Open Delta

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Hv&Lv

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I understand the two pot open delta, the 87% efficiency, even the 58% of three pots calculation. What I am wondering is where does the 30? current shift come from. Why is it there? This isn't the same as the IEEE 30? phase shift correct?
Remember, I am not an engineer...:)
 

bob

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The 30 degree of the secondary is with respect to the primary. As a user you are not aware of thr shift.
 

mivey

Senior Member
In non-engineering terms: Consider the primary and secondary like the legs of a triangle. Two primary legs can support the third secondary leg but they will be aligned in different directions.
 

Hv&Lv

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In non-engineering terms: Consider the primary and secondary like the legs of a triangle. Two primary legs can support the third secondary leg but they will be aligned in different directions.

okayyy... Try that in engineering terms then, lets see if it makes sense that way.(or at least doesn't sound so condescending)
 

mivey

Senior Member
okayyy... Try that in engineering terms then, lets see if it makes sense that way.(or at least doesn't sound so condescending)
Didn't mean for it to be condescending as it is a fair representation of the truth.

Draw the wye primary in the shape of a "Y". The secondary windings will be laying right along side of the "Y" legs. Instead of connecting them in a "Y" shape on the secondary, we are going to connect the legs end-to-end in a delta shape. This new shape will have an "orientation" different from the old shape.

In other words, if we took our original "Y" and drew three lines that connected the ends, we have our triangular-shaped delta. As you can see, the legs of the "Y" are pointing in a different direction from the legs of the delta. That is where the angle difference comes from.

Still not up to engineering specs. If you want more, let me know.
 

Hv&Lv

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Is this the arrangement you are refering to?
 

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Hv&Lv

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Ok then. Is this transformer also only 87% efficient? I was under the impression that this transformer had an IEEE mandated 30 degree phase shift.

I am also under the impression that a two transformer open delta bank is only 87% efficient because of a 30 degree current shift, where P=VI cos(?). So the 87% would come from √3/2. The 30 degree current shift, is it lagging?
 

dkarst

Senior Member
Location
Minnesota
Ok then. I was under the impression that this transformer had an IEEE mandated 30 degree phase shift.

There are a couple of misunderstanding here... the IEEE is an organization and can't mandate a 30 degree phase shift, that happens due to the physics of the windings on a Y-Δ connection. The IEEE or ANSI CONVENTION is that the high voltage side leads the low voltage side (by 30 degrees due to above). Please note which side is leading/lagging is a practice and not absolutely guaranteed by physics.

Ok then. Is this transformer also only 87% efficient?

I am also under the impression that a two transformer open delta bank is only 87% efficient because of a 30 degree current shift, where P=VI cos(?). So the 87% would come from √3/2. The 30 degree current shift, is it lagging?

The transformer's efficiency has nothing to do with the relative phase angles from primary to secondary. The 87% (and 58%) address the capacity of the combination, and really has nothing to do with efficiency. The efficiency of the transformer (for any reasonable size and loading) would probably be 95+%. The equation you are using P=VI cos(?) provides the real power given the angle between the voltage and current in a load. For example, it would apply to a load on the load side of a Y-Y just as well as on open delta bank.
 

mivey

Senior Member
Ok then. Is this transformer also only 87% efficient?
Not for the one in your picture. I believe you mean the reduced power-handling capability in the open configuration. That comes about because we are missing a transformer and the remaining two must work harder to make up for the missing pot for 3-phase loads. The combined rating of the two pots must be at least 115% of the three-phase load. (1/115% = 87%).

I was under the impression that this transformer had an IEEE mandated 30 degree phase shift.
The direction of shift is standardized (lead/lag). The fact of the shift is due to the physical arrangement.

I am also under the impression that a two transformer open delta bank is only 87% efficient because of a 30 degree current shift, where P=VI cos(?). So the 87% would come from √3/2.
The load handling capability has to do with the angles between the three-phase voltages and that three-phase is the most efficient way to deliver power. The 115% current has to do with the phase shift.

For a 100 kVA load, we have a need for 33.333 kVA to be supplied from each of the three phases. With two pots, we no longer are operating in the most efficient manner because the load is not evenly dispersed. The two pots are having to carry extra load during parts of the cycle because the third pot is not there to pick up the slack. That extra work is the extra 15% that they carry.

The 30 degree current shift, is it lagging?
The phase-to-neutral on the primary leads the phase-to-neutral on the secondary (that is the physical neutral point, which can be different than the grounded point).
 

mivey

Senior Member
Noticed Dennis answered first. I often open several threads and reply before refreshing.
 

Hv&Lv

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For a 100 kVA load, we have a need for 33.333 kVA to be supplied from each of the three phases. With two pots, we no longer are operating in the most efficient manner because the load is not evenly dispersed.

I understand this part.
The two pots are having to carry extra load during parts of the cycle because the third pot is not there to pick up the slack. That extra work is the extra 15% that they carry.
Here though the two pots will not pick up the extra slack. rather than carrying 66 2/3 of the load, they will only carry 58%(57.73%) of the load.( 66.666*(√3/2))
 

mivey

Senior Member
I understand this part. Here though the two pots will not pick up the extra slack. rather than carrying 66 2/3 of the load, they will only carry 58%(57.73%) of the load.( 66.666*(√3/2))

For the open configuration each pot will carry 57.73% of the load. The open bank with two pots must be sized to carry 2 x 57.73% or 115% of the three phase load. In other words, for a 100 kVA load, the combined bank rating must be 115 kVA or 57.73 kVA per pot.
 
