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
$I_{c}$
(
$B$
,
$\theta$
), leading to a nontrivial 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
$I_{c}$
(
$B$
,
$\theta$
). 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
$I_{c}$
(
$B$
,
$\theta$
) 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
$I_{c}$
(
$B$
,
$\theta$
) can lead to a nontrivial reduction in ac loss, even incorporating flux diverters.