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In order to calculate stray losses and predict temperature distribution as accurately as possible in power transformer tank walls and other mental constructive parts, an analysis method of the electromagnetic-thermal coupling is presented. TEAM Problem 21 (P21-B, P21-2, P21-M1) are adopted to verify the validation of the stray losses calculation by MagNet FEM software, showing that the calculation results are in good agreement with measured results. Then, taking the material properties affected by temperature into consideration, the two-way coupling analysis method is used to the calculation of stray losses and temperature distribution in power transformer. Introduction In the design of large power transformers, it is very important to determine as accurately as possible the amount and space distribution of stray losses due to leakage of magnetic flux, occurring in the winding and other metal parts. Many researchers and transformer manufacturers have proposed different analysis methods to calculate and reduce the stray losses in flitch plates, frames, and tank walls for eliminating the local overheating and efficiency decreasing [1][2][3][4]. Most transformer manufacturers still use the semi-empirical formulae to calculate stray losses and predict the maximum temperature rise. However, these methods are too oversimplified so that the results are inaccurate, or based on empirical techniques with questionable validity. Due to the complicated construction of power transformers, the stray losses in structural parts of power transformers, especially for the concentration regions of eddy current which are induced by leakage field, may cause local overheating. Most of the power transformers are strongly restricted by thermal regulation specifying hot-spot temperature inside the transformer tank [5]. Therefore, it is essential to predict those temperature rises accurately and take some measurements to reduce the eddy current losses and eliminate the hot spots. Based on electromagnetic analytical formulation linked with thermal finite element method, Paper [6] proposed 3-D methodology for the heating hazard assessment on transformer covers. Taking account of the temperature dependence of the heat transfer coefficient and of the conductivity, the paper [7] established a strong coupling between the thermal and electromagnetic equations to calculate eddy loss and predict temperature distribution of bushing adapters in three-phase generator step up transformer. In this paper, taking the material properties affected by temperature into consideration, the electromagnetic-thermal coupling analysis for the temperature rise prediction in power transformer structural parts is adopted. This strong coupling analysis method is able to improve the calculation accuracy and give important guiding suggestions for reducing eddy current losses and avoiding local overheating. Method Description and Simulation Model To determine stray losses, a full three-dimensional (3D) finite-element-method (FEM) analysis International Power, Electronics and Materials Engineering Conference (IPEMEC 2015) © 2015. The authors Published by Atlantis Press 317 of the power transformer is required. The three-phase five column transformer simulation model of OSFPS9-360MVA/330GY is shown in Figure1. MagNet FEM software, which has a time-harmonic analysis modular, is used to calculate the stray losses by Ω − T method. This coupling analysis has two steps. Firstly, the stray losses of electromagnetic model should be calculated to obtain the heat source. Next, the thermal model is solved with the heat source obtained from electromagnetic analysis. Considering the material properties affected by temperature, such as: heat transfer coefficient, the conductivity and the curve of B-H or B-W, heat sources are recalculated according to flow chart of coupled method iteratively in Figure2.

Calculation of Stray Losses and Temperature Distribution in Power Transformer Using Coupled Electromagnetic-Thermal Field Analysis