Parralleling Transformers

[H1H2N3F4]

I am reviewing a project where the design contractor is intending to parallel a 30MVA , delta-Y transformer with a 30 MVA Y-delta transformer.
I think that this can't be done because of the difference in the angular displacement between the two transformer windings.
He insisted that this can be done and is taking it too high.
I appreciate any suggestions.
. MMQ

 

[jwerthman]

The transformer phase shifts may be the same. Unless specified otherwise, the ANSI standard for delta-wye or wye-delta transformer is for the secondary winding to lag the primary winding by 30 degrees, regardless of which is delta and wye. If this is true for both transformers, then they could be paralleled with no phase angle problems.

However, I would be worried about the fact that there are obviously differences in the grounding between the two transformers. Loss of one transformer could result in ungrounding part of the system, unless this is taken care of by some other means.

 

[busbar]

The proposed may look good on paper, but needs to be proven by careful phasing checks on initial energization. There will be an inherent high-to-low-side 30° phase shift within each bank, but are both in the same direction? If not the same, change would require access to individual transformer coils, which is not likely. Added to the different grounding aspects, unbalance and fault currents, and the associated protective scheme will almost certainly change with either or both transformers in service.

I would insist on a detailed commitment from the contractor—with resolution spelled out in detail if the proposed system does not work to the end user’s contractually defined functional and safety specifications.

 

[codling]

You need to verify with the contractor that the turns ratio, phase shift and impedances will be properly matched. Then run the calculations based on the numbers provided to see what the circulating current would be.