Yoshihiro Iwata

Uniform current distribution conductor of HTS power cable with variable tape-winding pitches

In the multilayer conductor, the inner layers have higher impedance than the outer layers. As a result, the current concentrates in the outer layers. Our early study showed that its AC losses were reduced to one-tenth by making the current of each layer uniform. From such a point of view, a trial to realize the uniform current distribution was made by adjusting the winding pitches of high-temperature superconducting (HTS) tapes layer by layer. A 1 m long conductor was fabricated, where the inner layer had longer winding pitch than the outer layer. Experimental results showed that the currents flowing in individual layers were almost the same and that this conductor had lower AC losses than the nonuniform current distribution conductor.

50-m long HTS conductor for power cable

The long conductor fabrication is one of key technologies to realize the High-T/sub c/ superconducting power cable. A 50-m long conductor was fabricated by helically winding the High-T/sub c/ superconducting tape onto a former with a winding machine. The conductor consisted of ten layers of Ag-sheathed Bi-2223 tape which had a high critical current density of 10,000 A/cm/sup 2/ (at 77 K). AC losses and layer-by-layer current distribution were measured, feeding AC current of 100 to 2,000 Arms to the conductor cooled by LN2. The results showed that most of the current flowed in the outer layers where the impedance was low, and that AC losses were remarkably reduced by making the current distribution uniform.

HTS large scale application using BSCCO conductor

The basic property of high-Tc superconducting cables (HTS cables) using Bi-2223-based Ag-sheathed multifilamentary wire (Ag-sheathed wire) have been investigated for the realization of large-scale and compact cables, these being replaceable with conventional cables in existing ducts or tunnels. The AC performance of multi-layer HTS conductors, and three-phase HTS cables with coaxial superconducting magnetic shielding structure was evaluated. The characteristics of the HTS conductor and cable models of long length was investigated on a 50 m scale.

Electromagnetic interfilament coupling of silver-sheathed Bi-2223 multifilamentary tapes in transverse AC magnetic fields

Abstract To discuss the electromagnetic interfilament coupling of silver-sheathed multifilamentary Bi-2223 (HTS) tapes, AC losses of HTS tapes with various twist pitches and sample lengths in transverse magnetic fields were measured at 77 K by a magnetization method. Both hysteresis loss and eddy current loss in fields perpendicular to the HTS tape surface were approximately one order of magnitude greater than those in parallel fields. While AC losses in parallel fields were extremely reduced with a decrease in the twist pitch or the sample length, those in perpendicular fields scarcely decreased. AC losses of the untwisted HTS tape, which were proportional to the frequency in a perpendicular field of 29 mT, were slightly smaller below 70 Hz and larger above 150 Hz than those of the HTS tape twisted at a pitch of 8.6 mm. It is concluded that the coupling loss of untwisted HTS tapes is much smaller than the eddy current loss at commercial frequencies because of complete electromagnetic interfilament coupling.

Development of non-filled EPR insulation system for power lead of 66 kV-class superconducting fault current limiter

Development of ethylene-propylene rubber (EPR) insulation system has been advanced for a compact power lead of 66 kV-class high temperature superconducting fault current limiter (SCFCL). It is well known that insulation layers of superconducting cables are molded by extruding EPR containing inorganic filler. The power leads are not as long as the superconducting cables, making it difficult to mold their insulation layers by extrusion. When molding EPR by heat treatment, voids tend to remain in the insulation layer because of the low viscosity of filler-containing EPR. Partial discharge will occur and insulation deteriorates if voids exist in the insulation layer. Fluidity of EPR without filler (non-filled) was experimentally investigated to be used for insulating power leads. It was found that the viscosity of non-filled EPR is sufficiently low. It was also found that non-filled EPR has excellent crack resistance. An insulation model was easily constructed with simple dies. No voids were found in the insulation layer of this model, and no cracks occurred by thermal cycling between 333 K and 77 K. Consequently, it was confirmed that non-filled EPR is a suitable material for insulating power leads.

AC losses to be reduced by twisting filaments in a silver-sheathed Bi-2223 multifilamentary tape

An investigation is carried out to manifest types of AC losses in silver-sheathed Bi-2223 multifilamentary tapes that can be reduced by twisting filaments. The AC losses, in external magnetic fields parallel to the tape surface, consist mainly of the AC losses due to macroscopic screening currents in the multiple filaments and the AC losses due to the microscopic screening currents in the grains. The former loss depends greatly on the filament twist pitch, while the latter does not. Below the critical changing rate of a magnetic field at which screening currents at the outer filaments begin to saturate, the inter-filament coupling losses are added to the hysteresis losses for the equivalent filament size. Consequently, an analytical model can successfully explain experimental results that the macroscopic AC losses are considerably reduced by twisting filaments.

Spiral Pitch Adjusted Multi-layer Conductor with Alloy-sheathed Wire

Current distribution of spiral pitch adjusted multi-layer conductor was investigated with rogowski coils. Condition of the spiral pitch to suppress the unbalanced current distribution was determined by circuit model regarded the multi-layer conductor as a circuit network with mutual and self inductances. The conductor was fabricated with alloy-sheathed tapes. It was confirmed unbalanced current at each layer was reduced and the conductor loss was reduced in spiral pitch adjusted conductor.

Numerical Approach for a Description of Electromagnetic Phenomenon in Superconducting Composites

A numerical simulation code was developed to deal with electromagnetic phenomenon in type II superconductors and their composites with a modification to a commercial 3-dimensional FEM code. Two kinds of description for type II superconductors were considered, the critical state model and the power law current-voltage characteristics model. The validity of the algorithm was confirmed by an analysis of a uniform cylindrical conductor carrying a forced current where a analytical treatment is possible. As an initial investigation using this code, we analyzed a multi-layer conductor focusing on a relationship between AC loss and a distribution of current among different layers.

AC Loss of Transposed HTS Cable Conductors

We have developed new type HTS conductors in which all strands are transposed to balance the current distribution. Ic of the transposed conductor assembling subcables which consists of stranded Bi-2223 round wires was up to 1.2kA AC loss of the conductor was one order lower than that of the non-transposed conductor which has the multi-layer structure, assembling Ag sheathed tapes.

A Design of HTS Cable Conductors with Low AC Losses

A design to reduce ac losses of the cable conductors was investigated by using Bi-2223 Ag-sheathed strands with round shape. These new cable structures were designed and evaluated concerning with productivity based on the production experience of conventional metallic ac superconducting wires. Properties required for the conductors were also discussed regarding with characteristics of the strands. It was found that a design of practical cable conductors with low ac losses less than 1W/m was possible by realizing Bi-2223 Ag-sheathed round wires with Jc reaching 10,000A/cm2 at 77K.