Adv/Disadv DD, DY, YD or YY

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Natfuelbilll said:
Does anybody have a summary of these?
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Seriously, a few more details would help. Such as:

Step-up or step-down?

kVA or MVA size?

one side MV one side LV?
 
Looking for the basics.

I don't have a specific application in mind, but generally my applications are step down.

What is the impact of relative size?

Is there a size wherre the advantages /disadvantages change?

Which are most common?

Why are the others not so common?
 
If utility line is 3 phase what are reasons to specify each of the four types of transformers?

Does one have to "match" the transformer primary to the utility?

Do utitities offer wye-grounded primary service?
Do utitities offer wye primary service?
Do utitities offer delta primary service?

On the secondary side decision. If I need 480/277 I go wye-grounded. If i need just 480V why would I choose to go either wye (and not use the forth wire), wye-ungrounded, or delta?
 
No, you do not have to match the utility. Although I would always follow the utilities requirements.

Other than large distribution systems and utilities almost no one uses Y-Y transformer connections. D-Y or D-D depending on the load requirements are by far the most common arrangements. D-D is becoming rare, giving way to D-Y. A Y secondary provides a very easy method to establish a system reference point (ground) even if the neutral is not used by the load. Another nice thing about D-Y connections is the "noise" isolation it provides between the primary and secondary systems.
 
For reasons i am not clear on, I have been told Comed is now starting to supply only 480Y at the pole, although there seems to be a certain amount of fudging on this practice.

This is a bit of a nuisance since many MCCs do not come with any place to put the neutral wire, since it is rarely used.
 
Delta wye transformers often provide higher secfondary fault currents due to the connection being a separately derived system. A grdwye-grdwye includes the high side zero sequence impedance in the caulculation of ground faults, usually providing a lower fault current.

Jim T
 
petersonra said:
For reasons i am not clear on, I have been told Comed is now starting to supply only 480Y at the pole, although there seems to be a certain amount of fudging on this practice.

This is a bit of a nuisance since many MCCs do not come with any place to put the neutral wire, since it is rarely used.
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They don't need a place to put the neutral, however as Listed service entrance equipment (i.e. SUSE labeled) they must have a place to put the "grounded" reference wire.
 
I always heard there was zero advantage for a wye-wye transformer, ever. I typically never say never, but I've yet to prove that addage wrong.
 
bcorbin said:
I always heard there was zero advantage for a wye-wye transformer, ever. I typically never say never, but I've yet to prove that addage wrong.
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I'll offer two advantages. First lower cost in a wye primary. I specified medium voltage transformers for a medium sized utility, and always used wye in the 25 kv class. We often used wye in the 15 kv class although we could vary. Wye insulations are cheaper than delta.

Grounded wye primaries are not suseptable to ferroresonance. Ungrounded wye and delta primaries are more suseptable, particularly in higher voltages such as 25 kv class.

There are others, these are just two. If you aren't doing utility or at least medium voltage work, these points won't usually make a difference.

Jim T
 
In a building I just inspected, I saw an interesting transformer installation.

The building has a long footprint. So to power up the back end of the building, they installed a delta/delta transformer in the service room. They then installed just the 3 phase conductors to a delta/wye transformer in the back part of the building.

I thought that was pretty interesting.
 
petersonra said:
For reasons i am not clear on, I have been told Comed is now starting to supply only 480Y at the pole, although there seems to be a certain amount of fudging on this practice.

This is a bit of a nuisance since many MCCs do not come with any place to put the neutral wire, since it is rarely used.
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The solution is to simply spec. a MCC with a neutral bus. I don't think it has to have the full ampacity of the phase busses. I guess it also depends on the size of the MCC. You are much more likely to need the neutral on the smaller ones, and it doesn't really cost a lot more. But for a 3000A MCC, you are probably less likely to have a L-N load, and getting a fairly large size neutral buss could add a lot to the price.

I think it's even more of a nuisance to need a neutral wire and not have one in the MCC.
 
A delta to wye transformer suppresses the harmonics in the primary. So all those 120V computer load harmonics don't show up on the utilities power lines.

Also, with a wye secondary, you automatically get a symetrical ground point for 208/120V or 480/277V. Try to ground a delta secondary, and you usually wind up with high legs or corner grounded systems and other somewhat odd systems. (Although the POCO sometimes likes the open delta's since they can basically run a 3 phase service with 2 pole mounted transformers.)

Steve
 
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