High Tc Superconducting Cable with Bi-2223 Transposed Segment Conductors Shigeo Nagaya*, Naoji Kashima*, Kenji Goto, Chikashi Suzuki, Shoji Ajimura, Takashi Saitoh, Osamu Kohno, Kohichi Ohno, Kunio Hanabusa and Hajime Kato We developed the 15m-long, 2.6kA-class and new type superconducting cable conductor, which was made by winding a number of transposed segments around a stainless steel flexible former. The transposed segment was consisted of 5 Bi-2223 tapes, which were strengthen with Ag alloy sheath and insulated individually, to realize high strength and low A.C. loss of the tape. The conductor had the 3-layer structure, which was taken a method of equalizing impedance of each layer by adjusting spiral pitches. In the results of transport current tests, current distributions in each segment for the conductor were unified below changing rate of ±5%. Further, current distribution in 3 layers was homogenized below ±6% in the range from 200A to 2,800A. As the results, we successfully carried out a development of the low A.C. loss high-Tc superconducting (HTS) cable, for which A.C. loss was 0.1W/m at flowing 1,000A. 1. Introduction The development of High-Tc Superconducting (HTS) power cable having an even higher current density would allow the conductor to have a smaller diameter and would promise high-capacity transmission at relatively low voltage 77kV. We have suggested several prototype HTS cables consist of Agsheathed Bi2Sr2Ca2Cu3Ox (Bi-2223) tapes 1). On the other hand, we have investigated liquid nitrogen pressure loss of the simulated cooling system for HTS power cable system 2). In HTS cable for high current transmission system, it was confirmed in the multi-layered cable using Bi2223 Ag-sheathed tapes that the unbalanced current distribution generated by the difference of impedance of each layer. As a result, a transport current flows in outer layers and A.C. loss of the conductor increases. One of methods to solve this problem is to adjust each spiral pitch in order to have the same impedance in each layer 3) 4). In order to solve this problem, we have developed the transposed segment conductor consist of insulated Bi-2223 tapes 5). In this cable, an impedance of each tape for the conductor is equivalent because each tape is transposed in the segment. As the result, it could be expected that current distribution in the conductor becomes uniform. In the transposed segment, there is a problem that critical current property of Bi-2223 tapes was reduced because of impression of edgewise bending strain at the crossing part of the segment. Therefore, it is necessary to find the tape for a satisfactory tolerance against edgewise bending strain. To satisfy this requirement, the narrow width tape, half width of a conventional tape, was selected in order to make edgewise strain low. Further, Bi-2223 tapes were strengthened with Ag alloy sheath. We developed the 15m-long, 2.6kA-class conductor, which was fabricated by winding a number of transposed segments around a stainless steel flexible former. The segment conductor was consisted of 5 Agalloy sheathed Bi-2223 tapes. We investigated the current distributions of the wires and A.C. loss property for the conductor. Following is the result of this study. 2. Mechanical Property of Bi-2223 Tape Table 1 shows characteristics of the Bi-2223 tapes sheathed with pure Ag and Ag alloy. The graph in Fig. 1 illustrates critical current property for 2 sorts of tapes deferent in sheath material as a function of tensile load. Critical current property of the tape was measured at free stress condition in liquid nitrogen, Table 1. Characteristics of Tapes for Segment Conductors Superconductor Sheath material Tape size Core number * Chubu Electric Power Co., Inc. Fujikura Technical Review, 2002 Bi2 Sr2 Ca2 Cu3 Ox (Bi-2223) Pure Ag Insulation Ag alloy 1.60 × 0.25 mm 7 61 Coated 83 after tensile stress was loaded at room temperature. In Fig. 1, it was clear that tolerance of tensile stress was improved remarkably in the tape sheathed with Ag alloy. The region of edgewise