High current bi-2223 conductors with innovative wire geometry for power applications (BIG-POWA)

An equivalent circuit model for superconductors was developed, describing the behaviour of the conductor as viewed by an external user, for whom the global variables voltage, current and power losses are of interest. It can be used in time-continuous simulations with arbitrary input current or voltage waveforms and treats well both the sub-critical and super-critical current regime. The model is based on Maxwell¿s equations, measurement results, as well on the physical structure of the superconducting wires. Both current and voltage can be used as driving sources. It was implemented in the engineering software MATLAB, in whose environment a user-friendly graphical interface was created, facilitating the exploitation of the model. A new, modified power-law was used for describing the voltage-current relation of the superconducting material for currents above the critical one. The outputs of the model are the instantaneous current, voltage and power loss waveforms. The model is fast, compared to finite element simulations or other similar methods, and easy to apply. It may be used for modelling of superconducting devices, such as fault-current limiters and power cables.

Square/round wires enable an easy assembly when fabricating high current capacity conductors compared with the existing flat tapes. By using a drawing machine and a two-axial rolling machine, we have succeeded in preparing round wires and square wires. However, it is still a challenge to obtain high textured and uniform Bi-2223 filaments in the wires. A number of square/round wires have been fabricated by using two-axial rolling machine. Due to special deformation method, the properties of square/round wires are quite different to the standard flat tapes. We have experimentally showed that, for same filamentary configuration, the critical current of round wire always is lower than that in square wires. Therefore, our studying in this project mainly focuses on the square wires, with respect to the deformation, heat treatment, AC losses and so on. Long Ag-sheathed multifilamentary Bi(2223) square wires(more than 200 meters) fulfilling the specifications set out in the project have been successfully fabricated. The 40-filament square wire has a 90o-rotation symmetrical filament arrangement and therefore exhibits an almost angle-insensitive behavior with respect to the applied magnetic field. A critical cuurent density of 18kA/cm2 at 77K, 0T has been obtained, which is very close to the planned value of 20kA/cm2. This represent a very high level, being only a factor ~2 below the highest values obtained for flat Bi,Pb(2223) tape.

Two double-pancake coils were wound of the bundle conductor consisting of 7 parallel transposed mutually insulated HTS tapes. Two other double-pancake coils were wound of the bundle conductor consisting of 20 parallel non-transposed non-insulated HTS square wires. Critical current, current distribution, power loss and temperature were measured. Each transposed coil has 2 x 10 turns and dimensions: dia. 264/244 mm, length 37 mm. Critical currents of the individual coils are 115 A and 113 A. The limit of thermal stability is 220 A DC and 180 A RMS. Each non-transposed coil has 2 x 3 turns and dimensions: dia. 292/244 mm, length 16 mm. Critical currents of the individual coils are 176 A and 179 A. The limit of thermal stability is 320 A DC at least. The critical current of the whole experimental winding (2 transposed coils in the centre, 2 non-transposed coils at the ends) is 122 A. The winding withstands the AC current pulse 677 A pk / 4 s at least. Obtained results and experience will be used for further research and development particularly of superconducting transformers.

The silver sheath that constitutes the largest volume fraction in Bi-2223 superconducting tapes also represents the most important limitation to the predictable future cost reduction for these otherwise promising conductors to be employed in power applications. The present result is based on an original technique that allows for a substantial replacement of silver sheath in fully reacted conductors. The technique is fully scaleable and applicable to conductors manufactured by the standard PIT-process. More than 50% of the entire silver amount as been replaced by a low-cost alloy that also presents substantially higher electrical resistance and lower thermal conductivity than silver, together with enough mechanical strength to sustain tape handling. A performance reduction due to the post processing has been estimated to be below 10%. The effectiveness of the processing as been tested on conductors provided by a number of industrial manufacturers as well as by research laboratories. The silver replaced conductors are ideal for a number of applications, e.g. to be used as detectors for Medical Resonance Imaging.

