R.1.92
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Permissive Overreaching Transfer Tripping Scheme
POTT
Use the conventional POTT scheme for an application with a single
communications channel.
For details about implementing a conventional POTT scheme, see POTT Trip
Scheme on page A.1.36.
POTT2
Use the POTT2 scheme for applications with two communications channels,
one for single-phase fault identification and one for multi-phase 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
Identification on page A.1.41.
POTT3
Use the POTT3 scheme for phase-segregated applications with three
communications channels. In this scheme, each channel indicates permissive
trip for single-phase. Multi-phase fault detection results in all three channels
transmitting a permissive trip.
For details about implementing a POTT3 scheme, see Three-Channel POTT
Scheme, POTT3 on page A.1.44.
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 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
Current Reversal
Guard Logic
Use current reversal guard for parallel line applications if the Zone 2 reach
extends beyond the midpoint of the parallel transmission line. With current
reversal guard, the relay does not key the transmitter and ignores reception of
a permissive signal from the remote terminal when the reverse-looking
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