Combined Impact of Asymmetric Critical Current and Flux Diverters on AC Loss of a 6.5 MVA/25 kV HTS Traction Transformer

A 6.5 MVA/25 kV high temperature superconducting (HTS) transformer for the Chinese Fuxing high-speed train has been proposed to replace the oil-based transformers while achieving higher efficiency, lighter weight, and minimized volume. The high targeted efficiency (> 99%) makes AC loss reduction a vital issue. HTS coated conductors generally exhibit asymmetric critical current characteristics as a function of magnetic field angle I_c(B, θ), leading to a non-trivial influence on the AC loss of coil windings. The fast-computing T-A homogenization method is proposed to calculate the AC loss of the 6.5 MVA/25 kV traction transformer with large turn numbers. The variables, T and A, are the current and magnetic vector potentials, respectively. The AC loss of the transformer windings is analyzed for various coil configurations with and without flux diverters considering I_c(B, θ). At the rated current and 65 K, employing the flux diverters with a square-shape cross-section, the total AC loss is decreased by 73.7% and an extra 150 W loss reduction was also obtained. Moreover, an additional reduction of 37 W is realized upon utilizing the asymmetric I_c(B, θ) characteristic. The reduced 187 W in AC loss at 65 K corresponds to a reduction in ambient power requirement of over 5.6 kW. Therefore, considering asymmetric I_c(B, θ), can lead to a non-trivial reduction in AC loss, even incorporating flux diverters.