3 phase transformer bank impedance when 120V windings are paralleled

[Distribution Grid Don]

Apologies if this question has been covered before. I am doing a short circuit calculation for an arcflash study.

If we have a 3 phase bank of single phase 240V transformers, connected to provide 120/208V service (with the 120V windings paralleled in each transformer), and each transformer has a 2% impedance, do we still assume a 2% impedance for the bank? Or does the paralleling of the secondary windings reduce the impedance?

 

[ron]

I don't quite understand the reference to paralleling the secondary windings, but this graphic is typically how 3 single phase transformers are used to make a delta-wye 3 phase transformer bank. Calculate the secondary FLA and divide by 0.02 to do an infinite primary calc.

 

[jim dungar]

I don't quite understand the reference to paralleling the secondary windings, but this graphic is typically how 3 single phase transformers are used to make a delta-wye 3 phase transformer bank. Calculate the secondary FLA and divide by 0.02 to do an infinite primary calc.
View attachment 2575701

You are showing single voltage windings.

I think the OP is asking about two 120V winding secondaries that are paralleled (X1-X3 and X2-X4) and the connected into a 208Y/120V bank.

 

[Distribution Grid Don]

I don't quite understand the reference to paralleling the secondary windings, but this graphic is typically how 3 single phase transformers are used to make a delta-wye 3 phase transformer bank. Calculate the secondary FLA and divide by 0.02 to do an infinite primary calc.
View attachment 2575701

Thank you for the reply, Ron. When banking 3 single phase 120/240V transformers to get a 120/208 3 phase service, you only need to get 120V from each transformer. Rather than just using one half of the center tapped winding on the secondary (and only getting half of the transformer's capacity), the transformers are usually "cut" so that both ends of the secondary winding are tied to the same bushing and the center tap stays grounded, thus putting both 120V windings in parallel with each other. I am just wondering if that impacts the impedance of each transformer.

 

[Distribution Grid Don]

You are showing single voltage windings.

I think the OP is asking about two 120V winding secondaries that are paralleled (X1-X3 and X2-X4) and the connected into a 208Y/120V bank.

I think that's correct, Jim, if X2 and X3 are assumed both tied to the grounded center tap of each transformer. Internally, X4 would be cut over to X1 put both windings in parallel.
I'm new to these boards...how do I paste an image into the discussion? I only see a way to insert an image from a link.
Thanks.

 

[Hv&Lv]

Parallel inductors..
All the reciprocal calculations..

Never really thought about this situation although we do it at least once a month.
In Windmill..

 

[LarryFine]

I think that's correct, Jim, if X2 and X3 are assumed both tied to the grounded center tap of each transformer. Internally, X4 would be cut over to X1 put both windings in parallel.

Not quite. You would connect X1 and X3 together, and connect X2 and X4 together.

 

[ron]

I'm new to these boards...how do I paste an image into the discussion? I only see a way to insert an image from a link.
Thanks.

You paste the image in where the text goes. I use CNTL-C for copy from the source, and CNTL-V to paste it.

 

[CoolWill]

That's a good question, and I don't know the answer myself. But I would think that the impedance rating would be at the maximum voltage the transformer could output, which would mean the windings are in series for 240 volts. So putting them in parallel would cut the impedance in half?

 

[jim dungar]

That's a good question, and I don't know the answer myself. But I would think that the impedance rating would be at the maximum voltage the transformer could output, which would mean the windings are in series for 240 volts. So putting them in parallel would cut the impedance in half?

The majority of the %IZ is inductance not resistance.

I cannot remember ever making an adjustment to a transformer's %Z based on changes to the winding configuration. Too busy shoveling snow to dig out my old reference books.

 

[electrofelon]

Apologies if this question has been covered before. I am doing a short circuit calculation for an arcflash study.

If we have a 3 phase bank of single phase 240V transformers, connected to provide 120/208V service (with the 120V windings paralleled in each transformer), and each transformer has a 2% impedance, do we still assume a 2% impedance for the bank? Or does the paralleling of the secondary windings reduce the impedance?

