5.133
Date Code 20171021 Instruction Manual SEL-421 Relay
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 communica-
tions channel is less than perfect. Communications channel failures are unlikely
to occur during external faults. You can use the DCUB trip scheme with conven-
tional communications channels such as power line carrier (PLC). Use improved
methods such as M
IRRORED BITS communications to implement the DCUB trip-
ping 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 loss-of-guard (LOG) 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 per-
missive 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) out-
puts 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 Channel 2) in three-terminal line applica-
tions (e.g., setting ECOMM to DCUB2).
Enable setting ECOMM (when set to DCUB1 and DCUB2) determines the rout-
ing 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 con-
trol 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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