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 Ic(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 Ic(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 Ic(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 Ic(B, θ), can lead to a non-trivial reduction in AC loss, even incorporating flux diverters.