wye-delta transformer in MV distribution 1

[jgillbham]

I'm in the process of design of an upgrade of an existing 4160v 3 ph distribution system to a 12470 v 3ph system in order to take advantage of primary service rates. we have selected a 12470v:4160v (wye:delta) transformer to feed a part of the existing 4160v system that is in place for awhile before we can upgrade that part of the campus. Our assumption was that the existing transformers downstream were 4160v(delta):120/208Y, but recent investigation reveals that they are Y:Y. Can I still serve them from the delta secondary of the upstream transformer? should I make the primary Y ungrounded at either of the transformers?

 

[rbulsara]

1. Technically you can feed Y:Y from a delta transformer, for that matter any combination of delta/wye can be fed from each other. As long as you are not paralleling units, phase shifts etc does not matter.

2. I am more curious as to why you selected Y:delta 12.4kV/4.16kV transformer. It is more common and advantageous to have delta/wye unit (secondary wye) so that you have a grounded system at the source. Perhaps you were thinking feed downstream delta:wye units from a delta secondary, which by itself is not a good idea or misplaced thinking. How did you plan to ground the derived 4160V system?

Please reconsider our plans.

 

[alehman]

What rbulsara said...

 

[busbar]

The 12470Y/4160? transformer configuration is susceptible to problems with primary-side circulating currents if the neutral [HO] terminal is grounded. For 12470Y or 4160Y primaries, they may possibly be operated ungrounded if so designed. ANSI/IEEE standards [C57.12.70] allow for either, whether via internal tank strap or a fully insulated neutral terminal. Another discussion of the tradeoffs in MV distribution systems for neutral grounding is IEEE standard C62.92.4. It’s 50 years old, but Donald Beeman’s Industrial Power Systems Handbook has useful info on MV systems for campus/plant environments.

I sounds like it is going to be a non-mainstream 4160V system. The description is reversed from most systems—in that it is far more common to ground the ‘source’ 4160V (wye) neutral terminal. With grounded-wye-primary 4160/208V transformers, there is likelihood of 4160V overcurrent protection operation in an less-predictable manner during ground faults, with even more variability if one or more transformers are out of service.

 

[jgillbham]

we thought that the existing downstream transformers were delta:wye...everything I've read discourages the use of wye:wye transformers in distribution systems because of voltage instability and harmonics. Now that we know we've inherited the wye:wye stuff downstream, how about grounding the secondary delta of our 12470:4160 xfmer w/ a zig-zag grounding transformer to provide a grounding point for the downstream neutrals?

 

[rbulsara]

Sounds like you have already purchased the 12470:4160 Y/D transformer. If that is the case grounding its seconday with a zig-zag xfmr will be perfectly acceptable, in my opinion.

You only need to ground the neutral at the main transformer secondary, you need NOT to 'carry' it to each subdistribution Y:Y transformers.

 

[jghrist]

GrdY-GrdY is the most common connection for 3Ø distribution transformers in the US.

I don't understand why you would use a Y-&[ignore]Delta;[/ignore] 12470-4160 transformer with a zig-zag grounding transformer instead of using a &[ignore]Delta;[/ignore]-GrdY 12470-4160 transformer, considering the potential problems listed by others with the Y-&[ignore]Delta;[/ignore] connection.

 

[PWR]

If you already have the transformer with the 4160V delta secondary, the recommendation to go with a grounding transformer is probably the best bet. In the original question it was suggested that it might work to unground the primary wye on one or more of the transformers. You definitely don't want to do that. Independent of whether your source is grounded or ungrounded, a wye wye transformer with an ungrounded primary winding will have unstable secondary line to neutral voltages. Been there, done that.

 

[rbulsara]

I agree with PWR's last post. You want to tie the primary and secondary neutrals together of a Y:Y transformer and ground the secondary N. Normally the primary neutral is internally bonded to the secondary N and the casing.

More importantly, if you have operated them without any problems so far leave them the way they are as you are not effectively changing them by revising the service upstream .

 

[jgillbham]

Thanks for your input. When the system was energized, some secondary voltage fluctuation was observed. I'm thinking that maybe the existing downstream Y:Y transformer(s) might not have been grounded the way you describe and in some fashion the primary ground was interrupted during the new work.

 

[jbartos]

Comment on the previous posting marked ///\\When the system was energized, some secondary voltage fluctuation was observed.
///How has been the system implemented? By using Y-? 12470-4160 transformer? Is there any system grounding on the 4160V side?\\ I'm thinking that maybe the existing downstream Y:Y transformer(s) might not have been grounded the way you describe and in some fashion the primary ground was interrupted during the new work.
///By breaking the system grounding on the primary side of 4160v(Y):120/208Y transformer, the zero sequence current path is open. This should enable the 208Y/120V be without zero sequence current component in the neutral from the high voltage side. Also, any lightning strike might elevate the secondary neutral potential to high levels if the primary and secondary neutrals are tied and grounded. Are there voltage regulators being used?\\