Step Up Configuration

[smarteng]

Happy Friday Everybody,

We are need a 225 KVA 208 to 480 VAC step up transformer. I went through multiple treads and understand, best practice is to have Delta on primary. But I am still not sure about secondary. Since My load is a MCC which requires 3 Wire, I was planning to go with 480 Delta secondary. But then I went through Mike Holt Transformer calculation DVD and understand Delta Delta is most rare configuration and should be avoided. What is the problem with Delta-Delta configuration. Is there Code which restrict use of Delta-Delta. I would really appreciate if you could help me here with your expertise.


Thanks you so much.

 

[Besoeker]

Happy Friday Everybody,

We are need a 225 KVA 208 to 480 VAC step up transformer. I went through multiple treads and understand, best practice is to have Delta on primary. But I am still not sure about secondary. Since My load is a MCC which requires 3 Wire, I was planning to go with 480 Delta secondary. But then I went through Mike Holt Transformer calculation DVD and understand Delta Delta is most rare configuration and should be avoided. What is the problem with Delta-Delta configuration. Is there Code which restrict use of Delta-Delta. I would really appreciate if you could help me here with your expertise.


Thanks you so much.

Brit here.
I don't know what the code requirements but I'd want a star (wye) output so that I could earth/ground the star point. That fixes the voltages WRT ground at a lower level than a corner grounded delta arrangement.

 

[Jraef]

Brit here.
I don't know what the code requirements but I'd want a star (wye) output so that I could earth/ground the star point. That fixes the voltages WRT ground at a lower level than a corner grounded delta arrangement.

Yes, even if the MCC is not going to use any neutral loads, use a Wye on the 480V side and ground the Wye point, especially if your MCC has any VFDs in it or feeds external VFDs. A 208-480Y277V transformer will not be a common off-the-shelf item however, you will likely need to have it made for you. Still, there are a few specialty transformer suppliers that will do that fairly quickly.

The 208V side of the transformer can be Wye or Delta, it doesn't matter but if it is Wye, do NOT ground the Wye point on that side.

 

[electrofelon]

Basically all low voltage (<1000) transformers are delta wye. One could use a delta delta if there was a specific need for an ungrounded system. I cant think of a single instance of why one would use a wye primary (other than because "it's what we had lying around").

 

[ggunn]

Yes, even if the MCC is not going to use any neutral loads, use a Wye on the 480V side and ground the Wye point, especially if your MCC has any VFDs in it or feeds external VFDs. A 208-480Y277V transformer will not be a common off-the-shelf item however, you will likely need to have it made for you. Still, there are a few specialty transformer suppliers that will do that fairly quickly.

The 208V side of the transformer can be Wye or Delta, it doesn't matter but if it is Wye, do NOT ground the Wye point on that side.

I'm pretty sure that many sizes of 208V delta to 480/277V wye transformers are readily available off the shelf.

 

[infinity]

Even if you only needed 3Ø, 3W, 480 volts would there be any reason not to use a Wye secondary?

 

[Besoeker]

I'm pretty sure that many sizes of 208V delta to 480/277V wye transformers are readily available off the shelf.

Have be a pretty hefty shelf for a 225 kVA jobby...

 

[smarteng]

Brit here.
I don't know what the code requirements but I'd want a star (wye) output so that I could earth/ground the star point. That fixes the voltages WRT ground at a lower level than a corner grounded delta arrangement.

Thank you Besoker.

 

[smarteng]

Basically all low voltage (<1000) transformers are delta wye. One could use a delta delta if there was a specific need for an ungrounded system. I cant think of a single instance of why one would use a wye primary (other than because "it's what we had lying around").

Hi Electrofelon,

No there is no specific need of Delta as well as Star. Transformer just have one load of MCC which is 3 w.

 

[smarteng]

Even if you only needed 3Ø, 3W, 480 volts would there be any reason not to use a Wye secondary?

No sir. In what Scenario, delta is mandatory or recommended on secondary.

 

[jumper]

Hi Electrofelon,

No there is no specific need of Delta as well as Star. Transformer just have one load of MCC which is 3 w.

He was just saying that Delta -Wye is our most common setup.

 

[electrofelon]

Even if you only needed 3Ø, 3W, 480 volts would there be any reason not to use a Wye secondary?

No sir. In what Scenario, delta is mandatory or recommended on secondary.

Even if you dont need the neutral, Wye's are still almost always used because it is generally viewed as simpler to just make it a grounded system. Grounded systems are strongly preferred here in the states. You could corner ground a delta, but that is considered somewhat of an odd duck and getting panelboards and breakers for it can be a bit more of a hassle.

 

[Jraef]

Basically all low voltage (<1000) transformers are delta wye. ...

Yes, BUT... the Wye is on the wrong side. A "Delta-Wye" 480-208V 3 phase transformer is 480 DELTA to 208Y120V. If back feeding it to step UP from 208 to 480V, the 480V side is going to be Delta. That then REQUIRES that it either be corner grounded, or he has to add a ground monitoring system.

Even if you only needed 3Ø, 3W, 480 volts would there be any reason not to use a Wye secondary?

Yes, a delta power source it is potentially harmful to any power electronics in or fed from the MCC. It tends to turn their line side protection MOVs into little bombs eventually.

The other alternative is to use 3 separate single phase 208-480V transformers and create your own 480Y277V secondary.

 

[electrofelon]

Yes, BUT... the Wye is on the wrong side. A "Delta-Wye" 480-208V 3 phase transformer is 480 DELTA to 208Y120V. If back feeding it to step UP from 208 to 480V, the 480V side is going to be Delta. That then REQUIRES that it either be corner grounded, or he has to add a ground monitoring system.

No dispute there. I don't believe I ever called it out that way.

 

[LarryFine]

My friend the EE says that nobody would purposely want to install a system using a corner-grounded Delta.

He explained using the reasoning for a non-grounded Delta is to avoid equipment shut-down on the first fault, relying on fault detection, and that one would only ground a Delta because an arcing fault on one phase had caused cascading insulation failure on the other phases.

I countered it is, or was, often done, that subsequently grounding a phase would require work more easily done on initial installation, such as grounded-conductor coloring and non-fusing.

 

[beanland]

Avoid Ungrounded Supply

Avoid Ungrounded Supply

The code is full of references to allowing ungrounded systems where reliability is desired but that is archaic. If the system is ungrounded, you must install a ground detector. If you have a phase-ground fault, the remaining unfaulted phases have elevated voltages, risking damage. Further, there is a risk of a phase-ground arcing fault that goes undetected but causes extensive damage. If you have a solid phase-ground fault, finding the fault is a nightmare, divide and conquer is the only approach. A voltage spike on the primary, having no ground reference on the secondary, can be elevated risking damage to 480V equipment.

Another option is to use 208V delta to 480Y/277V wye; these are widely available. You can also use an autotransformer, 208Y/120V to 480Y/277V; they are not good at handling harmonics but are fine for motor (no VFD) and heater loads.