Multi-Physics Numerical Research in Oil-Immersed Three-Phase Transformer Under Load Unbalance

Transformers are susceptible to the influences of complex power grid systems, which may induce three-phase unbalance in transformers, thereby threatening their safety and stable operation. To better understand multiphysics interactions within a transformer under a three-phase load unbalance, a coupled multiphysics model is established and validated for an oil-immersed transformer based on the finite element method. The electromagnetic characteristics, conjugate heat transfer, and thermal stress of the transformer under three-phase load unbalance are analyzed, and the impact on the transformer’s relative aging rate is further assessed. The results show that under three-phase load unbalance, winding losses are significantly influenced by the degree of unbalance, while core losses remain almost unaffected. The maximum difference in winding losses between phases can reach 9.6 times, with a total loss increase of approximately 17.31% at a 30% unbalance degree for Case 3. The mutual heating effect between adjacent windings intensifies with the degree of unbalance, leading to higher temperatures in low-loss windings and sustaining high thermal stress and expansion. Severe three-phase unbalance (e.g., 30% unbalance degree in Case 3) can be mitigated by reducing the transformer load rate to 90%, thereby reducing the relative aging rate to about 20% of that under full load and significantly extending the transformer’s insulation life.