ABB RELION REX640 Technical Manual page 321

1MRS759142 C
REX640
Technical Manual
The differential protection is available only if the communication between the units is
working properly and no CT failure situation is detected. If a communication failure
exists, the protect area or unit is left out from the primary protection scheme.
Therefore it is practical to use overcurrent protection as a local backup functionality.
In the standard configuration of the protection relay, the backup overcurrent
protection is implemented with four overcurrent stages so that under normal
conditions, that is, when the line differential communication is healthy, only two
lowest stages are available for the remote backup protection. In case a line differential
communication failure exists, two more stages are released for rapid local backup
overcurrent and short circuit protection. These stages are blocked in normal situation
and automatically unblocked when the communication failure is detected. The setting
of the overcurrent stages for local backup protection has to be considered carefully in
order to achieve the best possible protection performance under abnormal conditions.
Two situations, case A and B, are shown in
media is valid and therefore the line differential protection is in operation. In this case,
the two lowest stages of overcurrent protection are in operation simultaneously. In
case B, a communication failure causes a situation where the line differential function
is not able to work properly. Unblocking of the two highest overcurrent protection
stages releases these functions to protect the line against overcurrents and short-
circuits.
GUID-936EDBC6-03A1-4372-982D-140050589F15 V1 EN
Figure 178: Backup overcurrent protection for line differential applications
Protection functions
Figure 178. In case A, the communication
Section 4
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