Thermal Design of a 2-Phase Flow Cooled Medium-frequency 140kVA Transformer for Railway Applications

In this paper, a cooling scheme and the corresponding thermal design method is proposed for high power high frequency transformer, which is used in power electronic transformer device for railway traction applications. Particularly, to address the overheating problem resulting from dimensional restriction and electrical insulation in railway applications, the cooling scheme based on actively cooled system with R245fa refrigerants is further enhanced by 2-phase flow cooling. The mathematic model of transformer copper and iron loss is first introduced. Subsequently, the required cooling area of transformer core is quantitatively calculated using thermal network method and the total pressure loss and heat transfer in two-phase flow is determined accordingly. Finally, the proposed thermal design method and corresponding cooling scheme are verified through the experiment on a 5.5kHz 140kVA transformer.