paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
(OP)
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.






RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
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.
RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs
RE: paralleling of 480 v Xfmrs downstream of 13.8KV Xfmrs