Selection of Transformer Primary Configuration

[jcdentonag]

We have a 3-P system that is 21600 VLL or 12740 VLG. We typically use delta-wye transformers, but in this case we are considering a wye-wye due to availability. There is a 12470Y-480Y/277 transformer available that the vendor suggested we use. The idea is that the transformer may be connected to the 21600 system since the line to ground voltage will only be 12470.

Is it possible to use a wye 12470 V primary transformer on a 21600 VLL system? The vendor states that 21600 rated windings are only required if we are using delta.

Are there any references or codes that directly address this directly?

Thanks,
JC

 

[waross]

You should have a solid neutral connection for the wye primary. If your system uses 12740 V line to neutral transformers for single phase loads then you will have a dependable neutral. If single phase loads are served with line to line connected transformers you may not have a primary neutral conductor available.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jcdentonag]

waross,

I'm not completely sure I understand what you are saying here. Is it valid to connect a 12470Y transformer to a 21600 volt system?

 

[davidbeach]

If you're happy with an 831/480V secondary, go ahead.

 

[Marmite]

The transformer rated voltage for a three phase transformer is always stated as line-line voltage, regardless of the internal winding configuration. You can not connect a transformer rated at 12470V to a 21600V source. The vendor is an idiot. The transformer will be severely damaged, as potentially too, will be anyone standing near it.
Regards
Marmite

 

[Mbrooke]

12470Y, if the actual label, is good for a 7,200/12470 solid grounded wye system. The primary must be solidly grounded as well.

Wye-wye is different than delta wye in a few ways:

1. No place for harmonics to circulate as well as transients. Basically you just complete a circuit.

2. A blown cutout can cause inductive tank heating. A 3 legged core is the most concern by far.

3. Zero sequence currents pass through. Any neutral current on the secondary shows up on the primary neutral.

A plus for some is a greatly reduced ferroresonacne risk where single phase switching takes place on circuits that are susceptible to causing it.

 

[Power0020]

what will be the difference between delta and wye connected transformer for ferro-resonance?, I think both will suffer core saturation in case of single phase failure?

 

[jcdentonag]

Thanks for the feedback. I did see a response from an engineer at the company who stated that it won't work.

To guard against ferroresonance we have usually used wye-wye. In power transformers you can put a delta in the secondary for harmonics. Another thing that can be done is the site's individual equipment can be isolated with delta transformers to prevent harmonics from entering the overhead system.

JC