bending strain on a segment conductor was represented schematically in Fig. 2. Fig. 3 shows critical current property for the tapes as a function of edgewise bending strain. The result of this measurement clearly shows that toler- ance of edgewise bending strain was increased remarkably in the tape with Ag alloy. The reason why tolerance of the strain for the tape became higher is that a tensile stress, which was loaded on the outside of a tape in bending, was dispersed all over the tape by an effect of the reinforcement. Consequently, it became possible that a transposed segment was fabricated in less than 100mm for a crossing pitch. 1.2 3. HTS Cable 1.0 Ic / Ico 0.8 0.6 Ag alloy sheath Pure Ag sheath 0.4 0.2 0 0 20 40 60 80 100 120 140 160 Tensile stress (MPa) Fig. 1. Critical Current Property for Tapes as Function of Tensile Load. – Comparison in Deference of Sheath Material – Table 2 shows characteristics of a transposed segment conductor. The conductor was consisted of 5 Bi-2223 tapes, which were sheathed with Ag alloy and insulated with resin individually. A crossing pitch of the conductor was 95mm and the transposed pitch was 950mm. A cross-sectional view of the conductor was represented in Fig. 4. The 15m-long cable was fabricated by winding a number of transposed segments around a stainless steel flexible former. A schematic of the cable is shown in Fig. 5. The cable had a 3-layer structure, which was taken a method of equalizing impedance of each layer by varying spiral pitches in order to realTable 2. Characteristics of Transposed Segment Conductor Tape Crossing part Edgewise-bending strain Tape sheathed with Ag alloy Number of tapes 5 Transposed pitch 950mm Transposed direction S Fig. 2. Schematic of Transposed Segment Conductor. 1.2 Fig. 4. Cross-section of Transposed Segment Conductor. Ag alloy sheath Pure Ag sheath 1.0 Transposed segment conductor with 5 tapes Ic / Ico 0.8 0.6 0.4 Insulation of inter-layer 0.2 0 0 0.2 0.4 0.6 0.8 1.0 Edgewise bending strain (%) Fig. 3. Critical Current Property as Function of Edgewise Bending Strain. 84 Former Fig. 5. Schematic of Cable with Transposed Segment Conductors. ize equivalent current distribution in the inter-layer 4). Characteristics of the cable are showed in Table 3 and then an outlook of the cable is shown in Fig. 6. current. The result illustrates that the critical current of the cable was 2.6kA, which defined by 1µV/cm, and n-value was 7.4. 4. Transport Current Test 4.2 Measurement of Current Distribution 4.1 Measurement of Critical Current D.C. transport current test was carried out to measure critical current for the cable at 77K. Fig. 7 shows voltage-current property at the introduction of D.C. Table 3. Characteristics of Cable with Transposed Segment Conductors Inner diameter 37mm Outer diameter 48mm Number of layer Fig. 8 shows a schematic of the terminal region on a measurement side for current distribution of tapes at the same segment. In this measurement, the error caused from resistance on current terminal was below 2%. Fig. 9 shows comparisons of flowing currents of wires when the transport current is at 1,000A and 2,000A. These results indicate that current distribution at the same segment is unified below changing Measurement of current into each tape 3 1st 2nd 3rd Number of segment 32 36 36 Spiral pitch length 805mm 440mm 255mm Spiral direction Z Z Z Transposed segment conductor Fig. 8. Schematic of Terminal on Measurement Side for Current Distribution of Tapes in the Same Segment. 3 2 Transport current 1,000A (60Hz) Current (A) 1 0 −1 −2 Tape 1~5 −3 0.05 0.055 0.06 0.065 0.07 0.075 Time (s) Fig. 6. Outlook of 15m, 2.6kA-class HTS Cable. 6 10 4 Ic=2.6kA (criterion:1µv/cm) N=7.4 Current (A) Voltage (V/m) 10−3 −4 10−5 10−6 102 2 0 −2 −4 103 D.C.current (A) 104 Fig. 7. Voltage-current Property at Introduction of D.C. Current. Fujikura Technical Review, 2002 Transport current 2,000A (60Hz) Tape 1~5 −6 0.03 0.035 0.04 0.045 0.05 0.055 Time (s) Fig. 9. Comparison of Flowing Currents for Wires, Which is at the Same Segment in the First Layer, at Flowing 1,000A, Upper, and 2,000A, Lower. 