Transformer neutral

[Razzap]

The new addition to our factory is fed from a pad-mounted 3-phase Wye-DELTA transformer. The secondary is 480/277 volts, 3-phase 4-wire. My understanding is that the neutral is connected to the center of the PRIMARY Wye. How can this type of connection work properly? It is contrary to everything that I was taught and my years of experience. Is it Code? Is is even possible that the system will function? Each leg measures 277 to ground and to the neutral. Please explain this.

 

[don_resqcapt19]

You can't have a 480/277 volt 4 wire if the secondary is delta.

 

[Razzap]

If the transformer is wired properly, then you are correct. Please re-read the post. The Neutral is connected to the PRIMARY wye. And I have confirmed the voltages. The lighting is all 277 volt, for example. The panels are all marked 480V 3-Phase, 4-Wire.

 

[GoldDigger]

If the transformer is wired properly, then you are correct. Please re-read the post. The Neutral is connected to the PRIMARY wye. And I have confirmed the voltages. The lighting is all 277 volt, for example. The panels are all marked 480V 3-Phase, 4-Wire.

If there is no neutral point on the secondary in the transformer, then you cannot run line to neutral loads on the secondary side, regardless of where you connect the neutral. And the delta output will be operating ungrounded.
If you connect perfectly balanced line to neutral loads to the secondary, you are creating a virtual neutral point by forcing the three phase lines to assume particular voltages with respect to ground. This is unstable, illegal and dangerous, since you do not have a real neutral as the grounded conductor.
You will have similar problems of unstable voltages on the secondary side to what you would have on a 120/240 split-phase MWBC with an open neutral. But instead of the neutral to ground voltage varying, you will have the line to ground/neutral voltages varying.

If you start with balanced lighting loads on all three line-neutral phases and turn off the breakers for two of the three phase load sets, you will no longer have a neutral point and you will do bad things to the remaining single phase loads. Three phase loads will not notice the difference except for the fact that the line to ground voltages may exceed the expected upper limit.

 

[Smart $]

If the transformer is wired properly, then you are correct. Please re-read the post. The Neutral is connected to the PRIMARY wye. And I have confirmed the voltages. The lighting is all 277 volt, for example. The panels are all marked 480V 3-Phase, 4-Wire.

Where are you getting the "wye-DELTA" info from. If the transformer, exactly what does it say?

 

[Razzap]

I cannot find any nameplate on the POCO transformer. I am getting my information from the Electrical contractor that installed the system, our in-house electrical engineer, and the as-built drawings. The service switchgear nameplate says 480 Volt, 3-phase, 4-wire, but that is very common and does not explain the transformer connections.

 

[delectric123]

IF THE SECONDARY IS DELTA, it is impossible to get 277 volts to ground on each phase. It would have to be a wye connection for that. And the primary windings are not electrically connected with the secondary windings. I don't know how current could flow from a secondary phase back into the primary.

 

[jim dungar]

I am getting my information from the Electrical contractor that installed the system, our in-house electrical engineer, and the as-built drawings.

They are wrong, or here is some mis-communication going on.

You are correct that the nameplate on the switchboard and panelboards have nothing to do with the transformer connections.
However, the laws of physics do.

And yes it is possible, and probable, that the secondary neutral point (X0) of your wye configured transformer is connected to the neutral point (H0) of a wye configured primary.

A delta configured padmount transformer is typically a very special order from a utility. Every time I have tried to get one it has never happened without an error occurring.

 

[GoldDigger]

I don't know how current could flow from a secondary phase back into the primary.

Through the balanced phase to ground loads on the secondary, of course.
Seriously, the current from one phase-to-"neutral" load could only flow back through phase-to-"neutral" loads on the other two phases. All the neutral wire is doing is providing a highly variable ground reference to an otherwise ungrounded delta, and is not carrying any current.
Think of it a three-phase MWBC with an open neutral wire.

 

[templdl]

Just curious. If the secondary is in fact a 480v delta then it would be safe to assume that it's 3ph/3w with no X0. If the primary is a eye then the the H0 is most likely grounded. As such could it be that the installer brought a EGC from the H0 to the secondary assuming that it would act as an X0 and EGC for the secondary and an the assuming that the 3ph3w now has an EGC?
That's the only way I can make any sense of this even that it will not work as the installer intended.
I'm trying to think outside the box a bit.

