Accurate calculation of winding resistance and influence of interleaving to mitigate ac effect in a medium-frequency high-power transformer

Medium/high-frequency transformer is an integral part of many power conversion systems. Switching at higher frequency results in lesser volume of magnetics but induces higher winding loss density, on account of increased eddy current effects in conductors. Thus winding resistance is a key parameter to characterize performance of a medium-frequency (MF) highpower (HP) transformer. In this paper, 10 kW, 0.5/2.5 kV, 1 kHz transformer designs are presented employing different winding dispositions (normal and interleaved) and conductor geometries (foil, round). Dowells' and Ferreira's methods, which are computationally cost-effective among other analytical methods, are discussed to calculate winding resistance at higher switching frequency. Nature of ac resistance factor distribution among multi-layer winding is examined in detail and shown that with adaption of interleaved winding, eddy-current effects at higher switching frequency is minimized. Analytically calculated results are validated with FEA (2D) simulation. At medium frequency, with proper selection of core and winding geometry, very close agreement is achieved between analytical and FEA (2D) results, which substantiates aptness and computational efficacy of Dowell's and Ferreira's methods as well as aiding effect of interleaving to reduce ac resistance in multi-layer wining.