172 QR of RTRI, Vol. 54, No. 3, Aug. 2013

Kenji SUZUKIAssistant Senior Researcher,

Masaru TOMITAAssociate Director,

Research and Development Promotion Division,also Laboratory Head,

High Temperature Superconducting Cable for Railway System

Yusuke FUKUMOTOAssistant Senior Researcher,

Applied Superconductivity Laboratory, Materials Technology Division

Atsushi ISHIHARAResearcher,

Tomoyuki AKASAKA Researcher,

The authors design and develop a prototype DC superconducting cable for railways. For the preliminary evaluation 2 m Bi-2223 DC cable is used. The maximum current flowing in the cable cooled by liquid nitrogen was 1720 A in the inner sheet wire and 2430 A in the outer sheet. The experimental results and simulations led to the conclusion important for practical applications that the leakage of magnetic field was negligible. This result is of crucial impor-tance, in particular for use in railway systems.

Keywords: DC HTS cable, railway systems, critical current(Ic), over current quality

1. Introduction

The high-Tc superconducting cable (HTS cable) is ex-pected to be used for bulk power transmission and features compactness, low cost, high efficiency and low loss [1-4]. Superconducting cables exhibit several environmental advantages including reduction of CO2 emission. High- Tc superconductor direct current (HTSDC) cables capable of carrying gigawatts of electric energy are ready for commer-cial applications [5,6]. The power loss in HTS transmis-sion lines is thirty times less than the transmission loss of ordinary alternating current (AC) power cables. In ad-dition, the lighter, thinner, higher-capacity superconduct-ing cables can be used like normal cables with a proper cryogenic cladding. Any new technology has to overcome a mental barrier of potential customers. Therefore, experi-mental tests and computer simulations have to be done to show advantages of the new system over the existing con-ventional technology.

We started first national project to develop a prototype DC superconducting cable for railway systems. The super-conducting cables will reduce the high resistive losses (6-8%) generated in the feeding system. The final goal of the project is to use the DC superconducting cable as a feeder of the railway overhead contact wire system. The power supply of Japan Railways in the metropolitan city of Tokyo and Osaka is DC current of 1.5 kV voltage [7,8]. The same voltage is also used in France, Netherlands, Australia, Ire-land, India (around the Mumbai area), New Zealand (Wel-lington), USA (Chicago area), UK, Slovakia, Portugal, etc. , while 3 kV DC current is used in Belgium, Spain, Poland, Italy, Czech Republic, South Africa, etc. The DC system is quite simple, however it requires thick cables and short in-tervals between feeder substations because of the notable resistive losses and high currents required. The substa-tions spacing along the rail track is normally 2-3 km. In a railway substation, the voltage is transformed to low voltage by a transformer and converted to direct current

by a rectifier (see in Fig. 1). The positive terminal of the rectifier is connected to the contact wire and the negative terminal of the rectifier is connected to the rail. Both the contact wire and the rail bring the power to the train and substations will feed the DC current and maintain the re-quired system voltage around 1500 V. Therefore, the sub-stations (feeder stations) are very important and require monitoring. If HTS cables were used as a feeder of the con-tact wire, the voltage decay could be significantly reduced and thus also the number of substations. Figure. 1 shows the structure of the contact wire system. The HTS cable should be capable of carrying the current with a safety margin of 5 kA-s and to ensure voltage up to 1500 V in the continuous power transmission line.

This report presents the motivation and design of a prototype high- Tc DC superconducting cable designed for feeding the railway system, the first kind in the world.

Yusuke KOBAYASHI Researcher,

Applied Superconductivity Laboratory, Materials Technology Division

Fig. 1 The structure of the DC feeding and overhead con-tact wire system.

