3. Development of basic technologies
      The Kunitachi institute has promoted research and development centering on the durability of superconducting magnets and ground coils over long periods, and has developed and tested a device to locate the point of ground fault in feeding circuits to support quick recovery operation.

(1) Verification of superconducting magnet durability
      In one of the most important areas for the commercialization of the superconducting magnetic levitation railway system, the RTRI verified the durability of a superconducting magnet over a period equivalent to the lifespan required for revenue service.
      The superconducting magnet is subjected to load fluctuations caused by magnetization/demagnetization, heat cycles and vibration during running. Durability over a period equivalent to the service lifespan taking into account the former two factors has been verified through running tests on the Yamanashi test line. To verify the effects of vibration during running, however, the frequency of loading was insufficient in past running tests. To compensate for this shortage, the RTRI implemented a superconducting magnet excitation bench test (Fig. 9).
      The effect of vibration on the superconducting magnet during running is the greatest at the resonance frequency, with the whole magnet subjected to a vertical torsional force from levitation coils and to a lateral bending force from propulsion coils. Since the durability of superconducting coils, load supporting members and other major components had already been verified by unit tests, the RTRI investigated the durability against vibration of the superconducting magnet as a whole including low-temperature piping. For this purpose, the RTRI performed exciting tests on levitation coils in a 60-degree pitch 4-square grid arrangement and on propulsion coils in 120-degree pitch single-layer arrangement. This simulated the vibration during actual running to a cumulative frequency equivalent to that expected during 15 years of operation on revenue service lines. Results showed no abnormalities in the degree of vacuum in the vacuum tank or low temperature piping, and no changes over time were observed in the vibration modes of the different parts. Summarizing the results of the running and element technology tests, the RTRI was able to confirm that the superconducting magnet had the durability required for operation on revenue service lines (Fig. 10).

Fig. 9 Durability test apparatus

Fig. 10 Cumulative frequency of vibration loading in the durability verification test

(2) Outdoor current feeding test on PLG coils over long periods
      One of the most important areas in ensuring the reliability of the levitation railway system is to verify the durability of ground coils, as they will be in service for more than 30 years in outdoor environments. As a means to verify their durability, therefore, the RTRI performed a long-period outdoor current feeding test on a PLG coil developed to incorporate the functions of propulsion, levitation and guidance to cut construction costs.
      During the 180-day test period, the RTRI applied a voltage to the PLG coil to a frequency equivalent to that of a 35-year service period on a revenue service line, and heat cycles of 18-hour current feeding and 6-hour non-feeding per day (between the ambient temperature and 90ºC) (Fig. 11). The results showed no conspicuous changes in the appearance of the coil or its insulation characteristics after the test (Fig. 12), demonstrating the thermal durability of the PLG coil.

Fig. 11 Scene of long-period outdoor current feeding test

Fig. 12 Changes in insulation characteristics over time

(3) A technique to locate the ground fault point in feeding circuits
      A number of propulsion coils are arranged on the ground-level guideway wall in the Maglev system or the linear motor system. If a ground fault occurs in the feeding circuit including the propulsion coils, it is important to locate the fault quickly to support prompt recovery operation. The RTRI therefore developed a fault point location technique applicable to the feeding circuit of the levitation railway system, and performed simulated ground fault tests to verify its validity.
      If a one-phase ground fault occurs in a propulsion coil group (or section) in the feeding circuit (Fig. 13), the substation detects it and immediately stops the power source inverter. A vibrating transient current flows between the filter set at the substation and the failed section, and the vibration frequency of this current depends on the distance from the substation to the section and also on the fault point in the section. If these characteristics are known in advance by simulation or other means, therefore, the frequency of transient current will give the location of the fault point in the section.
      To verify this fault-locating technique, the RTRI performed a simulated ground fault test on the Yamanashi test line. The test showed that fault points could be located with an error margin of about 5% of the entire section length.

Fig. 13 Phenomenon when a ground fault occurs in the feeding current