Study on Energy Transfer Mechanism Between Primary and Secondary Windings of Transformer Based on Mutual Energy Flow Theory

Transformers are core components of power systems, and their energy transfer mechanisms are typically explained physically relying on Faraday's law of electromagnetic induction and Ampère's circuital law. However, these laws primarily focus on the calculation of engineering parameters and have limitations in revealing the physical essence of how energy flows from the primary winding to the secondary winding. Based on Poynting vector analysis, this paper points out misconceptions present in traditional teaching and literature---mistakenly treating the self-field Poynting vector of the primary winding as the main entity for energy transfer. This paper proposes that the internal energy flow within a transformer should be determined by the total Poynting vector generated by the superposition of primary and secondary fields. By decomposing the total energy flow into self-energy flow and mutual energy flow, the paper defines their respective physical functions: the self-energy flow, related to self-inductance, corresponds to reactive power in the time-averaged sense and does not participate in energy transfer across windings; whereas the mutual energy flow, closely related to mutual inductive coupling, is the real carrier responsible for transmitting power from the primary "source" to the secondary "sink". The study further explores the particle-like attribute of mutual energy flow, establishing its physical connection with photon energy flow. Under quasi-static conditions, this paper provides, for the first time, quantitative numerical calculations of the mutual energy flow between long cylindrical transformer windings, verifying the validity of the mutual energy flow theorem as an energy conservation law in near-field quasi-static environments. Furthermore, this paper delves into the inconsistency between quasi-static theory and radiation theory in explaining energy transfer, pointing out the inherent internal inconsistency of classical Maxwell electromagnetic theory in describing energy flow. This research not only provides a new theoretical perspective for optimizing transformer electromagnetic design but also offers an important basis for understanding the unified energy flow transmission mechanism from quasi-static to radiation fields.