R.1.93
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
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 permissive 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-
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 Echo Time Delay Pickup (ETDPU) timer specifies the time a permissive
trip signal must be present.
The Echo Duration Time Delay (EDURD) limits the duration of the echoed
permissive 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 permissive 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 arc (i.e., the fault will restrike following auto-reclose at
the strong terminal). Because the strong terminal is beyond the Zone 1 reach,
it cannot trip for end-zone faults.
The faulted phase voltage(s) is depressed at the weak-infeed terminal, a
condition that generates significant residual voltage during ground faults. The
SEL-421 uses phase-to-phase undervoltage level detectors and a residual
overvoltage level detector to qualify a weak-infeed condition. If setting EWFC
equals Y, the relay enables the weak-infeed logic and settings 27PPW and
59NW are active. For single-pole tripping applications, set EWFC to SP and
setting 27PWI is active.
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