Electric powersubstation

Rectifier

Feeder

Contact wireAir

section

Feeding diverging device

flow of electric current

Rail

PAPER

173QR of RTRI, Vol. 54, No. 3, Aug. 2013

2. Results and discussion

2.1 Development of Superconducting wires

High performance is one of the most important require-ments for the practical cable, which is mainly governed by the carrying capacity of the superconducting tape. For this an estimation of critical current (Ic) at 77 K is very impor-tant. Therefore, the self field critical current (Ic) of various Bi-2223 superconducting tapes commercially available in the market were measured in a liquid nitrogen tempera-ture (77 K) using a 1 mm/cm criteria. The total length of the tape used for the tests was 10 cm. The tapes critical current (Ic) and protection layer details are given in Table 1. The Type HT is the high strength wire made by reinforc-ing Type H with metallic tapes such as stainless steel and copper tapes. The tapes laminated with 50 micro meter copper alloy (1wt % of Sn) named as a Type HT (CA50) and laminated with 20 micro meter stainless steel named as a type HT (SS20). It should be noted that all measured tapes indicated that the critical current (Ic) was around 180 - 196 A at 77 K (see Table 1). Further, the highest 196 A critical current (Ic) was recorded in the tape which lami-nates the 50 micro meter copper alloy, Type HT (CA50) (see Fig. 2). The results indicate that the performance of the silver sheathed Bi-2223 superconducting tapes are im-proved drastically and can be used for variety of industrial applications including the power cable.

For the over characteristics, selected data of all tapes are presented in Table 1. A square over current pulse with duration of 100 ms is applied to the sample tape and subse-quently the critical current characteristics are re-measured to observe if the tape degraded or not. The peak value of over-current pulse was increased by several increments until the tape degraded. The results are presented in Fig. 3. It is clear that over current quality of the type H which laminates the copper alloy (CA50), has improved. In addi-tion, the over current quality was remarkably improved by copper lamination including the 2 mm2 thick copper foil in parallel to the tape surface (see Fig. 3). On the basis of the experimental results presented above, it is clear that the superconducting cable is not damaged by the process when over current occurs because of copper alloy protective layer. The superconducting cable is made of multiple layers of the superconducting wire with protective layer. When an over current occurs, the current will pass and spread in the silver sheath as well as protective layer. As a result the superconducting cable is safe to use.

Table 1 Ic and special feature of different type of DI-BSC-CO)

Type ofDI-BSCCO

Ic (A) Remarks

Type H 181Type HT (SS20) 190 20 um Stainless steel laminationType HT (CA50) 196 50 um Copper alloy laminationType H + Cu 1mm2 181 Cu 1mm2 tape was soldered on the

type H tapeType H + Cu 2mm2 181 Cu 2mm2 tape was soldered on the

type H tape

Fig. 2 V-I relationship of variety of Bi-2223 superconduct-ing tapes at 77 K.

Fig. 3 Over current characteristics of variety of Bi-2223 superconducting tapes at 77 K.

0

10

20

30

40

50

60

0 50 100 150 200 250

Type H

Type HT (SS20)

Type HT (CA50)

Vol

tage

(µV

)

Current (A)

Fig. 4 Magnetic field versus critical current (Ic) characteristics of the type HT (CA50) tape in various temperatures. (left) Perpendicular magnetic field. (right) Parallel magnetic field..

10.01Perpendicular Magnetic Field (T)

0. 10

100

200

300

400

I c (A

)

63666972778290

100

T(K)

1Parallel Magnetic Field (T)

0. 10

100

200

300

400

I c (A

)

0.01

The effect of magnetic field on the critical current (Ic) of superconducting tape is an another important factor in their technological use for the development of supercon-ducting cable. The cable applications the HTS tapes are assembled into a conductor to acquire in high current car-rying capacity, the AC losses in the tapes in the assembled conductor are affected by the magnetic fields produced by

0 500 1000 1500 2000 2500

I c /I c

0

Pulse current (A)

1.0

0.9

0.8

0.7

0.6

Type H +

Cu2 mm2

Type H +

Cu1 mm2

Type H Type HT(SS20)

Type HT(CA20)

