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How is the transistion from 3 to single phase done on a 12.5kV feeder?

How is the transistion from 3 to single phase done on a 12.5kV feeder?

How is the transistion from 3 to single phase done on a 12.5kV feeder?

(OP)
Hi all, disclaimer: I'm not a power engineer or an EE but have a basic understanding of power transmission and distribution.

I'm trying to estimate the voltage impacts on a distribution line if we were to interconnect a small micro-hydro system (100-1500kW).

Our point of interconnection is ~30 miles from the nearest substation and the line is 12.5kV. We know the distribution line is 3-phase for the first half (~15 miles) and then single phase thereafter. We can see that the line goes from 3 conductors to 2 conductors at the point where it transitions to single phase.

So what is on the two conductors in the single phase part of the system? Do those conductors have 2 of the original 3 phases, and therefore 12.5kV between them? Or should we assume that those conductors have one of the original phases and a neutral return wire and therefore 7.2kV across them?

I realize it may be impossible for you to know for sure without more information, but is there a common practice or is either situation possible / likely?

Thanks!

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

Could be either. Why not ask the utility?   

David Castor
www.cvoes.com

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

And while you're asking the utility about the phase/voltage available, ask them whether they will accept a 100 to 1500kW source as single phase. And 30 miles away from the substation.

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

30 miles from the nearest substation?  Is that for real?  Anyway, I think we would be willing to bet on the utility answer!

Alan

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

(OP)
Thanks all.

I took your advice and just heard back from someone familiar with the system (not the utility but a guy from ILB who worked on a recent upgrade).  Looks like the system is delta configuration on the 3-phase side and the two conductors on the single phase side are 2 of the original 3 phases.

The question of whether we could install such a large single phase generator is what we're ultimately trying to answer.  We can't ask the utility without going through a full interconnection process which we're not ready for yet (and don't have the $).  

So doing our own first order calculations of the impact on voltage is how we're trying to estimate what the limits are to the system.  Based on your responses, it sounds like we're on the right track and we can assume 12.5kV across the single phase lines.

BTW - We're also looking into a system upgrade to 3-phase, but the costs will likely be too high for this small of a system.

Thanks again.

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

"...We can't ask the utility without going through a full interconnection process which we're not ready for yet (and don't have the $)..."

You may not be able to "officially" ask the utility, but you surely have enough $$ in hand to ask a couple of their people to share thoughts on the matter while they consume lunch and a few of what ever beverage they favor. Doing so might save a lot of time, money and adult language later on.     

RE: How is the transistion from 3 to single phase done on a 12.5kV feeder?

The last question I had for the local utility cost me lunch and yielded some good answers as well as a good meal and some good general conversation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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