Can anyone please explain this transformer configuration????

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jay_the_gemini

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Hi, I have been looking at some electrical drawings for a structure, and I see something quite puzzling.

If someone can please explain all the connections in a detailed fashion, that would be very appreciated.

The service coming from the Utility 480/277V WYE, and the end goal is to step it down to 208/120V WYE.

However, upon looking at one of the details, I see they are using a delta-wye transformer. Can someone expain how on earth this works?

Where does the Neutral from the 480/277V side go? Is there a Neutral Bar inside the transformer housing as the diagram seems to indicate?

xfmer.jpg

Thank you!
 
Welcome to the forum.

There is no neutral on the primary side of the transformer, there is no reason for one since there is no electrical connection between the primary and secondary windings. You derive a whole new system on the transformer secondary, NEC calls it a separately derived system or SDS for short. 250.30 tells you what to do with them.

You don't need a neutral on the secondary. You could could use a delta - delta transformer and ground the corner. Usually we like wye secondaries because we like to use 120V for some things and we like the lower phase to neutral voltage. There is also a high leg delta but and auto transformers, but those aren't important at this moment.
 
Thanks for the reply. I still have a bunch of lingering questions. Namely, to begin with, where does the Neutral from the 480Y side go? I mean, where does it physically attach? Do you just leave it dangling? Only kidding, but I see that L1, L2, and L3 all connect respectively to H1, H2, and H3. That still leaves the N unaccounted for connection wise...?

Thank you.
 
jay_the_gemini said:
Thanks for the reply. I still have a bunch of lingering questions. Namely, to begin with, where does the Neutral from the 480Y side go? I mean, where does it physically attach? Do you just leave it dangling? Only kidding, but I see that L1, L2, and L3 all connect respectively to H1, H2, and H3. That still leaves the N unaccounted for connection wise...?

Thank you.
Click to expand...

You don't use the 480V neutral. You don't need it. You don't bring it to the transformer. If you were hooking up a three phase motor it would be the same thing, three hots and an equipment ground. Neutral not invited to this party.
 
Got it. I just spent the last few hours reading about separately derived systems, and it really answered all my questions, then your answer to the neutral simply not being brought out to the transformer sealed the deal on understanding the concept. What an elegant concept, whoever developed the idea of creating a new Neutral to Ground bond point via the transformer secondary is very crafty. It is very satisfying to tackle these concepts and get a hold of them.
 
While you are researching the delta-wye configuration, there is a whole other discussion to not forget -- in a delta-wye configuration, since the triplet harmonics are all in phase they are short circuited in the delta primary. The triplet harmonics current on the secondary serve to turn the white wire brown :eek: . This is goodness in that it reduces system harmonics on the supply side, bad for your transformer as there is a lot of heating from the shorted harmonics.
 
Don't take this the wrong way, but this is probably the most popular three phase transformer configuration out there, delta primary to wye secondary.

Most popular for single phase is in many ways similar to what you are asking - two wire primary to multiwire secondary such as 480 x 120/240. Same thing there- on the primary no neutral is needed, all you have is a coil that needs 480 volts across it to do what it was designed to do.
 
180306-1023 EST

By 1878 Edison had evolved his understanding of how to build an electrical distribution system, that for lighting a high resistance lamp would be required, and that connection in parallel was required. Further he understood that a high efficiency dynamo (generator) was necessary, and that maximum power transfer (source resistance = load resistance) was not the correct design criteria.

I don't have a reference, but something about 100 V was probably about the highest voltage a practical light could be made at that time. Obviously from Edison's experiments. This was a high enough voltage that a somewhat practical (meaning economic) system could be built. Note that the concept was a parallel distribution system, whereas other experts in the electrical field were only considering series circuits.

Amount and cost of copper was a consideration. By the time of the Pearl Street station, or somewhat after, it was understood that three wires could be used for distribution and reduce copper use. Thus, the invention of the neutral wire. Was it called that at the time? I don't know.

Neutral from dictionary definitions does not provide any help that I found.

From an extension of the meaning of neutral in general it can be looked at as somewhat of a mid point.

For a broad understanding of the evolution of the electric power system and incandescent bulb see "Menlo Park Reminiscences", by Francis Jehl. I can remember Jehl telling us about things at Menlo Park, but I was too young at the time to have asked any questions that today I would want to ask.

