Open Delta ?

[TwoBlocked]

Help my confusion, please. An open delta/open delta (V/V) supplies 3 phase power on the secondary with 120 degrees difference between the primary feeds. But when I think of a typical 480V single phase to 120/240 split phase transformer, I am having trouble visualizing the real difference. I imagine the V/V primary as the same as a center tapped single phase transformer, without the center tap being used anywhere. And I imagine the V/V secondary as the same as a center tapped single phase transformer with the center tap used as neutral. But somehow the one provides 3 phase on the secondary and the other split phase. Something about two transformers being in series is different than one transformer? (center tapped or not...)

 

[winnie]

The key difference is that in the open delta system you have two separate magnetic cores, with the required different phase angles in the induced voltages.

With a single phase transformer, you might have two coils connected in series, but they are both on the same magnetic core, and both get magnetic flux with the same time phasing.

If you took the two transformers of the open delta setup, but fed them from a single phase supply, then the output of the pair would still be single phase. You need the two separate cores to carry out of phase magnetic flux, and the three phase input to set up the out of phase flux.

Jon

 

[TwoBlocked]

..... You need the two separate cores to carry out of phase magnetic flux, and the three phase input to set up the out of phase flux.

Jon

You are certainly right, but I am have a hard time imagining it. It must be the currents in the two transformers are not be in phase even though the primary voltage would be "seen" by the transformers as single phase. Is there (perhaps) a current going back and forth between the two transformers?

 

[gar]

230409-2116 EDT

TwoBlocked:

A single phase transformer with a high permeability iron core with tight magnetic coupling between a single primary winding, a single isolated secondary winding, and a 1-to-1 ratio will have an output voltage nearly the same as the input voltage. The output phase will be nearly in phase with the primary, or 180 degrees out of phase depending on how the output wires are connected.

As an idealization we can assume the transformer has perfect coupling between primary and secondary, and that there is zero internal impedance between primary and secondary.

Consider one ideal transformer is connected to a source with a defined phase angle to some reference of 0 degrees.

Then consider a second ideal transformer that is fed by a source that is 120 degrees shifted in phase from the first source. The second ideal transformer will have an ouput phase angle relative to first transformer that is either 120 or 240 degrees shifted from the phase of the first transformer..

Connect one wire of each of the seconaries together. Now you have a two phase source with a shift of 120 degrees between their outputs.

Between the two open output terminals you have a 240 degree phase difference. Thus, you really have a three phase source with only two actual physical secondaries. This is a very common utility connection. If more three phase power is required, then the power company will then add a third transformer.

. . .

 

[jaggedben]

You are certainly right, but I am have a hard time imagining it. It must be the currents in the two transformers are not be in phase even though the primary voltage would be "seen" by the transformers as single phase. Is there (perhaps) a current going back and forth between the two transformers?

An open delta has three primary wires, not two. Each pair has a voltage that's 120deg out of phase with the others. The primary source for the A-B transformer is not in phase with the primary for the A-N-C transformer.

A single-phase 480V to 120/240V transformer only has two primary wires.

Yes, there is a current across both transformers in the open delta, if say you hook up a load from B to C.

Hope that helps.

 

[LarryFine]

An open delta has three primary wires, not two.

They are often driven by two primary lines and the neutral.

 

[TwoBlocked]

An open delta has three primary wires, not two. Each pair has a voltage that's 120deg out of phase with the others. The primary source for the A-B transformer is not in phase with the primary for the A-N-C transformer.

A single-phase 480V to 120/240V transformer only has two primary wires.

Yes, there is a current across both transformers in the open delta, if say you hook up a load from B to C.

Hope that helps.

They are often driven by two primary lines and the neutral.

OH! Then if you lose the neutral, would it become like a single primary transformer with the secondary being a split phase? I am sure it happens ...

 

[winnie]

OH! Then if you lose the neutral, would it become like a single primary transformer with the secondary being a split phase? I am sure it happens ...

Yup.

You must have 3 primary 'phases' for correct phase displacement.

Each transformer is sitting between 2 of the primary 'phases'.

If you lose the 'center' phase then all you have left is a single phase system.

With the correct transformer arrangement the primare 'center' phase could be the utility multi grounded neutral conductor. This setup is often called "open wye: open delta".

If you have a full three leg delta:delta transformer but you feed only two of the input terminals (single phase primary) then the output will also be single phase.

