F-6 Setting Negative-Sequence Overcurrent Elements Date Code 20001006
SEL-351 Instruction Manual
Figure F.5: Negative-Sequence Overcurrent Element Derived from “Equivalent” Phase
Overcurrent Element, 51EP
( )
51 3 300 520QF pickup A A: = × =
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 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.
Courtesy of NationalSwitchgear.com
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