Effects of Asymmetric Coupling on Winding AC Resistance in Medium-Frequency High-Power Transformer

Medium-frequency high-power (MFHP) isolated dc/dc converter has been extensively employed in various applications, including the high-voltage dc, distributed generation, high-speed railway, more electric ship, and more electric aircraft. The MFHP transformer is one of the most important components of the dc/dc converter. In MFHP application, the circulating current often occurs in the parallel secondary windings for transformer with foil-wound winding, together with primary-series secondary-parallel (PSSP) structure. It is because of the existence of asymmetric mutual inductance, described as asymmetric coupling between the primary and the secondary winding, that the absolutely symmetry cannot be achieved. Thus, a knee frequency exists, which in turn leads to significantly increased ac resistance if the working frequency of transformer is set above it. To predict the knee frequency caused by asymmetric coupling in windings, an analytical model together with a scaled-down experimental transformer to obtain the asymmetrical coupling coefficients is developed to predict the knee frequency for real-scaled transformer. The accuracy of developed analytical model is validated by the measured result. Then, a prototype 980 kVA megawatt-level MFHP transformer having PSSP structure is been built based on the analytical and experimental optimized knee frequency.