Coordination with two transformer feeds

[mull982]

Another coordination question I have been pondering:

If I have a bus and feeding this bus is two identical transformers fed from different sources what is typically required for coordination of the breakers on this bus.

Lets say that on the secondary of each transformer there is a breaker which is connected to the bus and feeding power to the bus. Then tapped off of the bus there are several feeder breakers.

Do you simply need to coordinate each of the transformer secondary breakers with the largest feeder breaker on the bus?

Do you need to account for both transformers operating as well as only one transformer operating? For both transfoerms operating is there anyting special reqired to account for only portion of feeder fault current (1/2) being seen by each transformer secondary breaker?

 

[mull982]

From what I've read and saw the two transformer secondary breakers should be set with the same settings and the two feeder breakers on the bus should be set with the same settings (feeding similar loads) with the feeder breakers being coordinated with the secondary transformer breakers? Do you agree?

Would you need to verify that the feeder breakers coordinate when only one of the transformers is in service since the fault current through each breaker for a fault downstream of the feeder breakers would be the same? If they coordinated with only one transforemr in service wouldn't they then coordinate with both transformers in service since fault current through secondary breakers would only be half downstream feeder breakers?

 

[augie47]

I was recently one of the inspectors on a job where two and three transformers were feeding one buss. This was at an auto manufacturer and the arrangement was such to avoid downtime in the event the primary was lost to one transformer.
(There were actually unit subs. 12kv to 480)
There was much discussion on the dangers of back-feeding, proper OCP for the buss, etc.
The job was eventually approved after significant engineering input and a few of the details I recall were:
(a) The fault current was calculated considering input from all transformers (it ended up at 100KAic)
(b) The transformers had inter-linked logic that prevent the buss (2500 amp) from overloading as the system monitored the sum of current from each transformer.
(c) The transformer logic circuit included "back-feed" relays that prevented back-feed to the primary.

Not sure if that helps answering.