6.40
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
500 kV Parallel Transmission Lines With Mutual Coupling Example
Direct tripping is also implemented for reliability and to decrease the overall trip-
ping time of the SEL-421 at Station S for cross-country faults beyond the reach
of local Zone 1 ground-distance protection. The logic PT1 and PT3 requires that
the circuit breakers at Station R single-pole trip the external fault on line 2 first
before the SEL-421 at Station S can single-pole trip for the case of cross-country
faults beyond the reach of Zone 1 ground-distance protection at Station S. Direct
tripping for cross-country faults is faster since the SEL-421 at Station S do not
have to wait for the remote circuit breakers to single-pole trip.
PT1 := RMB1A. General Permissive Trip Received (SELOGIC Equation)
PT3 := RMB2A. Three-Pole Permissive Trip Received (SELOGIC Equation)
Trip Logic
Trip logic configures the relay for tripping. These settings consists of the following:
➤ Trip equations
➤ Trip unlatch options
➤ Single-pole trip options
➤ Trip timers
➤ Enable single-pole tripping
Trip Equations
Set these six SELOGIC control equations for tripping:
➤ TR (unconditional)
➤ TRCOMM/TRCOMMD (communications-assisted)
➤ TRSOTF (SOTF)
➤ DTA, DTB, and DTC (direct tripping)
The TR SEL
OGIC control equation determines which protection elements trip
unconditionally. Set TR to Zone 1 instantaneous distance protection, Zone 2
time-delayed distance protection, and inverse time overcurrent protection for
backup.
TR := Z1T OR Z2T OR 51S1T. Trip (SELOGIC Equation)
The TRCOMM/TRCOMMD SEL
OGIC control equation determines which ele-
ments trip via the communication-based scheme logic. In this example, only use
the TRCOMM setting (i.e., set TRCOMMD = NA). Set instantaneous Zone 2
distance protection in the TRCOMM logic equation.
TRCOMM := Z2P OR Z2G. Communications-Assisted Trip (SELOGIC Equation)
The TRSOTF SEL
OGIC control equation defines which elements trip when SOTF
protection is active. Set instantaneous overcurrent element 50P1 and Zone 2 dis-
tance protection in the TRSOTF SEL
OGIC control equation.
TRSOTF := Z2P OR Z2G OR 50P1. Switch-Onto-Fault Trip (SELOGIC Equation)
The DTA, DTB, and DTC SEL
OGIC control equations receive single-pole direct
transfer trips from the remote terminal whenever the remote SEL-421 single-pole
trips. Use this tripping logic for reliability and to decrease SEL-421 operating
time during cross-country faults beyond the reach of local Zone 1 ground-dis-
tance protection.
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