5.138
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Trip Logic
Trip SELOGIC Control Equations
You select the appropriate relay elements for unconditional, direct transfer trip-
ping, 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/TRCOMMD—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 equa-
tion. 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 uncondition-
ally. You would typically set all instantaneous and time-delayed tripping ele-
ments (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 protec-
tion is active. These elements trip instantaneously if they assert during the
SOTFD time.
TRCOMM
The TRCOMM and TRCOMMD 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. Normally, you need only
one equation, but if you want to separate distance and directional elements, use
both equations. For example, enter the distance elements in the TRCOMM equa-
tion and the direction elements in the TRCOMMD 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
To Next Page
Loading...
