Liquid-Immersed Distribution Transformers' Thermal Analysis With Consideration of Unbalanced Load Current Effect

Distribution transformers (DTs) are the most costly and crucial power grid equipment, and accurate investigation of their thermal status can prevent their failure, since insulation condition directly correlates with hotspot temperature (HST). In this article, an accurate and nonuniform magnetic-thermal analysis of DT is proposed for precise HST prediction. In the magnetic analysis, the DT is modeled as a 2-D axial symmetry, and the losses calculation of the windings has been fulfilled as a nonuniform. In the thermal analysis, the DT is modeled as 3-D and nonuniform, and the conservator and core stacking, which has a considerable effect on the HST, are precisely modeled. By taking advantage of optical fiber sensors (OFSs) in the understudied 500-kVA DT, the accuracy of the proposed nonuniform 3-D computational fluid dynamic (CFD)-based modeling during the temperature rise test (TRT) is validated. The empirical evaluation results depict that the presented nonuniform CFD-based thermal analysis for HST prediction is very precise, and there is an appropriate vicinity to the experimental values. The error percentage of the proposed 3-D CFD-based thermal analysis is 0.11% (0.1 °C) compared with the OFSs measurements, which demonstrates the precision and effectiveness of the presented modeling. Also, the verification of the results of nonuniform 3-D CFD-based thermal analysis in top-oil temperature (TOT) and bottom-oil temperature (BOT) during the experimental TRT is fulfilled via thermography. According to the attained evaluated results, temperatures of 3-D CFD-based thermal analysis and thermography in the noted two points are in good accordance with each other. In short, the error percentage is less than 0.65%, which indicates the correctness and proper performance of the proposed 3-D CFD-based modeling. Finally, the proposed nonuniform 3-D model was subjected to the unbalanced load currents of 0.95, 1.05, 1.1, 1.15, and 1.2 of rated current, which increased the HST by -1.6 °C, 1.3 °C, 2.8 °C, 4.3 °C, and 6.0 °C, respectively, over the original model without unbalanced load currents.