Utility & Customer fault coordination

[UtilityGy]

Hello ALL,

This question is succession to my earlier question regarding reclosing. I appreciate the input.

On my 27 kV system, station bus has a fault 3 ph fault of 631 MVA, approx. 13000 A. The feede is a mix of O/H and U/G, run of 6000 m. Fault at customer main incoming primary 27 kV fused gear is 5300 A.

Customer primary gear feeds three transformers each, 5 MVA, 27/600V, %Z=6. A customer secondary 3 phase fault will reflect as 1340 A fault on primary.

Here is my question, as a utility protection engineer, am I supposed to pick a customer secondary 3 ph fault on 600 V on timed overcurren of my primary 27 kV station feeder breaker. Whom do I want this fault current to be seen. Secondary 600 V breaker clear or 27 kV primary fuse, what about my primary breaker ?

If yes, then do i want a secondary fault( 120/208V) on a 100 KVA pole top transformer unit to be seen by the station breaker the fault as well ?

I am sure I am missing something utility based protection application ? Thanks

 

[jghrist]

Generally, you will not worry about coordination with customer protection. Size the transformer protection to protect the transformer and make it the customer's responsibility to coordinate with that.

 

[farmape]

Indeed, from a customer's representative perspective, it seems the customer is responsible for coordination with the utility, and the utility may dictate (or strongly advise) the customer's mains settings. This is just my experience I don't know if it is the norm.
But looking at the bigger picture, in general, a primary protective device should not be the primary method to clear a secondary fault.

 

[cranky108]

farmape, why not? Not to be a heal, but such a scheme can reduce the cost of a main breaker. This is important on smaller loads to keep the cost reasonable.
However haveing said that, the protection has better resolution on the secondary side. So put the relays on the secondary, and trip the primary circut switcher.

Fuses don't protect much of anything. The best they do is remove a faulted devices.

All I have said should be scaled to the size of the customer, and the value of the equipment. So for residentual transformers, just use fuses because the transformers are so in expencive.