As of my last knowledge update in September 2021, the term "transformer zero-sequence flux" doesn't refer to a well-known or established concept in the field of transformers or electrical engineering. Transformers are devices used to transfer electrical energy between two or more circuits through electromagnetic induction. The concept of zero-sequence flux doesn't fit within the typical framework of transformer theory.
It's possible that the term has gained significance or been introduced after my last update, or it might refer to a specialized or niche concept that is not widely covered in general transformer literature.
If you have specific context or additional details about where you came across this term, I'd be happy to try to help you understand it better or provide relevant information based on the information available up to September 2021.