Transport AC Losses Research Articles

Development and Characterization of a 2G HTS Roebel Cable for Aircraft Power Systems

Lightweight megawatt-range power cables in electric aircraft propulsion systems will allow reducing fuel consumption by more than a half. Taking into account the low voltage limitations for onboard use, superconductivity providing high current-carrying capacity becomes an enabling technology for this application. In the frame of a preliminary study of an onboard superconducting power distribution grid, we fabricated and tested a high-temperature superconducting (HTS) Roebel cable. The work was done by a consortium of institutions and supported by Airbus Group Innovations. SuperOx provided a 400 A-class 12-mm-wide 2G HTS wire. The team at the Karlsruhe Institute of Technology prepared 5.5-mm-wide Roebel strands with a transposition length of 226 mm and assembled the strands into a 6-m-long Roebel cable. The team at the Russian Scientific Research and Development Cable Institute performed characterization of the cable. Short samples of the cable were tested to determine their critical currents, mechanical properties, and stability during thermal cycling. For a 4-m-long section of the cable, we measured the critical current, as well as the transport ac losses at frequencies in the range from 50 to 400 Hz. This paper presents the cable design and test results. The feasibility of using 2G HTS Roebel cables in electric aircraft systems is discussed.

Transport ac losses of a second-generation HTS tape with a ferromagnetic substrate and conducting stabilizer

The current–voltage curve and transport ac loss of a second-generation HTS tape with a ferromagnetic NiW substrate and brass stabilizer are measured. It is found that the ac loss is up to two orders of magnitude larger than what is expected by the power-law E(J) determined by the current–voltage curve and increases with increasing frequency. Modeling results show that the overly large ac loss is contributed by the ac loss in the HTS strip enhanced by the NiW substrate and the magnetic hysteresis loss in the substrate, and the frequency-dependent loss occurs in the brass layer covering the substrate but not in the ferromagnetic substrate itself as assumed previously. The ac loss in the brass layer is associated with transport currents but not eddy currents, and it has some features similar to ordinary eddy-current loss with significant differences.

Scaling law and general expression for transport ac loss of a rectangular thin strip with power-law E(J) relation

Transport ac loss Q of a superconducting rectangular thin strip obeying a power-law relation E∝Jn as a function of current amplitude Im may be, following Norris, expressed by normalized quantities as q(im). A scaling law is deduced that if Icf, Ic and f being the critical current and frequency, is multiplied by a positive constant C, then im and qm are multiplied by C1/(n−1)and C2/(n−1),respectively. Based on this scaling law and the well-known Norris formula, the general function of q(im, n, f) is obtained graphically or analytically for any practical purpose, after accurate numerical computations on a set of q(im) at several values of n and a fixed value of f.

Transport ac loss of elliptical thin strips with a power-law E(J) relation

The transport ac loss Q of an elliptical thin strip of critical current Ic with a power-law relation is accurately computed as a function of current amplitude Im and frequency f. The resulting is normalized to following the Norris critical-state formula, and converted to at a critical frequency fc based on a transport scaling law. Having a set of at several values of n as a base, a general expression of is obtained, which can be used to easily calculate for any practical purposes.

Transport AC Losses in Soldered Joint of the YBCO-Coated Conductors

In this paper, we present a direct measurement of the transport AC loss in solder joint of the YBa 2Cu3O7−x -coated conductor (YBCO CC) at liquid nitrogen temperature. The method is based on measuring the temperature rise of the thermally insulated joint which is fabricated by soldering the YBCO CC samples face to face without removing the metallic stabilizer. The contact resistances in these joints are in the range of 10 to 40 n Ω. It is found that (1) the AC losses in the sample with lap joint dependence of I/I c can be described by use of a third-order power law, where I and I c denote the transport current peak and critical current of the YBCO CC sample, respectively. (2) The loss in sample with joint is greater than that of the original YBCO CC sample in case of I/I c<0.6, while there are no much in the loss in two kinds of sample when I/I c is in the range from 0.6 to 0.7. Once I/I c>0.7, the loss in sample with joint is less than that in the original tape.

