In recent years, the clean vehicles that emit no greenhouse gases, such as electric vehicles, storage battery electric railway vehicles and electric propulsion ships, are attracting attention for a carbon-free society by 2050. On the other hand, the charging time for a storage battery is much longer than that of the driving the clean vehicles. In order to solve this problems, it is important to develop the rapid charger of several 100 kW-class that can charge a storage battery in a few minutes. Although a plug-in type rapid charger has been investigated for the electric vehicles and the electric propulsion ships, there is a risk of electric shock when charging a storage battery in a wet area (in the rain). Also, a fast charging method of installing the overhead contact lines using pantographs at the station has already been adopted for the storage battery electric railway vehicles. However, there are problems of the disconnection in the overhead contact lines due to the generation of heat by large current flowing and the maintenance on the overhead contact lines. Therefore, we have been focusing on a wireless power transmission (WPT) system having a high capacity and safety. However, since a conventional WPT system using copper coils for the clean vehicles is required to transmit the electric power of several 100 kW in a short time, it is difficult to suppress heat generation by the internal resistance of a copper coil. Therefore, we have been investigating the high-efficiency and high-capacity WPT system using a high-temperature superconducting (HTS) coil.

The previous researches have clarified that the cryocooler power loss cannot be neglected because it is several tens of times larger than the AC loss of the HTS coils. Therefore, the increase in size of the HTS coils and the cryocooler was the problem, and it is necessary to increase the transmission power density per the HTS coil. However, the magnetic flux density of the HTS coils increases with increasing the transmission power density. Thus, the AC loss in the HTS coils is expected to increase by not only the self-magnetic field but also the mutual induction between the transmitting and receiving coils. Therefore, we need to clarify the low loss coil structure for the high-power density with high Q-value and the effect of the high-power density HTS coil on the electric power transmission characteristics of the large-capacity WPT system. In this study, we focused on the WPT system using the HTS coil for the storage battery electric railway vehicle in the non-electrified section, and investigated the low loss coil structure for the high-power density with high Q-value. Also, we clarified the AC loss characteristics in the HTS coils generated by the mutual induction between the transmitting and receiving coils, and investigated the electric power transmission characteristics in the WPT system using the high-power density HTS coil for the storage battery electric railway vehicle.

As a result, we found that the receiving power of the WPT system using the HTS coil structure with the narrow HTS wires parallel in the radial direction was about 10 times higher than that using copper coils. Although the AC loss of the high-power density HTS coil was about 2 times higher than that of the single HTS coil by the mutual induction between the transmitting and receiving coils, the several 100 kW-class WPT system using HTS coils can transmit the electric power with a high-efficiency of more than 95%.

Keywords: HTS coil, AC loss, WPT system, railway vehicle