Schweitzer Engineering Laboratories SEL-351A, Manual

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F.6
SEL-351A Relay Instruction Manual
Date Code 20080213
Setting Negative-Sequence Overcurrent Elements
Coordinating Negative-Sequence Overcurrent Elements
Figure F.5 Negative-Sequence Overcurrent Element Derived from
“Equivalent” Phase Overcurrent Element, 51EP
Equation F.1
Having achieved coordination between the feeder relay negative-sequence
overcurrent element (51QF) and the downstream line recloser phase
overcurrent element (51R) for phase-to-phase faults, coordination between the
two devices for other fault types is also achieved.
Negative-Sequence Overcurrent Element Applied at a Distribution Bus
(Guideline Step 5 )
The preceding example was for a distribution feeder. A negative-sequence
overcurrent element protecting a distribution bus provides an even more
dramatic improvement in phase-to-phase fault sensitivity.
The distribution bus phase overcurrent element pickup must be set above the
combined load of all the feeders on the bus, plus any emergency load
conditions. The bus phase overcurrent element pickup is often set at least four
times greater than the pickup of the feeder phase overcurrent element it backs
up. Thus, sensitivity to both bus and feeder phase faults is greatly reduced.
Feeder relay backup by the bus relay is limited.
Negative-sequence overcurrent elements at the distribution bus can be set
significantly below distribution bus load levels and provide dramatically
increased sensitivity to phase-to-phase faults. It is coordinated with the
distribution feeder phase or negative-sequence overcurrent elements and
provides more-sensitive and faster phase-to-phase fault backup.
Ground Coordination
Concerns
If the downstream protective device includes ground overcurrent elements, in
addition to phase overcurrent elements, there should be no need to check the
coordination between the ground overcurrent elements and the upstream
negative-sequence overcurrent elements. The downstream phase overcurrent
Time (
Sec
onds
)
10.0
5.0
2.0
1.0
0.5
0.2
0.1
Negative-Sequence Current (Amps: 3I
2
)
125 250 500 1000 2000 4000
51QF
where:
51QF: pickup 3 300 A
( )
= 520 A • =
Courtesy of NationalSwitchgear.com