See https://en.wikipedia.org/wiki/Pearl_Street_Station . This was started with one generator. So at that point there was no neutral.

At the moment I can not find a reference on Edison and the word neutral that relates back to its origin.


An excellent discussion relating to neutral follows, for the most part I would agree with what is said. But I think it lacks some important history.
http://apc.starin.biz/?file=White_Paper_-_Neutral_Wire_Facts_and_Mythology.pdf

The following is also good.
https://en.wikipedia.org/wiki/Ground_and_neutral

There is some useful perspective in the following.
https://en.wikipedia.org/wiki/Electrical_wiring_in_North_America

.
 
gar said:
180306-1023 EST

By 1878 Edison had evolved his understanding of how to build an electrical distribution system, that for lighting a high resistance lamp would be required, and that connection in parallel was required. Further he understood that a high efficiency dynamo (generator) was necessary, and that maximum power transfer (source resistance = load resistance) was not the correct design criteria.

I don't have a reference, but something about 100 V was probably about the highest voltage a practical light could be made at that time. Obviously from Edison's experiments. This was a high enough voltage that a somewhat practical (meaning economic) system could be built. Note that the concept was a parallel distribution system, whereas other experts in the electrical field were only considering series circuits.

Amount and cost of copper was a consideration. By the time of the Pearl Street station, or somewhat after, it was understood that three wires could be used for distribution and reduce copper use. Thus, the invention of the neutral wire. Was it called that at the time? I don't know.

Neutral from dictionary definitions does not provide any help that I found.

From an extension of the meaning of neutral in general it can be looked at as somewhat of a mid point.

For a broad understanding of the evolution of the electric power system and incandescent bulb see "Menlo Park Reminiscences", by Francis Jehl. I can remember Jehl telling us about things at Menlo Park, but I was too young at the time to have asked any questions that today I would want to ask.

See https://en.wikipedia.org/wiki/Pearl_Street_Station . This was started with one generator. So at that point there was no neutral.

At the moment I can not find a reference on Edison and the word neutral that relates back to its origin.


An excellent discussion relating to neutral follows, for the most part I would agree with what is said. But I think it lacks some important history.
http://apc.starin.biz/?file=White_Paper_-_Neutral_Wire_Facts_and_Mythology.pdf

The following is also good.
https://en.wikipedia.org/wiki/Ground_and_neutral

There is some useful perspective in the following.
https://en.wikipedia.org/wiki/Electrical_wiring_in_North_America

.
Click to expand...
The neutral is not somewhat of a midpoint. It is precisely tapped on the center of the coil. That is why it is called neutral

Sent from my A574BL using Tapatalk
 
Or in a wye distribution, the neutral is the ends of all three coils tied together at one end. The other ends form the three phases. This gives you a voltage of the phase voltage divided by the sqrt of 3.

Example:
480/√3=277

Sent from my A574BL using Tapatalk
 
180306-1336 EST

DrSparks:

The word neutral in English derives from around 1500. The various dictionary definitions don't particularly address its usage in electrical distribution systems.

The word neutral is used in various ways electrically. Some usage is probably very incorrect, and other uses may be confusing.

Most will probably agree that a neutral wire is one that is earthed, and may carry load current. But, most will probably also required neutral be something connected to a point where there is a type of balanced load such that neutral carries less current than the sum of all load currents.

I don't consider earthing to be an absolute requirement. I could take a tapped transformer and ground one outer end of the transformer, and that one end would not likely be a neutral. The tap point would probably be the neutral. Or if the transformer is not grounded anywhere, then a secondary endpoint may or may not be a neutral. Grounding location does not define whether or not a wire is a neutral

On the other hand:

1. Take one transformer with two separate secondaries, and connect the two secondaries in series. With one phasing that common point can be a neutral. Or.

2. Take two separate transformers, and do as in (1). The result could be the same.

I would not want to say that a center tapped secondary is always a neutral. Consider a two or three transformer three phase wild leg delta with one secondary center tapped and that secondary used for single phase loads. For the loads on that secondary the center tap is likely a neutral, and also likely grounded. But that center tap is not a neutral for three phase loads.


jay_the_gemini:

A three phase source does not necessarily mean there is a neutral or grounding anywhere.

