Thermal behavior of a buoyancy-driven oil-immersed transformer and coolant performance assessment

The dependable operation and effective thermal management of transformers are essential to power-system stability. To tackle this, a numerical model for the buoyancy-driven oil-immersed transformer cooling system is established, focusing on the heat transfer and flow characteristics within the transformer and its influencing factors. A figure of merit (FOM) is proposed to guide the coolants’ selection and development for immersion liquid cooling systems under natural convection conditions. The results show that due to the competitive interaction mechanism between local natural convection and global natural convection, the convective heat transfer capability near the left windings decreases, resulting in the left windings’ temperature being significantly higher. In static mode, the oil-immersed transformer’s hotspot temperature is positively correlated with the load, with a critical load of 107%. Furthermore, the magnitude of FOM is positively correlated with the mineral insulating oil’s heat dissipation performance, and its evaluation of insulating oil performance in static mode is reliable. Notably, based on the FOM’s importance, the thermal properties for different mineral insulating oils are ranked as dynamic viscosity > density > thermal expansion coefficient > thermal conductivity > specific heat capacity. With the increase in temperature, only the dynamic viscosity’s weight factor rises, while the other thermal properties’ weight factors reduce. Moreover, compared to mineral insulating oil 2 and mineral insulating oil 3, the service life of the transformer using mineral insulating oil 1 is increased by 1.11 and 1.53 times, respectively.