IEEE Power & Energy Magazine - May/June 2021 - 54
maintained within specified limits during the transition and
in which the network never " goes dark. " If, for example, a
closed transition was wanted for microgrid D, then all the
switches shown in green would have to open simultaneously.
Various delays may make such an operation difficult, and
there is also the possibility that switches may stick (fail to
open). Furthermore, a closed transition requires that some
fast-acting resource within the microgrid (usually an energy
storage resource) must quickly assume the voltage and frequency regulation, and there are generally costs associated
with achieving this functionality. Closed transitions are certainly achievable and needed in some situations, but open
transitions are typically simpler, and the equipment and
design costs are often lower. During an open transition, one
must deal with the resulting outage by placing any highly
sensitive loads on uninterruptible power supplies.
The situation for secondary spot networks is a bit simpler,
but the challenge is similar. Figure 6 shows the secondary
spot network from Figure 3 sectionalized into two microgrids
energized by DERs. Microgrid A includes a single load and
DER on the same secondary bus, and microgrid B involves
DERs on one secondary bus serving a load on another secondary bus. The secondary bus breakers labeled O are open
in this configuration. The same two situations arise here as
for the secondary grid network: if the DERs are large enough,
there is the possibility of a reverse power flow through the
NPs, and the fault current available while off grid will be significantly different from that available in the grid-connected
DER
CB
Main Service (MV)
CB
DER
Primary Feeder 1 (MV)
Primary Feeder N (MV)
NU
NU
NU
NU
DER
C
Secondary
Main 1 (LV)
DER
Loads
B
Loads
A
Loads
Loads
DER
D
DER
Loads
Loads
DER
Secondary
Main K (LV)
Loads
figure 5. A secondary grid network sectionalized into four microgrids, labeled A, B, C, and D. The dot-dash lines indicate the microgrid boundaries. The gray portions of the system are outside the microgrids.
54
ieee power & energy magazine
may/june 2021
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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