# Xformer theory Q



## chicken steve (Mar 22, 2011)

Hi all, 
I'm trying to understand the pros / cons of this wye/wye (or star/star) system arrangement

you'll have to pardon the rudimental drawing, best i can do (can't figure how to draw a white /grey nuetral to be seen and all

now i haven't found any code violations for either scenario , but that may just mean_ i can't find them_

as to issues, i suppose the top pix could invite OCPD faults on the 480 side into the 208 side

but then the bottom method , done repetitively throughout a steel structure, basically turns the whole building into a nuetral

your commenst please, which would you choose and why?

~S~


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## BBQ (Nov 16, 2010)

chicken steve said:


>


Asparagus


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## Chris Kennedy (Nov 19, 2007)

They are both wrong. Choppin broccoli.


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## BBQ (Nov 16, 2010)

chicken steve said:


> I'm trying to understand the pros / cons of this wye/wye (or star/star) system arrangement


OK, the top arrangement is not NEC compliant and the neutral from the primary has no relationship with the hots on the secondary side so you can't do that.

The bottom set up looks correct and NEC compliant to me


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## Chris Kennedy (Nov 19, 2007)

BBQ said:


> The bottom set up looks correct and NEC compliant to me


The grounded conductor from the secondary panel is connected to the primary and secondary EGC's. If chicken steve meant to connect that to the secondary wye also I would agree.


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## BBQ (Nov 16, 2010)

Chris Kennedy said:


> The grounded conductor from the secondary panel is connected to the primary and secondary EGC's..


Of course it is, it has to be.



> If chicken steve meant to connect that to the secondary wye also I would agree


I believe that is what the XO indicates even though it is not drawn perfectly.





Platypus.


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## Big John (May 23, 2010)

My understanding is that a a solidly grounded wye-wye setup is inherently unstable and it's in your interest to go through great lengths to avoid that connection. You get a lot of problems with triplen harmonics and fault current from your secondary will be seen on your primary so it makes coordination difficult.

We have put them in as a floating wye secondary, but for reasons I don't remember it's a good idea to have a fairly balanced load in those circumstances, has something to do with floating the voltage of the neutral point.

-John


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## Chris Kennedy (Nov 19, 2007)

BBQ said:


> I believe that is what the XO indicates even though it is not drawn perfectly.


Yes, I see he indicates bonding jumper which by definition would indicate its connected to the secondary XO. 







> Platypus.


 Thats a great idea but I already pulled out chicken breasts with skin and ribs to grill. (no offense intended chicken steve)


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## BBQ (Nov 16, 2010)

Big John said:


> We have put them in as a floating wye secondary, but for reasons I don't remember it's a good idea to have a fairly balanced load in those circumstances, has something to do with floating the voltage of the neutral point.


If you float the XO connection you can't use it for a load and would have to consider it to be a delta output.


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## chicken steve (Mar 22, 2011)

BBQ said:


> OK, the top arrangement is not NEC compliant and the neutral from the primary has no relationship with the hots on the secondary side so you can't do that.
> 
> The bottom set up looks correct and NEC compliant to me


ok, 450? 250? i'm missing it BBQ


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## chicken steve (Mar 22, 2011)

Chris Kennedy said:


> Yes, I see he indicates bonding jumper which by definition would indicate its connected to the secondary XO.
> 
> 
> > yeah Chris, main bonding jumper in the Xformer to XO, grounding electrodes, building steel, h20 pipe, basicaly an SDS
> ...


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## chicken steve (Mar 22, 2011)

BBQ said:


> If you float the XO connection you can't use it for a load and would have to consider it to be a delta output.


now i'm confused....


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## chicken steve (Mar 22, 2011)

Big John said:


> My understanding is that a a solidly grounded wye-wye setup is inherently unstable and it's in your interest to go through great lengths to avoid that connection. You get a lot of problems with triplen harmonics and fault current from your secondary will be seen on your primary so it makes coordination difficult.
> 
> We have put them in as a floating wye secondary, but for reasons I don't remember it's a good idea to have a fairly balanced load in those circumstances, has something to do with floating the voltage of the neutral point.
> 
> -John


_Floating _the voltage of the neutral point?

sounds like lack of a solid one to me, i mean, if you've got one connected to the service, wouldn't that solve the _float _part?


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## Chris Kennedy (Nov 19, 2007)

chicken steve said:


> yeah Chris, main bonding jumper in the Xformer to XO, grounding electrodes, building steel, h20 pipe, basicaly an SDS


If indeed a SDS then by definition that would be a system bonding jumper. 




> i was just under the impression that _not all_ xformers were required to be an SDS


Auto transformers (buck-boost) would not be considered a SDS because there is a direct electrical connection between primary and secondary.........


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## erics37 (May 7, 2009)

Is there ever really any reason to wire a 3-phase transformer primary other than Delta?


