2G Tapes Research Articles

Development of a Novel Soldered-Stacked-Square (3S) HTS Wire Using 2G Narrow Tapes With 1 mm Width

Based on the pre-existing assembled, stacked, or twisted structures of high temperature superconducting (HTS) wires, a novel soldered-stacked-square (3S) HTS wire is suggested and developed through narrowing, stacking, and soldering processes in this study. For the fabrication processes of the 3S wire, a narrow 2G tape with 1 mm width is applied, and the stacking and soldering processes are completed simultaneously. To confirm the performance of the 3S HTS wire, several 3S samples are fabricated. For the 3S HTS samples, we evaluate typical electrical and mechanical properties, and investigated for the feasibility of the 3S HTS wire in the HTS applications. The test results show that measured critical currents of two 3S wire samples are 170 and 255 A, and their curves are typical ones of HTS tapes. The measured self-field ac loss is also reasonable, which is analyzed from both theoretical models of the elliptical and thin strip equations. Moreover, the minimum bending diameter for the 3S samples is approximately 60 mm, and the allowable axial tensile force is 350 N. These mechanical properties of 3S wire are satisfied for the HTS applications significantly. Finally, the smallest round structure of the HTS wire is conceptually suggested for the 3S HTS wire.

H-Formulation FEM Modeling of the Current Distribution in 2G HTS Tapes and Its Experimental Validation Using Hall Probe Mapping

One of the most widespread mathematical formulations applied to simulate the electromagnetic phenomena of coated conductor in the recent literature is the H-formulation. However, the only validation of the model has been indirect by using measurements taken from the applications, as measurements of the energy losses in ac fields, forces developed in levitation systems, or any other parameter related to a specific application. Direct validation of the calculation requires the observation of the local out-of-plane magnetic field over the surface of the sample, and it is only accessible under magneto-optical observations and, in a larger scale and better dynamic range, by the Hall scanning microscopy. We propose here the experimental validation of the H-formulation by comparing the simulated results with measurements made by a Hall probe mapping in a second-generation (2G) tape sample for several dc transported currents at 77 K. This paper presents a methodology to simulate the 2G tape by using only measured data obtained from a sample and its normalized $J(B)$ experimental curves. Some boundary conditions that allow a faster convergence of the problem are investigated. Simulated results of the 2G tape modeled considering only the 1- $\mu\text{m}$ high-temperature superconductor (HTS) layer were compared with others that represent the most important layers of the coated conductor structure in the calculations. The simulated and measured results present a good agreement, proving that this model can calculate precisely the magnetic field and, hence, the current distribution in HTS samples.

Magnetoresistance, Irreversibility Fields, and Critical Currents of Superconducting 2G Tape

The magnetoresistance, irreversibility fields, and critical current density were studied for a commercial 2G tape at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape of Tc50 % = 91.5 K and Tc0 = 90 K and the width of superconducting transition of ΔT = 1.5 K were obtained. The widths of the transition from the normal to the superconducting state do not increase at the applied magnetic field up to 90 kOe and do not depend on the orientation of the magnetic field with respect to the tape plane. The irreversibility field values were obtained and successfully fitted as a function of temperature using the formula: H_{text {irr}} =H_{text {irr0}} left ({1-frac {T}{T_{text {c0}} }} right )^{n}. The irreversibility fields show an anisotropy, and at the liquid nitrogen temperature, they reach Hirr = 430 kOe and Hirr = 106 kOe for the parallel and perpendicular directions, respectively. The anisotropy ratio amounts to γ = 4 at 77 K and is small in comparison with other high-temperature superconducting materials. The critical current density of this tape was found to be of the order of 106 A cm−2 at 77 K in the self-magnetic field.

Experimental Research on Bending Characterization of 2G Tapes and Strand Consisting of 2G Tapes

In order to improve the anisotropic characteristics of critical current and current-carrying capacity, a high-temperature superconducting (HTS) strand made from four substacks consisting of the second-generation (2G) HTS tapes was designed and prepared since a single HTS tape has a limited current-carrying capacity. The strand with a round cross-sectional aluminum sheath is composed of two components with stacked 2G tapes and outer sheaths which are normal metals as jackets. A strand model with a round cross section and an aluminum sheath was designed and fabricated for the evaluation of the bending characteristics. In order to demonstrate its bending characteristics, the dependence of critical current on in-plane bending radii for 2G tapes and bending radii for the strand sample was tested in self-field and at 77 K. The results show that the minimum bending radius of the strand sample is larger than the minimum in-plane bending radius of the 2G tapes.

