Transformer Connections 1

[tmet]

We are looking to add a new power transformer to municipal electric system. Currently, they have two (2) delta-to-delta transformers (with load tap changers), each with it's respective zig-zag grounding transformers to provide the ground on the wye distribution system. Upon talking with some transformer manufacturers, they suggested making the third transformer a ungrounded y-to-y ground connection, which would maintain the 0 degrees phase shift and would be more economical. I believe this would work, but I am cautious and still doing research to make sure this would work. Does anyone have a reason why this third transformer connected ungrounded y-to-y ground would not work with the existing delta-to-delta transformers (with separate grounding zig-zags)?

Thank you for any input to the topic.

 

[mersin33]

If the loads are unbalanced, you may have ground potential problems in a Yyn connection, i.e. the unbalanced load on the secondary does not have any path on the ungrounded Y connected primary.

 

[jghrist]

The ungrd wye-ungrd wye connection should work the same as delta-delta, but I don't think you can use reduced neutral insulation without grounding the neutral, so there may not be any cost savings.

 

[busbar]

A delta/zig-zag configuration will also yield zero phase shift, and may serve as an operational backup to the existing grounding autotransformer(s).

Standalone operation on a contingency basis may preserve lowside zero-sequence characteristics and triplen-harmonic reduction.

Similar to what jghrist has pointed out for the Yy case, ungraded lo-side winding insulation would permit upgrounded operation, but may not be a necessary feature.

 

[jwerthman]

The Yyn connection will phase out OK with the delta-delta connection that you have now, but it will not eliminate the grounding transformer unless the Yyn transformer includes a delta tertiary winding sized to provide an effective ground and to carry the continuous neutral unbalance current.

 

[rbulsara]

Are you going to parallel these transfomer secondaries?

If yes, you are better off keeing the identical arragement for the sake of consistency.

If not, it does not make a difference if you use the identical configuration or not or whether there is a phase shift.

Obvioulsy someone had added, and rightly so, grounding to an originally ungroudned system. If not paralleling, I would rather go with a delta primary and grounded Y secondary transformer, which is a much simpler, safer and reliable system at the same cost as an ungrounded system.