GE 750 Instruction Manual page 293

CHAPTER 5: SETPOINTS
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
FIGURE 5–80: Autoreclosure Shots 1 to 4 Logic
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-
S7 CONTROL
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