Numerical Simulation of AC Loss in 2G High-Temperature Superconducting Coils with 2D-Axisymmetric Finite Element Model by Magnetic Field Formulation Module

In this paper, numerical simulation of the second generation (2G) high-temperature superconducting coils has been developed in the module of magnetic field formulation with 2D-axisymmetric model. The brevity of expressions and the consistency between the 2D model and 2D-axisymmetric model of this approach make it easier than the method of partial differential equations (PDEs) and magnetic fields module to simulate the high-temperature superconductors. The accuracy of this technique was certified through the comparison with the results of infinite long tape solved by the analytical equation and PDE method. Then, the simulation about a high-temperature superconducting coil with 40 turns was conducted, in which the anisotropic characteristics of 2G tapes expressed with the fitting formulation of J(B, 𝜃) was considered. The distribution of current density and magnetic field at different time steps, and the voltage variation for each turn in the coil were studied. It could be seen that the electromagnetic quantities at the inner turns of a coil must be especially noticed. And finally, the AC loss of our model was calculated.

Breakdown Characteristics of Liquid Nitrogen in Strongly Inhomogeneous Electric Fields

Resistive superconducting fault current limiters (R-SCFCLs) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium-voltage level. To develop R-SCFCLs for the high-voltage application exceeding 100 kV, it is necessary to know the dielectric properties of liquid nitrogen more in detail, particularly if sharp edges, occurring for example at 2G tapes or at connection conductors, cannot be avoided. Consequently, in this work, the breakdown characteristics of strongly inhomogeneous electrode systems in liquid nitrogen were investigated. An electrode configuration of a plane electrode and a needle electrode with a tip radius r of approximately 0.1 mm and sphere electrodes with radii r from 1 to 3 mm were used. The distance d between the electrodes was adjusted in the range of up to 15 mm. The electrode configuration was immersed in liquid nitrogen within an open isolated vessel, and the ac breakdown voltage was measured in dependence on the rounding radius r of the high-voltage electrode and the electrode distance d. For each combination of r and d, the mean breakdown voltage was calculated from 15 single measurements. The measuring results using sphere electrodes reveal that the mean breakdown voltage VBD approximately follows the empirical law V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BD</sub> = c · d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sup> with b = 0.46. For the needle electrode with a radius of roughly 0.1 mm, the mean breakdown voltages are remarkably smaller and increase with distance d only with an exponent b = 0.28.

Torsional Dependence of the Critical Current in 2G Tapes

Since the second-generation (2G) superconducting wires started to be produced in a large scale almost a decade ago, the quality of the materials manufactured has been continually improved. Every year, the manufacturers present new materials with better electrical properties, increasing the critical current density, critical magnetic field, and the total length produced with continuous homogeneity. The mechanical properties of the coated conductor should be taken into account since electro-dynamical forces appear in working conditions as stress in the cooling and heating processes. In applications such as fault-current limiters, superconducting magnetic energy storage, transformers, electromagnets, motors, cables, etc., the consideration of them is mandatory to the project of those devices. In this context, this paper studies the electrical and mechanical properties of 12-mm 2G wires samples under torsion without axial tension. Critical current is measured as a function on the torsion angle. Details of the experiment and their results are here presented.

Status of Superconducting Fault Current Limiter in Italy: Final Results From the In-Field Testing Activity and Design of the 9 kV/15.6 MVA Device

Ricerca sul Sistema Energetico—RSE S.p.A. has recently accomplished a more than two-year-long in-field activity on the first Italian superconducting fault current limiter (SFCL). In order to carry out the in-field activity, a 9-kV/3.4-MVA SFCL device was installed as single feeder protection inside the Milan MV distribution grid; further to this first success, the development of a new device to be installed in the same substation, but as a transformer protection, has been already initiated. After having briefly summarized the final result of the in-field activity on the 9-kV/3.4-MVA device, this paper is mainly focused on the new 9-kV/15.6-MVA upgraded device: in particular, the first small-scale HTS winding prototypes will be described and the power tests implemented to validate their performances will be discussed. The test results allowed to identify the best layout, consisting in two parallel-connected coaxially arranged 2G windings, each made of two layers anti-inductively wound on a fiberglass cylinder, and each featuring a total 2G tape amount of 96 m. The actual research plan foresees to install the upgraded device in the live grid in 2016.

