Primary phase loss on delta Xfmr

[ptonsparky]

I'm having fun trying to calculate the secondary voltages of a Delta-Wye transformer as the primary line to line voltages change, for whatever reason. Primary is closed Delta 480 volt

Assume I lose L2, a fuse opens. What will the voltage be from H1 to H2 and H3 to H2? 240?

 

[ron]

Interesting opportunity to test ChatGPT5. I pasted your exact text:

Short answer (ideal case): about 240 V from H1–H2 and 240 V from H3–H2.

Why​

• In a closed-delta 480 V primary, the three windings form the triangle sides H1–H2, H2–H3, and H3–H1. Each side normally has 480 V across it.
• If the L2 line (the corner at H2) opens, only L1 and L3 are still driving the delta. That applies 480 V between the other two corners (H1 and H3).
• The path from H1 to H3 is two series windings (H1–H2 and H2–H3). With equal impedances and no load at H2, the 480 V splits evenly, so each winding sees ~240 V.
• Hence, a meter reads ~240 V from H1→H2 and ~240 V from H3→H2 (opposite polarity, same magnitude).

Real-world caveats​

• Any secondary loading will pull the floating corner (H2) around, so those numbers can drift—sometimes a lot.
• You can also see “mystery” full voltage across the open fuse itself with a high-impedance meter because of backfeed through the other two transformers, but that is a different measurement than across a winding.
• Running a closed delta while single-phased can overheat the bank quickly; currents become unbalanced and ugly.

 

[jim dungar]

I'm having fun trying to calculate the secondary voltages of a Delta-Wye transformer as the primary line to line voltages change, for whatever reason. Primary is closed Delta 480 volt

Assume I lose L2, a fuse opens. What will the voltage be from H1 to H2 and H3 to H2? 240?

You need to calculate using positive, negative, and zero sequence components.

Your first reaction might be L1-L2 =240V, L2-L3, =240V, AND L3-L1=480V, but that does not taking account what is happening in the transformer core.


I know from experience that typical under voltage relays on the secondary might not see the open primary fuse.

 

[ptonsparky]

For my situation I was assuming no load on the secondary.
Does this look about right?
Actual

Transformer	Designed	A-B	B-C	C-A
Delta Primary Voltage	480​	240​	240​	480​
Wye Secondary Voltage	208​	104.00​	104.00​	208.00​
Ratio	2.31​
		A-N	B-N	C-N
Line to Neutral or EG		60.12​	60.12​	120.23​

 

[ptonsparky]

You need to calculate using positive, negative, and zero sequence components.

Your first reaction might be L1-L2 =240V, L2-L3, =240V, AND L3-L1=480V, but that does not taking account what is happening in the transformer core.


I know from experience that typical under voltage relays on the secondary might not see the open primary fuse.

Whooosh...that sound I hear over my head.

 

[jim dungar]

Whooosh...that sound I hear over my head.

Yeah. Hard core power systems engineering.

 

[Deltaforce]

For my situation I was assuming no load on the secondary.
Does this look about right?
Actual

Transformer	Designed	A-B	B-C	C-A
Delta Primary Voltage	480​	240​	240​	480​
Wye Secondary Voltage	208​	104.00​	104.00​	208.00​
Ratio	2.31​
		A-N	B-N	C-N
Line to Neutral or EG		60.12​	60.12​	120.23​

In three phase Line voltage= 1.73 *phase Voltage. op case loss of one phase, single phase Transformer then. How 60.12*1.732=104 then?

 

[jim dungar]

Does this look about right?

You won't have a true 120° phase shift on the transformer secondary because you lost the phase shift on the primary.
A standard single core transformer cannot create a true three phase secondary from a single phase primary.

 

[ptonsparky]

You won't have a true 120° phase shift on the transformer secondary because you lost the phase shift on the primary.
A standard single core transformer cannot create a true three phase secondary from a single phase primary.

Ok.

Now 'if' the L2 fuse was good and for whatever reason the voltage was 240. Does that work?

 

[ptonsparky]

In three phase Line voltage= 1.73 *phase Voltage. op case loss of one phase, single phase Transformer then. How 60.12*1.732=104 then?

I didn't think of it as 1 phase, but you're right.

 

[jim dungar]

Ok.

Now 'if' the L2 fuse was good and for whatever reason the voltage was 240. Does that work?

So, you have unequal primary side line voltages?
You have a choice to make; either the Secondary wye point can float, like the wye point in a motor, or it is a fixed relation, like when you have a 4 wire secondary.

 

[wwhitney]

For a 480D : 208Y/120 transformer, energized only with 480V single phase on the C-A primary coils, when it is unloaded, won't the voltages need to be symmetric about the B primary side connection? So 240V A-B and 240V B-C on the primary side, as there is nothing to drive the floating B point closer to A vs C. Or can simple manufacturing variations break the symmetry enough to significantly move the B voltage point away from the midpoint?

Then if symmetry does hold, the L-L voltages on the secondary become 180V, 180V, and 0V, while the L-N voltages become 120V, 60V, 60V. You can visualize the standard delta-wye diagram of voltage vectors, and as you collapse one point of the primary side triangle to the midpoint of the opposite side, two legs of the secondary wye flatten out to be along the same line as the third leg, opposite in direction and half as long.

Cheers, Wayne