Textured materials were developed. These models can be used in 2D numerical simulations of non-twisted multifilamentary conductors with various filament arrangements in applied transport current and/or magnetic field of different orientation. AC loss optimization for use of Bi-2223/Ag wires and tapes was achieved for various operating conditions. A new 3D formulation was implemented in the software package Flux3D, and was used for a detailed study of the coupling effect between superconducting filaments via the resistive (Ag) matrix, induced by an externally applied varying magnetic field. These new results go beyond the theoretical predictions, which are valid only for the particular case of fully penetrated infinite slab geometries. Investigated was the dependence of the inter-filamentary coupling on several physical and geometrical parameters. The 3D formulation, coupled with circuit equations for simulating an external current source, was successfully tested on typical 3D geometries, such as a corner-shaped conductor and a mono-layer cable, composed of eight twisted wires. The developed advanced numerical methods represent powerful state-of-the-art tools for modelling of high-Tc superconductors in 2D and 3D.

A high-speed field-mapping system with 7 and 15 Hall probes, using 8 or 16-channel digital lock-in amplifier, was designed, constructed and is operational at frequencies up to 50Hz. The system is now able to simultaneously generate the Hall excitation current, to acquire the data simultaneously, to compute the lock-in product on 8 or 16 channels, and to store the results in adapted files. The data for the measured dynamic field on the surface of the superconductor with applied transport current was then used in a specially developed inverse problem algorithm for finding the corresponding current distribution. The method is based on the application of Ampere¿s theorem to a superconductor in the Critical State Model. The reconstructed current distribution was compared to results from finite-element-method simulations in order to mutually validate the two methods. It was demonstrated that the current reconstruction method gives sufficiently accurate description of the current distribution in the superconductor. The developed novel measurement system and reconstruction algorithm represent state-of-the-art characterization technique for describing dynamically the field and current distribution in high-Tc superconductors.

An electrical model for multi-layer superconducting cables was developed, which can be used for finding the optimal geometrical configurations (winding pitch and direction of each layer), leading to a uniform current distribution among the layers and giving the lowest AC losses. The model is implemented in the common engineering software MATLAB. This model can take into account the three-dimensional structure of the cables and its rapidity of calculations allows investigating a great number of combinations of the geometrical parameters. The model can be applied for cables with different number of layers. The finite element method was used in addition for computing in detail the current and magnetic field distribution inside the tapes composing the cable, in order to precisely evaluate the AC losses, which are strongly influenced by the geometry of the tapes and their arrangement in the cable structure. The dependence of the transport properties of the cable on the generated self magnetic field was also taken into account, since for high applied currents the self-field effects are significant and cannot be neglected. The developed models may be of interest to manufactures and end-users of superconducting power cables.

Any electrical device experiences transient over-currents and the response of a superconducting winding to over-currents is an important issue. The behaviour of superconducting elements (tape alone and coils) under currents above their critical value has been experimentally carried out. If a single tape can withstand very high over-currents (5-10 Ic), a superconducting coils with several non-cooled layers is much more sensitive to over-currents. For a nine layer coil, the temperature of 200 K is reached after a delay of only 0.29 s for an over-current of 2.65 Ic (Je = 34 MA/m2). Several coils with temperature sensors were built and tested. The runaway logically occurs at lower currents compared to the critical one when the wire critical current density increases. The thermal runaway even appears very closed to the critical current, 1.4 Ic (Je = 34 MA/m2). Some numerical simulations (finite element method) in steady state show a good agreement between the measured and calculated temperatures taking into account the inaccuracy for some thermal data. Some temperature calculations have been made also in transient considering adiabatic conditions. The agreement with the measured temperatures is good if the tape volume is multiply by a factor two. Any electrical device experiences transient over-currents and the response of a superconducting winding to over-currents is an important issue. The behaviour of superconducting elements (tape alone and coils) under currents above their critical value has been experimentally carried out. If a single tape can withstand very high over-currents (5-10 Ic), a superconducting coils with several non-cooled layers is much more sensitive to over-currents. For a nine layer coil, the temperature of 200 K is reached after a delay of only 0.29 s for an over-current of 2.65 Ic (Je = 34 MA/m2). Several coils with temperature sensors were built and tested. The runaway logically occurs at lower currents compared to the critical one when the wire critical current density increases. The thermal runaway even appears very closed to the critical current, 1.4 Ic (Je = 34 MA/m2). Some numerical simulations (finite element method) in steady state show a good agreement between the measured and calculated temperatures taking into account the inaccuracy for some thermal data. Some temperature calculations have been made also in transient considering adiabatic conditions. The agreement with the measured temperatures is good if the tape volume is multiply by a factor two.