 

[Hv&Lv]

A delta configured padmount transformer is typically a very special order from a utility. Every time I have tried to get one it has never happened without an error occurring.

Sort of what I was thinking. We don't supply them at all. In 25 years only installed one on another system and its installation was marked on every map around, along with special labeling on the PMT.
The other one was shipped to us mistakenly from Ermco, it was specially built for another company.

 

[kwired]

A delta configured padmount transformer is typically a very special order from a utility. Every time I have tried to get one it has never happened without an error occurring.

Sort of what I was thinking. We don't supply them at all. In 25 years only installed one on another system and its installation was marked on every map around, along with special labeling on the PMT.
The other one was shipped to us mistakenly from Ermco, it was specially built for another company.

There are many delta secondary padmounts around here, usually 240/120 with a high leg, but the POCO's are preferring 208/120 wye transformers because they are not as much of a specialty item. Padmount with 480 volt delta secondary is likely nearly non existent around here though.

Just curious. If the secondary is in fact a 480v delta then it would be safe to assume that it's 3ph/3w with no X0. If the primary is a eye then the the H0 is most likely grounded. As such could it be that the installer brought a EGC from the H0 to the secondary assuming that it would act as an X0 and EGC for the secondary and an the assuming that the 3ph3w now has an EGC?
That's the only way I can make any sense of this even that it will not work as the installer intended.
I'm trying to think outside the box a bit.

Outside the fact that some of these configurations are pretty rare in a single unit padmount, a delta system can have the midpoint of one phase grounded regardless of the voltage. On a 480 volt system that means the high leg operates at about 415 volts to ground, and the other two phases are 240 volts to ground.

To the OP: if you are confused about tying the primary grounded neutral to the secondary grounded neutral (or any other combinations of grounded conductors together) remember those conductors just happen to be grounded and are of the same potential to each other. They are still two separately derived systems that have nothing to do with one another other than one common lead of each is connected together and is grounded in at least one location.

The current related to the high voltage side is/can be passing through anything connected to the high voltage grounded conductor, but (disregarding voltage drop) is still zero volts to ground and is zero volts to the secondary grounded conductor. Same with the secondary grounded conductor, any current flowing on it is seeking the secondary windings and could care less about the primary winding even though there is continuity to the primary winding via the primary grounded conductor. When things fail is when you come up with some undesired current paths, depending on what paths remain intact, but is also why we have some of the rules we do with grounding and bonding.

 

[templdl]

There are many delta secondary padmounts around here, usually 240/120 with a high leg, but the POCO's are preferring 208/120 wye transformers because they are not as much of a specialty item. Padmount with 480 volt delta secondary is likely nearly non existent around here though.

It is not unusual that I have seen the incorrect system voltages stated such case 277/480, 120/208 for 3ph/4w systems as well as 120/240 for a 3ph/4w delta with a lighting tap which is consistent with those who are not accustomed to the correct notation of system voltages. As noted in my post I was specific in stating 480v 3ph3w when referring to the secondary delta.
And the "high leg" is only present when there is a lighting tap between the 'A' and 'C' where the "high" leg is the 'B' phase. But I am intrigued with you reference to a 480v delta with a 'high' leg where it appears that the A-C phase is center tapped and the center tap grounded which I'm to assume that you are getting the 415v to growing from. However I have provided transformers for over 20 year and I have personally never provided a transformer with a secondary as such.
And to Jim's point ,having dealt with those who are inexperienced with specifying transformers I always try verify their specifications. Once someone placed an order with me for a 1ph dry type pad mount for an airport installation where the insisted rt hat the secondary was 240v. I asked the numerous times if they wanted 120/240 1ph3w because I knew that the actual application would need 120v. Nope, just 240v. OK if that's what you want until the installer called me to ask where the neutral was. They ordered a new core and coil.
And considering that there are those out the that do questionable things I refer back to my previous post regarding the transformer that Iam to understand has a wye PRI, and a simple and basic delta 3ph3w secondary. Did the installer brand the H0 connection which was most likely grounded and expect it to be a neutral point for the secondary because there was no secondary neutral and no point or ground. Was his intent for that grounded H0 connection to be the EGC for the secondary. From the way the configuration is being described it wouldn't surprise me as it is not uncommon to find those who don't have the slightest clue about what to do with a delta.-

