Laureline Porcar

Current limitation with bulk Y-Ba-Cu-O

The fault current limiter is a very attractive device for electric networks. Meander pattern conductors cut from bulk melt textured YBCO were studied for this application. The meanders are put in series and/or in parallel to match the required current and voltage. The YBCO materials are attractive because they show a very effective limitation with a relative low volume (high engineering current density and normal state resistivity). However, they are sensitive to hot spots. To avoid these destructive hot spots the operating temperature is chosen very close to the critical temperature (above 90 K). This temperature range is reached using a pressurised liquid nitrogen bath. Working close to Tc has two major advantages. The first is reduced values of Jc which limit the power dissipation. The Jc can be matched by changing the pressure on the nitrogen bath. The second is that the proximity of the normal state is favourable for homogeneous quenches along the whole meander as it can be experimentally recorded. Results obtained on single meanders and on the whole assembly are reported under steady state operation as well as during current limitation. Forty three meanders in series limited the current to 740 A (11 000 A unlimited value) under 1 kV.

Processing of large Y1Ba2Cu3O7-x single domains for current-limiting applications

Bulk-textured YBa2Cu3O7-x single domains could be used for current-limiting applications by cutting and assembling long meanders, which would be submitted to network electric fields before using a breaker to interrupt a fault current. For that purpose, large YBaCuO single domains up to 93 mm in diameter can be isothermally grown by using a standard melt texturing growth (MTG) process with a SmBaCuO seed. The essential parameters that have to be controlled in order to reach this size are the temperature growth window, the substrate reactivity and the temperature homogeneity in the sample. Standard 45 mm diameter single domains show excellent superconducting properties, such as Jc above 105 A cm-2 and a homogeneous superconducting-to-normal transition at 91.8 K for 20 cm long conductors cut in these pellets. These measurements demonstrate the long -range homogeneity of single domains regarding Tc and Jc. Nevertheless the high Jc values lead to a too large a dissipation in the normal state at T = 77 K. Different methods to reduce the critical current density are described in order to fulfill the conditions for a safe recovery of the material after undergoing a magnetothermal transition.

High critical transport currents of melt textured YBCO up to 6000 A

Abstract A melting zone processing has been developed to texture long bars of YBCO (10 cm length and 5 mm diameter). The misorientation of ( a , b ) planes with the sample long axis is weak. The transport current is produced by a capacitor discharge with period varying between 1.25 and 20 ms. Pulsed transport currents reaching 3000 A (15000 A/cm 2 ) and 6000 A (30000 A/cm 2 ) could cross resistanceless samples at 77 K over several centimeters length. These results show that YBCO can transport high nominal currents even up to its critical current thanks to high quality of contancts. Above the critical current, the material has been protected against the development of the magnetothermal transition.

Large piezoresistance and magnetoresistance effects on Ni45Co5Mn37.5In12.5 single crystal.

We report resistivity measurements as a function of magnetic field or uniaxial stress across the martensitic transformation on Ni45Co5Mn37.5In12.5 Heusler single crystal. The resistivity and transformation temperatures are strongly affected by the application of an uniaxial stress or a magnetic field and consequent magnetoresistance and piezoresistance that can reach up to 60% or 122%, respectively, are measured. This behavior opens up a large range of possible applications with a relatively small pressure or magnetic field as a control parameter to tune the resistivity.

Fabrication and characterization of thin-wall YBCO single-domain samples

Thin-wall geometr yi mplies a regular network of holes in a bulk, centimetric sample so that its effective thickness can be considered to be smaller than 1.5 mm. Thin-wall geometry offers a significant potential for helping the oxygenation process, mechanical reinforcement and thermal stabilization. Thin-wall geometry was obtained by drilling holes in slightly sintered pellets or by pressing pellets with embedded needles. The growth of a single domain, up to 50 mm in diameter, on such thin-wall geometry pellets was confirmed, despite the presence of many-hole patterns, by in situ high-temperature video monitoring. A significant decrease of porosity and cracks is observed, associated with the reduction of diffusion paths produced by thin-wall geometry. The improvement of the material quality is established by a significant increase (about 40%) of the magnetic trapped field in thin-wall samples. (Some figures in this article are in colour only in the electronic version) M This article features online multimedia enhancements

Magneto-Optical Investigations of LZO Buffer Layer Thickness Effects on YBCO Microstructure in the Simple NiW/LZO/YBCO Structure

The NiRABiTS/La2Zr2O7MOD/YBa2Cu3 O7MOCVD structure is a promising architecture for (YBCO) coated conductors. We succeeded in growing highly textured superconducting YBCO films on LZO-buffered Ni-5 at%W (NiW) substrates by metalorganic chemical vapor deposition (MOCVD). A single LZO buffer layer grown by metalorganic decomposition (MOD) is sufficient to ensure structural compatibility between YBCO and Ni, and to protect the substrate from oxidation during YBCO deposition. Textured Ni substrates obtained by rolling technique have intrinsic defects such as grain boundaries and rolling scratches. It is not easy to highlight their effects on LZO and YBCO films by usual measurements like XRD or SEM. The combination of Electron Backscattered Diffraction (EBSD) and Magneto-Optics imaging (MO) is useful to image grain boundaries and flux distribution in coated conductors. It provides complementary information on superconducting film qualities. These two techniques allowed us to understand how the microstructure of the LZO buffer layer and thus of the YBCO film is linked to the current density in this simple heterostructure.

