paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs

[ldh04]

I have a distribution system described as follows(basics only) (3) 115Kv/13.8Kv Xfmrs with the three 13.8 buses tied together. two of the 13.8 buses feed (4) 13.8Kv/480v transformers that feed 4 seperate buses. These 480v buses have cross-ties that form a ring bus. The normal arrangement is for all 4 of the 480 v buse to be operating independently (cross ties open) although all four are normally being fed from the same 13.8Kv bus. My question: Is there a problem feeding two of the 480 v buses from one 13.8Kv bus and the other two from the second one(remember the 13.8 Kv buses are tied together noramlly)? What if the 13.8 Kv buses were not tied together. Would a problem exist if I attempted to parallel my 480v bus across the cross tie? I have been told that this would effectively parallel the separate 13.8 Kv buses and potential result in an out of phase condition.

 

[stevenal]

If phasing is correct normally, there will be no phasing problem created just because you've opened the 13.8 tie. Yes, this effectively parallels the 13.8 buses. This parallel current path is now through the two transformers increasing losses and possibly overloading them. A load flow study is required to see if there is a problem.

 

[gordonl]

As noted for phase it will be fine because the transformers are phase matched because they are already paralleled, BUT:

1)If you parallel the 13.8KV busses through the 480V you can potentially overload your 480V systems if the 13.8KV bus potentials are not equal, you could hog most of the load to one transformer, or a worst end up back feeding on to a 13.8KV bus from the other through the 480V tie. (Voltage difference at 13.8KV would have to approach the PU 480V transformer drop) A load flow study would answer this question. (With your breif description I wouldn't expect a problem unless tap changers can get involved.)

2)If for any reason one of the 13.8KV busses was tripped off of the utility while the 480V tie was closed the other bus would try and totally back feed it through the 480V tie. In similar situations I use directional power relaying on the 480V mains to isolate the 480V system from the faulted or dead 13.8KV bus and keep the entire 480V up.

3)The most common problem with paralleling transformer (particularly on 480V systems) is exceeding the fault ratings of the gear or MCB panels fed from the sub. This would require a short circuit study to evaluate.

 

[jbartos]

Suggestion: Loops in power distribution system always cause concerns. Specifically, circulating currents may develop, e.g. harmonics. They can elevate temperature of the loop conductors due to overloading. The system grid is being monitored for the power flow and in some spots compensated, i.e. swing bus, SVARs, Capacitor Banks, Synchronous Condensers, etc. This power flow is usually modeled on computer.

 

[ldh04]

Thanks for the responses. I wasn't sure if the existing setup gave us redundancy. I mean with the 13.8KV being fed to the 13.8kv/480 volt transformers via a single feeder breaker. A loss of that breaker would result in a total loss of 13.8KV. If I understand then there is no problem feeding the 13.8KV/480v transformers via two seperate feeders as long as the 13.8kv bus is paralleled?

 

[stevenal]

A "no problem" answer is too much to ask for after a few lines in a forum. Best we can do is give you a heads up on a few things to watch for. For example: available fault current will go up with paralleled transformers. Make sure your protection is adequate.

 

[jbartos]

I concur with the previous posting. There appears to be a lot at steak at 115kV to try to solve it by a pencil and paper approach; although some senior electrical engineers can achieve reasonably good results by such approach.