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 POTT Trip
Scheme on page 6.38.
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 Cross-Country Fault Iden-
tification on page 6.43.
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 Three-Channel POTT
Scheme, POTT3 on page 6.46.
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
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