IEEE Power & Energy Magazine - May/June 2021 - 53
needs to be a way to disconnect the NT from the secondary
network if it becomes separated on its primary side.
The second challenge involves large differences in the available fault current from a secondary network and that available
from an IBR-sourced microgrid. Figure 5 depicts the grid network in Figure 4 sectionalized into four microgrids. Microgrid
A is a single DER feeding one facility on a dedicated bus. Thus,
it might not contain any secondary network conductors within
its boundary. Microgrids B, C, and D involve a DER in one part
of the secondary network serving a load in another part, and
as a result, there are secondary network conductors that form a
portion of each of these microgrids. The fault current availability on these conductors will be dramatically different between
the grid-connected and off-grid modes, yet it remains necessary
to have a protection system that isolates and deenergizes the
fault in both cases.
Microgrids B, C, and D have more than one point of
interconnection with the larger grid. If they were combined
into one networked microgrid, the resulting system would
enjoy the reliability benefits of the local sources and of having redundant power paths at the distribution level. However,
there would be additional challenges in the coordination of
the transition from on grid to off grid and back. Grid-tooff-grid transitions can be characterized in two ways. Open
transitions are those in which a microgrid is deenergized
and then black-started in the off-grid mode. Closed transitions, which are sometimes called seamless transitions, are
those in which the voltage and frequency in a microgrid are
DER
P, Q
DER
CB
P, Q
P, Q
Main Service (MV)
CB
Primary Feeder 1 (MV)
Primary Feeder N (MV)
P, Q
NU
P, Q
NU
NU
P, Q
P, Q
NU
DER
Secondary
Main 1 (LV)
DER
Loads
Loads
DER
Loads
Loads
DER
Loads
Loads
DER
Secondary
Main K (LV)
Loads
figure 4. A secondary grid network with DERs added (indicated by the red arrows). The DERs are larger than the loads
served, and thus the power flow through the NUs can now be bidirectional under normal operating conditions, as
indicated by the P, Q arrows.
may/june 2021
ieee power & energy magazine
53
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
IEEE Power & Energy Magazine - May/June 2021 - Cover4
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