Long run of 480v or step up/dn to MV?

[Todd0x1]

Wasn't sure if this should be posted here or in engineering, but since the discussion centers around cost I put it here.

Hypothetical scenario:

Need to run 600 amps of 480v 3ph 4w 500-600ft (along a somewhat challenging path, vertically through a building, across a roof, and back down the other side). Is this enough current and a long enough run to where it would make sense to step up to 4160v and back down to 480v at the other end? This would cut three conduits down to one smaller one, eliminate a ton of coring, and get rid of the neutral needed with a 480v feeder. The unit substations would be located indoors.

If my quick assumptions are correct, the 480v circuit would require 3 parallel sets of 4x 4/0 cu (lets say specs forbid aluminum). Thats $25,000 in wire not counting a EGC. The 4160v using 3x #6 MV-105 is $3600 in wire again not counting a EGC.

How much does a 500kva unit substation cost?

At the step down end if there are substantial loads at both 120/208 and 480v does it make sense to use two smaller unit substations to step down directly to the utilization voltage?

On the step up end, all that would be needed is the transformer and a 4160v disconnect on its output correct? The breaker in the switchgear supplying it can be the primary disco / ocpd correct?

How would the transformer core losses compare, say 500kva 480-4160 + 300kva 4160-120/208 + 112kva 4160-277/480 VS: 300kva 480->120/208?

 

[electrofelon]

I dont think it is worth it. IIRC a 500 KVA is about $14,000. Losses 24/7 forever. 3X 500 MCM AL in 3" is sounding pretty good, but then again, sometimes its hard to explain the most logical cost effective solution when someone wants to spend 18k more for copper. :happysad:. Someone needs to listen to a moody blues album (in search of lost cord is good) in the woods and do some soul searching. :happyyes:

 

[infinity]

You also need to keep in mind that a 500 kva substation may weigh between 4000 and 5000 pounds. That will require some highly skilled electricians or paid riggers to set in place. I think that given the choice I would opt for the 480 volt risers.

 

[PaulMmn]

You can't convince them to run the line through the dining room and the grand ballroom? Where I work, we actually moved the computer UPS, and the wall it was near, back about 3 feet so they could widen the hallway and make a Grand Entrance into the learning center (ie a nest of conference rooms).

Originally they wanted to route the hallway through the telephone switch room!

 

[Todd0x1]

You also need to keep in mind that a 500 kva substation may weigh between 4000 and 5000 pounds. That will require some highly skilled electricians or paid riggers to set in place. I think that given the choice I would opt for the 480 volt risers.

Placement of the substations isn't an issue. In looking at the SQD VPI transformers, the 300kva w/ fan option would likely be ok (but that is a matter for the engineers).

I am just trying to get a sense of what the MV gear would cost and how that compares to the 480v feeders. Enormous labor and material savings running those MV feeders vs 480v.

And also what the transformer core losses would be.

If the MV gear costs ~30K it makes sense to use the MV gear and then it comes down to the annual costs of core losses. If the MV gear costs $100k then not so much. There is a big solar install on the site which keeps the utility bill really low. I need to see what what the rate is for the surplus solar sold to the utility.

How can I figure what the 24/7 core losses are for the three MV transformers?

 

[Todd0x1]

You can't convince them to run the line through the dining room and the grand ballroom? Where I work, we actually moved the computer UPS, and the wall it was near, back about 3 feet so they could widen the hallway and make a Grand Entrance into the learning center (ie a nest of conference rooms).

Originally they wanted to route the hallway through the telephone switch room!

That run length is pretty much a straight shot. Its just far away.

 

[electrofelon]

I don't know how to model overall transformer losses. I am sure some on here can help with that . No load losses are traditionally assumed to be .5% of KVA, but in my experience new units are a bit lower, maybe .4%. load losses are the hard part.

One mistake I find people make when looking at transformers is they ignore the voltage drops of two transformers - that is they look at the step up step down scheme as having negligible VD, but then when pricing out the no transformer scheme, they price it out with a low VD and it's not a fair comparison. The transformer scheme will easily have 5% VD. Don't think you will have real stable voltage with the step up step down.

 

[Cow]

480v still makes sense at that distance and amperage.