85 rate of ±5%. It follows from this that an impedance of each tape for the segment is almost equivalent because each tape is transposed in the segment. Fig. 10 shows a schematic of the terminal region on a measurement side for current distribution of individual layer. The currents of individual layer were shown in Fig. 11. These results indicate that current distribution in 3 layers is homogenized below ±6% in the range from 200A to 2,800A. Consequently, we succeeded in making current distribution uniform in the conductor by means of transposing tapes and adjusting spiral pitch. Power supply 60Hz Rogowsky coil 10m Sample Cryostat Voltage signal Reference signal Lock-in-amp 4.3 Measurement of A.C. Loss Fig. 12. Schematic Circuit for Measurement of A.C. Loss. 102 101 A. C. loss (W/m) A.C. loss of the conductor has been measured with 4-terminal transport method. After A.C. current at a frequency of 60 Hz was fed into the conductor, voltage (V) of the conductor and a phase (θ) between voltage and current were measured by a lock-inamplifier. Fig. 12 shows the schematic circuit for measuring A.C. loss of the cable. Fig. 13 shows A.C. loss of the cable as a function of transport current. In 100 Norris eq. 10−1 10−2 10−3 2 10 Measured 103 Transport current (Arms) 104 Fig. 13. A.C. Loss Characteristics of Cable with Transposed Segment Conductors. – Comparison of Measured Value with Calculated Value – Fig. 10. Schematic of Terminal on Measurement Side for Current Distribution of Each Layer, First, Second and Third. the same figure, a solid line indicates calculated value by a Norris’s equation (1) 6). Current into each layer (Arms) 1,000 W = (µo · f · Ic2/π) · [(Ip/Ic) − (Ip/Ic)2/2 +(1 − Ip/Ic) ln (1 − Ip/Ic)] ········· (1) First layer 800 Second layer f : frequency Ip : peak value of transport current Ic : critical current of a cable Third layer 600 400 200 0 0 1,000 2,000 Transport current (Arms) 3,000 Fig. 11. Introducing Current into First, Second and Third Layer, Respectively. 86 It has been considered that A.C. loss of a cable is good agreement with the equation. But in this experimental result, A.C. loss of the conductor was smaller than that by the equation. It is considered that A.C. loss of the conductor is reduced by homogenized effect for the transposed segment conductor. As a result, A.C loss at flowing 1,000A was 0.1W/m7). 5. Conclusion We have developed the HTS cable with transposed segment conductors, which were consisted of insulat- ed Bi-2223 tapes, in order to make a current distribution uniform in the cable. To satisfy a requirement of tapes for the segment conductor, the narrow width Bi-2223 tape, which was sheathed with Ag alloy, was selected. Consequently, it became possible that a transposed segment was fabricated in less than 100mm for a crossing pitch. The 15m-long conductor was fabricated by winding a number of transposed segments around a stainless steel flexible former. D.C. critical current of the conductor was 2.6kA, which defined by 1µV/cm, at 77K. As the individual wire of the segment was transposed, current distributions in the same segments for the conductor was unified below changing rate of ±5%. Further, by adjusting spiral pitches for each layer, current distribution in 3 layers is homogenized below Fujikura Technical Review, 2002 ±6% in the range from 200A to 2,800A. Throughout these experiments for realizing homogeneous current distribution of wires in the conductor, we carried out a development of the low A.C. loss HTS cable, which A.C. loss was 0.1W/m at flowing 1000A. References 1) A. Kume, et al.: Adv. In Supercond. VIII Vol.2, p.1307, 1995 2) K. Ohno, et al.: Adv. In Supercond. XII Vol.1, p.833, 1999 3) J. Fujikami, et al.: Adv. in Supercond. XI Vol.2, p.903, 1999 4) S. Mukoyama, et al.: Adv. in Supercond. XI Vol.2, p.1373, 1999 5) N. Futaki, et al.: Adv. In Supercond. XII Vol. 1, p.736, 1999 6) W. T. Norris: J. Phys. D (Applied Physics) Vol.3 p.489, 1970 7) K. Goto, et al.: Physical vol. 357-360, p.1255, 2001 87