Transformer capacity based on physical size

[Skaperen]

I once saw three very large transformers in a fenced area smaller than a typical substation fencing. They were close enough to read some numbers on a metal nameplate which gave a list of about 6 primary tap voltages in the 12000 to mid 13000 volt range. There were some other stamped numbers but it was too far to read them. The wiring appeared to be a wye/star secondary because there were 4 conductor bundles coming from them on the far side. The secondary conductors were very large and looked like they could support a few to many thousands of amps. I could not see the secondary bushings because they were on the far side facing the building. I could not see the primary bushings because there was some kind of enclosure assembly over the top of them (had it not been there, they would have been within object tossing distance from outside the fence). The transformers were three separate units, as opposed to one big one with an E-core. They were styled like round pole transformers, but much larger, with about a dozen huge heat sink fins on two sides of each. The actual tank was about 7.5 feet tall, and about 3.5 feet diameter. They were spaced at about an 8 foot interval. Any guesses as to the volt-amps capacity of these things based on that physical size?

What's the largest transformer made that has 208/120 or 120/240 output?

 

[0xDEADBEEF]

If you can guess the weight than this site might help:
http://www.bigpowertransformers.com/database/btransformerLIST.asp

 

[Skaperen]

If you can guess the weight than this site might help:
http://www.bigpowertransformers.com/database/btransformerLIST.asp

Unfortunately, I couldn't get through the fence to lift one up to see how heavy it was

Actually, I just gave up on the list as it was so many to click on just to see if it might even be closed based on the size. Their picture links didn't work. And I don't know if on three phase transformer the size of three single phase transformers comes out to the same total capacity.

 

[I_am_learning]

7.5 feet by 3.5 feet. I guess its range is from 200 to 400 KVA.
Crudely speaking, the Capaicty of Transformer increases as Fourth Power of Linear Dimension.

 

[Skaperen]

7.5 feet by 3.5 feet. I guess its range is from 200 to 400 KVA.
Crudely speaking, the Capaicty of Transformer increases as Fourth Power of Linear Dimension.

Hmmm. I've seen a three phase pad mount (fed from underneath, so no above ground wires visible) transformer rated 2.5 MVA that was, in terms of volume, maybe about the same as one of these big single phase tanks. So my first rough estimate would have been maybe 2 MVA per phase, to give it some reserve due to apparent age (looked like maybe 40 to 50 years old).

Oh, and I would have figured it would increase in proportion to the third power.

 

[dlgoff]

Do you know what (company, residential area, etc,) the transformers are feeding? If so, why don't you try asking who actually knows instead of speculating. I know that you're interested in physical size vs. capacity, but if you really want to know, try to find out what the name plate says (without going inside the fence i.e.).

 

[Skaperen]

Do you know what (company, residential area, etc,) the transformers are feeding? If so, why don't you try asking who actually knows instead of speculating. I know that you're interested in physical size vs. capacity, but if you really want to know, try to find out what the name plate says (without going inside the fence i.e.).

I tried to find out around 1995 when the building was being torn down. By then the transformers were gone. The building was from the late 1800s. No one knew anything about the transformers.

One of my curiosities was why 3 separate single phase transformers instead of a big giant three phase one.

 

[dlgoff]

One of my curiosities was why 3 separate single phase transformers instead of a big giant three phase one.

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Delta, Delta-Y, Delta-Delta and Y-Y.

http://en.wikipedia.org/wiki/Transformer_types"

I'm guessing they were using a Y-Y configuration like this:

 

[Skaperen]

http://en.wikipedia.org/wiki/Transformer_types"

I'm guessing they were using a Y-Y configuration like this:

That could be. But is that a reason to not use a single 3 phase E-core transformer? I guess if all the single E-core ones are only made as D-Y, that could be a reason. It could be cheaper to buy 3 single phase transformers instead of a custom made E-core. Are there any other technical issues where an E-core has problems, like whatever motivates a Y-Y?

Also, while looking at your diagram, another question pops up that I've wondered about. Are the single phase pole transformers which have dual 120 volt windings internally grounded on the center tap, or can they be grounded externally on one end LV bushing to make them be 240V L-N (or three of them yield 416Y/240)?

 

[dlgoff]

It could be cheaper to buy 3 single phase transformers instead of a custom made E-core.

I think this is what you are seeing; lower cost for a "small" power user (as compared to say a city distribution substation).

The building was from the late 1800s.

Single phase transformer designs back then were probably physically much larger than today's.

 

[Skaperen]

Single phase transformer designs back then were probably physically much larger than today's.

Although the building was from the late 1800's or so, the transformers did not appear to be anything that old. They looked to be circa 1950 or so. Still, I'm sure today they could build them even more compact than in 1950, so today's sizes might not give clear info. The wires looked even larger than what is feeding the building I work in now (4200 amps at 208Y/120). But even those could be different.

So if one were to make a single phase pole-style transformer today for 500 kVA (and the largest I have seen in any catalogs is a 1/3 that capacity), how big might that need to be? I'd guess about 1.44x\ times each of the 3 dimensions of the 166 kVA ones.