Voltage for Single Phase Transformer

[Smart $]

Thank you for your reply!
product name is AUD (Autodesk Utility Design)
The standard we are using is "UNITED STATES DEPARTMENT OF AGRICULTURE---Rural Utilities Service", so maybe 24940V is used for urban and 12470 is used for rural, is it right?

USDA RUS is based on cooperatives, which are mostly "rural" community uitilities..

There are lots of info on their website, but the spec's pertinent to your issue are not...

Bulletin 50-37 RUS Specifications for Distribution Transformers (Overhead Type)
Bulletin 50-73 RUS Specifications for Pad- Transformers (Single and Three-Phase)

Found in Index of Electric Issuances, which has an order form on last page if you or anyone else is interested.

USDA RUS Electric Program Homepage

 

[Smart $]

Thank you for you reply!
But I'm still confused that how could a single phase system use a Y structure.....:ashamed1:

IIRC, mivey and Hv&Lv are utility guys. All they're saying is the transformer's primary is designed to be connected to a 12470Y/7200V distribution system. It don't actually have a wye connection.

 

[mivey]

so maybe 24940V is used for urban and 12470 is used for rural, is it right?

Not really, although you usually see 25 kV in real dense areas like small metro. It can be a matter of distance or load density.

Some old school employees fear the higher 25 kV voltage and lobby to keep 12 kV even in dense areas (not too different than what happend with the 4kV vs 12 kV change). In addition to the lower costs for certain load/system profiles, you also get better protection with 25 kV.

 

[mivey]

IIRC, mivey and Hv&Lv are utility guys. All they're saying is the transformer's primary is designed to be connected to a 12470Y/7200V distribution system. It don't actually have a wye connection.

Correct. That is just terminology as I have heard it called delta or wye connections, even with single-phase transformers. It is like that in some manufacturer transformer guides. Probably not the best wording but everyone knows what you mean. It has been that way for a long, long time.

 

[steve66]

Thank you for your reply!
Yes, We also has 24940Y/14400v voltage setting.
The standard we are using is "UNITED STATES DEPARTMENT OF AGRICULTURE---Rural Utilities Service", so maybe 24940V is used for urban and 12470 is used for rural, is it right?

No, the various utilites use any of the standard voltages (or even non-standard voltages) anywhere they want.

I've seen 12KV ran right under higer voltage systems.

 

[weressl]

OK. I looked and it agrees with what I said.

Add:
In particular, see Table 1 of http://www.cooperindustries.com/con...urces/library/201_1phTransformers/G201201.pdf

Exactly: "Dual voltage primaries shall utilize an externally operable switch. Unit must be de-energized prior to changing voltages."..and that supports what I stated, no?

 

[mivey]

Exactly: "Dual voltage primaries shall utilize an externally operable switch. Unit must be de-energized prior to changing voltages."..and that supports what I stated, no?

No. The "12470Y/7200" is the L-L/L-N voltage. The primary winding is for the L-N 7200V.

The dual voltage primary transformer would be something like "24940Y/14400 x 12470Y/7200". With a series connection you have a primary winding for 14400V and with parallel primary windings you can use 7200V. Both configurations use the L-N voltage, not the L-L voltage.

 

[Hv&Lv]

Thank you for your reply!
Yes, We also has 24940Y/14400v voltage setting.
The standard we are using is "UNITED STATES DEPARTMENT OF AGRICULTURE---Rural Utilities Service", so maybe 24940V is used for urban and 12470 is used for rural, is it right?

Like others have stated, it all depends on who and where. We are quickly moving to all 24940Y/14400 (25kV). We are in the process, and have been for some years, of converting all our 12470 to 24940.
There are some places that are actually converting from 25kV back to 12kV, hard for me to understand why...

 

[weressl]

No. The "12470Y/7200" is the L-L/L-N voltage. The primary winding is for the L-N 7200V.

The dual voltage primary transformer would be something like "24940Y/14400 x 12470Y/7200". With a series connection you have a primary winding for 14400V and with parallel primary windings you can use 7200V. Both configurations use the L-N voltage, not the L-L voltage.

OK, that makes sense. Do you have an explanation why both L/N and L/L voltages are listed when the transformer is clearly inteded to be connected to the L/N in other word to a single voltage? Isn't it clear that one of the connection - the one w/o the HV bushing - is the 'grounded' conductor? Or is it to assure and clarify that it is NOT to be connected to an ungrounded system?

 

[Hv&Lv]

OK, that makes sense. Do you have an explanation why both L/N and L/L voltages are listed when the transformer is clearly inteded to be connected to the L/N in other word to a single voltage? Isn't it clear that one of the connection - the one w/o the HV bushing - is the 'grounded' conductor? Or is it to assure and clarify that it is NOT to be connected to an ungrounded system?

What if you had a 7200 volt delta system? What would be on the XF nameplate then?
We do have double bushing XF's. We ground one side now. We could use a 14400 XF and connect one 7200 to one bushing, then connect another 7200 to the other bushing. Here the data plate would list 14400∆

 

[mivey]

OK, that makes sense. Do you have an explanation why both L/N and L/L voltages are listed when the transformer is clearly inteded to be connected to the L/N in other word to a single voltage? Isn't it clear that one of the connection - the one w/o the HV bushing - is the 'grounded' conductor? Or is it to assure and clarify that it is NOT to be connected to an ungrounded system?

It shows that it can be connected with 7200V_L-L from a delta system or 7200V_L-N from a wye system voltage across the winding. The standard would be to show this label as "7200/12470Y" (my transformer guide references ANSI C57.12.00).

Not sure why a straight "7200" label would not be sufficient unless the grounded system adds voltage stress or something. My guide just shows "7200" for a connection to a delta system and does not mention the wye system. It was not in my guide, but I remember reading about voltage stresses between the windings and the tank/neutral or something but I don't recall the exact details and would have to look it up to get it straight. If you are interested, I've got a reference in my office somewhere and could look for it.

If the poster has the correct voltage order, I suspect the poster left off the "Grd" part of the label which would indicate that it has reduced insulation at the neutral end and must be a grounded neutral. If the "Grd" voltage is on the left, it means the neutral end may be connected to the tank and would be labeled as "12470GrdY/7200".

Without the neutral connection to the tank allowed, it would be on the right as "7200/12470GrdY" but still indicating that the neutral must be grounded because of reduced insulation.