6.78
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
You have enabled three levels of residual ground overcurrent elements. The
Level 2 residual ground directional overcurrent element provides communica-
tions-assisted tripping for internal unbalanced faults. The Level 3 residual ground
overcurrent element provides nondirectional start (50G3) and directional start
(67G3).
The Relay Word bit DSTRT asserts if any of the following elements pick up:
➤ Zone 3 phase-distance elements (Z3P)
➤ Zone 3 ground-distance elements (Z3MG)
➤ Level 3 residual ground directional overcurrent element (67G3)
Relay Word bit DSTRT is useful when a bolted close-in three-phase fault occurs
behind the relay. If the polarizing voltage for the distance elements collapses to
zero, the corresponding Zone 3 supervisory phase-to-phase current level detec-
tors latch the Zone 3 phase-distance elements. Therefore, the Zone 3 phase-
distance characteristics do not need a reverse offset for this particular situation.
Assign Relay Word bit DSTRT (Directional Start) to OUT102 to start transmis-
sion of the blocking signal.
OUT102 := NSTRT AND NOT STOP OR DSTRT.
OUT103 stops the local transmitter from sending the blocking signal to the
remote terminal.
OUT103 := STOP OR 3PT.
Time delay on pickup prevents transmission of the blocking signal if a transient
causes a reverse-looking element to pick up momentarily. Set the corresponding
timer to 1 cycle.
Z3XPU := 1.000. Zone 3 Reverse Pickup Time Delay (0.000–16000 cycles)
You can also extend the blocking signal during current reversals. Set the corre-
sponding dropout timer to 5 cycles.
Z3XD := 5.000. Zone 3 Reverse Dropout Delay (0.000–16000 cycles)
Coordination Timers
The forward-looking elements that provide high-speed tripping at Station S must
be delayed momentarily so the local circuit breaker does not trip for external
faults behind Station R. This time delay provides time for the nondirectional and
reverse-looking blocking elements at Station R to send a signal to Station S
during out-of-section faults. This particular time delay is the coordination time
for the DCB trip scheme. There are separate coordination timers for Zone 2 dis-
tance elements (21SD) and Level 2 residual directional overcurrent elements
(67SD).
The recommended setting for the 21SD timer is the sum of the following three
times:
➤ Control input recognition time (including debounce timer)
➤ Remote Zone 3 distance protection maximum operating time
➤ Maximum communications channel time
If the control input time delay on pickup debounce timer is zero, the maximum
recognition time for the control input is 0.125 cycles. Assume a remote Zone 3
distance protection pickup time of one cycle; the remote Zone 3 distance protec-
tion should operate faster than the local Zone 2 distance protection because the
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