Design Study of Onboard Superconducting Transformer

Hiroshi HATA
Chief Researcher
Senior Researcher
Chief Researcher
Traction control G., Vehicle control technology Div.Superconducting materials G., Materials technology Div.

1. Introduction
  High Tc superconducting wires such as Bi2223 have made great progress in recent years. As a result, a prototype superconducting transformer rated around 1 MVA was manufactured. Its merits are lightweight and high efficiency. We have carried out a rough design study of a superconducting traction transformer to evaluate its merit. This work was carried out in conjunction with Kyushu University .

2. Premise for a design study
  We assumed the specification of the transformer as Table 1 in consideration of the current design for Shinkansen type rolling stock.

Table 1 Specification of transformer
Input power3 MVA
Voltage (prim./second.)25 kV / 1 kV
Current (prim./second.)120 A / 1 kA~3
Frequency60 Hz

  The weight of conventional transformer according to Table 1 is estimated to be 2800kg. We used the specification of the high Tc superconducting wire that presently has the best performance. When AC current flows in a superconducting wire, an AC loss is generated, which must be compensated for by a refrigerator. We show an image of superconducting transformer in Fig.1

Fig.1 Basic structure of superconducting transformer (excluding refrigerator)

  We assumed four cases for the design study. When compared with the present wire, the Ic and AC loss are:
Case1:Ic the same, AC loss the same
Case2:Ic twice, AC loss the same
Case3:Ic the same, AC loss 1/10
Case4:Ic twice, AC loss 1/10

  Ic means the critical current. "Ic twice" and "AC loss 1/10" assume the progress of superconducting wire technology in the near future. For all these four cases, we calculated five cases of 1-turn voltage.

3. Results
  In all cases, we first determined the dimensions of windings and core. Then we carried out magnetic field distribution analysis for the windings and calculated the AC loss, which is a function of the magnetic field applied to the windings. Finally we calculated the weight of transformer including a refrigerator. The results are shown in Fig.2. As for the cases 1 and 2, the weight is around 2600 kg which is slightly lighter than that of conventional transformer. In the case 4, the minimum weight is around 1800 kg, 30% lighter.

Fig.2 Transformer weight in all cases

  We calculated the improvement of efficiency in Case3. Result is shown in Table2.
Table 2 Efficiency of the superconducting transformer
AC loss----299 W
Other loss131 kW170 W
Refrigerator input----12 kW