Wilfried Goldacker

Status of high transport current ROEBEL assembled coated conductor cables

Assembling coated conductors (CC) into flat ROEBEL bars (RACC cable) was introduced in 2005 by the authors as a practicable method of reaching high transport currents in a low AC loss cable, which is a cable design suited for application in windings. The transport current of 1.02?kA in self-field at 77?K achieved so far, however, is still too low for several applications in electrical machinery such as larger transformers and generators/motors. A new cable concept for further increased currents was presented just recently. The goal of the new design was primarily to demonstrate the possibility of strongly increased transport currents without changing the important cable features for low AC losses. such as, for example, the transposition length of the strands. We present detailed investigations of the properties of this progressed cable design, which has threefold layered strands, an unchanged transposition pitch of 18.8?cm and finally the application of 45 coated conductors in the cable. A 1.1?m long sample (equivalent to six transposition lengths) was prepared from commercial Cu stabilized coated conductors purchased from Superpower. The measured new record DC transport current of the cable was 2628?A at 77?K in self-field (5??V?cm?1 criterion). The use of three slightly different current carrying batches of strand material (? 10%) was a special feature of the cable, which allowed for interesting investigations of current redistribution effects in the cable, by monitoring a representative strand of each batch during the critical current measurement. Although current redistribution effects showed a complex situation, the behaviour of the cable was found to be absolutely stable under all operational conditions, even above the critical current. The high self-field degradation of the critical current reached the order of 60% at 77?K, and could be modelled satisfactory with calculations based on a proven Biot?Savart-law approach, adapted to the specific boundary conditions given in this new cable design.

High critical current densities in Bi(2223)/Ag tapes

Bi(2223)/Ag monofilamentary tapes have been characterized by measurements of jc vs. B and vs. theta , the angle between the applied magnetic field and the c axis. High degree of texturing, high density and Bi(2223) single phase conditions were found to be the major effects leading to high jc values. A further condition for reaching the highest jc values is a very low overall carbon content, the presence of carbon at the grain boundaries acting as a strong limiting factor. The highest value of jc in the present work is 4.104 A/cm2 at 77K,0T, corresponding to more than 2.105 A/cm2 at 4.2K,0T.

Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables.

Accelerator-Quality HTS Dipole Magnet Demonstrator Designs for the EuCARD-2 5-T 40-mm Clear Aperture Magnet

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The Enhanced European Coordination for Accelerator Research and Development (EuCARD-2) Work Package 10 is a collaborative push to take high-temperature superconductor (HTS) materials into an accelerator-quality demonstrator magnet. The demonstrator will produce 5 T stand alone and between 17 and 20 T when inserted into the 100-mm aperture of a Fresca-2 high-field outsert magnet. The HTS magnet will demonstrate the field strength and the field quality that can be achieved. An effective quench detection and protection system will have to be developed to operate with the HTS superconducting materials. This paper presents a ReBCO magnet design using a multistrand Roebel cable that develops a stand-alone field of 5 T in a 40-mm clear aperture and discusses the challenges associated with a good field quality using this type of material. A selection of magnet designs is presented as the result of the first phase of development.

The EuCARD-2 Future Magnets European Collaboration for Accelerator-Quality HTS Magnets

EuCARD-2 is a project supported by FP7-European Commission that includes, inter alia, a work-package (WP10) called “Future Magnets.” This project is part of the long term development that CERN is launching to explore magnet technology at 16 T to 20 T dipole operating field, within the scope of a study on Future Circular Colliders. The EuCARD2 collaboration is closely liaising with similar programs for high field accelerator magnets in the USA and Japan. The main focus of EuCARD2 WP10 is the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, The cable will be used to wind a stand-alone magnet 500 mm long and with an aperture of 40 mm. This magnet should yield 5 T, when stand-alone, and will enable to reach a 15 to 18 T dipole field by placing it in a large bore background dipole of 12-15 T. REBCO based Roebel cables is the baseline. Various magnet configurations with HTS tapes are under investigation and also use of Bi-2212 round wire based cables is considered. The paper presents the structure of the collaboration and describes the main choices made in the first year of the program, which has a breadth of five to six years of which four are covered by the FP7 frame.

