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5-22
Applications
Date Code 20011026
SEL-321/321-1 Instruction Manual
detect a fault at Bus C after an LOP condition is detected. The maximum expected load for this
circuit is 450 A. The minimum fault current level for a fault at Bus C is 491 A. Faults at Bus C
do not drop the positive-sequence voltage at Bus B below 14 V. With a small margin for fault
study and CT performance error, the 50M setting for this example is 475 A.
50M secondary amps =
475
200
2 37
A primary
A
=
.
secondary
For the relay at Breaker 3:
ELOP = Y
LOPD = 3.00
50M = 2.37
59QL = 14.00
59PL = 14.00
- Relay Elements and Other Settings Affected
All set distance and directionally controlled overcurrent elements.
Miscellaneous Scheme Logic
In our application, we are using three-pole tripping only and pole-discordance is not required. To
disable the Pole-Discordance logic, set EPOLD = N.
The relay is required to trip three-pole only for this application. Setting ESPT = N and ESPO =
N disables the single-pole tripping logic.
The 3POD dropout delay timer is used to prevent operation of the ground distance element when
all three poles do not close simultaneously. When a breaker is closed, the ground distance
elements are blocked for 3POD time. The 3POD dropout time delay should be set for the worst
case pole scatter upon breaker closure. For our application a setting of 1.5 cycles is required.
In applications where a portion of the bus (or stub of the bus) is energized by a closed breaker,
yet the line is deenergized and isolated by an open disconnect switch, the relay provides definite-
time phase overcurrent protection. Distance protection is not applicable if line-side PTs are
employed. This logic can only be performed if LOP is enabled.
The stub protection may also be used for definite-time overcurrent protection when the relay
detects a loss-of-potential. With ESTUB = Y, set the 50MFD timer to the appropriate delay.
The Stub Protection logic is only allowed to trip when the relay detects an LOP condition and the
phase current exceeds the 50M pickup threshold.
The relay also provides four methods of trip unlatch (open the trip contact) logic. If no trip
unlatch logic is selected, trip output remains closed for the greater duration of the fault or the trip
duration timer. The four trip unlatch methods are as follows:
•
Option 1 allows the trip contact to open if the trip condition has reset, the minimum trip
duration (TDURD) expires, and the lowest set 50L, 50QF, or 50QR element is not picked
up.
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