IEEE Power & Energy Magazine - May/June 2021 - 30

Circuit
Breaker Closed
L
WT
B

Load

Circuit
Breaker Open
PV

Bus 5

Photovoltaic

CB4

Bus 6

CB5 CB7

CB9 CB10

Utility Grid

Wind Turbine
Batteries Energy
Storage
Fault Current

PEs
CB11

CB6 CB8

Fault

Backup
Protection

PV
CB1

CB2

CB12

Bus 3

Bus 4

CB13 CB15

CB17 CB18

Utility Grid
Bus 1
Grounding
Transformer
Distribution Grid

CB14 CB16

CB20

CB19
L

WT
Bus 2

CB3
B

Microgrid

(c)

figure 7. (Continued) (c) The operation of the backup feeder protection.

status signal to the relay at the other end of the line when
a forward in-zone fault (or a blocking signal for out-of-zone
faults) is detected. However, a trip signal is issued to the associated circuit breaker of the relay only if it detects a forward
fault, e.g., the relay at CB15, and at the same time receives a
signal from a remote-end relay, e.g., the relay at CB17.
For example, if a fault occurs between buses 3 and 4, the
relays at CB15 and CB17 will detect a forward fault. Both
will send a command to each other that will allow the relays
to trip their circuit breakers and disconnect the faulty feeder
[Figure 7(b)]. On the contrary, relays controlling CB10 and
CB12 will also detect a forward fault. However, they do not
receive a signal to trip from the relays at the other end of their
respective lines (relays at CB13 and CB18) because those
relays detect a reverse fault. Therefore, CB12 and CB10 will
not trip, and the faulted feeder only will be isolated.
Communication between the devices can also assist in
the deployment of backup protection schemes that address
breaker failures or topology changes. In the previous example, if CB15 or CB17 fails to open, the respective relays will
transmit a tripping command to the upstream relays that
control CB13 and CB18 and to relays controlling the generation that will remain in the faulty region, e.g., the relay
controlling CB14. The operation is presented in Figure 7(c).
If a fault occurs in the zone of the relays protecting distributed
generators or loads, e.g., the relays controlling CB14 and CB16,
30

ieee power & energy magazine

respectively, and their respective breaker fails to operate, those
relays can send a signal to the upward relays, e.g., the relays controlling CB13 and CB15, to trip and clear the fault. Topological
changes can also affect the overall differential protection scheme.
For example, if CB10 and CB18 are open, the relays controlling
CB9 and CB17 will not experience fault currents, and they will
maintain blocking to their pairs even if faults inside their protected zone (lines) occur. The differential protection of a line can
be enabled or disabled based on the status of its respective circuit
breakers. In the previous case, the relays of CB10 and CB18 can
send their status to the relays of CB9 and CB17, respectively, to
stop or start the blocking procedure of their pair. Therefore, if
the relay controlling CB10 informs the relay controlling CB9 to
stop blocking the relay controlling CB7, the line between buses
5 and 6 is solely protected by the overcurrent element of the relay
controlling CB7.
Differential schemes increase the protection reliability if
proper communication infrastructure is used. Also, they require
digital devices that can execute basic protection functions, support communication protocols, and offer the flexibility to design
the logical equations on which they operate.
Industrial substation communication protocols, mainly
IEC 61850 and especially its generic object-oriented substation event (GOOSE) feature, can be deployed for such
schemes. GOOSE messaging can be deployed for the peerto-peer communication required for differential protection
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