Transport ac loss in a rectangular thin strip with power-law [formula omitted] relation

Transport ac losses of a rectangular thin strip obeying relation E/Ec=(J/Jc)n with a fixed critical current Ic and n=5,10,20,30, and 40 are accurately computed at a fixed frequency f as functions of the current amplitude Im. The results may be interpolated and scaled to those at any values of Ic,f, and 5⩽n⩽40. Normalized in the same way as that in Norris’ analytical formula derived from the critical-state model and converting f to a critical frequency fc, the modeling results may be better compared with the Norris formula and experimental data. A complete set of calculated modeling data are given with necessary formulas to be easily used by experimentalists in any particular case.

The scaling of transport AC losses in Roebel cables with varying strand parameters

A Roebel cable is a good candidate for low-voltage windings in a high-temperature superconductor (HTS) transformer because of its high current-carrying capability and low AC loss. Transport AC loss measurements were carried out in 1.8 m long 15/5 (fifteen 5 mm wide strands) and 15/4 Roebel cables. The results were compared with those in many Roebel cables composed of 2 mm wide Roebel strands. Comparison of the AC losses hinted that the intrinsic difference in normalized transport AC losses is due to differences in the g/w (ratio of the horizontal gap between the Roebel strands over the Roebel strand width) values. The intrinsic difference was confirmed by measuring transport AC loss in a series of horizontally arranged parallel conductor pairs with various g values. A method to scale transport AC losses in Roebel cables with varying strand parameters was developed. The scaling method will be useful for a rough assessment of AC loss in one-layer solenoid winding coils, such as in a HTS transformer.

Numerical Investigation on Thermal Stability of Conduction-Cooled Bi-2223/Ag Coil Under AC Ripple Current for Space Applications

This study investigated the transport ac loss in a high-temperature superconducting (HTS) coil under ac ripple currents with dc offsets for space applications of the HTS coil. We developed an analysis method to evaluate the effective ac loss in the HTS coil on the basis of the percolation depinning model. Our analysis clarified that larger dc offsets greatly increase the effective ac loss even under a smaller ac current. In addition, we investigated the effect of the ac loss with the ripple current on the thermal behavior of a conduction-cooled Bi-2223/Ag coil. As a result, the ac loss decreased the thermal stability of the conduction-cooled coil in case that HTS tapes in the coil are in the flux-flow state such as the load factor of 80%. However, at a lower load factor such as less than 60%, the ripple current did not have much effect on the thermal stability of the conduction-cooled coil because most of the ac power were consumed as a reactive power. These results suggested that, in order to apply the ac ripple current to HTS coils, the operational load factor must be properly selected. This study leads to a design of the light-weight HTS coil system for space missions.

AC loss properties of single-layer CORC cables

In this work, an experimental and theoretical study of the transport and magnetization AC losses of a single layer ReBCO cable is presented. Examined cable consisted of five coated conductor tapes wound in parallel helically on the round core with 40 mm pitch and six full transposition resulting in the 24 cm total cable length. Transport currents up to 600 A amplitude with frequencies 36 Hz and 72 Hz were used in the transport loss measurement. AC fields up to 50 mT were applied during the magnetization loss measurement at 36 Hz and 72 Hz, respectively. Very good agreement between the experimental results and the simulations for magnetization loss was achieved. However, the experimental results for transport loss are markedly greater than the prediction reached from simulation. Nevertheless, they are lower than the transport AC loss theoretically assumed for a straight superconducting tape with the identical critical current.

Numerical simulations of the alternating current loss in round high-temperature superconducting wire with a hole defect

This paper presents a finite element model to solve the electromagnetic behavior and the AC loss in round high-temperature superconducting wire with a hole defect both in external field condition and self-field condition. The hole defect is assumed to be infinitely long along the wire. The model is based on the H formulation and the highly nonlinear E−J characteristic. The simulation results for the round superconducting wire with a hole defect and the one without defect are compared. It is found that the existence of the hole defect causes small reduction for the magnetization AC loss and large enhancement for the transport AC loss. The influences of the position and shape of the hole on the AC loss are also investigated. We find that the AC loss in external field condition decreases when the hole defect moves towards the edge of the superconducting wire from its center. However, the feature is opposite in self-field condition. Meanwhile, the influence of the shape of the hole on the AC loss is not strong in both conditions.

