Schweitzer Engineering Laboratories SEL-321 - Communications-Assisted Tripping Schemes; Permissive Overreaching Transfer Trip (POTT) Logic

Date Code 20011026 Specifications 2-9
SEL-321/321-1 Instruction Manual
Timer Inputs
and Outputs
Phase 21 Gnd 21 67N 67Q
In Out In Out In Out In Out
Zone/Level 1 67N1 67N1 67Q1 67Q1
Zone/Level 2 M2P M2PT Z2G Z2GT 67N2 67N2T 67Q2 67Q2T
Zone/Level 3 M3P M3PT Z3G Z3GT 67N3 67N3T 67Q3 67Q3T
Zone/Level 4 M4P M4PT Z4G Z4GT 67N4 67N4T 67Q4 67Q4T
COMMUNICATIONS-ASSISTED TRIPPING SCHEMES
The relay includes tripping schemes which provide unit protection for transmission lines with the
help of communications. No external coordination devices are required. The three scheme
selections from the relay setting procedure are:
• Permissive-Overreaching Transfer Trip (POTT)
• Directional Comparison Unblocking (DCUB)
• Directional Comparison Blocking (DCB)
All three schemes require Zone 2 set forward and Zone 3 set reverse. Develop other
communications-assisted tripping schemes using logic inputs and the programmable output logic.
Examples are discussed later in this section.
To use a communications scheme, select and set the scheme and program inputs and outputs to
monitor and control the communications receiver and transmitter.
Permissive Overreaching Transfer Trip (POTT) Logic
This logic performs the following tasks:
• Keys the communications equipment to send permissive trip when any element you
include in the MTCS logic variable 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.
External Input
Received Permissive
Trip Signal PT Assign PT to a programmable input. Drive with
receiver output.
Timer Settings
Zone 3 Rev. Block Delay Z3RBD Current-reversal guard timer.
Echo Block Delay EBLKD Prevents echoing of received PT, for settable delay,
after dropout of local permissive elements. Set to
OFF to ignore EBLKD and defeat echo input to
ATB logic.
Echo Time Delay P/U ETDPU Sets minimum time requirement for received PT,
before echo begins. Set to OFF for no echo.