Sizing 120/240 Delta Transformer (Mixed Three Phase and Single Phase Load)

[ElectricalEngineer!]

Greetings All,
I am trying to size/validate the rating on the medium voltage transformer for a project. For some reason I can't find many resources that indicate how to properly do this when you have mixed loads (Three Phase and Single Phase). What Size transformer will be needed?

The Need:
1000 AMP SES (600 Three Phase and 400 Single Phase)
Based on my calculation, I am thinking that I have demand load of (343.8kVA) (This total three and single phase load combined) which means I need 300kVA

I am planning to use 120/240 Padmount Delta Transformer

Any guidance would be greatly appreciated!

 

[augie47]

I think you are going to encounter a problem in that 240/120 3 phase transformer have a significant reduced neutral capacity.

 

[ElectricalEngineer!]

I think you are going to encounter a problem in that 240/120 3 phase transformer have a significant reduced neutral capacity.

Augie47,
What do you mean? Would you please expand on that a little more?
Thank You!

 

[jim dungar]

240/120V delta is not a typical configuration for customer owned padmount transformers.

Why not use the more common 208Y/120V wye?

A transformer wire a common three legged core does not like an appreciable single phase center tapped load. These configurations are often limited to 5% loading on the center tap.

 

[ElectricalEngineer!]

240/120V delta is not a typical configuration for customer owned padmount transformers.

Why not use the more common 208Y/120V wye?

A transformer wire a common three legged core does not like an appreciable single phase center tapped load. These configurations are often limited to 5% loading on the center tap.

Jim,
The transformer is utility owned. I am using 120/240 Delta due to the transformer scarcity going on now. I agree with your input...

 

[augie47]

Definitely a rare bird here..utility 240/120 3 phase padmount... any chance there is a misunderstanding ?

 

[topgone]

@ElectricalEngineer!,
If you are conversant with the split-phase, open delta configuration to feed mixed loads (3-phase and 1-phase loads), maybe you could save some doing that.
If my brain didn't fail me, what you need according to my calculations are two, single-phase transformers (total capacity = 417 kVA);

T1, sized at 250 kVA (secondary voltage = 240 V), and​

T2, (another transformer) sized at 167 kVA (secondary winding is center-tapped, voltage = 120 -0 120 V).​

Please do a cost comparison (comparing with having a 300 kVA pad-mounted transformer versus having 2 -250 & 167 kVA open-delta, split-phase bank).

 

[jim dungar]

Jim,
The transformer is utility owned. I am using 120/240 Delta due to the transformer scarcity going on now. I agree with your input...

The utility should be the ones doing the sizing. Give them your loading and let them supply the proper unit.

Utilities in this area purchase padmount's with triplex cores, which are similar to three individual single phase units in one common tank.

 

[ElectricalEngineer!]

@ElectricalEngineer!,
If you are conversant with the split-phase, open delta configuration to feed mixed loads (3-phase and 1-phase loads), maybe you could save some doing that.
If my brain didn't fail me, what you need according to my calculations are two, single-phase transformers (total capacity = 417 kVA);

T1, sized at 250 kVA (secondary voltage = 240 V), and​

T2, (another transformer) sized at 167 kVA (secondary winding is center-tapped, voltage = 120 -0 120 V).​

Please do a cost comparison (comparing with having a 300 kVA pad-mounted transformer versus having 2 -250 & 167 kVA open-delta, split-phase bank).

topgone,
I am familiar with the open delta configuration and I appreciate your analysis. I am unfortunately using a 120/240 Delta padmount transformer for this project. So wanted to know for a 1000 AMP (600A Three Phase and 400A Single Phase) Service, what size 120/240V Delta padmount transformer would be sufficient?

 

[curt swartz]

topgone,
I am familiar with the open delta configuration and I appreciate your analysis. I am unfortunately using a 120/240 Delta padmount transformer for this project. So wanted to know for a 1000 AMP (600A Three Phase and 400A Single Phase) Service, what size 120/240V Delta padmount transformer would be sufficient?

Do you work for the utility or the customer? If you work for the customer the utility is going to tell you what voltages they offer for your loads and they will size the transformer. They could care less what size you want.

 

[topgone]

topgone,
I am familiar with the open delta configuration and I appreciate your analysis. I am unfortunately using a 120/240 Delta padmount transformer for this project. So wanted to know for a 1000 AMP (600A Three Phase and 400A Single Phase) Service, what size 120/240V Delta padmount transformer would be sufficient?

Utilities have their transformer sizing programs and we're in the dark as to how they do it. You can observe how they are betting on having pole pigs so undersized that a regular engineer would be scratching his head. Their idea is that transformers can be loaded beyond what their ratings say they should. The revenues a transformer provides them could be maxed, sacrificing the transformer's useful life. As long as the terminal voltage provided stays in the green, they'd continue using undersized distribution transformers. I dare not hazard a guess.

 

[hillbilly1]

My last project, the poco set a 750 kva for a 1600 amp 480 volt service, they didn’t want to, they wanted to set a 500, but the customer said he was fine paying extra for the 750. He has a new machine that’s very large and voltage sensitive. If the voltage goes out of wack for just a short period of time, the whole batch of product has to be tossed. It even has a 300 amp UPS to provide a short term hold over.

