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5.125
Date Code 20171021
Instruction Manual
SEL-421 Relay
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
POTT
Use the conventional POTT scheme for an application with a single communica-
tions channel.
For details about implementing a conventional POTT scheme, see
POTT2
Use the POTT2 scheme for applications with two communications channels, one
for single-phase fault identification and one for multiphase fault identification.
This scheme is useful in applications where there is a high likelihood of cross-
country faults.
For details about implementing a POTT2 scheme, see
.
POTT3
Use the POTT3 scheme for phase-segregated applications with three communica-
tions channels. In this scheme, each channel indicates permissive trip for single-
phase. Multiphase fault detection results in all three channels transmitting a per-
missive trip.
For details about implementing a POTT3 scheme, see
POTT Scheme Logic
The POTT scheme logic performs the following tasks:
➤
Keys the communications equipment to send permissive trip (PT)
when any element you include in the TRCOMM/TRCOMMD
SEL
OGIC
control equation asserts and the current reversal logic is not
asserted
➤
Prevents keying and tripping by the POTT logic following a current
reversal
➤
Echoes the received permissive signal to the remote terminal
➤
Prevents channel lockup during echo and test
➤
Provides a secure means of tripping for weak- and/or zero-infeed
terminals
➤
Ensures proper tripping at both terminals during cross-country faults
(via special logic implemented with SEL
OGIC
control equations)
The POTT scheme logic consists of the following:
➤
Current reversal guard logic
➤
Echo
➤
Weak infeed logic
Summary of Contents for SEL-421-4
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