I have an induction furnace that is fed by two padmount transformers in parallel - one having a wye secondary and the other delta.

The transformers were new in 2020 and the delta one has come back with a bad oil sample. I am struggling to determine why. I have been told by the transformer manufacture that in this configuration and feeding a 12 pulse rectifier, harmonic current would circulate in the delta windings.

Can someone explain this in somewhat laymans terms? I am an electrician, so I have some knowledge of transformer theory but am not very knowledgeable about the more complex theory and math that you get into with transformers.

If I was to put a power analyzer on these, should I expect to see a different result between the delta and wye units?

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