R.1.106
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Trip Logic
Trip SELOGIC Control
Equations
You select the appropriate relay elements for unconditional, direct transfer
tripping, switch-onto-fault, and communications-assisted tripping. Set these
SEL
OGIC control equations for tripping:
➤ TR—Unconditional tripping
➤ DTA, DTB, DTC—Direct transfer tripping
➤ TRSOTF—SOTF tripping
➤ TRCOMM—Communications-assisted tripping
Include the instantaneous and time-delayed tripping elements in the TR
SEL
OGIC control equation. You would typically set instantaneous high-set
current level detectors and Zone 2 distance protection in the TRSOTF
SEL
OGIC control equation. You would also set instantaneous Zone 2 distance
protection in the TRCOMM SEL
OGIC control equation.
TR
The TR SELOGIC control equation determines which elements trip
unconditionally. You would typically set all instantaneous and time-delayed
tripping elements (step-distance protection plus instantaneous and time-
overcurrent protection) in the TR SEL
OGIC control equation.
DTA, DTB, and DTC
The DTA, DTB, and DTC SELOGIC control equations determine which
elements directly trip the remote terminal. Each equation is phase selective. If
you are applying three-pole tripping only, set DTA, DTB, and DTC to the
same Relay Word bit expression.
TRSOTF
The TRSOTF control equation defines which elements trip while SOTF
protection is active. These elements trip instantaneously if they assert during
the SOTFD time.
TRCOMM
The TRCOMM SELOGIC control equation determines which elements trip via
the communications-based scheme logic. You would typically set the
overreaching Zone 2 distance elements or Level 2 directional overcurrent
elements in the TRCOMM SEL
OGIC control equation.
Trip Unlatch Options
Unlatch the trip contact output after the trip to remove dc voltage from the trip
coil. The SEL-421 provides two settings to unlatch trip contact outputs after a
protection trip has occurred:
➤ TULO—following a protection trip, phase selective
➤ ULTR—following a protection trip, all three poles
TULO
Tab le 1.71 shows the four trip unlatch options for setting TULO.
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