ABB RELION REX640, Техническое руководство, Страница: 333 / 1960

The operating times of the main and backup overcurrent protection of the above
scheme become quite long, this applies especially in the busbar faults and also
in the transformer LV-terminal faults. In order to improve the performance of the
above scheme, a multiple-stage overcurrent protection with reverse blocking is
proposed.
Figure 202
shows this arrangement.
Overcurrent protection with line differential protection
The line differential protection relay has also four separate overcurrent functions
which can be used as the backup protection of line differential function for lines and
cables. There are three stages available with definite or inverse time characteristics
and an instantaneous stage.
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
Figure 201
. In case A, the communication
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.
1MRS759142 F Protection functions
REX640
Technical Manual
333