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## Meadow (Jan 14, 2011)

erics37 said:


> Is there ever really any reason to wire a 3-phase transformer primary other than Delta?


Outside of commercial and industrial, pocos use primary wye grounded as much as possible:laughing::no: The theory is interesting and has been debated profusly since the invention of 3 phase power.


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## RIVETER (Sep 26, 2009)

Chris Kennedy said:


> They are both wrong. Choppin broccoli.


Choppin...I like his music.


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## Meadow (Jan 14, 2011)

Here are a few advantages of primary wye grounded (at least for pocos) (there are more): As core is wound the insulation on the primary can be reduced as you head tward the star point (V potential get closer to earth) 
Only a single bushing is required if the neutral is earthed repeatedly. 
Easier to fuse, fuses only see current on 1 winding instead of 2. 
If the secondary is delta, and the single units are used and if 1 phase goes 3 phase power can still be provided, at reduced capacity of the bank (57.7 I think)
There is no risk of Ferroresonance in the transformer as primary GrY.
Easier to conect, lowest cost of any other conection.


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## RIVETER (Sep 26, 2009)

meadow said:


> Here are a few advantages of primary wye grounded (at least for pocos) (there are more): As core is wound the insulation on the primary can be reduced as you head tward the star point (V potential get closer to earth)
> Only a single bushing is required if the neutral is earthed repeatedly.
> Easier to fuse, fuses only see current on 1 winding instead of 2.
> If the secondary is delta, and the single units are used and if 1 phase goes 3 phase power can still be provided, at reduced capacity of the bank (57.7 I think)
> ...


Maybe you are a lineman...I am not. Explain why the insulation can be reduced as you get closer to the ground potential. It will be the same current that is needed for the service...at least, I think. And, in the event of a aberrant voltage, why would the POCO risk a transformer failure because they were too cheap to plan for the worst? Just asking.


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## Meadow (Jan 14, 2011)

RIVETER said:


> Maybe you are a lineman...I am not. Explain why the insulation can be reduced as you get closer to the ground potential. It will be the same current that is needed for the service...at least, I think. And, in the event of a aberrant voltage, why would the POCO risk a transformer failure because they were too cheap to plan for the worst? Just asking.



It has to do with transformer theory. When the Primary of a transformer is wound and the connection will be a permanent wye, as you get closer to the coil lead that will be attached to the tank you can reduce the insulation due to lower potiential (ie being closer to earth and the grounded core) Think auto transformers, as you tap closer to the winding connected to the neutral lead the voltage goes down (see pic) The same theory applies to the primary and secondary of isolated transformers. The current remains the same through out the winding. As for transformer overvoltages generaly not a concern, there are suppressors on the primary bushings.

On a side note pocos try to be as economical as possible and sacrifice other things with it (ie pole mounted transformers theoretically can last over a hundred years but are sized to run at thermal over loads that limit there life to about 25-30 years. (The savings are in a thinner pole easier installation and less money spent on the unit)


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## Big John (May 23, 2010)

meadow said:


> ...When the Primary of a transformer is wound and the connection will be a permanent wye, as you get closer to the coil lead that will be attached to the tank you can reduce the insulation due to lower potential....


 You can, but are they actually wound that way? Like I said, I know we've got a bunch of floating wyes, seems like a heck of a gamble on the manufacturers part to assume that neutral point won't ever see system voltage.

-John


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## chicken steve (Mar 22, 2011)

Gentlemen,

obviously my Xformer theory lacks, is there something i should be reading?

any advice appreciated, and thanx in advance

~CS~


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## Big John (May 23, 2010)

These two sources can both be saved as PDFs. Both of them focus on pole-mount distribution transformers but much of the theory and practice can be directly applied to LV transformers:

This one is a great list detailing a huge variety of possible transformer connections.

And this one is a great basic transformer tutorial from McGraw Hill.

-John


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## chicken steve (Mar 22, 2011)

Saved & Thanx Big John

from the first one pdf>









YY Closed / Neutral = Prim yes- Sec-yes
　
　
The Primary Nuetral should be tied firmly to the system nuetral, otherwise exessive voltage may develop on the secondary side

If 3ph load is unbalanced, part of the load current flows in the primary neutral

The 3rd harmonic componet of the Xformer exciting current also flows in the primary neutral

For these reasons, it is necessary that the netrals be tied together as shown

If this tie were omitted, the line to neutral voltages on the secondary would be very unstable

That is, if the load on one phase were heavier than on the other two, phases would rise

Also, large 3rd harmonic voltages would appear bettween lines and neutral, both in the xfomer, and in the secondary system, in addition to the 60HZ component of voltage. 

This means that for a given value of RMS voltage, the peak voltage would be much higher than that for a pure 60Hz voltage

This overstreeses the insulation both in the Xformer and in all apparatus connected to the secondaries


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