 

[electrofelon]

Ju

topgone,
I am familiar with the open delta configuration and I appreciate your analysis. I am unfortunately using a 120/240 Delta padmount transformer for this project. So wanted to know for a 1000 AMP (600A Three Phase and 400A Single Phase) Service, what size 120/240V Delta padmount transformer would be sufficient?

Just reiterating what others have said, but you do not spec the utility's transformer. That is not how it works. last time I expressed concern about some utility equipment serving my client , they said "let us worry about our equipment". Also I don't know of a utility that will do a 120/240 three phase pad mount that large. Have they said they will supply this?

 

[Joethemechanic]

That large, 120/240 delta with a high leg, What is this supplying? Seems like you are going to spend a fortune on copper, not to mention controls with that low of voltage

 

[winnie]

I think you are going to encounter a problem in that 240/120 3 phase transformer have a significant reduced neutral capacity.

What do you mean? Would you please expand on that a little more?

See:

Is there a 5% limit for the single phase loading on Delta-Delta connected Transformers with 240/120 Center Tapped secondaries? | Schneider Electric USA

One might assume that a 240/120V delta transformer could handle 1/6 of its kVA as a pure L-N 120V load. But the maximum they are generally rated to handle in this fashion is 1/20 of its rating.

As others have said, the Utility will be in charge of picking the transformer. But it would pay for you to calculate the maximum expected unbalanced 120V load and supply that information to the utility. If the utility is expecting a dominant 3 phase load with a smidge of 120V incidental loading they will use one transformer design, if they are expecting lots of 120V unbalanced loading then they will use a different design.

Separate issue, tangential curiosity:

What is not clear to me is how much of the total transformer capacity is available when the neutral is maximally loaded.

Say you have a 100kVA 240/120V delta transformer. For a balanced 3 phase load, the rated load current is 240A. If I place a 5kVA 120V load on this transformer (42A L-N at 120V), and a balanced 3 phase load, how big can the 3 phase load be? My _guess_ is about 83kVA, on the assumption that the transformer can handle a maximum of 240A per phase terminal, but I'd appreciate better info on the topic.

-Jon

 

[topgone]

See:

Is there a 5% limit for the single phase loading on Delta-Delta connected Transformers with 240/120 Center Tapped secondaries? | Schneider Electric USA

One might assume that a 240/120V delta transformer could handle 1/6 of its kVA as a pure L-N 120V load. But the maximum they are generally rated to handle in this fashion is 1/20 of its rating.

As others have said, the Utility will be in charge of picking the transformer. But it would pay for you to calculate the maximum expected unbalanced 120V load and supply that information to the utility. If the utility is expecting a dominant 3 phase load with a smidge of 120V incidental loading they will use one transformer design, if they are expecting lots of 120V unbalanced loading then they will use a different design.

Separate issue, tangential curiosity:

What is not clear to me is how much of the total transformer capacity is available when the neutral is maximally loaded.

Say you have a 100kVA 240/120V delta transformer. For a balanced 3 phase load, the rated load current is 240A. If I place a 5kVA 120V load on this transformer (42A L-N at 120V), and a balanced 3 phase load, how big can the 3 phase load be? My _guess_ is about 83kVA, on the assumption that the transformer can handle a maximum of 240A per phase terminal, but I'd appreciate better info on the topic.

-Jon

@winnie,
Sorry, you got it wrong. The 5% single-phase load limit is only if you load on the delta high-leg terminals (the terminal opposite the center-tapped phase of the delta paired with the center-tap neutral).

 

[curt swartz]

@winnie,
Sorry, you got it wrong. The 5% single-phase load limit is only if you load on the delta high-leg terminals (the terminal opposite the center-tapped phase of the delta paired with the center-tap neutral).

That is not what is says.

The amount of the imbalance allowed when using the Center Tap (CT) to feed 120V loads is limited to 5% of the overall rated KVA of the Transformer to help prevent excessive circulating currents.

120 Volt loads would not be connected to the high leg.

 

[winnie]

@winnie,
Sorry, you got it wrong. The 5% single-phase load limit is only if you load on the delta high-leg terminals (the terminal opposite the center-tapped phase of the delta paired with the center-tap neutral).

Tell Schneider.

AFAIK one is not supposed to connect high leg to neutral loads at all.

Jon

 

[jim dungar]

Tell Schneider.

AFAIK one is not supposed to connect high leg to neutral loads at all.

Jon

I dont think I have never seen this prohibition in writing.
The typical reason is the problem with finding 1pole 208V rated breakers.
This connecion would also put loading one of the 240V legs as well as one of the 120V windings thereby increasing the apparent load on the transformer with a resultant impact on voltage regulation.
In the end a 240:208V buck- boost transformer is usually more cost effective.

 

[Joethemechanic]

In Philly lots of customers got high leg delta services because you could run your old 2 phase equipment from it. One phase was a-c and the other was n-b. Motors were 220v. I guess if you wanted to do it right you could use a boost/buck to even out the voltages, but I never saw it that way. Never really saw the voltage imbalance be a problem either, but then again most of those motors were nowhere near continuously loaded