IEEE Power & Energy Magazine - May/June 2021 - 51

Because the available fault currents in secondary networks
are so high, arc flashes can be a significant concern,
particularly in systems operating at 480 V.
current. In fact, the reverse power function is often set to be
able to detect the magnetizing current of the associated NT
so that it will trip the NP if its associated primary feeder
CB opens for any reason. When planning and implementing a secondary network, the NTs must be sufficiently well
matched so that circulating currents between them are kept
to a minimum, partly to avoid heating and also to enable
the use of reverse power flows for protection, as described.
Because the available fault currents in secondary networks
are so high, arc flashes can be a significant concern, particularly in systems operating at 480 V. For this reason, some
NPs offer a feature called active arc flash mitigation that
can be turned on when maintenance is being performed.
Essentially, this feature sensitizes the NPs to forward current, reducing the forward current rating to be just barely
above the load current to limit the incident energy that might
be delivered to an arcing fault.

Microgrids
A microgrid is an intentional power island that 1) has a clearly
defined electrical boundary and 2) is planned, -controlled, and
protected so that it can operate in either a grid-parallel or an autonomous (off-grid, or intentional islanding) mode. Microgrids are
typically deployed to improve the reliability of electric service
to areas that, for one reason or another, suffer from below-average reliability, and they are installed in facilities where there is
an on-site energy source that can be readily adapted to provide
power if the main grid supply is lost. Microgrids have demonstrated their reliability enhancement benefits in multiple applications. For example, the Borrego Springs microgrid serves a
community at the end of a long radial circuit across difficult
terrain that is frequently impacted by storms, and the combined
heat and power (CHP)-based microgrids at Princeton University and New York University enabled the campuses to keep the
lights on during Hurricane Sandy.

Main Service (MV)

NU 1

NU 2

NU 3

NC

NC

Tie

NC

Tie
Loads

Secondary
Main 3 (LV)

Secondary
Main 2 (LV)

Secondary
Main 1 (LV)

Primary
Feeder 3
(MV)

CB 3

Primary
Feeder 2
(MV)

CB 2

Primary
Feeder 1
(MV)

CB 1

NC
Tie

Tie
Loads

Loads

figure 3. A spot network with a fault on primary feeder N, showing fault current paths sourcing current from both sides
of the fault.
may/june 2021	

ieee power & energy magazine 	

51



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
IEEE Power & Energy Magazine - May/June 2021 - Cover2
IEEE Power & Energy Magazine - May/June 2021 - Contents
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IEEE Power & Energy Magazine - May/June 2021 - Cover3
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