My gut says that the impedance stays the same. The difference is you have the transformer now wired for twice the current so that is why your available fault current doubles. Remember in regards to transformers, the number we use is "% impedance", which is not "impedance". %Z is the percentage of nameplate voltage that when applied to the transformer with a shorted secondary, will result in nameplate current at the secondary.

 

[wwhitney]

%Z is the percentage of nameplate voltage that when applied to the transformer with a shorted secondary, will result in nameplate current at the secondary.

And so, by the turns ratio, equivalently the rated (nameplate) current on the primary.

So the question is, if we have a rated %Z for the 240V (series) secondary coil configuration, and we do this test using %Z * primary voltage, and then we reconfigure the secondary to the 120V (parallel) secondary coil configuration, will the primary current change? Equivalently will the total impedance the primary voltage source sees change?

Seems like just looking at the two coils themselves, the secondary coil impedance is less in the parallel configuration (the current in each coil is unchanged). Because the resistance of the wires and the inductances of the coils themselves add in the series configuration, but their reciprocals add in the parallel configuration.

But I have no sense of how large a component of the total impedance the primary voltage source sees the secondary coil impedance is. And hence if any change in %Z is significant.

Cheers, Wayne

 

[Hv&Lv]

but their reciprocals add in the parallel configuration.

But I have no sense of how large a component of the total impedance the primary voltage source sees

Yeah, this is what I was thinking in post 6

 

[kwired]

The majority of the %IZ is inductance not resistance.

You have same coils with same voltage across each individual coil and same current for a given VA of demand This should keep the inductive effects to nearly the same level. Seems any resistive component is likely what may be different though it likely won't be all that significant?

 

[Distribution Grid Don]

Not quite. You would connect X1 and X3 together, and connect X2 and X4 together.


View attachment 2575703
View attachment 2575702

So, when banking these for a 120/208V service, you would connect X2-X4 to the neutral, I assume? For a 3 bushing secondary, I am familiar with internally connecting X3 to X1, and then taping over the exterior X3 bushing, for each transformer.

 

[Distribution Grid Don]

You have same coils with same voltage across each individual coil and same current for a given VA of demand This should keep the inductive effects to nearly the same level. Seems any resistive component is likely what may be different though it likely won't be all that significant?

This is kinda where I was leaning.

 

[kwired]

So, when banking these for a 120/208V service, you would connect X2-X4 to the neutral, I assume? For a 3 bushing secondary, I am familiar with internally connecting X3 to X1, and then taping over the exterior X3 bushing, for each transformer.

View attachment 2575749

Normally to parallel the two coils you need to tie X1 and X3 together and tie X2 and X4 together. And you have the lower voltage rating of the unit available between X1/X3 point and X2/X4 point. If you interchanged say X1 and X2 you put the two coils in opposition to each other and would leave you with a high level current loop through both coils. How those connect to the external side, I not really familiar with them but one external terminal would not be used as you only will have two possible points to connect to when all is said and done.

 

[electrofelon]

I hope when the transformers are wired for 120V output for wye banks the impedance doesn't change, if it does me and a lot of other people have been calculating wrong values for years

 

[LarryFine]

So, when banking these for a 120/208V service, you would connect X2-X4 to the neutral, I assume? For a 3 bushing secondary, I am familiar with internally connecting X3 to X1, and then taping over the exterior X3 bushing, for each transformer.

Right. You're first configuring each transformer for 120v, then wiring three in a wye.

 

[LarryFine]

Normally to parallel the two coils you need to tie X1 and X3 together and tie X2 and X4 together. And you have the lower voltage rating of the unit available between X1/X3 point and X2/X4 point.

Right. We parallel them to obtain full transformer capacity at the lower voltage.

A center-tapped secondary is equivalent to one being fixed at the higher-voltage.

If you interchanged say X1 and X2 you put the two coils in opposition to each other and would leave you with a high level current loop through both coils.

Right again. It's a matter of polarity, like when paralleling two batteries.

How those connect to the external side, I not really familiar with them but one external terminal would not be used as you only will have two possible points to connect to when all is said and done.

There are internal jumpers moved around among the four secondary terminals and two or three bushings.