB1, CB2,
CB7, CB8, and CB9) between the existing load circuit breakers in the community facilities and the upstream transformer
protective fuses. Although the existing overcurrent protection was selectively coordinated using time grading, there
was an insufficient time interval to coordinate these new
protective devices with the utility's distribution system and
existing load overcurrent protection. Using only time grading to selectively coordinate the protection, especially if
Utility
R
44 kV
Legend:
12 kV
52
R
Multifunction Relay
(Microprocessor Based)
K
Key Interlock
CB52-1
12 kV
12 kV
600 V
600 V
CB1 (Point of Interconnection) CB2
R
R
Microgrid
R
K
600-V Switchgear Bus
R
CB3
R
CB4
CB5
R
CHP
265 kW
CB7 R
CB8
R
CB9
dc
dc
CHP
265 kW
R
ac
ac
Battery
System
250 kW
CB6
R
Solar
8 kW
Memorial
Gardens
YMCA
Thomson
Park
Customer Loads
figure 2. A simplified single-line diagram of the CEP microgrid. CB: circuit breaker; CHP: combined heat and power.
may/june 2021
ieee power & energy magazine
73
IEEE Power & Energy Magazine - May/June 2021
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021
Contents
IEEE Power & Energy Magazine - May/June 2021 - Cover1
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IEEE Power & Energy Magazine - May/June 2021 - Cover3
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