Jon

 

[TwoBlocked]

.....

If you have a full three leg delta:delta transformer but you feed only two of the input terminals (single phase primary) then the output will also be single phase.

Jon

Thanks! Are you sure about a delta/delta becoming single phase secondary if losing a primary feed? I thought that was one of the advantages.

 

[hillbilly1]

The advantage of an open delta is for the poco. They only have to bring in two phases from the main line, and they save buying and installing one more transformer. Usually for customers with small three phase loads.

 

[jim dungar]

Thanks! Are you sure about a delta/delta becoming single phase secondary if losing a primary feed? I thought that was one of the advantages.

Try not to use the industry slang of calling a single conductor a 'phase'. Voltages are measured between two different points, therefore a phase voltage requires two phase,or line, conductors not simply two 'phases'.

Delta connected transformer performance, during the loss of an input conductor, depends on if the transformer is connected using line-line or line to neutral voltages.

 

[winnie]

If you have a delta:delta transformer, and you lose 1 primary phase, then you lose the necessary phase difference to generate 3 phases.

When the poco runs an open delta with two 'phases', they must also use the neutral. If they only had the two phases and didn't have something serving as the neutral, then they could only have single phase. But at poco voltages, the earth can sometimes be used as that third conductor.

-Jon

 

[Besoeker3]

If you have a delta:delta transformer, and you lose 1 primary phase, then you lose the necessary phase difference to generate 3 phases.

When the poco runs an open delta with two 'phases', they must also use the neutral. If they only had the two phases and didn't have something serving as the neutral, then they could only have single phase. But at poco voltages, the earth can sometimes be used as that third conductor.

-Jon

I have have only one experience of open delta and that was in Taiwan and that was around 50 years ago. It's not something we need or use here in UK as far as I am aware.

 

[TwoBlocked]

Thanks! Are you sure about a delta/delta becoming single phase secondary if losing a primary feed? I thought that was one of the advantages.

What I meant by "delta/delta" was closed delta, 3 transformers. If you lose a primary feed, you still have 3 phase power on the secondary, right?

 

[winnie]

What I meant by "delta/delta" was closed delta, 3 transformers. If you lose a primary feed, you still have 3 phase power on the secondary, right?

I am pretty sure no.

Say you have a delta:delta 480V:480V transformer, with primary A,B,C and secondary X,Y,Z. If you disconnect the supply to terminal B, then the secondary voltages will be 480V X:Z, 240V X:Y and 240V Y:Z. The voltages X:Z, X:Y, and Y:Z will all be in phase.

-Jon

 

[JoeStillman]

Three-Phase, "open delta" transformer banks are only open delta on the secondary. The primary is open wye. Each of the two primaries are connected line-to-neutral.

 

[LarryFine]

OH! Then if you lose the neutral, would it become like a single primary transformer with the secondary being a split phase? I am sure it happens ...

Yup.

You must have 3 primary 'phases' for correct phase displacement.

Each transformer is sitting between 2 of the primary 'phases'.

Actually, every open delta I have seen uses two single-bushing transformers, and is supplied by only two primary lines and the neutral, maintaining the phase angles.

The main unit with the grounded center tap is identical to a typical 120/240v 1ph supply. In fact, the open delta began as a 3ph modification to existing 1ph services.

 

[gar]

230410-1046 EDT

winnie:

480 yes. The other two are undefined until you place loads on their outputs.

For example put a short between X and Y, then Y to Z will be 480.

.

 

[winnie]

Actually, every open delta I have seen uses two single-bushing transformers, and is supplied by only two primary lines and the neutral, maintaining the phase angles.

The main unit with the grounded center tap is identical to a typical 120/240v 1ph supply. In fact, the open delta began as a 3ph modification to existing 1ph services.

Agreed. I put the word phase in quotes because I was using it slightly differently from its common usage, to mean any primary circuit conductor. I then said that the quote phase unquote could be the poco earthed neutral conductor or even possibly the soil ( for some sort of polyphase earth return setup)

-Jon

 

[winnie]

230410-1046 EDT

winnie:

480 yes. The other two are undefined until you place loads on their outputs.

For example put a short between X and Y, then Y to Z will be 480.

.

I see what you are saying. Because the series coils (X to Y and Y to Z) don't share a common core, the seeming centerpoint of this series arrangement is high impedance, and thus will not necessarily divide voltage evenly.

-Jon