12,000ft main feeder - 120/240V service stepped up to 480V, then back down 240V -Transient concerns

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ChargedUp

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We have a customer that requires 120/240V for their equipment. Initially 30kVA possibly increasing in the future to 90kVA for a remote celltower. The utility will provide 120/240V service at their closest single phase line which is 12,000’ from the tower location.

We plan to take the 240V service, step it up to 480V, run with multiple sets of 4/0 Aluminum, make use of transformer taps, and install a solid state voltage regulator at the 120/240V equipment location.

We have done our voltage drop calculations and can address that aspect. Our concern is the system operation during transients such as inrush or faults. Has anyone had experience with a long run at 480V?
 
In no particular order:

Have you priced having the POCO bring power to the site?

I would consider using a much higher transmission voltage.

You can use a single-ended (one line grounded) method.
 
tom baker said:
what would cause transients?
Why not go to a mv instead of 480? For that distance would be less expensive
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At very least see if you can't get single phase 480 service and eliminate one transformer, better yet if they would let you have MV service voltage, or at very least let you install cable or conduit and let them acquire it for future maintenance. Though runs that long are not that common for customer to install around here, there are many cases where POCO will pull MV cable through owner/owner's contractor installed conduits to get power to places that don't have it close by.
 
Why on earth would you go to 480. Just get a couple pad Mount 12.47/7200 grndY transformers. You would need one concentric neutral cable, probably 3.50-4 dollars a foot. I know many electricians get scared off with medium voltage, but really all you have to do is put a load break elbow on the cable, it really is not hard. You will need a BG crimper and a semicon stripper. PM me I can send you a link to a YouTube video. Just watch it, practice stripping the cable a couple times you'll be totally fine. PM me if you want to talk about it, I do these around here fairly often.
 
electrofelon said:
Why on earth would you go to 480. Just get a couple pad Mount 12.47/7200 grndY transformers. You would need one concentric neutral cable, probably 3.50-4 dollars a foot. I know many electricians get scared off with medium voltage, but really all you have to do is put a load break elbow on the cable, it really is not hard. You will need a BG crimper and a semicon stripper. PM me I can send you a link to a YouTube video. Just watch it, practice stripping the cable a couple times you'll be totally fine. PM me if you want to talk about it, I do these around here fairly often.
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Depending on POCO, have a service point with CT metering on primary, then customer primary to step down xfmr and service panel.
The 3 miles of 4/0 does not seem feasable
 
I’ve done that at 2000’, 45 kva, but as others have said, at that distance, MV would be much better. There are plenty of private MV and HV contractors that you could possibly sub to, and still come out ahead. I will probably do MV on the next one that I have to do.
 
hillbilly1 said:
Another thing to consider, is the fault current at that distance may so low, that the ocp may take a lot longer to trip at the user end.
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250.122 in the 2020 NEC would require a calculation to determine the size of the EGC. This is the program to determine the fault current
steeltubeinstitute.org

GEMI Analysis Software | Steel Tube Institute

Download the free GEMI Analysis Software to:– confirm wire-type equipment grounding conductors provide an effective ground-fault current path– confirm steel...
steeltubeinstitute.org steeltubeinstitute.org
 
For less stress on the conductors and equipment to ground, I used the center tap as the grounded conductor, even though the only time any current would flow would be on a fault. That way voltage to ground would be 240 volts, not 480.
 
hillbilly1 said:
For less stress on the conductors and equipment to ground, I used the center tap as the grounded conductor, even though the only time any current would flow would be on a fault. That way voltage to ground would be 240 volts, not 480.
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You could, but then you have to run 3 conductors instead of 2. The electrical stress at 480 volts on the conductors is extremely low, I wouldnt worry at all about putting 480V to ground.
 
electrofelon said:
A setup such as the one in the OP would typically just use two conductors, one being a grounded conductor bonded at both ends, also used as the fault path, (250(A)(2) exception?)
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I agree. Some may remember the drawing I have posted in the past to make this compliant and cost efficient.
But at this distance, I'm with you, this should be done way above 480 or 600 volt.
 
Hasn't been mentioned, but....

Buried or overhead?
The original sounds like buried, but that's a LOT of trench and a LOT of fault possibilities. And up-sizing the wires vs overhead. Overhead will be a lot easier and cheaper if it's an option.

Maintenance.
If the PoCo owns the line, they'll charge something for the privilege of having it but will also have everything needed to repair when it gets hit by lightning. And they get to size the wires as they see fit (NESC) and not be constrained by NEC sizing.

I wouldn't consider taking a service at the end utilization voltage for this if there was any other option. If the PoCo won't extend their line to the site, maybe you can take a primary-voltage service connection from them onto customer-owned lines and transformer at the end site. The cost of MV switches and metering might well be offset by the cost of the lines themselves.

Have you talked to an engineer at the PoCo about options? If all they're offering is 240/120v that kinda sounds like the business office answer.
 
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