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You are the protection engineer assigned to update protections on a given distribution feeder illustrated in the 1-line diagram below. Details of the feeder sections beyond the tap fuses are omitted as they are not needed for this exercise.

The feeder short term emergency (STE) rating is 850A.

  • Breaker CTs are 2000:5A multi-ratio which have been set on their 600:5A tap.
  • All fuses indicated in the 1-line diagram (including the mystery fuse) are T-speed type expulsion fuses.
  • Relay types are as follows:
    • Phase (Qty 3): GE Type IAC-53 (2-16A TOC and 10-80A IOC ranges)
    • Ground (Qty 1): GE Type IAC-53 (0.5-4A TOC and 10-80A IOC ranges)

Your task is as follows.

  1. Select an appropriate fuse size for the mystery fuse near the recloser symbol that best coordinates with the downstream 100T fuse as well as the upstream feeder relays. Full coordination with both is not possible so you’ll have to make some compromises.
  1. Determine if the recloser indicated in the diagram is needed to sectionalize the feeder. If it is then select appropriate 51 and 51N element delay curves from those in the recloser curve sheet. Otherwise, omit it. Assume fixed recloser 51 and 51N element pickups of 240 and 80 primary amps respectively.
  1. Provide feeder 50 and 50N relay element pickup settings to trip the feeder and block reclosing for faults in the initial underground section of the feeder. Take care to avoid blocking reclosing for faults beyond the underground section.
  1. Provide feeder 51 and 51N relay element pickup and time dial settings. Coordinate your relays as best you can with downstream devices. It is generally expected that the 51 element will fully coordinate with all downstream devices whereas the 51N element may not. Therefore, you’ll need to make some compromises.

Hints:

  • Read and consider the entire project before starting, as the numbered items above are not intended to specify the sequence of your work.
  • A common practice for relay-to-fuse coordination is to consider a fictitious coordinating fuse of a predetermined size located at the first overhead point on the feeder (see crooked fuse in the diagram). Here the available fault current is the max current that both the relays and a fuse will see. By coordinating feeder relays with this coordinating fuse, downstream fuses the same sizer or smaller can be ignored. Coordination with larger downstream fuses must still be checked.

The table below is not needed for this problem since I already gave you fault currents at each of the buses. It is strictly for information and possible future use. It’s not uncommon for distribution engineers to have tables like this in their toolbox. The particular below table lists pre-calculated circuit impedances per unit distance for typical 13.8kV feeder conductors and construction arrangements.

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