A single phase generator or other source does not have to be tied to ground (earth). In most cases it is. The electrical system in your car is not grounded. The car sits on four insulators (tires) with no low impedance electrical connection to earth,

.
 
I prefer the sense of the original definition in the NEC before they adapted it to allow the center tap of the low winding of a high leg delta to be called a neutral:
The neutral wire is a circuit conductor that is at the same voltage with respect to all of the other circuit conductors. (This only applies when there are at least three circuit conductors, not counting parallel wires.)
Graphically, on a phasor diagram, the neutral is the conductor which is at the geometric center of the phasor pattern.

Even this definition leaves some room for discussion about whether the usually grounded conductor of a two out of three phase circuit is still a neutral since one of the phases it should be at equal voltage from is missing. My inclination is to still call it a neutral, as I would the common conductor of two single secondary windings in a true three phase system. One then has to be more careful in defining when a neutral should or should not be counted as a current carrying conductor for ampacity calculations.
 
GoldDigger said:
I prefer the sense of the original definition in the NEC before they adapted it to allow the center tap of the low winding of a high leg delta to be called a neutral:
Click to expand...
It makes sense to me, since most of the usage of a high leg delta is typically 120 - 0 - 120 and as far as most implementation is concerned, the B phase doesn't exist. At the wall outlet there is no difference between single phase and 240 high leg delta; why should the grounded conductor be called a neutral for one and not the other?
 
ggunn said:
It makes sense to me, since most of the usage of a high leg delta is typically 120 - 0 - 120 and as far as most implementation is concerned, the B phase doesn't exist. At the wall outlet there is no difference between single phase and 240 high leg delta; why should the grounded conductor be called a neutral for one and not the other?
Click to expand...

Just that you cannot create a four wire MWBC for three phase using the "neutral" as one conductor. Other than that, you are correct that for the purposes of 120/240 only, you have an effective neutral even though you do not when using the B phase.
 
IMO it is neutral only to the 120/240 side of the delta. It is not neutral to the entire supply though.

Before they did change NEC definition - we certainly did have delta banks that served multiple customers - some only received single phase service. Nobody thought twice at calling the grounded conductor neutral on those services.
 
180306-2429 EST

kwired:

I don't understand your last comment. You know what you said, but I don't. What follows you know, but I am including it for others.

For a plain full delta or an open delta with only three wires there is no neutral. Theoretically for calculation purposes I can assign a point in vector space as a neutral. Back at the substation said delta may be produced by a wye source. If this is done, then each of the hot lines will have a well defined relationship to earth, assuming the wye source was grounded.

Many industrial plants are supplied with totally floating deltas, and only the magnitude of leakage paths defines the hot lines relationship to earth.

If a one secondary on a delta has a center tap on its winding and that center tap is grounded, then no other secondary from that transformer(s) can have its center tap grounded. But you could have other center tapped secondaries floating and provide split-phase loads.

.
 
gar said:
180306-2429 EST

kwired:

I don't understand your last comment. You know what you said, but I don't. What follows you know, but I am including it for others.

For a plain full delta or an open delta with only three wires there is no neutral. Theoretically for calculation purposes I can assign a point in vector space as a neutral. Back at the substation said delta may be produced by a wye source. If this is done, then each of the hot lines will have a well defined relationship to earth, assuming the wye source was grounded.

Many industrial plants are supplied with totally floating deltas, and only the magnitude of leakage paths defines the hot lines relationship to earth.

If a one secondary on a delta has a center tap on its winding and that center tap is grounded, then no other secondary from that transformer(s) can have its center tap grounded. But you could have other center tapped secondaries floating and provide split-phase loads.

.
Click to expand...
All I said was for a high leg delta system the grounded conductor is only neutral to that one side of the delta, so for 120/240 4 wire delta the neutral is only neutral to the two 120 volt corners. And that was how I see it, others may not see it that way.

I did further point out that if the utility had a high leg bank in the middle of the block and supplied multiple customers from it - some of those customers may only have 120/240 single phase supply run to them. In those cases nobody ever questioned whether they should call the grounded conductor on those single phase services "neutral".
 
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