IEEE Power & Energy Magazine - May/June 2021 - 55
mode, especially if the DERs are IBRs. In addition to the
fault detection and isolation challenges, there are some needs
in the area of overvoltage protection. For example, most NTs
are connected with a delta winding on the primary feeder
side. With this configuration, it becomes possible for DERs
on the secondary network to support a ground fault overvoltage on the primary feeder side of the NT. Means must be
provided to prevent this, such as including a primary side
zero-sequence overvoltage function in the NP.
Potential Solutions to the
Protection Challenges of
Microgrids in Secondary Networks
At present, the most widely discussed alternative to the
reverse power protection challenge described previously is a system of communications-linked digital relays
-implementing -various forms of differential defense. Differential protection uses measurements of all current inflows
to (or outflows from) a piece of equipment or circuit section.
The vector sum of all of these current flows should be zero,
according to Kirchoff's current law. If it is not zero, then
some current is being " lost " from the zone defined by the
current transducers, indicating a fault or other problem and
enabling appropriate action to be taken.
Differential relaying is routinely used on equipment
such as transformers and transmission lines, and it is a
mature and reliable technology. It can be applied in radial
distribution in the form of a fault location, isolation, and it
is a mature and service restoration system, but in the case
of a secondary network, multiple zones must be protected.
For example, looking at the system in Figure 4, the protection system must prevent reverse current from back-feeding
through the secondary network into any fault on any primary
feeder but also into any fault on any section of the main service. However, the basic principle is the same, and, at least
in theory, differential protection could be applied regardless of the types and distributions of microgrid sources. The
primary drawback is a high cost because the differential
scheme must have transducers on all current inflow/outflow
points from all zones along with communications to take
DER
Main Service (MV)
CB
DER
Primary
Feeder N
(MV)
Primary
Feeder 2
(MV)
CB
Primary
Feeder 1
(MV)
CB
Secondary
Main 2 (LV)
Secondary
Main 1 (LV)
NU
Secondary
Main N (LV)
NU
NU
DER
DER
O
O
Tie
Loads
A
Tie
DER
NC
Loads
O
Tie
Loads
Tie
B
figure 6. A secondary spot network sectionalized into two microgrids, labeled A and B. The dot-dash lines indicate the
microgrid boundaries. The gray portions of the system are outside the microgrids. O: open secondary bus breakers.
may/june 2021
ieee power & energy magazine
55
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
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IEEE Power & Energy Magazine - May/June 2021 - Cover3
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