Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Set the Z3RBD timer to accommodate the following:
➤
Remote Station R circuit breaker maximum opening time
➤
Maximum communications channel reset time
➤
Remote Station 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 circuit breaker is open, or a weak infeed condition exists at the local termi-
nal, 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 trip signal back to the
remote terminal after specific conditions are satisfied. The echo logic includes
timers for blocking the echo logic as well as timers for qualifying the permissive
signal.
Use Echo Block Time Delay (EBLKD) to block the echo logic after dropout of
local permissive elements. The recommended setting for the EBLKD timer is the
sum of the following:
➤
Remote Station R circuit breaker opening time
➤
Communications channel round trip time
➤
Safety margin
Assume a circuit breaker opening time of 3 cycles, a communications channel
round trip time of 2 cycles, and a safety margin of 5 cycles. The sum of these
times gives a conservative setting of 10 cycles for a three-cycle circuit breaker.
EBLKD := 10.000 .
Echo Block Time Delay (OFF, 0.000–16000 cycles)
The echo time delay, setting ETDPU, makes certain that the reverse-looking ele-
ments at the receiving end have sufficient time to operate and block the received
echo signal for external faults behind the remote terminal. The delay also guards
the echo and weak infeed logic against noise bursts that can occur on the commu-
nications channel during close-in external faults.
Because of the brief duration of noise bursts and the pickup time for the reverse-
looking elements, a received signal must be present for a short time to allow the
POTT scheme to echo the permissive signal back to the remote terminal. The
ETDPU timer specifies the time a permissive trip signal must be present. The
ETDPU setting depends upon your communications equipment, but a conserva-
tive setting for this timer is 2 cycles.
ETDPU := 2.000 .
Echo Time Delay Pickup (OFF, 0.000–16000 cycles)
The setting EDURD (Echo Duration Time Delay) limits the duration of the
echoed permissive signal. Once an echo signal initiates, it should remain for a
minimum period of time and then stop, even if a terminal receives a continuous
permissive signal. This cessation of the echo signal prevents the permissive trip
signal from latching between the two terminals. Assume a 3-cycle circuit breaker
at the remote terminal and a 1-cycle channel delay. The sum of these two is a set-
ting of 4 cycles.
EDURD := 4.000 .
Echo Duration Time Delay (0.000–16000 cycles)