R.1.100
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Directional Comparison Unblocking Scheme Logic
Directional Comparison Unblocking Scheme Logic
The directional comparison unblocking (DCUB) tripping scheme in the
SEL-421 provides a good combination of security and reliability, even when a
communications channel is less than perfect. Communications channel
failures are unlikely to occur during external faults. You can use the DCUB
trip scheme with conventional communications channels such as PLC (power
line carrier). Use improved methods such as M
IRRORED BITS communications
to implement the DCUB tripping scheme efficiently and economically.
M
IRRORED BITS communications and the DCUB tripping scheme give secure,
high-speed operation.
Through a control input programmed to the LOG (loss-of-guard) function, the
relay monitors the LOG output from the communications receiver. If LOG
asserts, and no trip permission is received, the relay can high-speed trip during
a short window using selected overreaching elements. The relay then asserts
permissive trip blocking signal UBB and locks out permissive trip Relay Word
bit PTRX. The typical DCUB application is a POTT scheme with the addition
of a frequency shift-keying (FSK) carrier as the communications medium.
Enable the DCUB logic by setting ECOMM to DCUB1 or DCUB2. You must
provide the relay all POTT settings plus the settings exclusive to the DCUB
scheme. The following is an explanation of the differences between setting
choices DCUB1 and DCUB2:
➤ DCUB1—directional comparison unblocking scheme for two-
terminal lines (i.e., communication from one remote terminal)
➤ DCUB2—directional comparison unblocking scheme for
three-terminal lines (i.e., communication from two remote
terminals)
The DCUB logic takes the loss-of-guard and permissive trip outputs from the
communications receivers and makes permissive trip (PTRX1 and PTRX2)
outputs and permissive trip (unblock) blocking (UBB1 and UBB2) outputs.
PTRX1 asserts for loss of channel or for an actual received permissive trip in
two-terminal line applications (e.g., setting ECOMM to DCUB1).
PTRX1 or PTRX2 assert for loss of channel or for an actual received
permissive trip (for the respective Channel 1 or 2) in three-terminal line
applications (e.g., setting ECOMM to DCUB2).
Enable setting ECOMM (when set to DCUB1 and DCUB2) determines the
routing of Relay Word bits PTRX1 and PTRX2 to control Relay Word bit
PTRX. Relay Word bit PTRX is the permissive trip receive input into the trip
logic.
Three-Terminal Lines
If you apply the DCUB scheme to a three-terminal line, program SELOGIC
control equation PT1 and PT2 as follows:
PT1 := IN105 General Permissive Trip Received (SELOGIC Equation)
PT2 := IN106 Channel 2 Permissive Trip Received (SELOGIC Equation)
Relay control inputs IN105 or IN106 assert when the relay receives a
permissive signal from one of the two other terminals. The relay cannot high-
speed trip until both inputs assert. These two control inputs were chosen for
this example. Use control inputs that are appropriate for your application.
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