Design and optimization of medium frequency, medium voltage transformers

Medium voltage, medium frequency transformers (MFTs) are much smaller in size and weight compared to conventional low frequency transformers. The MFTs are very attractive for applications where full control of the power flow and high power density are required, such as power electronic interface in smart grids and traction converter system. With reduced size, the optimal design of MFTs becomes more challenging due to the high isolation requirement and thermal stress. This paper presents a design and optimization methodology for MFTs with emphasis on thermal and insulation design. The improved thermal model for multi-layer windings consisting of litz-wire and the analytical calculation of maximum electric field in core window area are included in the optimization process. By using the methodology, the optimal designs of a 25kW/4 kHz MFT with two commonly used core materials are compared. The possibility of volume reduction by increasing the operating frequency is also investigated.