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5.8 S7 CONTROL
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 recloses the feeder breaker after a short time delay. If the fault was tran-
sient, the entire feeder is returned to normal service and customers experience a very short disturbance. If the fault is per-
manent 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 'loset' overcurrent elements are programmed to be blocked after the first reclosure. This gives down-
stream devices, such as fuses, time to interrupt a permanent fault, especially on a feeder tap. Loset elements would then
be re-enabled after the scheme is reset. 'Hiset' elements may also be blocked on a subsequent shot with a faster time over-
current curve selected to allow the fault to burn off. In this case a permanent fault would then cause a time overcurrent trip.
A typical autoreclose scheme as implemented in the 760 would respond to faults as follows:
1.
With the breaker closed and protection enabled, a transient fault produces a current above the pickup of both Instanta-
neous Overcurrent 1 (loset) and Time Overcurrent 1 elements.
2.
The Time Overcurrent element begins to time, and the Instantaneous Overcurrent element operates, signaling the
breaker to trip and initiate a reclosure.
3.
The breaker trips and signals the autoreclose scheme that it is now open. The Instantaneous Overcurrent 1 and Time
Overcurrent 1 elements automatically reset because the breaker is open.
4.
If all requirements of the autoreclose scheme are fulfilled, autoreclose signals the breaker to close and advances the
shot counter. Shot 1 setpoints block the Instantaneous Overcurrent element from further operation.
5.
The breaker closes resulting in an inrush current of the feeder loads.
5
6.
If the fault was transient, the current reduces to the load level before a trip occurs. The autoreclose scheme eventually
resets and the cycle begins again at Step 1.
7.
If the fault is permanent, a current above the pickup of both the Instantaneous Overcurrent 1 and the Time Overcurrent
1 elements is produced. However, the Instantaneous Overcurrent 1 element is blocked from operating.
8.
While the Time Overcurrent 1 element is timing, any protection devices between this relay and the fault location are
provided an opportunity to isolate the fault.
9.
If no protection downstream from the relay clears the fault, the Time Overcurrent 1 element will time-out, signaling the
breaker to trip and initiate the autoreclose scheme again.
10. If the shot counter has not reached its maximum allowed value then the cycle begins at step 6 using protection set-
points for the current shot.
11. If the shot counter has reached its maximum then the scheme goes to lockout.
5-128
Courtesy of NationalSwitchgear.com
750/760 Feeder Management Relay
5 SETPOINTS
GE Multilin
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