 

[kwired]

I did start that post with this:

Outside the fact that some of these configurations are pretty rare in a single unit padmount

But I am intrigued with you reference to a 480v delta with a 'high' leg where it appears that the A-C phase is center tapped and the center tap grounded which I'm to assume that you are getting the 415v to growing from. However I have provided transformers for over 20 year and I have personally never provided a transformer with a secondary as such.

I would say that every such system I have seen (and there are a lot of them at field irrigation services around here) was made up of single phase pole top mounted transformers. I have seen padmounts supplying this kind of system but they are all older installs and are also made from multiple single phase units to make a three phase bank. Some are open delta some are a full delta.

 

[delectric123]

I don't know how current could flow from a secondary phase back into the primary.

Through the balanced phase to ground loads on the secondary, of course. Seriously, the current from one phase-to-"neutral" load could only flow back through phase-to-"neutral" loads on the other two phases. All the neutral wire is doing is providing a highly variable ground reference to an otherwise ungrounded delta, and is not carrying any current. Think of it a three-phase MWBC with an open neutral wire.

It is impossible if the secondary is DELTA to get a Ground Reference from the primary to get 277 volts. The OP said the secondary had 277 volts to ground on all three phases

 

[GoldDigger]

It is impossible if the secondary is DELTA to get a Ground Reference from the primary to get 277 volts. The OP said the secondary had 277 volts to ground on all three phases

Wrong. It is not possible to get a stable ground reference on a delta secondary, but you can produce a ground-referenced voltage of 277 on each line if you connect equal impedances from each line to ground. I suspect that is what has happened in the OP's case. (You can also create a more or less stable center point by connecting just the primary windings of a zig-zag transformer.)
You do not have a neutral point on the transformer side, but you do have a virtual neutral point created by a wye-connected load. If the center point is not connected to anything else, the delta will be ungrounded, but will have equal voltages from each line to the center point. If you ground that center point, then the line voltages will become ground referenced by that action even though no load current will be flowing through that ground.
Once the line-to-center loads become unequal for any reason, the center point will still be at ground, and still no current will flow through the ground. But the line to ground voltages will no longer be equal and some or all of the line to center loads will be damaged.
If you increase the load on just one line, the other two 277V loads will see increased voltages, potentially up to 480V.
If you decrease the load on one line, the other 277V loads will see reduced voltages and the load that you decreased will see a voltage above 277V, up to 480V.

 

[delectric123]

Wrong. It is not possible to get a stable ground reference on a delta secondary, but you can produce a ground-referenced voltage of 277 on each line if you connect equal impedances from each line to ground. I suspect that is what has happened in the OP's case.

Its beyond my knowledge why somebody would set up something like that and call it "safe". There must be a misunderstanding somewhere between the OP and whoever gave him the info..

 

[GoldDigger]

Its beyond my knowledge why somebody would set up something like that and call it "safe". There must be a misunderstanding somewhere between the OP and whoever gave him the info..

If the OP's description is correct, it certainly looks like whoever designed/installed this thought that a primary neutral also automatically served as a secondary neutral, and it is pure accident that the system is working at all. Definitely not code and not safe. But the set of measurements he describes are not impossible.

 

[BPoindexter]

I would be willing to bet that someone somewhere got their wires crossed (to make a pun). I am guessing that the WYE-DELTA is actually a DELTA-WYE. If they are feeding 277V Lighting loads there is no way they could get that voltage Phase-Phase on a Delta.

 

[delectric123]

I would be willing to bet that someone somewhere got their wires crossed (to make a pun). I am guessing that the WYE-DELTA is actually a DELTA-WYE. If they are feeding 277V Lighting loads there is no way they could get that voltage Phase-Phase on a Delta.

i agree