New HTS 2G Round Wires

Key components for the power transmission and distribution for the future grids are superconducting cables. The energy network in highly populated areas will soon fully benefit from this technology, in particular for the retrofitting of the existing cables. However retrofitting the existing grid in underground structures (pipes, pits, turn), requires cables as small a diameter as possible. The HTS cable core is generally manufactured by laying tapes around a former with a diameter of about 25 mm to prevent any damage to the tape during the cabling and the handling operations. However in this configuration, the former cannot be used to transport the current. This triggered a new approach in cable design involving round 2G HTS wires with a diameter from 1 to 2 mm. They can be assembled to build a small cable core with a high ampacity. The benefits of this new wire design on power cables are discussed. The first results on different manufacturing processes and the characterizations of the 2G HTS wires are presented.

Superconducting fault current limiter with bulk materials

Abstract Superconducting materials offer the unique possibility to limit fault currents by their self-triggering quench to a resistive state above a given threshold current. Their use in power networks would improve the power quality, which is a real need today. YBCO or BSCO bulk materials are good candidates for current limitation. Their behaviours are rather different, in particular the electric field versus current characteristics. BSCO compounds are protected by their homogeneous quench due to the numerous defects (grain boundaries) distributed along the material. An assembling of 60 sintered Bi bars was tested under 1 kV. The current limitation is effective (a 5000 A short-circuit has been limited to 1080 A) but it occurs at 24 times the critical current. The increase of the critical current density of Bi compounds by their texturation reduces the current excursion above the critical value. In YBCO bulk elements, the defects are localized and make YBCO very sensitive to hot spots. Those are unavoidable and the operating conditions should be adapted in order to make them non-destructive. That is the self-protection concept. To fulfil it, we chose to operate near the critical temperature (above 90 K) to reduce the critical current density. The small difference with T c is very favourable for the quench. A fault current limiter based on 43 YBCO meanders working at 90.5 K in a pressurized liquid nitrogen bath is presented. The meanders are cut in 45 mm in diameter single domain pellets elaborated using the top-seeding technique. Supplied under a maximum voltage of 1 kV, the current was limited at 740 A instead of the theoretical value of 6500 A without quench.

Current limitation based on bulk YBaCuO meanders

Abstract This work aims at testing the performance of melt textured YBaCuO to limit fault currents. To do so, a 1 kV resistive fault current limiter (FCL) has been built and tested. The basic elements of this demonstrator were meanders cut out of textured pellets. Preliminary tests on meanders showed that the operating temperature of the limiter had to be above 90 K in order to obtain a homogeneous quench. The upper limiting capabilities of YBaCuO single bars and meanders were measured at 90.5 K. Bars reached a maximum electric field of 500 V/m and an average temperature of 280 K after a 20 ms short-circuit, while meanders attained values of 320 V/m and 210 K. The 1 kV demonstrator consisted of 43 meanders connected in series, with a shunt resistor added in parallel to each meander. The operating temperature was 90.5 K and the nominal current 100 A peak . Although this demonstrator succeeded to limit a fault current of 5.1 kA peak at a value of 730 A, several of the meanders broke during limitation.

Non destructive quench of bulk YBaCuO in liquid argon

Abstract The superconducting fault current limiter (FCL) is particularly attractive since no conventional limiting device exists for high voltage networks and thus corresponds to a real need. YBCO is a good candidate for current limitation due to its large critical current density and its large normal state electric field. This material is, however, very sensitive to hot spots in liquid nitrogen. An operation at higher temperatures decreases the critical current density and consequently reduces local heating (hot spots). Tests in liquid argon (87.3 K) were carried out. The critical current density remained large (about 50 MAm −2 , 0 T). For instance the limiting current is much higher (11–15 times) than the critical current but to our knowledge, it is the first time that a non destructive quench was observed in bulk YBCO mono-domain sample. The power and energy per unit volume developed during the transition reached 75 GWm −3 and 350 MJm −3 respectively. These values are large enough to induce an effective current limitation and weak enough to restrict overheating and resulting destruction. In this paper we report measurements and their analysis and show a new and hopeful way to improve YBCO limitation efficiency.