M Vojenčiak

Phenomenological description of flux pinning in non-uniform high-temperature superconductors in magnetic fields lower than the self-field

Critical currents measured on a commercial Bi-2223/Ag tape in applied magnetic fields ranging from zero to 100 mT are analysed in order to demonstrate the validity of the phenomenological description proposed by the authors for flux pinning in polycrystalline high-temperature superconductors in low magnetic fields. The four-parameter model is a generalization of the Kim formula describing the magnetic field dependence of the critical current density. When combined with the finite-element calculation, it is able to predict the critical currents measured in different field orientations with an average accuracy better than 0.2%. The volume density of the flux pinning force is easily evaluated from the parameters of the model. Particular attention is paid to the problem of a possible sample non-uniformity. In fact, the experimental data can be better explained assuming the hypothesis of better filament texturing in outer filaments. This also means that the accurate determination of critical currents in low magnetic fields can be used to reveal a non-uniformity in the current-carrying capability of the superconducting material.

Experimentally determined transport and magnetization ac losses of small cable models constructed from YBCO coated conductors

Several short cable models were constructed from YBCO coated conductor (YBCO CC) to study their basic dc and ac electrical properties. They were prepared using superconducting tapes helically wound on fiberglass former of different diameter (5, 8 and 10 mm) with different twist pitch (from 1.7 up to 2.4 cm). The number of parallel-connected tapes ranged from 1 up to 6. The standard length of the models was 11 cm. In one case a 35 cm long model has been manufactured in order to perform a bending test. We observed that the critical currents of the models were proportional to the number of tapes used for their construction. Transport and magnetization ac loss were measured at 36 and 72 Hz.

Coated conductor arrangement for reduced AC losses in a resistive-type superconducting fault current limiter

The basic element of a resistive superconducting fault current limiter (FCL) can consist of coated conductor tape exceeding a few meters in length and compacted into a cryogenic envelope. This paper is focused on optimizing the arrangement of coated conductors with a non-magnetic substrate for a resistive superconducting FCL. Several configurations have been tested experimentally and theoretically. Two low-loss arrangements have been identified, both utilizing the bifilar configuration, i.e. the currents in two adjacent tapes are identical in amplitude but opposite in direction. The separation between two adjacent tapes s varied from 0.07 up to 2.10 mm. For the lowest examined separation s the AC transport loss of the straight bifilar model decreased by more than one order with respect to the AC transport loss in the single-tape configuration. Further AC loss decrease is achieved when the pair of tapes carrying opposite currents forms a flat pancake coil. We developed a numerical model in order to analyze the influence of distance between adjacent tapes. To achieve agreement between experimental and numerical results it was necessary to incorporate a lateral distribution of critical current density in the tape. The remaining differences between the results of experiment and calculation can be explained by analysis of experimental imperfections. Finally we suggest an empirical fit for the prediction of AC loss of a practical superconducting fault current limiter.

Performance Improvement of Superconducting Tapes Due to Ferromagnetic Cover on Edges

Improvement of critical current and reduction of AC loss is presented for a common multifilamentary Bi-2223/Ag tape with thin nickel layer electroplated at the edges. Numerical calculations using a commercial finite-element code have been carried out to find the distributions of electrical current and magnetic field when the whole section is filled with the critical current density. The dependence of critical current density on local magnetic field and its orientation was taken into consideration, allowing to understand the mechanism of Ic increase. Optimization of the cover thickness and width would be possible due to these calculations. AC loss was investigated experimentally in the condition of AC transport and simultaneous action of AC magnetic field in phase with transport current. Reduction of both the transport loss and the magnetization loss has been observed, as well as the loss at simultaneous action of transport AC and applied AC field.

AC Loss and Voltage Signal in a Pancake Coil Made of Coated Conductor With Ferromagnetic Substrate

The voltage signal and the AC loss in a pancake coil made of a coated conductor tape with magnetic substrate is measured. The experimental data have been analyzed with the help of numerical calculations. It is found that while for a single tape the AC loss is dominated by the substrate, for the coil it is dominated by the superconducting layer. Moreover, the substrate increases the AC loss generated in the superconducting material, making it similar to a slab. A compensation method for the voltage signal is also described, thanks to which the contribution from the substrate and the superconducting layer can be distinguished.