For reduction of ac losses of superconducting composites by optimizing geometry, filament configurations and normal matrix materials, it is desirable that individual loss components can be determined independently, whereas established techniques for ac loss measurements can only give the total loss. Bases on our previous research on ac loss measurements, SOTON has developed and refined within the Big-POWA project a novel technique, which enables the measurement of individual loss components. This technique involves two experiment protocols: - Direct measurement of the superconductor loss using the 3RD harmonic of the pick-up voltage, and indirect measurement of coupling current loss as the difference between the total and superconductor losses. This method is applicable to standard loss measurement technique using pick-up coils. - Direct measurement of coupling current losses using a pair of voltage taps long the direction of the applied field and transverse to the longitudinal direction. Protocols are normally applied in tandem for better accuracy. This novel measurement technique is relevant to all cases of ac loss in superconductors. It will not only aid the reduction of losses in conductor development, but also help to understand previously unknown behaviours of novel conductors. SOTON has a state-of-the-art ac loss measurement system integrated with this novel technique, and is ready to serve the community working on material development and power applications of superconductors.

The losses could be reduced by increasing the resisitivity of the metal matrix and by twisting the bi-2223 filaments. With round/square wires, smaller twist pitches than for flat tape can be obtained without damage to the filaments. Different square Bi,Pb(2223) wires with and without barriers SrZrO3 were prepared by two-axial rolling technique. The ac losses in several series of square wires were measured, and the results showed that there are considerably lower ac losses at 50Hz compared to the standard flat Bi(2223)/Ag tapes in a perpendicular magnetic field. The AC losses in the square wires are considerably smaller than in flat tapes, due to the small aspect ratio(1:1) of square wires.

One of the main technical-economical issue in the commercialization of cable for transmit power energy using HTS material is represented by the electrical losses in AC regime. Currently ¿standard¿ HTS materials commonly utilized for this application are composite tapes with superconductor straight filamentary BSCCO enclosed in a silver matrix. To demonstrate a reduction of cable AC losses special wires were developed by the consortium and a cable sample was realized. Tapes have 15-mm twisted BSCCO filaments (instead that straight), in a pure silver matrix. The 1.5-m, >1000 Amp IcDC Power Link model was manufactured in Pirelli laboratory. The cable was realized by a full industrial scalable process. Special end termination cable were also developed. Cable details: - Metallic flexible tubular former, - 2 layers helically wrapped with 15 tapes each, - AC/DC sensors, - External protection, - End copper terminations/connections. AC losses were experimentally measured by electric method on the twisted Power Link, and compared with a AC losses calculated by FEM analysis on an ¿equivalent¿ cable made with standard ¿straight¿ filament tape architecture. Results show a sensible reduction of losses in the low current regime with tape-twisted cable. At higher current, the coupling between filaments, as responsible for higher losses. More effective magnetic decoupling is auspicated.

Merck¿s responsibility in this working task is the manufacture and development of encapsulated precursor rods with high homogeneity in density and establishment of quality control test. The main goals for this task was to avoid powder contamination, to work on better batch to batch reproducibility leading to improved tape filament homogeneity and more Ic uniformity. To improve the reduction of powder contamination (esp. carbon dioxide from air) we have developed: - Fully encapsulated rod manufacturing process - Fully encapsulated rod handling - Fully encapsulated rod transportation - Qualified packages for protection of powder rods. Two designs for packages of CIP rods were tested: -- Aluminium tube with thread and cap with rubber O-ring -- Glass tube with plastic plugs and aluminium foil inside For the qualification of the new packages different properties have to be checked: - Transportation and handling properties - Protection against breakage - Sealing of package and long term stability - Potential contamination sources of all material used in packages.

The efforts have been focused on twisting round conductors (up to 7 wires) and making flat tapes with various twist pitches. In order to increase the critical current, a combination of optimisation runs were carried out. A twisting machine setup was developed. From the results it was concluded that there is no specific procedure, which results in a great improvement of the critical current. As a conclusion, this method should not be pursued.

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