5.126
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
Current Reversal Guard Logic
Use current reversal guard for parallel line applications if the Zone 2 reach
extends beyond the midpoint of the parallel transmission line. With current rever-
sal guard, the relay does not key the transmitter and ignores reception of a per-
missive signal from the remote terminal when the reverse-looking protection sees
an external fault. The Zone 3 Reverse Block Delay (Z3RBD) timer extends these
two actions after a current reversal ceases and the reverse-looking elements drop
out.
Echo
If the local circuit breaker is open, or a weak infeed condition exists, the remote
relay permissive signal can echo back to itself and 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. This echo logic includes timers for qualifying the per-
missive signal and timers to block the echo logic during specific conditions.
Use the Echo Block Time Delay (EBLKD) to block the echo logic after dropout
of local permissive elements. The recommended time setting for the EBLKD
timer is the sum of the following:
➤ Remote terminal circuit breaker opening time
➤ Communications channel round-trip time
➤ Safety margin
An echo delay ensures that the reverse-looking elements at the receiving end
have sufficient time to operate and block the received echo signal for external
faults behind the remote terminal. This delay also guards the echo and weak
infeed logic against noise bursts that can occur on the communications channel
during close-in external faults. Typically, these noise bursts coincide with faults
external to the line section.
Because of the brief duration of noise bursts and the pickup for the reverse-look-
ing 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
Echo Time Delay Pickup (ETDPU) timer specifies the time a permissive trip sig-
nal must be present.
The Echo Duration Time Delay (EDURD) limits the duration of the echoed per-
missive signal. Once the echo signal begins, 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.
Weak-Infeed Logic
The SEL-421 provides weak-infeed logic to high-speed trip both line terminals
for internal faults near the weak terminal. The weak terminal echoes the permis-
sive signal back to the strong terminal and allows the strong terminal to trip.
After satisfaction of specific conditions, the weak terminal trips by converting the
echoed permissive signal to a trip signal.
In some applications, one terminal might not contribute enough fault current to
operate the protective elements, even with all sources in. It is important to trip the
weak-infeed terminal to prevent low-level fault current from maintaining the fault
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