Enhanced thermal modeling of three-phase dry-type transformers

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This work presents an enhanced thermal modeling for dry-type three-phase transformers, addressing the heat exchange mechanisms through conduction, radiation, and convection. Additionally, an equivalent thermal network is proposed to analyze the thermal behavior of these transformers. The thermal model includes several improvements, such as the differentiated representation of the core and windings through different nodes, and the consideration of primary and secondary windings as distinct nodes. Furthermore, the effects of convection and radiation in the inner areas of the core windows are accounted for, providing higher accuracy to the model. This modeling allows for a detailed analysis of the transformer under unbalanced load conditions, considering the complete geometry of the equipment. The results demonstrated that even with just a 5% load imbalance, there was a change in the hot spot temperature (HST). Moreover, the evaluation of temperatures in the coils, both internal and external to the core windows, revealed variations of up to 7 °C in the analyzed load unbalance scenarios. The study also highlighted that the phase of the harmonic component has an impact on the final equipment temperature. Additionally, the overvoltage and overload analyses showed that voltage has a greater impact on increasing the average core temperature, while overload mainly affects the winding temperature. Another relevant finding is that the internal coils near the core windows are most vulnerable to temperature rise when subjected to overload.