174 QR of RTRI, Vol. 54, No. 3, Aug. 2013

the currents flowing in the surrounding tapes. In order to realize the critical current (Ic) degradation with magnetic field on BISCCO superconducting tape laminated the cop-per alloy type HT (CA50) was selected and studied around liquid nitrogen temperature. The external magnetic field of 0.1 T step up to 1 T was applied parallel and perpen-dicular to the tape surface and measured the critical cur-rent (Ic) in varying temperatures from 63 K to 100 K. The experimental results are presented in Fig. 4. It is clear that the critical current (Ic) decreased at higher tempera-tures and increased at lower temperatures. These results indicate that the critical current (Ic) strongly decreased with external magnetic field perpendicular to the tape surface (see Fig. 4). At 77 K the Ic was decreased to 50 % and 100 % when the external field is changed from 0.1 T and 1T perpendicular to the tape surface. These results demonstrate the technical importance of proper selection of the operating temperature and the type of tape before designing the superconducting cable. The performance of cable can be improved considering the above parameters including optimization of former diameter and number of layers in case of multi-layer cable. The numerical analysis and further experiments are under the way.

2.2 Development of Superconducting cables

Design of the cable core is of great importance for fab-rication of the high-Ic cable. One of the objectives of cable conductor design is to minimize quantity of the high- Ic wire and simultaneously to meet the requirements of the cable capacity. Figure. 5 shows the structure of this supercon-ducting cable. It is composed of ten-strand Bi-2223 wires rolled around a copper core. The electrical insulation is made of polypropylene laminated paper (PPLP) impregnat-ed by liquid nitrogen. The insulation system has good elec-trical and mechanical properties, and long-term reliability. The insulation thickness is 2 mm. Further, 14-strand su-perconducting wires were rolled onto the electrical insula-tion (PPLP). The superconducting cable allowed transmis-sion of 1720 A DC current under the liquid nitrogen cooling at the ambient atmospheric pressure as shown in Table 2. The photograph of the superconducting cable system is in Fig. 6. The envelope of the system was made of stranded copper wire. The gap between the wires was 0.5 mm.

In order to estimate the critical current (Ic) of the superconducting cable, both the conductor-layer and the shieldlayer current/voltage characteristics were measured at liquid nitrogen temperature (77.3 K). In order to obtain the lower temperatures, measurements were carried out with liquid nitrogen under reduced pressure. The results are presented in Fig. 7. When using only the inner sheet

wire, the maximum current at 77.3 K was 1720 A. The critical current (Ic) value increased with decreasing temper-ature and reached 2260 A at 72 K. In the outer sheet wire the current reached 2430 A at 77.3 K. Like in the conduc-tor layer, shield layer’s critical current increased to 3250 A when temperature decreased to 71.4 K. In this design, the nominal Ic of each superconducting wire at liquid nitrogen temperature was 160 A. Therefore, the total nominal cur-rent-carrying capacities of the inner and outer layers were 1600 A and 2240 A, respectively. The experimental data confirmed these nominal values and showed even higher currents, namely 172 A and 173 A per wire of the inner and outer sheets, respectively. Moreover, the n values were very high, 17 and 18, respectively. The critical current (Ic)

Table 2 Specifications of the 2 meter long prototype DC superconducting cable.

Part Size Characteristics

Former F16 mm Filled-core fine-stranded copperHTS tape 0.35 mm × 4.5 mm Bi-2223, Ic ＝ 160 A at 77.3 KHTS conductor F17 mm Ic ＝ 1720 A at 77.3 K, 1 layer with 10 tapesElectrical F22 mm Polypropylene Laminated Paper “PPLP”HTS shield F23 mm Ic ＝ 2430 A at 77.3 K, 1 layer with 14 tapes

Fig. 5 Schematic cross section of the superconducting cable structure.

Fig. 6 Prototype 2 m long Bi-2223 DC superconducting cable for railway system.

Electric Insulation (2mmt): φ22mm(PPLP)

Protection (1mmt): φ25mm

Former (Cu): φ16mm

HTS Conductor: φ17mm

HTS Shield Layer: φ23mm

175QR of RTRI, Vol. 54, No. 3, Aug. 2013

and n values for various temperatures around the boiling point of liquid nitrogen are summarized in Table 3.

In Japan the railway tracks are very close to the resi-dential areas. Therefore, the effect of leaked magnetic field on the society should be evaluated. For example, due to leaked magnetic field picture of electron microscope may

Fig. 7 Measurement result of critical current Ic of 2 m long Bi-2223 type superconductor around liquid nitrogen temperature (77.3 K).

