Temperature Rise Test and Thermal-Fluid Coupling Simulation of an Oil-Immersed Autotransformer Under DC Bias

Loss and temperature increase due to DC bias occurring in power transformers may lead to damage and reduced lifespan. To study the influence of different levels of DC bias on the temperature rise in transformer structural components, 0, 8, and 16 A DC were introduced into the MV side of a test transformer. A 2D axisymmetric finite element model was also established to calculate and analyze the distribution of winding loss under DC bias. Combined with the 3D field-circuit coupling model, the core loss under DC bias was calculated on the basis of the half-wave average algorithm. The eddy loss of the steel structure was also obtained using a 3D field-circuit coupling model. On the basis of the thermal-fluid coupling model, the transient temperature changes of typical points were simulated. Results showed that the calculation error of loss and temperature are small when the DC current is 0 A. Moreover, the error of loss and temperature increases when the DC current is 8 or 16 A. The methods used in this study lay the foundation for subsequent research on the temperature rise of large-capacity power transformers under DC bias, especially for the single phase transformers with ONAN cooling mode.