Influence of the voltage taps position on the self-field DC and AC transport characterization of HTS superconducting tapes

The current–voltage (I–V) curve is the basic characteristic of a superconducting wire or tape. Measuring I–V curves is generally problematic when samples have poor stabilization. Soldering voltage taps to an active part of the conductor affects the effectiveness of the local cooling and/or can be difficult to do in certain devices such as fault current limiters and cables where the tapes are closely packed. In order to overcome these problems, voltage taps can be placed outside the active area of the superconductor. We proved both by simulations and experiments that this arrangement leads to the same results as the standard four point method and it provides more detailed information for sample protection. The same arrangement can also be used for AC transport loss measurement. However in this case particular care has to be taken because the eddy current loss in the current leads contributes to the total measured loss. We used numerical simulations to evaluate the contribution of the eddy current loss to the measured AC loss. With help of simulations one can determine whether the contribution of the eddy current loss is significant and possibly optimize the current leads to reduce that loss contribution.

Electrical measurements of AC losses in high temperature superconducting coils at variable temperatures

Measurements of AC losses in high temperature superconducting (HTS) coils wound from two different types of YBa2Cu3O7 (YBCO) coated conductors are reported. AC loss measurements by different arrangements of voltage loops and pick-up coils were investigated to propose accurate and convenient techniques to measure the AC losses in HTS coils, especially for large coils with the measurement signals significantly higher than the input range of typical lock-in amplifiers. A new and simple sub-cooling technique with an open liquid nitrogen bath was developed to measure AC losses in the sample coils at variable temperatures between 65 and 77 K. The temperature dependence of the losses in these coils was qualitatively explained based on the data on transport and magnetization AC losses in isolated tapes at variable temperatures.

Influence of Cracks in the Superconductor Layers Caused by Mechanical Stresses on Critical Currents and AC Transport Losses in YBCO CCs

Influence of mechanical stresses on critical currents and ac transport losses in YBCO CCs with copper stabilization layers is investigated by ac loss measurement and SEM observation of surfaces of YBCO layers. Tested YBCO strips were made by slitting mechanically a wide tape. Cracks caused by the slitting were found in the YBCO layer even in the virgin strips in the edge side areas. Mechanical stresses to strips widen and grow those original cracks. In the paper, the characteristics of critical current and ac transport current losses depending on mechanical stresses are explained in relation to features of the cracks.

The Dependence of Transport AC Loss on Temperature and DC Parallel Magnetic Field in an Eight-Strand YBCO Roebel Cable

We measure transport ac losses in an 8/2 (eight 2-mm-wide strands) YBCO Roebel cable at 77 and 65 K without dc parallel magnetic field, and at 77 K under dc parallel magnetic fields of 0-1.5 T. The transport ac loss in the Roebel cable scales with temperature and dc parallel magnetic field as the square of the cable's critical current.

Transport AC Loss Properties of a Bi-2223 Superconducting Coil From 0.1 Hz to 10 Hz

We propose a method to evaluate transport AC losses in Bi-2223 superconducting coils by integrating the local external-field losses of the stacked taped wires over the coil's windings. We measured the transport AC losses of a 16 double-pancake coil of twisted Bi-2223 Ag-sheathed tapes, in the frequency range from 0.1 to 10 Hz at liquid nitrogen temperature. We also measured the external-field losses of short specimens of 6-stack Bi-2223 wires both in perpendicular and parallel field configurations through a saddle-shaped pickup coil method. The external losses are mainly due to hysteresis losses in the superconducting filaments and coupling losses among the filaments. The local external-field loss in the test coil is approximately calculated as the sum of perpendicular- and parallel-field losses of the short specimen. In the calculation, geometrical effects because of the neighboring turns in the coil are also considered, in addition to the magnitude and direction distributions of the local magnetic field. We compared the numerically evaluated external-field losses to the measured transport AC losses, and discussed the applicability of the proposed method.

Numerical Analysis of AC Loss Reduction in HTS Superconducting Coils Using Magnetic Materials to Divert Flux

In this paper, the use of magnetic materials to divert flux in high-temperature superconductor superconducting coils and reduce transport ac loss is investigated. This particular technique is preferred over other techniques, such as striation, Roebel transposition, and twisted wires because it does not require modification to the conductor itself, which can be detrimental to the properties of the superconductor. The technique can also be implemented for existing coils. The analysis is carried out using a coil model based on the H formulation and implemented in comsol multiphysics. Both weakly and strongly magnetic materials are investigated, and it is shown that the use of such materials can divert flux and achieve a reduction in transport ac loss, which, in some cases, is quite significant. This analysis acts to provide a foundation for further optimization and experimental work in the future.

