R.1.105
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Trip Logic
Trip Logic
Use the SEL-421 trip logic to configure the relay for tripping one or two
circuit breakers. You can apply the SEL-421 in single-pole tripping
applications, three-pole tripping applications, or both. Set the SEL-421 to trip
unconditionally (as with step distance) or with the aid of a communications
channel (as with the POTT, DCUB, DCB, and DTT schemes).
Three-Pole Tripping
The relay uses three-pole tripping logic if Relay Word bit E3PT (three-pole
trip enable SEL
OGIC control equation) equals logical 1. You can set E3PT to 1
or assign a control input so that an external condition changes the state of this
Relay Word bit.
There are separate three-pole tripping SEL
OGIC control equations for two
circuit breakers, E3PT1 and E3PT2, respectively. When you set E3PT1 or
E3PT2 to 1, the corresponding circuit breaker trips three pole only. For details
on setting E3PT, E3PT1, and E3PT2, see Trip Logic and Reclose Sources on
page R.2.26.
Single-Pole Tripping
The relay uses single-pole tripping logic if Relay Word bit E3PT, three-pole
trip enable SEL
OGIC control equation, equals logical 0. You can either set
E3PT to 0 or assign a control input so that an external condition changes the
state of this Relay Word bit.
The SEL-421 automatically single-pole trips for the following conditions
when the single-pole tripping logic is active:
➤ Zone 1 ground distance protection asserts for a single phase-to-
ground fault
➤ Zone 2 ground distance protection asserts for a single phase-to-
ground fault and is permitted to trip via the communications-
assisted tripping logic
➤ Any one of three SELOGIC control equations, DTA, DTB, or
DTC, is assigned to an input and asserts (per-phase direct
transfer trip)
You can also set the SEL-421 to single-pole trip through the following three
options:
Table 1.70 Additional Settings for Single Pole Tripping (SPT)
Setting Description Selection
Z2GTSP Zone 2 Ground Distance Time Delay Single-Pole Trip Y
67QGSP Zone 2 Directional Negative-Sequence/Residual Ground
Overcurrent Single Pole Trip
Y
EWFC
a
a
In POTT and DCUB settings
Weak-Infeed Trip SP
b
b
SP = single pole
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