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Transformer Connections

Transformer Connections

Transformer Connections

(OP)
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.

RE: Transformer Connections

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.

RE: Transformer Connections

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.

RE: Transformer Connections


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.
  

RE: Transformer Connections

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.

RE: Transformer Connections

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.

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