5.122
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Directional Comparison Blocking Scheme
If the control input time delay on pickup debounce timer is zero, the maximum
recognition time for the control input is 0.125 cycles.
Starting Elements
You can select nondirectional elements, directional elements, or both to detect
external faults behind the local terminal. These elements send a blocking signal
to the remote station to prevent unwanted high-speed tripping during out-of-sec-
tion faults. Nondirectional elements do not process a directional decision, so non-
directional elements are always faster than directional elements.
Nondirectional Start
Relay Word bit NSTRT (Nondirectional Start) is assigned to a contact output to
start transmitting the blocking signal. NSTRT asserts if either 50Q3 or 50G3
pickup.
Directional Start
Relay Word bit DSTRT (Directional Start) asserts if any of the following ele-
ments pick up:
➤ Zone 3 phase-distance elements
➤ Zone 3 ground-distance elements
➤ Level 3 negative-sequence directional overcurrent element
➤ Level 3 zero-sequence directional overcurrent element
Relay Word bit DSTRT is useful when a bolted close-in three-phase fault occurs
behind the relay. Zone 3 phase-distance characteristics do not need a reverse off-
set. Should the polarizing voltage for the distance elements collapse to zero, the
corresponding Zone 3 supervisory phase-to-phase current level detectors will
cause the Zone 3 phase-distance elements to latch.
Use timer Z3XD (Zone 3 Reverse Time Delay on Dropout) to extend the block-
ing signal during current reversals. Use timer Z3XPU (Zone 3 Reverse Time
Delay on Pickup) to prevent extension of the blocking signal resulting from
Z3XD if a reverse-looking element picks up during a transient. This pickup delay
ensures high-speed tripping for internal faults.
Extension of the Blocking Signal
The directional comparison blocking scheme typically uses an on/off carrier sig-
nal to block high-speed tripping at the remote terminal for out-of-section faults.
Connect the carrier receive block signal output contact from the teleprotection
equipment to a control input assigned to Relay Word bit BT (Block Trip
Received). This input must remain asserted to block the forward-looking ele-
ments after the coordination timers expire. If the blocking signal drops out
momentarily, the distance relay can trip for out-of-section faults.
Timer BTXD (Block Trip Extension) delays dropout of the control input assigned
to Relay Word bit BT so that unwanted tripping does not occur during momen-
tary lapses of the blocking signal (carrier holes). This timer maintains the block-
ing signal at the receiving relay by delaying the dropout of Relay Word bit BT.
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