Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
The setting 3POD establishes the time delay on dropout after the Relay Word bit
3PO deasserts. This delay is important when you use line-side potential trans-
formers for relaying. Use the 3POD setting to stabilize the ground-distance ele-
ments in case of pole scatter during closing of the circuit breaker(s).
3POD := 0.500 .
Three-Pole Open Dropout Delay (0.000–60 cycles)
POTT Trip Scheme
The permissive overreaching transfer trip (POTT) scheme is selected to provide
high-speed tripping for faults along the protected line.
The POTT scheme logic consists of four sections:
➤
Current reversal guard logic
➤
Echo
➤
Weak infeed logic
➤
Permission to Trip Received
Current Reversal Guard Logic
You need current reversal guard for this parallel line application. When a reverse-
looking element detects an external fault, the relay does not key the transmitter
and ignores reception of a permissive signal from the remote terminal. The
Zone 3 Reverse Block Delay (Z3RBD) timer extends these two conditions after a
current reversal occurs and the reverse-looking elements drop out.
Set Z3RBD timer to accommodate for the following:
➤
Remote terminal R circuit breaker maximum opening time
➤
Maximum communications channel reset time
➤
Remote terminal R Zone 2 relay maximum reset time
Assume a circuit breaker opening time of 3 cycles, a communications channel
reset time of 1 cycle, and remote Zone 2 relay reset time of 1 cycle. The sum of
these times gives a conservative setting of 5 cycles for a three-cycle circuit
breaker.
Z3RBD := 5.000 .
Zone 3 Reverse Block Time Delay (0.000–16000 cycles)
Echo
If the local circuit breaker is open, or a weak infeed condition exists at the local
terminal, the received permissive signal can echo back to the remote relay and
cause it to issue a high-speed trip for faults beyond the remote relay Zone 1
reach. The SEL-421 includes logic that echoes the received permissive signal
back to the remote terminal after specific conditions are satisfied. The echo logic
includes timers for qualifying the permissive signal as well as timers for blocking
the echo logic during specific conditions.
Use setting EBLKD (Echo Block Time Delay) to block the echo logic after drop-
out of local permissive elements. The recommended setting for the EBLKD timer
is the sum of the following:
➤
Remote terminal R circuit breaker opening time
➤
Communications channel round trip time
➤
Safety margin