0.0010

0.0008

0.0006

0.0004

0.0002

0.0000

Voltag

e (

V)

0 500 1000 1500 2000 2500 3000 3500 4000

Current (A)

1µV/cm1µV/cm

Conductor layer

Shield layer

77.6 K

73.6 K72.0 K

73.7 K

77.6 K74.4 K

74.2 K73.2 K

71.4 K

Table 3 Performance of the 2 m long DC superconduct-ing cable around 77 K.

T (K) Critical current value (A) n value

Conductor layer

Shield layer

77.3 1720 1873.7 2080 1673.6 2110 1772.0 2260 1777.3 2430 1774.4 2840 1774.2 2890 1873.2 3000 1871.4 3250 18

Fig. 8 Leaked magnetic field result of shield layer and conductive layer at liquid nitrogen temperature (77.3 K). (inset) The simulation results for leaked magnetic field of shield layer and conductive layer at liquid nitrogen temperature (77.3 K).

Lea

ked m

agnet

ic fie

ld (

G)

0

200

150

100

50

0 500 1000 1500 2000 2500 3000

Current (A)

Lea

ked m

agnet

ic fie

ld (

G)

Radial distance (mm)

200

150

100

50

0

250

0 10 20 30 40 50

by energizing to a shield layerby energizing to a conductive layerby energizing to both layers

get distorted, the image of TV screens may appear blur-ring, or electrocardiogram data may get disturbed. The stray field may also affect patients with heart stimulator. The magnetic field profile around the superconducting cable was checked using a Hall probe. The Hall probe was attached to a measuring arm that could move in 3 direc-tions of translation and rotation around the axis. Eventu-ally, the point by point field mapping was done. First, we applied the DC current of 0-1800 A at 77.3 K in the seal layer of the superconducting cable and mapped the leaked magnetic field at 45 mm radial distance around the cable. A similar experiment was repeated for the conductor layer. Finally, we sent the current through the both layers in the opposite direction and measured the leaked magnetic field value at the same, 45 mm radial distance. The leaking magnetic field results are presented in Fig. 8. It is clear that the individual layers show a negligible leaked mag-netic field. When the same current was sent through both layers in opposite directions, the leaked magnetic field was zero. The results suggest that the DC superconducting ca-ble can be used for the railway system without any hazard for public.

We also performed numerical simulations of the field leakage using the ELF/MAGIC software and the results are presented in the inset of Fig. 8. In this case, we as-sumed all details of the real cable as given in Table 2 and set DC current flow to 1000 A. The leaked magnetic field was calculated for several radial distances varying from 0 to 50 mm. The result for both the conductor layer and the shield layer showed very small leaked magnetic field. In addition, the radial distance around 45 mm showed a value similar to the experimental one (see the inset of Fig. 8). The results prove that the DC superconducting cable can be used for railway systems without any special precau-tions.

3. Summary

In summary, we have developed a prototype DC super-conducting cable for railway systems. The main goal of the project was to up-grade the feeder of the overhead contact wire system between the substation and the electric train.

The highest 196 A DC critical current has been ob-served at 77 K in an HTS tape limited by copper alloy. Over current analysis shows that copper alloy lamination and copper protection layers are important to improve the over current quality. The critical current (Ic) magnetic field dependence of the tapes offers several advantages includ-ing selection of the operating temperature and estimation of magnetic field effect. Knowing the basic characteristics of the tape is very important for designing a superconduct-ing cable.

Our preliminary tests of the DC superconducting cable showed that the cable is applicable for feeder of the overhead contact wire. Ic of each superconducting wire at liquid nitrogen temperature exceeded 170 A, which is suf-ficient for railway system in Japan. Further, the critical current (Ic) performance of the cable could be significantly improved by lowering the operating temperature from 77 K to 71 K. In addition, the magnetic field experiments in-dicated that the leaked magnetic field was negligible. The

176 QR of RTRI, Vol. 54, No. 3, Aug. 2013

simulation results showed high consistency with the cor-responding experimental data. The present results clearly demonstrate that the developed DC superconducting cables are reliable and capable of use in the railway system.

Acknowledgment

This research was partially supported by Japan Sci-ence and Technology Agency, JST, under Strategic Promo-tion of Innovative Research and Development Program.

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