Schweitzer Engineering Laboratories SEL-421-4 - Page 345

6.9
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Applications Examples
230 kV Overhead Distribution Line Example
Apply SOTF when using line-side potentials for relaying. Use nondirectional
overcurrent protection to clear close-in faults. Also use instantaneous overreach-
ing distance protection to clear faults along the line. Assign instantaneous Zone 2
mho phase and ground-distance protection plus Level 1 phase overcurrent ele-
ment to TRSOTF.
TRSOTF := Z2P OR Z2G OR 50P1. Switch-Onto-Fault Trip
(SEL
OGIC Equation)
Single-Pole SOTF
This is a three-pole tripping application example; confirm that the SOTF protec-
tion is for three-pole tripping.
ESPSTF := N. Single-Pole Switch-Onto-Fault (Y, N)
Voltage Reset
You can configure the logic such that the SOTF enable duration resets within at
least 5 cycles after it first asserted, but before the SOTFD timer expires. To
quickly reset the SOTF period, the relay must sense that the positive-sequence
voltage is greater than the VRSTPU setting multiplied by the nominal voltage.
Use setting EVRST (Switch-Onto-Fault Voltage Reset) to enable fast reset. The
advantage of resetting SOTF protection quickly is that unwanted tripping does
not occur for subsequent faults external to the remote terminals during the SOTF
period; these trips can occur if you set instantaneous Zone 2 distance protection
elements in the TRSOTF SEL
OGIC control equation. Enable the voltage reset
option, and leave the VRSTPU setting at default (0.8).
EVRST := Y. Switch-Onto-Fault Voltage Reset (Y, N)
SOTF Initiation
The SOTF logic asserts via one or both of the following methods:
➤ A change in the normally open auxiliary contact 52A status showing
that the circuit breaker has just opened
➤ Assertion of the relay control input assigned to the circuit breaker
close bus
The 52A method works well for both single and multiple circuit breaker applica-
tions and does not require an input from the close bus. However, the close bus
method only enables SOTF protection immediately following the close command
to the circuit breaker. For more information, see Switch-Onto-Fault Logic on
page 5.117.
Turn off 52AEND, 52A Pole-Open Time Delay.
52AEND := OFF. 52A Pole-Open Time Delay (OFF, 0.000–16000 cycles)
Select the close bus option for this application and set the close enable delay
(CLOEND) shorter than the shortest reclose open interval.
CLOEND := 10.000. CLSMON or Single-Pole Open Delay
(OFF, 0.000–16000 cycles)