R.1.89
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Directional Comparison Blocking Scheme
The recommended setting for the 67SD timer is the sum of the following three
times:
➤ Control input recognition time (including debounce timer)
➤ Remote Level 3 nondirectional low-set overcurrent element
maximum operating time
➤ Maximum communications channel time
The output of Level 2 delay timer 67SD is Relay Word bit 67QG2S (Negative-
Sequence and Residual Directional Overcurrent Short Delay).
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-section faults. Nondirectional elements do not process a directional
decision, so nondirectional 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
elements 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 offset. 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
blocking 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
signal 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 elements after the coordination timers expire. If the blocking signal
drops out momentarily, the distance relay can trip for out-of-section faults.
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