Current on each phase of a T-T transformer

[mbrooke]

How is secondary current reflected on the primary of a TT connected transformer? Nothing in the NEC on how to treat them or fuse them.

 

[gar]

190526-0847 EDT

mbrooke:

A TT is two single phase transformers.

The primary current of a single phase transformer is approximate determined by the transformer turns ratio times the secondary current.

The wild leg current is straight forward. The other currents have to be determined by phasor addition.

https://en.wikipedia.org/wiki/Scott-T_transformer

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[mbrooke]

190526-0847 EDT

mbrooke:

A TT is two single phase transformers.

The primary current of a single phase transformer is approximate determined by the transformer turns ratio times the secondary current.

The wild leg current is straight forward. The other currents have to be determined by phasor addition.

https://en.wikipedia.org/wiki/Scott-T_transformer

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How to calculate for code wire sizing and primary OCPD? Nothing in Mike Holts material.

 

[iceworm]

How to calculate for code wire sizing and primary OCPD? Nothing in Mike Holts material.

From the outside looking in, it is a Delta-Wye transformer. The primary and secondary conductors and OCP can't tell any different.

As for the currents in the coils and the winding conductor sizes, that is up to the xfm mfg to size to meet spec

 

[jim dungar]

190526-0847 EDT

A TT is two single phase transformers.

The NEC treats a transformer with T windings the same as any other three phase transformer bank. Just like it does open-delta and open-wye configurations.
Most(?) three -phase transformers smaller than 15kVA and maybe half of all 15kVA units (480:208Y120) are made with T connected windings, and have been for close to 50 years or so.

 

[LarryFine]

Agreed. From the little bit I read on gar's wiki link, the phase currents are equal, to the extent the load currents are.

What I find fascinating is the way it can be used to transpose 3ph to 2ph or 2ph to 3ph. Wow. I must read more.

 

[mbrooke]

From the outside looking in, it is a Delta-Wye transformer. The primary and secondary conductors and OCP can't tell any different.

As for the currents in the coils and the winding conductor sizes, that is up to the xfm mfg to size to meet spec

So that means if all currents are equal on the secondary, the will be equal on the primary?

 

[LarryFine]

So that means if all currents are equal on the secondary, the will be equal on the primary?

Yes; read gar's wiki link.

 

[gar]

190526-1318 EDT

Look at the WIKI 3 phase drawing.

For a resistive load from X1 to X0 its current reflects to its primary as a current in phase with its primary voltage H1 to the midpoint of H2-H3.

Resistive loads on X2 and X3 to X0 reflect to their primaries as currents in phase with synthetic voltages from H2 and H3 to an imagined neutral point H0. A real H0 does not exist.

If the secondary resistive loads are balanced, then all primary currents are balanced and each primary current is of the same value.

Any primary circuit protection needs to tolerate transformer inrush current.

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[jim dungar]

What I find fascinating is the way it can be used to transpose 3ph to 2ph or 2ph to 3ph. Wow. I must read more.

No.
It takes a somewhat special Scott-Tee connection to go between 3ph and 2ph. A standard run of the mill T connection is strictly 3ph to 3ph.
You can tell the difference by looking at the position of the taps on the windings.

 

[mbrooke]

Yes; read gar's wiki link.

I'm skeptical of wiki

 

[gar]

1905226-2221 EDT

In a wild leg 3 phase connection what is the phase angle between the voltage of the X1-X3 single phase winding, and the X2-CT voltage? It is 90 degrees. That is fundamentally a 3 phase to 2 phase converter. A Scott T.

A T-T configuration is basically one Scott T converter driving a second Scott T in reverse. Taking 3 phase to 2 phase and 2 phase back to 3 phase.

Take Wiki with some question. Do your own analysis. Wiki has a lot of correct discussion and information.

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[LarryFine]

No.
It takes a somewhat special Scott-Tee connection to go between 3ph and 2ph. A standard run of the mill T connection is strictly 3ph to 3ph.
You can tell the difference by looking at the position of the taps on the windings.

I did note that when I read the entire page. My comment still stands, correct equipment used.

 

[mbrooke]

1905226-2221 EDT

In a wild leg 3 phase connection what is the phase angle between the voltage of the X1-X3 single phase winding, and the X2-CT voltage? It is 90 degrees. That is fundamentally a 3 phase to 2 phase converter. A Scott T.

A T-T configuration is basically one Scott T converter driving a second Scott T in reverse. Taking 3 phase to 2 phase and 2 phase back to 3 phase.

Take Wiki with some question. Do your own analysis. Wiki has a lot of correct discussion and information.

.

And a lot that I take with a grain of salt.

 

[gar]

190526-2337 EDT

mbrooke:

Back to your post #1 question.

For a balanced load the input currents are a reflection of the secondary currents based approximately on the inverse of the voltage ratio of the transformer combination.

Input circuit protection, in part, is determined by inrush current as well as steady state transformer current rating. True of all transformers.

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[Tony S]

 

[gar]

190527-0959 EDT

Tony S:

You have drawn a standard Scott T for 3 phase to 2 phase. That which is the first Wiki drawing. The Wiki drawing is better than yours because it provides a somewhat intuitive impression of the phase angle relationships.

I believe that mbooke's original question relates to the second Wiki circuit, 3 phase to 3 phase. It is probably referred to as a T-T because it is really two Scott Ts in series. A different layout of the schematic might make this clearer.

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[mbrooke]

190527-0959 EDT

Tony S:

You have drawn a standard Scott T for 3 phase to 2 phase. That which is the first Wiki drawing. The Wiki drawing is better than yours because it provides a somewhat intuitive impression of the phase angle relationships.

I believe that mbooke's original question relates to the second Wiki circuit, 3 phase to 3 phase. It is probably referred to as a T-T because it is really two Scott Ts in series. A different layout of the schematic might make this clearer.

.

Yes, 3 phase to 3 phase.

These are also common in 3 phase pole pigs, I'll dig for some pics.

 

[iceworm]

From the outside looking in, it is a Delta-Wye transformer. The primary and secondary conductors and OCP can't tell any different.

As for the currents in the coils and the winding conductor sizes, that is up to the xfm mfg to size to meet spec

So that means if all currents are equal on the secondary, the will be equal on the primary?

Yep - unless the mfg is doing something strange and not telling us.

Draw a box around the transformer and just look at the vectors. They will look like a delta primary and a wye secondary.

Direct observation (as in empirical data) - and opinion:
Outfits that are buiding these, are doing so because they can save a buck-46 on copper and steel (or aluminum/steel in this case). And the xfm tend to run really hot, as in too hot to touch. Which wasn't really part of the question - but perhaps interesting

 

[mbrooke]

Yep - unless the mfg is doing something strange and not telling us.

Draw a box around the transformer and just look at the vectors. They will look like a delta primary and a wye secondary.

Direct observation (as in empirical data) - and opinion:
Outfits that are buiding these, are doing so because they can save a buck-46 on copper and steel (or aluminum/steel in this case). And the xfm tend to run really hot, as in too hot to touch. Which wasn't really part of the question - but perhaps interesting

Allright, I'll buy it. Just difficult for me to visualize as I can't stop confusing it with an open delta.