Impact of Combined Current and Voltage Harmonics on the Thermal Behavior of Distribution Transformers: A Coupled Nonuniform Magnetic–Thermal Approach

Since the insulation state and hotspot temperature (HST) are closely linked, monitoring the thermal condition of distribution transformers (DTs)—critical and costly power grid components—can help prevent failures. This study focuses on accurately predicting the HST in DTs. During the temperature rise test, optical fiber sensors (OFSs) are used to evaluate proposed nonuniform 3-D computational fluid dynamic (CFD)-based models. In contrast to the OFS measurements, the new 3-D CFD-based thermal study displays an error percentage of 0.11% ( $0.1~^{\circ }$ C), indicating the high accuracy and efficiency of the model. Furthermore, the results of nonuniform 3-D CFD-based thermal assessments are validated using thermography for both top-oil temperature and bottom-oil temperature. With an error percentage of <0.65%, the findings indicate an acceptable degree of correlation between thermography and thermal analysis of 3-D CFD at the designated two locations, indicating the new nonuniform 3-D CFD-based model’s acceptable correctness. Finally, the new nonuniform 3-D model is exposed to total harmonic distortions (THDs) for voltage and current of 5%, 10%, and 15%. This causes an HST increase of $3.3~^{\circ }$ C, $7.1~^{\circ }$ C, and $10.3~^{\circ }$ C more than the previous model without harmonics.