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mivey

Senior Member
... rather than carrying 66 2/3 of the load...
Also, you would normally expect two transformers to share the load equally so you would expect each to carry 50% of the load. In the open-bank configuration, they each carry 57.73% of the load.

So we have 57.73%/50% =115% which tells us that they carry 15% more than what we would normally expect.
 
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Hv&Lv

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There are a couple of misunderstanding here... the IEEE is an organization and can't mandate a 30 degree phase shift, that happens due to the physics of the windings on a Y-Δ connection. The IEEE or ANSI CONVENTION is that the high voltage side leads the low voltage side (by 30 degrees due to above). Please note which side is leading/lagging is a practice and not absolutely guaranteed by physics.
That makes sense now. When parallelling XF's I can see where that would be important. I know when we build a bank we can control the shift based on how the leads are connected.



The transformer's efficiency has nothing to do with the relative phase angles from primary to secondary. The 87% (and 58%) address the capacity of the combination, and really has nothing to do with efficiency. The efficiency of the transformer (for any reasonable size and loading) would probably be 95+%. The equation you are using P=VI cos(?) provides the real power given the angle between the voltage and current in a load. For example, it would apply to a load on the load side of a Y-Y just as well as on open delta bank.
I understand that the total efficiency would be dependant on the impedance. and that the real power equation is used for AC loads. I've always been told that the voltage and current are out by 30 degrees on this particular bank configuration. I was just curious as to why. I assumed recirculating currents, but I've never found or been told the exact reason.
 

Smart $

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Ok then. Is this transformer also only 87% efficient? I was under the impression that this transformer had an IEEE mandated 30 degree phase shift.
No. It is, shall we say, fully efficient. The transformer of the label posted has all three sets of windings. The 30? phase shift is because windings are configured delta-wye. Look at the diagram of primary and secondary windings. X0-X1 has same angle as H3-H1, X0-X2 same as H1-H2, and X0-X3 same as H2-H3. As such, each pair of windings are the same phasing.

Now you have to remember that secondary voltage phase relationships are determined by line-to-line voltage, not neutral-to-line voltage. X3-X1 angle is 30? "removed" from that of X0-X1/H3-H1. The same is true of X1-X2 to X0-X2/H1-H2 and X2-X3 to X0-X3/H2-H3.

I am also under the impression that a two transformer open delta bank is only 87% efficient because of a 30 degree current shift, where P=VI cos(?). So the 87% would come from √3/2. The 30 degree current shift, is it lagging?
Let's start with the more basic explanation...

All the load current developed across the open secondary terminals must pass through both secondary windings. Let's say for example you have a balanced 100A 3? load. The current across each winding (1) of a full compliment of secondary windings (3) would be 100A?√3 or 57.74A. However, when you only have two secondary windings in open delta configuration, all 100A must pass through each of the two windings.

The 30? phase shift between winding voltage and winding current is a result of the preceding. Note that when you have full delta secondary windings, the line current splits at the terminal to 57.74A each in my example. Phasing of that current also shifts +/- 30? to align with the voltage across the respective winding. However, when you have an open delta configuration, the current's phasing is not shifted to align with the voltage, and thus the 30? shift... lagging at one open terminal, leading at the other. The windings realize a pf of .87 even when the load pf is 1.
 
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Hv&Lv

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No. It is, shall we say, fully efficient. The transformer of the label posted has all three sets of windings. The 30? phase shift is because windings are configured delta-wye. Look at the diagram of primary and secondary windings. X0-X1 has same angle as H3-H1, X0-X2 same as H1-H2, and X0-X3 same as H2-H3. As such, each pair of windings are the same phasing.

Now you have to remember that secondary voltage phase relationships are determined by line-to-line voltage, not neutral-to-line voltage. X3-X1 angle is 30? "removed" from that of X0-X1/H3-H1. The same is true of X1-X2 to X0-X2/H1-H2 and X2-X3 to X0-X3/H2-H3.


Let's start with the more basic explanation...

All the load current developed across the open secondary terminals must pass through both secondary windings. Let's say for example you have a balanced 100A 3? load. The current across each winding (1) of a full compliment of secondary windings (3) would be 100A?√3 or 57.74A. However, when you only have two secondary windings in open delta configuration, all 100A must pass through each of the two windings.

The 30? phase shift between winding voltage and winding current is a result of the preceding. Note that when you have full delta secondary windings, the line current splits at the terminal to 57.74A each in my example. Phasing of that current also shifts +/- 30? to align with the voltage across the respective winding. However, when you have an open delta configuration, the current's phasing is not shifted to align with the voltage, and thus the 30? shift... lagging at one open terminal, leading at the other. The windings realize a pf of .87 even when the load pf is 1.

Thank you. You actually answered another question I hadn't asked with that last paragraph. I also want to thank mivey and dkarst for their explanations also.
 
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