6.112
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
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 if
specific conditions are satisfied.
This application example does not use the weak-infeed feature.
EWFC := N. Weak Infeed Trip (Y, N, SP)
Permission to Trip Received
Assign a control input to receive trip permission from the remote terminal.
PT1 := IN103. General Permissive Trip Received (SELOGIC Equation)
Trip Logic
Trip logic configures the relay for tripping. There are four trip logic settings com-
ponents:
➤ Trip equations
➤ Trip unlatch options
➤ Trip timers
➤ Three-pole tripping enable
Trip Equations
Set these three SELOGIC control equations for tripping:
➤ TR (unconditional)
➤ TRCOMM/TRCOMMD (communications-assisted; in this example
we use only TRCOMM)
➤ TRSOTF (SOTF)
TR
The TR SELOGIC control equation determines which protection elements cause
the relay to trip unconditionally. Set TR to the Zone 1 instantaneous distance pro-
tection (Z1T), Zone 2 time-delayed distance protection, and the inverse-time
overcurrent element (51S1) for backup protection. For information on setting
51S1, see Selectable Operating Quantity Time Overcurrent Element 1 on
page 6.10.
TR := Z1T OR Z2T OR 51S1T. Trip (SELOGIC Equation)
TRCOMM
The TRCOMM SELOGIC control equation determines which elements trip via
the communication-assisted tripping logic. In the TRCOMM SEL
OGIC control
equation, set Zone 2 mho phase-distance protection for phase faults, and Level 2
negative-sequence directional overcurrent element (67Q2) for ground faults.
TRCOMM := Z2P OR 67Q2. Communications-Assisted Trip (SELOGIC Equation)
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