POWER FUSE SAVING SCHEMES

Fuse saving schemes
Fuse saving schemes are used as a strategy to attempt to prevent permanent outages when transient faults occur beyond tap fuses on a distribution system. Such schemes typically utilize instantaneous overcurrent relays on the feeder breaker, which are set to be capable of sensing faults beyond tap fuses on the associated line.

As such, faults beyond these fuses can be cleared by the feeder breaker prior to the fuse being damaged. The low-set instantaneous relay typically is removed from service prior to the first or second autoreclose of the breaker.

If the fault is permanent in nature, the fuse operates after the breaker autorecloses since the low-set instantaneous relays are no longer in service and the time overcurrent relays are set to coordinate with the tap fuses.

The coordination allows the fuse to blow without interrupting the whole feeder the second time. If the fault is transient, all customers are restored, including those beyond the fuse, when the breaker initial autorecloses.

When fuse saving schemes are used, it is beneficial to autoreclose the feeder breaker as rapidly as practical. Since, by action of these relays, the feeder breaker is allowed to operate for faults beyond downstream protective devices, and on a significant portion of the feeder, fast autoreclosing will mitigate the impact of more frequent breaker trips.

It is common practice at many of the utilities utilizing fuse saving schemes to employ immediate autoreclosing (20 cycles of dead time to allow for deionization) on the first shot.

Often the low-set instantaneous overcurrent relays, or elements, are enabled just before resetting the reclosing relay. By doing this, a low-magnitude fault, for which the time delay relays have not completed their timing before the reset period of the reclosing relay expires, will be cleared.

If this is not done, the reclosing relay will reset and the reclosing sequence will be repeated a number of times. Alternatively, many microprocessor relays take care of this problem by blocking the reset timer whenever an overcurrent element is timing.

Fuse blowing schemes
Fuse blowing schemes are used to minimize the impact of a fault on the total feeder by allowing a fuse time to interrupt a faulted lateral if the fault is on the customer side of the fuse.

In these schemes, a time delay sufficient for the fuse to operate before the upstream breaker is tripped is added to the instantaneous overcurrent elements, or these instantaneous overcurrent elements are not in service during the initial trip of the feeder, and the time overcurrent elements are set to coordinate with the fuse.

The instantaneous overcurrent elements are enabled into the tripping circuit following the initial autoreclose attempt.