ENSYSTROB—Resistive Fault Current Limiter Based on Coated Conductors for Medium Voltage Application

A new German government funded project for a resistive fault current limiter has started in September 2009 (ENSYSTROB) and is presented. The consortium includes partners from industry, research centers and utilities. It aims the construction of a 3-phase medium voltage current limiter (12 kV, 800 A) for the protection of the domestic supply in a power plant. A special feature of this application is the presence of large in-rush currents (4100 Ap for 50 ms, 1800 A for 15 s). The superconducting components are bifilar pancake coils consisting of pairs of YBCO-tapes in face to back configuration. Successful limitation experiments on prototype components with prospective currents in the whole range are shown. Also the stability with respect to high voltage could be demonstrated. The AC-losses under normal operation are shown, by simulation and experiment, to be smaller than the heat input of the current leads. The current limiting components are compared with similar elements based on BSCCO 2212 bulk material (Nexans SuperConductors).

Experimental assessment of the current-limiting mechanisms in BSCCO/Ag high-temperature superconducting tapes

The current - voltage characteristics and the magnetic field dependence of the critical current of a range of mono-core BSCCO 2223 tapes are presented, illustrating the complementary use of transport and magnetization experiments in determining and analysing the current-limiting dissipation processes in these HTS conductors. Below a magnetic cross-over field the samples resemble a Josephson-linked current network, with the dominant dissipation at the weakest grain boundaries. In this regime, increasing field leads to a gradual fragmentation of the network. The network homogeneity and connectivity can be inferred from screening current length-scale measurements and comparison of transport and magnetization measurements. Above the cross-over field , flux motion within the surviving strongly linked backbone dominates the dissipation. The details of the dependences of the critical current and flux creep rate on magnetic field can be used to examine the intragranular pinning potential in the tapes. We show that, despite the wide range of transport critical-current values of the samples examined, the intragranular pinning proves to be remarkably sample independent. We conclude that while at low fields may be increased by improved processing that yields better intergrain connectivity, the high-field can be enhanced only by strengthening the pinning within the BSCCO 2223 crystallites themselves.

Design and Production of the ECCOFLOW Resistive Fault Current Limiter

The European project ECCOFLOW aims at a versatile resistive-type superconducting fault current limiter. For the first time, such a device will be tested at two different sites. The project partners have developed a superconducting fault current limiter design based on REBCO tapes with respect to the specifications provided by two hosting utilities. The limiter will operate at 1 kA at a rated voltage of 24 kV, and will be tested in both a busbar and a transformer feeder application. The design started with extensive investigations on the superconductor tape properties, especially with respect to the limitation behavior in all possible scenarios, and subsequent wide-range modeling of its in-grid behavior. The general integrated layout provides a limitation time of up to 1 s. The limiter is cooled using Gifford-McMahon cryocoolers to recondense the evaporated nitrogen. The present status of production and testing will be presented as well.

Cube-textured nickel and Ni alloy substrates for YBCO coated conductors

Abstract This paper deals with the texturing of nickel and NiCr, NiW and NiCrW alloys. Concurrently with measurements of the primary recrystallization kinetics, the recrystallization texture of these materials was investigated in dependence on the annealing temperature and time. An increase of the recrystallization temperature or time sharpens the texture due to grain growth, limited however by the occurrence of secondary recrystallization when a sample specific temperature is exceeded. The most favorable recrystallization temperature was found to correlate with the recrystallization kinetics, varying with the sample composition. The reduced stacking fault energy (SFE) in binary Ni alloys in comparison with pure Ni favors the formation of recrystallization twins {1 2 2}〈2 1 2〉 . The effect of a ternary alloy on the SFE is not easy to predict. So we tried to find, experimentally, a ternary alloy with medium to high SFE to produce the cube texture.