Adding single phase banks when one is different

[MarvinM]

My first post here, be gentle, please.

To begin with, I know that 3 single-phase transformers can be wired together to make one three-phase transformer and that the kVA of each transformer is cumulative. For example, 3 50kVA single-phase transformers make one 150kVA three-phase transformer.

My question is; Can I simply add 3 single-phase transformer banks if one bank is larger?

What I'm looking at is a three-phase 4 wire Delta system (secondary) consisting of two 25kVA single-phase transformers and one 50kVA single-phase transformer. Is this the same as a 100kVA transformer? Or is there some more complicated math I would understand better if I were an actual engineer?

 

[powerpete69]

Going by general theory:
If you were using balance three phase loads you would only have 75KVA worth of transformer available with your proposed set up. If you wanted to load up the single phase bus of your panel (call it bus C for example) from the 50 KVA transformer with an extra 25KVA worth of single phase loads I suppose you theoretically could assuming you kept each single pole breaker on the C bus. This could increase your output to 100 KVA.
I'm not sure an inspector would love this. Not sure this would meet code. Not sure I would do this.
But theoretically you likely could.

 

[Hv&Lv]

What you’re describing sounds like a 120/240 closed Delta bank. The larger transformer with Supply roughly 25 kVA of single phase loading and 25 kVA of three phase loading to match the other 2 25 kVA transformers.
Are you wanting to parallel a bank with this one, or are you just wanting to make this bank bigger?

 

[MarvinM]

Thank you for the responses.

I'm only trying to model what already exists in Easypower. The set-up described above is what is provided by the utility. I've modeled it a number of ways already, and the worst case is when modeling it as a 50kVA transformer, but I want to be accurate and modeling it as a 50kVA transformer isn't.

Since this is a utility set up I have no idea what codes they fall under, but that is outside my purview.

 

[Hv&Lv]

Utility is under NESC.
The modeling software should have transformer bank with one XF size per phase.
I’ve never used Easypower. We use windmill.
I assume they are about the same if it’s system modeling software.
They should total it up to a 100kVA

 

[MarvinM]

I have dug into Easypower's online "knowledge database". They have some kind of workaround for modeling single-phase transformers, but no official support. So I've implemented their workaround and parallelled the transformers, but I'm going to see if I can get one of their engineers to agree with what I've done.

Thanks again for the assistance.