GE 750 Instruction Manual page 227

750/760
S7 Control
Feeder Management Relay
FIGURE 5–77: Autoreclosure Shots 1 to 4 Logic
g) Autoreclosure Application Example
Utility statistics indicate that a large percentage of feeder faults (about 80%) are of
a transient nature. Typically, once the feeder is tripped an autorecloser
automatically reclosures the feeder breaker after a short time delay. If the fault was
transient, the entire feeder is returned to normal service and customers experience
a very short disturbance. If the fault is permanent and on the load side of another
protection point, the reclosure scheme delays another trip of the breaker until this
other device clears the fault so that service is disrupted only for loads beyond this
other protection point.
A common phase protection scheme uses instantaneous elements in conjunction
with automatic reclosing. The design goal is to select setpoints that will detect faults
out to the most distant (in impedance terms) point of the feeder and provide fast
operation. Typically 'lo-set' overcurrent elements are programmed to be blocked
after the first reclosure. This gives downstream devices, such as fuses, time to
interrupt a permanent fault, especially on a feeder tap. Lo-set elements would then
be re-enabled after the scheme is reset. 'Hi-set' elements may also be blocked on a
subsequent shot with a faster time overcurrent curve selected to allow the fault to
burn off. In this case a permanent fault would then cause a time overcurrent trip.
GE Multilin
5–131
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