IEEE Power & Energy Magazine - May/June 2021 - 48

Secondary networks have been in use for more than a century,
and today they serve hundreds of urban downtown areas
and thousands of individual facilities.
is entirely different from that of secondary networks. IBRs
are often distributed throughout a system, their fault currents
are generally low, and the current flow is not unidirectional.
Thus, if it were ever contemplated to deploy a microgrid on a
secondary network, significant system protection challenges
would have to be overcome.
This article is the second in a two-part series about the
influence of IBRs on microgrid protection. It briefly discusses secondary networks and IBR-sourced microgrids and
then delves into the protection of both types of systems, the
difficulties that would need to be surmounted in protecting
a secondary network containing an IBR-sourced microgrid,
and potential approaches to addressing these issues.

Background
Secondary networks are a nonradial, three-phase power
system topology that provides redundancy in a distribution
system so that loads have a continuous connection to alternative feeders. Thus, secondary networks provide an extremely
high continuity of electric power service against disturbances
in the distribution network, although they are still vulnerable
to interruptions in the transmission systems above them.
Secondary networks are so named because they operate at a
" secondary voltage, " usually 120/208 V or 277/480 V in the
60-Hz world. Secondary networks have been in use for more
than a century, and today they serve hundreds of urban downtown areas and thousands of individual facilities worldwide.
Secondary network protection is a mature field. Because of
the redundant paths to the source, these networks tend to have
low source impedances and very high fault current availability, and secondary network protection is often designed
around (and to take advantage of) this fact. Also, because
secondary networks contain loops and meshes, most of their
faults must be isolated from both sides, which requires special considerations in the protection system design.
It is also possible to conceive of a " primary network " in
which loops and meshes at a medium voltage (MV), such as
15 kV, are used to provide redundancy in the power path and
achieve continuity-of-service improvements. Many of the
general principles discussed here concerning secondary networks would also apply to such a " primary network. " Since
almost no distribution systems today are designed to operate
in this way, this possibility is mentioned only as a future
consideration and discussion item.
Microgrids are another means to provide higher-reliability electric service and increased power system resilience.
According to IEEE Standard 2030.7-2018, a microgrid is
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ieee power & energy magazine

a collection of distributed energy resources (DERs) and
loads within a defined boundary that have been planned and
designed to operate in either a grid-parallel mode or to disconnect and operate in an off-grid mode when desired. Many
microgrids are in operation, with all known examples constructed on previously radial circuits (no loops and meshes).
Microgrids may be powered by any combination of DERs,
including rotating machines and IBRs. The use of local
sources reduces the level of vulnerability to disruptions in
the transmission system. Microgrid protection is an evolving
field in which several challenges must be solved. Microgrids
must be protected in both on- and off-grid modes, and the
available fault current and its properties may vary greatly
between those configurations through time. This is especially true for microgrids sourced entirely by IBRs, noting
that most microgrids deployed today fall into this category
for at least some portion of their off-grid operation. Also, if
a microgrid's sources are distributed instead of centralized,
there are additional protection challenges.
If one were to contemplate creating a microgrid on a secondary network, these two sets of protection requirements
would collide in a way that creates unique and particularly
difficult challenges. In this article, secondary networks and
microgrids, especially IBR-sourced microgrids, are briefly
described along with their protection challenges. The task of
deploying a microgrid on a secondary network is described,
and potential protection approaches are discussed. DC
microgrids, while important, are not addressed.

Secondary Networks
Topology of Secondary Networks
Secondary networks are grouped into two main categories:
grid and spot. These are briefly described in the following.
Secondary Grid Networks

Secondary grid networks are used to serve dense urban
areas, such as downtowns and business districts. They are
sometimes called street networks because the cables in the
grid are usually laid out under, and thus follow, the streets in
the area. They may also be referred to as area networks and
downtown networks. Figure 1(a) presents the topology of a
typical secondary grid network. A single main three-phase
service is shown at the top, but a grid network can have more
than one. This secondary grid network has N primary feeders
and K secondary mains, all three phase. The main service
and primary feeders are all operated at MV (a distribution
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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