Two 2000kVA tranformers in loop config vs radial config

[Topherball]

Electrical System infrastructure:
1) We have two 2000 kVA transformers that feed our campus 5kV underground distribution system they are redundant and each can handle the full campus.
2) We have three- 5 switch Sectionalizing Switches to distribute out to our buildings.
3) Both transformers and all lines are phased properly so we can either be in loop config or radial with one transformer taking care of two of the Sectionalizing Switches, and the 2nd one taking care of the other switch.

I have been running the system in a radial configuration, not a loop configuration. My thinking is if we lose something we do not lose the whole campus. I am now being asked why this is better than having a loop configuration with each transformer feeding from each end.

Is there a "best practices" application for this? I guess if each is sharing the load both transformers would run cooler, and if there was a failure of either transformer or on the feed to either transformer, and a fuse blows, the other transformer would then be able to handle the campus. Your thoughts?

 

[epimetheus]

Are the transformers rated for such service? Will your utility allow you to run in a loop configuration? Who owns the transformers? Are the primaries of both transformers fed from the same substation? If the primary feeds differ, you could circulate current through your system, though the transformer impedance would limit that affect to some extent. Do you have undervoltage tripping on your system? If you're running in a loop config and you lose one source, say to a fault elsewhere on the source feeder, your other transformer will back feed the fault through your system unless you (or the utility) trip the transformer that's lost its source.

 

[Topherball]

Are the transformers rated for such service? Will your utility allow you to run in a loop configuration? Who owns the transformers? Are the primaries of both transformers fed from the same substation? If the primary feeds differ, you could circulate current through your system, though the transformer impedance would limit that affect to some extent. Do you have undervoltage tripping on your system? If you're running in a loop config and you lose one source, say to a fault elsewhere on the source feeder, your other transformer will back feed the fault through your system unless you (or the utility) trip the transformer that's lost its source.

Yes our transformers are utility rated. We have a single 12kV utility line that feeds the campus and each transformer steps the system down to 5kV to distribute. We use Basler relays to monitor the quality of the utility feed which will open our main breaker from the utility if there is an undervoltage condition. You bring up a very good point about back feeding the fault, which points me towards keeping it as a radial system.

 

[epimetheus]

The best course of action, IMO, would be to have someone perform a power flow study to determine system stability and proper operation when looped.

 

[epimetheus]

Also, what I meant by asking if the transformers were rated for such service was are they designed for parallel operation?

 

[ron]

Often the downstream equipment isn't rated for the summation of the fault currents from both transformers.

 

[Phil Corso]

Topherball,

If you are looking for a Probability Risk Assessment of the two arrangements cited, additional information is required.

Can you provide a Single-Line Diagram or even a hand drawn sketch?

Regards, Phil Corso

 

[Tony S]

If you intend to run them in a loop configuration you need directional protection for the infeed transformers. A loop is normally run with an open point, two radials.

The open point is at #3

This is the open point in one of our 11KV loops.