Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.79
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
345 kV Tapped Overhead Transmission Line Example
apparent fault impedance is deeper inside the remote Zone 3 distance protection
characteristic. Finally, assume a communications channel time of 0.5 cycle. The
sum of these times provides a conservative setting of 1.63 cycles.
21SD := 1.625 .
Zone 2 Distance Short Delay (0.000–16000 cycles)
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
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 1-cycle pickup
for remote Level 3 nondirectional blocking elements; the remote Level 3 current
level detectors operate faster than the local Level 2 current level detectors
because the remote Level 3 current level detectors pickup is lower. Finally,
assume a communications channel time of 0.5 cycle. The sum of these times pro-
vides a conservative setting of 1.63 cycles.
67SD := 1.625 .
Level 2 Overcurrent Short Delay (0.000–16000 cycles)
Blocking Signal Extension
Assign a control input to recognize when the local terminal receives a blocking
signal from the remote terminal during external faults.
BT := IN103 .
Block Trip Received (SEL
OGIC
Equation)
The DCB trip scheme uses an on/off carrier signal to block high-speed tripping at
Stations S and R for out-of-section faults. Connect the carrier receive block sig-
nal output from the teleprotection equipment to a control input assigned to the
SEL
OGIC
control equation BT. This control input must remain asserted to block
the forward-looking tripping elements after the coordination timers expire. If the
blocking signal drops out momentarily, the distance relay can trip for out-of-
section faults.
A built-in timer, BTXD, delays dropout of the control input assigned to BT. This
timer maintains the blocking signal at the receiving relay by delaying the dropout
of BT. However, delayed tripping can occur for internal faults because this DCB
protection scheme employs nondirectional elements; the relay always sends a
blocking signal regardless of fault location. Therefore, set this timer to zero so
that high-speed tripping occurs when the nondirectional starting elements assert
for an internal autotransformer fault.
BTXD := 0.000 .
Block Trip Received Extension Time (0.000–16000 cycles)
Figure 6.18 illustrates the dc schematic for the DCB trip scheme.