Two Single Phase Transformers Connected in a Bank

[GoldDigger]

You can feed a three phase load, with an appropriate adjustment to the transformer capacities.
Yes, you could I suppose calm it an open wye, but it has all four wires.

There is, however, an asymmetry in that one phase wire is fed from two windings in series and so has a higher source impedance and voltage drop than the other two.

 

[ActionDave]

You can feed a three phase load, with an appropriate adjustment to the transformer capacities.
Yes, you could I suppose calm it an open wye, but it has all four wires.

I'm still confused. Does this mythical transformer bank work like a real world wye system, three phase loads and single phase loads, or not.

 

[GoldDigger]

It works with three phase loads and line to neutral loads just like any wye. And in the same way that an open delta works like any other delta.
The open delta has poorer voltage regulation with respect to changes in load on one line to line phase. And what I will call for now the open wye has poorer voltage regulation on two of the line to line phases and one line to neutral phase than a full wye. The unloaded voltages and phase angles are the same as a full wye.

 

[mivey]

I'm still confused. Does this mythical transformer bank work like a real world wye system, three phase loads and single phase loads, or not.

Nothing mythical, and can be verified by simple vector math. I could not find the other diagram I had but drew one and added vector notes (ignore the impedance symbols shown on primary side as that is for other notes):

 

[mivey]

It works with three phase loads and line to neutral loads just like any wye. And in the same way that an open delta works like any other delta.

Exactly

The open delta has poorer voltage regulation with respect to changes in load on one line to line phase. And what I will call for now the open wye has poorer voltage regulation on two of the line to line phases and one line to neutral phase than a full wye. The unloaded voltages and phase angles are the same as a full wye.

Yes.

This is commonly seen where we have a two-phase feeder serving an open-wye/delta or open-wye/wye bank. The customer normally would de-rate 3-phase motors to allow for the expected voltage unbalance. In some cases you would over-size the transformers if there are loads sensitive to voltage unbalance but this might require doubling the transformer size so the customer would pay for the larger transformers or size their equipment to handle the de-rating factor.

In the open-delta/wye case, it would be unusual to have an open bank when a 3-phase primary is available. At the moment I can think of three cases that might apply: very small 3-phase wye loads, limited availability of non-stock transformers, emergency conditions. You would want adequate single-phase protection.

 

[kwired]

As I see it, a major limiting factor for most POCO's ability to supply this would be the fact they don't stock any transformers with 4 secondary terminals. And quite a few only use single busing primary units.

The system still requires all three primary phases - but a open delta with delta secondary can use two phases and the neutral, and standard transformers they already have in stock for use with single phase services.

 

[mivey]

As I see it, a major limiting factor for most POCO's ability to supply this would be the fact they don't stock any transformers with 4 secondary terminals.

Probably true for small units.

And quite a few only use single busing primary units.

I would consider that unusual

The system still requires all three primary phases - but a open delta with delta secondary can use two phases and the neutral, and standard transformers they already have in stock for use with single phase services.

I think you mean open wye - delta but yes, that is why it is the common configuration most see.

 

[kwired]

Probably true for small units.

I would consider that unusual

I think you mean open wye - delta but yes, that is why it is the common configuration most see.

I can not think of any POCO in this region that uses two bushing transformers (primary) for single phase - they always connect phase to neutral.

I know there are places that use them though, and won't say you can't find an occasional two bushing around this area - but would be rare and likely has some age on it.

I did mean open wye - delta.

 

[mivey]

I can not think of any POCO in this region that uses two bushing transformers (primary) for single phase - they always connect phase to neutral.

That is the more common case for single-phase except for places (like California) where they use two-bushing primary to control NEV. However, we are talking about transformers for banking and most POCOs I've seen have some two-bushing pots for banking.

 

[gar]

151115-1556 EST

Around our area there are very many two bushing primary transformers. These are for single phase loads from a delta supply or three phase loads.

At my son's shop the 3 phase open delta secondary is fed from two phases of a primary wye. The transformer primaries are probably 2 bushings, but I will have to check.

A reason for some confusion on this thread is we have two groups with different backgrounds.

Fundamental to understanding how different transformer connections can produce different output results is the fact that the sum of two sine waves of exactly the same frequency produce a sine wave. Coupled with this knowledge and the concept of using vector analysis (later called phasor), concieved about 1900 for AC circuits, provides simplified means of circuit analysis.

I went looking for references to provide background. The following do not directly address what I wanted to find.

M. B. Stout in Analysis of A-C Circuits has a good discussion starting on page 10. Book still appears to be under copyright, and probably not in most libraries, except the University of Michigan Engineering Library.

http://www.math.mcgill.ca/labute/courses/133f03/VectorHistory.html
Possibly not too interesting to most.
https://en.wikipedia.org/wiki/Oliver_Heaviside
Interesting.
https://en.wikipedia.org/wiki/Electricity
https://en.wikipedia.org/wiki/Phasor
http://www.google.com/url?url=http:...sR8I_g&usg=AFQjCNEX_66heL5u50FFZWWE6g_S0yQ8vQ

Dinner time. Do your own analysis for interest.

.

 

[big john]

I can not think of any POCO in this region that uses two bushing transformers (primary) for single phase - they always connect phase to neutral...

I see a lot of two bushing cans up this way. I ain't for certain, but I assume it's because we've got a lot of remote customers so it reduces voltage drop.

 

[kwired]

I see a lot of two bushing cans up this way. I ain't for certain, but I assume it's because we've got a lot of remote customers so it reduces voltage drop.

We have a lot of remote customers as well, but the single phase customers are not the ones putting significant load on the system, it is the irrigation customers that are loading the system - but most of them are three phase services.