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6.39
Date Code 20171021
Instruction Manual
SEL-421 Relay
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
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
three times gives a conservative setting of 10 cycles for a 3-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
echo time delay pickup (ETDPU) timer specifies the time a permissive trip signal
must be present. The ETDPU setting depends upon your communications equip-
ment, but a conservative 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 termination 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)
Weak Infeed
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 causes the strong terminal to trip. The
weak terminal trips by converting the echoed permissive signal to a trip signal
after satisfaction of specific conditions.
This application does not require use of the weak-infeed feature.
EWFC :=
N
.
Weak Infeed Trip (Y, N, SP)
Permission to Trip Received
Two Relay Word bits identify receipt of permission to trip:
➤
PT1
—General permission to trip received
➤
PT3
—Three-pole permission to trip received
Cross-Country Fault Identification on page 6.43
tion of this particular communications-assisted tripping scheme logic.
If PT1 is asserted, the relay can high-speed single-pole trip via the communica-
tions channel. However, if PT3 is asserted, the relay high-speed three-pole trips
via the communications channel. This logic prevents the SEL-421 at Station S
from three-pole tripping for cross-country faults (for example, A-Phase-to-
ground fault on Line 1 and B-Phase-to-ground fault simultaneously on Line 2)
beyond the reach of local Zone 1 ground-distance protection.
Содержание SEL-421-4
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