Numerical analysis of transport AC loss in HTS slab with thermoelectric interaction

This paper presents an investigation of the AC loss in high temperature superconducting slabs, when applying an alternating transport current of sinusoidal form. The proposed model describing this AC loss is built upon the following coupled physics: the Maxwell’s equations governing the electromagnetic fields, the intrinsic nonlinear relations, and the thermal diffusion equation governing the temperature fields. The numerical results demonstrated that the temperature of the whole slab (interior or exterior) increases monotonously with the frequency and amplitude of the applied alternating current. After considering the local temperature changing effect, it is found that most of the magnetic induction, the critical current density, and the local current density in the slab periodically vary with time with non-sinusoidal form, while the period is either the same or half of that of the applied current, which fully exhibits a typical characteristic of nonlinear systems. Further, the AC loss varying with the amplitude and frequency of the applied current has the power law as almost same as the analytical results based on Bean’s critical state model at low amplitude I0. However, for the case of high current amplitude I0, the numerical results gradually deviate from the analytical results and the local temperature changing has a significant effect on the transport loss.

AC Losses in HTS Tapes and Devices With Transport Current Solved Through the Resistivity-Adaption Algorithm

Alternating current (ac) losses in high-temperature superconductor tapes and devices with transport current are solved by using the resistivity-adaption algorithm (RAA). The most advanced feature of the RAA is that it enables the simulation of any model derived from the flux motion theory on finite-element analysis (FEA) packages that have an eddy current solver. The principle of the RAA, as well as its realization on the ANSYS FEA package, is introduced. The simulation begins with the calculation of the ac loss of an ellipse and of strips with aspect ratios ranging from 50 to 2000. The accuracy and efficiency of the calculation are verified through comparisons with the Norris theoretical curves. The possible errors and the method to overcome such errors are discussed. The most significant improvement in the proposed RAA from that discussed in a previous study is that the RAA was proven to be valid for calculating the field-dependent critical state model by using the descendant process from + <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> to - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> . We then extend this method to calculate the transport ac loss of a stack of ellipses with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> ) characteristic from a typical Bi2223/Ag tape and the transport ac loss of a stack of strips with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> ) characteristic from a typical YBCO-coated conductor.

Complex study of transport AC loss in various 2G HTS racetrack coils

HTS racetrack coils are becoming important elements of an emerging number of superconducting devices such as generators or motors. In these devices the issue of AC loss is crucial, as performance and cooling power are derived from this quantity. This paper presents a comparative study of transport AC loss in two different types of 2G HTS racetrack coils. In this study, both experimental measurements and computer simulation approaches were employed. All the experiments were performed using classical AC electrical method. The finite-element computer model was used to estimate electromagnetic properties and calculate transport AC loss. The main difference between the characterized coils is covered inside tape architectures. While one coil uses tape based on RABITS magnetic substrate, the second coil uses a non-magnetic tape. Ferromagnetic loss caused by a magnetic substrate is an important issue involved in the total AC loss. As a result, the coil with the magnetic substrate surprised with high AC loss and rather low performance.

AC Losses of YBCO Coated Conductor with Ferromagnetic Substrate in DC Field

As YBCO coated conductors are usually prepared on nickel or nickel alloy substrates, additional ferromagnetic losses are generated in the substrates when the conductors are exposed to AC magnetic fields or carrying alternative currents in practical application. In this work, AC transport loss factors of YBCO tapes with Ni-W alloy substrates were studied in DC magnetic fields. The influence of ferromagnetic substrate on AC loss and the repressive effect of DC background magnetic field on ferromagnetic loss in the substrate were presented and analyzed. The results showed that the AC transport loss factor of YBCO tape with Ni-W substrate decreased gradually with the increasing background field, and the minimum value of AC loss factor appeared as the field reach to about 18 mT and 45 mT for parallel and perpendicular applied field respectively. Based on the analysis, the method to reduce AC transport current loss in coated conductor with nickel alloy substrate is proposed.