IEEE Power & Energy Magazine - May/June 2021 - 77
First, consider how a microgrid would respond to a utility outage if it could not seamlessly island in unplanned
events. A typical response is to trip the DERs but keep
the customer loads connected to the utility system, with
the expectation that the utility power may be restored
momentarily via reclosing or restoration. In this typical
response, if the utility outage persists, then the microgrid
will be isolated from the utility by opening the interconnect breaker. When the microgrid is isolated, it can be
energized in islanded mode by starting and paralleling
the DERs and then connecting microgrid loads. This
response can take a few minutes and includes waiting for
the utility power supply to be restored and black starting
the islanded microgrid.
By contrast, microgrids that can do seamless, unplanned
islanding in response to a utility fault do so not by tripping
the DERs but by immediately tripping the interconnection
breaker. When the interconnection breaker trips, the DERs
must continue to supply power while transitioning from grid
tied to islanded mode. In this way, the lights stay on, but the
microgrid is islanded.
How Seamless, Unplanned Islanding
Was Implemented
There were three key components necessary to successfully implement seamless, unplanned islanding at the North
Bay CEP, the first of which was the microgrid controller.
Because the microgrid controller has a complete picture of the
microgrid, it can determine whether a seamless, unplanned
transfer to island mode would be successful if attempted. This
is referred to as the determination of transfer readiness.
The second piece is the protective relays. The microgrid
needs the relays to quickly detect and isolate problems in the
utility supply. In this context, quickly means within a few
power system frequency cycles.
Finally, the DERs must be capable of switching from
their grid-tied mode of operation to their islanded mode
while continuing to supply power. At the CEP microgrid,
Utility Supply Substation
CB52-2
A portion of the utility system
may become unintentionally
islanded when the utility
feeder breaker CB52-2
(or any other device between
CB52-2 and CB1 or CB2)
is opened. System protection
must prevent unintentional
islanding.
44-kV Utility Feeder Line
52
44 kV
12 kV
52
Microgrid
CB1 (Point of Interconnection) CB2
K
ac
dc
ac
dc
Customer
Loads
figure 5. A diagram depicting the possibility of unintentional islanding.
may/june 2021
ieee power & energy magazine
77
IEEE Power & Energy Magazine - May/June 2021
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021
Contents
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