6.103
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Zone 3
Zone 3 has reverse-looking distance protection that you do not need to apply for
tripping in this application. Set Zone 3 for zero time delay.
Z3D := 0.000. Zone 3 Time Delay (OFF, 0.000–16000 cycles)
SOTF Protection
SOTF logic is enabled when the circuit breaker closes. This logic provides pro-
tection for a short duration (setting SOTFD) until other protection (such as trip-
ping from SEL
OGIC control equations TR, TRCOMM, and TRCOMMD) is
available. The TRSOTF SEL
OGIC control equation defines which protection ele-
ments cause the relay to trip when the SOTF scheme is active. Assertion of the
protection elements assigned to TRSOTF during the SOTFD time causes the
relay to trip instantaneously.
Use nondirectional overcurrent protection to clear close-in faults. Also use
instantaneous overreaching distance protection to clear faults along the line.
Assign instantaneous Zone 2 mho phase and ground-distance protection plus
Level 1 phase overcurrent element to TRSOTF.
TRSOTF := Z2P OR Z2G OR 50P1. Switch-Onto-Fault Trip (SELOGIC Equa-
tion)
Voltage Reset
You can configure the logic so the SOTF enable duration resets within at least 5
cycles after it first asserted but before the SOTFD timer expires. To quickly reset
the SOTF period before this time, the relay must sense that the positive-sequence
voltage V
1
is greater than setting VRSTPU times the nominal voltage.
Use setting EVRST (Switch-Onto-Fault Voltage Reset) to enable fast reset. The
advantage of resetting SOTF protection quickly is that unwanted tripping does
not occur for subsequent faults external to the remote terminals during the SOTF
period; these trips can occur if you set instantaneous Zone 2 distance protection
elements in the TRSOTF SEL
OGIC control equation. Enable the voltage reset
option.
EVRST := Y. Switch-Onto-Fault Voltage Reset (Y, N)
SOTF Initiation
The SOTF logic asserts via one or both of the following methods:
➤ A change in the normally open auxiliary contact 52A status showing
that the circuit breaker has just opened
➤ Assertion of the relay control input assigned to the circuit breaker
close bus
The 52A method initiation works well for both single and multiple circuit breaker
applications and does not require an input from the close bus. However, close bus
only initiation enables SOTF protection immediately following the close com-
mand to the circuit breaker. For more information see Switch-Onto-Fault Logic
on page 5.117.
Turn off 52AEND (52A Pole-Open Time Delay).
52AEND := OFF. 52A Pole-Open Time Delay (OFF, 0.000–16000 cycles)
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