ABB RELION REX640 Technical Manual page 671

1MRS759142 F
around 10...50 MVA) is protected with the line differential function. The protection
includes the transformer from the protection field. In case D, the connection
between two substations and a small distribution transformer is located at the
tapped load. The usage of LNPLDF is not limited to these applications.
Figure 380: Line differential applications
Communication supervision
A typical line differential protection application includes LNPLDF as the main
protection. Backup over current functions is needed if a protection communication
failure occurs. When the communication supervision function detects a failure in
the communication between the protective units, the safe operation of the line is
still guaranteed by blocking the line differential protection and unblocking the over
current functions.
When a communication failure is detected, the protection communication
supervision function issues block for the LNPLDF line differential protection and
unblock for the instantaneous and high stages (instance 2) of the over current
protection. These are used to give backup protection for the remote end feeder
protection relay. In situations where the selectivity is weaker than usually, the
protection should still be available for the system.
REX640
Technical Manual
Protection functions
671