Current Sharing Properties of Superconducting Parallel Conductors in Rectifier Transformers for Railway Electrification System

In Japan, superconducting cables for the direct current railway electrification systems have been progressively developed. When superconducting cables are used as a feeder of the railway electrification system, the voltage drop could be significantly reduced. However, fault current due to short-circuit and the like should be enhanced since superconducting cables have no resistance. In this study, it is proposed that superconducting transformers with current limiting function are applied as rectifier transformers so that the fault current can be suppressed. Although superconducting feeding system has many effects, the rated secondary current of the rectifier transformers is very large. So the current capacity of the secondary windings of the rectifier transformers should also be enhanced up to several kA. To realize that, transposed parallel conductors composed of HTS tapes are expected to be adopted. For low ac loss and uniform current sharing among the tapes, the constituent tapes of parallel conductors need to be transposed so that their inductances balance each other. In this paper, the optimum transposition method in the case where a 32-tape parallel conductor is wound into a single-layer solenoid is investigated by the numerical simulation of the current sharing among the constituent tapes and the validity is verified experimentally.