Schweitzer Engineering SEL-351P, Быстрый старт

Страница: 633 / 674

F-6 Setting Negative-Sequence Overcurrent Elements Date Code 990215
SEL-351P Manual Técnico
element remain the same (if the element is a definite-time element, the time delay remains the
same).
Figure APPENDIX F: .5: Negative-Sequence Overcurrent
Element Derived from “Equivalent” Phase Overcurrent Element, 51EP
( )
51 3 300 520 QF 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.