I'm not familiar with the specific term "residual flux" in the context of transformer operation. Transformers are devices that transfer electrical energy between two or more circuits through electromagnetic induction. They consist of two or more coils of wire, known as windings, which are usually wound around a common magnetic core.
If by "residual flux" you're referring to a term that might have emerged after my last knowledge update in September 2021, I don't have information on it. Transformer operation primarily revolves around concepts like magnetic induction, turns ratio, mutual inductance, and the conservation of energy between the primary and secondary windings.
If you can provide more context or information about what you mean by "residual flux" in relation to transformer operation, I'd be happy to try to help with the information available up to my last update.