AC Loss Measurement of YBCO Cable Model

AC transport loss of the cable model constructed from YBCO coated tapes has been investigated. The single layer model 0.5 m long consists of 14 tapes placed on the surface of the epoxy- fiberglass tube of outer diameter 2 cm in one layer with 15deg lay angle. In the center of the model the Cu rope with cross section 45 mm2 connected in parallel with the tapes was placed as an over critical current protection. The coaxial brass tube was utilized as a return wire. The critical current of the individual tape was 75 A. Two procedures for determination of the cable AC transport loss were utilized. In the first one 14 measurement loops of the length 0.36 m with taps soldered to the individual tapes and lifted 1 cm above the cable were symmetrically placed around the cable. After measurement of the individual tape voltages their average was used for calculation of the cable AC transport loss. Second one was the so-called secondary voltage method. This method is based on the fact that the cable is energized through a power transformer. The voltage from resistive parts of the secondary was subtracted from the total secondary voltage measured by contact-less loop embracing the transformer core. Afterwards, this difference was taken to calculate the cable AC transport loss. Due to non-stabilized YBCO tapes the visualization of the individual tape transition to the normal state within the period of the transport current was possible.

Modification of critical current in HTSC tape conductors by a ferromagnetic layer

In some applications of tape conductors from high temperature superconductors (HTSC) the magnetic field is created by the transported current itself. This is e.g. the case of power transmission cables or current leads. Quite complex distribution of local magnetic field determines then the ability of the superconducting element to carry electrical current. We have investigated how much the critical current of a tape conductor can be changed by putting a ferromagnetic layer in the vicinity of the HTSC material. Numerical procedure has been developed to resolve the current and field distribution in such superconductor-ferromagnet composite tape. Theoretical predictions have been confirmed by experiments on sample made from Bi-2223/Ag composite tape. The critical current of such tape can be improved by placing a soft ferromagnetic material at the tapes edges. On the other hand, the calculations show that the ferromagnetic substrate of YBCO coated tape reduces its self-field critical current.

Losses in Bi-2223/Ag tape at simultaneous action of AC transport and AC magnetic field shifted in phase

Investigation of AC loss under simultaneous action of transport AC and external AC field is of prime importance for reliable prediction of dissipation in electric power devices like motors/generators, transformers and transmission cables. The experimental rig allowing to perform AC loss measurement in such conditions on short (10 cm) samples of tapes from hightemperature superconductor Bi-2223/Ag has been designed and tested. Both the electrical and thermal method have been incorporated, allowing to combine better sensitivity of former one and a higher reliability of the latter one. Our main aim is to see how the AC loss depends on the phase shift between the current and the field. Such a shift could acquire rather different values in various applications. While in transformer winding, the maximum phase shift at full load will probably not exceed a few degrees, in a three phases transmission cable in tri-axial configuration it is around 120°. Therefore we explored the whole range of phase shifts from 0 to 360°. Surprisingly, the maxima of dissipation do not coincide with zero shift as expected from qualitative considerations.

Can resistive-type fault current limiter operate in cryogen-free environment?

Superconducting fault current limiters are unique devices that offer fast response to fault without need of an external triggering system. Therefore, they are interesting for industrial use, although their price is high. The principle of the resistive fault current limiter is based on the steep current-voltage characteristic of superconductors. When the current rises over the critical one, voltage on the superconductor rises steeply, and this mechanism blocks the increase of the current. Simultaneous appearance of voltage and current during this limiting period of operation generates a substantial amount of heat that is dissipated in the superconducting wire causing a rapid increase of its temperature. In this contribution, we further develop the idea that, during a limiting period, there is only a small difference between cooling by liquid nitrogen and adiabatic conditions. Using this approach, one can think about a current limiter free from a liquid coolant using a cryocooler. In this contribution, we discuss main differences in cooling conditions and test the idea on a short sample of the high-temperature superconductor REBCO conductor. We compare the behavior of identical samples cooled to the same temperature by liquid nitrogen and conduction cooled. If the realization can be achieved, the huge benefit would be an FCL with a tunable triggering current via its operating temperature.

Experimental and numerical study of influence of ferromagnetic cover on critical current of BiSCCO-2223/Ag tape superconductor

Samples of commercially available BiSCCO-2223/Ag tape have been partially covered by the ferromagnetic material. Improvement of the self-field critical current up to 15 % has been achieved by this procedure. A critical current of such tape strongly depends on geometric and magnetic properties of both, the superconducting tape as well as the ferromagnetic cover. Numerical simulations, based on the critical state model using commercial finite element method (FEM) code, have been performed. Properties of superconductor are characterized by anisotropic dependence of the critical current density on magnetic field as well as detail geometry of filaments. The ferromagnetic material is characterized by nonlinear magnetization curve. Nonlinear dependences of the critical current on selected parameters are shown in this work. Optimization of the cover parameters using these curves has been made. Samples with various parameters have been manufactured for the confirmation of numerical simulations results. Experimental results are in good qualitative agreement with results obtained by numerical simulations.