Tests of the 15-kV Class Coreless Superconducting Fault Current Limiter

This paper presents the design and tests of a superconducting fault current limiter (SFCL) for medium voltage as a device allowing for a more effective use of the existing power network infrastructure. The limitation of short-circuit currents by the SFCL to safe levels will result in the network elements being susceptible to smaller electrodynamic and thermal overloads. The application of an SFCL leads to an increase of the allowable short-circuit power at the point of connection of new power generating sources, which is determined by the short-circuit parameters of the power network. This, in turn, will result in an increase of the capability of the power network for connecting distributed generation energy sources based on renewable energy sources. Light and compact construction of the coreless SFCL consists of three magnetically coupled windings cooled in liquid nitrogen: primary and secondary windings, made of HTS 2G tape, and a parallelly connected primary copper winding. A high magnetic coupling between the superconducting primary and secondary windings gives a low voltage drop on the limiter at a nominal current. The presented solution reduces the size and the weight of the device. Tests performed at a high-power test facility show the limiting performance of the coreless inductive SFCL.

The magnetic behavior and mechanical properties of low resistance joints of GdBa2Cu3O7 − δ 2G tapes

The electrical properties of contacts of second-generation (2G) GdBCO superconducting tape conductors, which are soldered by Rose’s and PbSn solders, are investigated in an external magnetic field up to 5 T at a liquid nitrogen boiling temperature. The joint resistance at T = 77 K in the case of PbSn solder is approximately half that in the case of Rose’s alloy. The rise in the contact magnetoresistance with the field is weak and independent of the orientation of the magnetic field, and it is saturated in fields on the order of 3 T for both solders. The mechanical tensile strength of initial tape conductors and contacts is measured at room temperature. The ultimate strength of contacts produced by the PbSn solder is more than twice that for the contact made by Rose’s alloy. In the latter case, the ultimate tensile strength is lower than the critical stress of superconductor degradation.

Structural modeling of HTS tapes and cables

Structural finite element analysis (FEA) has been used as an insightful tool to investigate the electromechanical behavior of HTS REBCO tapes and twisted stacked-tape cables under tension, torsion, bending and combined loads. A novel technique was developed for modeling the layered composite structure of the 2G tapes with structural solid-shell elements in ANSYS®. The FEA models produced detailed strain information for the REBCO superconducting layer which was then paired with an analytical model to predict the critical current performance of the 2G HTS tapes under various loads. Two commercially available HTS tapes (SuperPower and SuNAM) under tension, torsion and combined tension–torsion were first analyzed with FEA and compared with available experimental results at 77K. A sharp critical current degradation was experienced at the yield strength of the tapes under tension and below a 100mm twist-pitch under torsion. Combined tension–torsion loads had a more gradual degradation of critical current for twist-pitches of 115mm or shorter but had a negligible difference compared to pure tension for longer twist-pitches. Using the structural solid-shell technique for modeling 2G tapes in ANSYS®, an FEA methodology for simulating full scale three-dimensional HTS stacked-tape cables under pure bending was created. A model of a Twisted-Stacked Tape Cable (TSTC), a configuration first proposed at MIT, was initially developed and then adapted to the slotted-core HTS Cable-In-Conduit Conductor produced by the ENEA laboratory in Italy. The numerical axial strain of the HTS REBCO tapes within the cables as calculated by FEA were found to agree with an analytical model for two cases: perfect-slip (frictionless) and no-slip (bonded). The ENEA CICC model was also compared with recent experimental critical current data at 77K and was found to match best using a low friction coefficient of 0.02 indicating that the tapes within the cable freely slide with respect to each other helping to reduce the axial strain during bending.

Investigation of the flashover and breakdown voltage of thin polymer films in liquid nitrogen

Resistive high-temperature superconducting fault current limiters (R-SFCL's) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium voltage level. To develop R-SFCL's for high-voltage applications exceeding 100 kV, the dielectric properties of liquid nitrogen as insulation material in different arrangements have to be investigated more in detail. Considering R-SFCL's, the 2G tapes are typically arranged in bifilar coils. Adjacent tapes are insulated by a combination of thin polymer foils and liquid nitrogen resulting in a surface discharge arrangement, which dielectric properties were investigated. Specimens of foil material were inserted between the two electrodes and immersed in liquid nitrogen within an open isolated vessel. The applied ac voltage was raised until flashover occurred, and the shortest overhang of the foil material was recognized as flashover length. The investigations were performed for the polymer materials polyimide, PTFE, and PET and thicknesses of the foils between 75 and 250 $\mu\textrm{m}$ in dependence on the specific properties of the electrode arrangement. The results reveal that the flashover lengths are rather small, so that in most cases, the breakdown of the polymer foil occurs before the flashover.

Twisting Characteristics and Tensile Effects of aQuasi-isotropic Strands made of Coated Conductors

With the development of second generation (2G) high-temperature superconducting (HTS) tapes, several cabling configurations, such as twisting stacked-tapes cable and conductor on round core cables, have been proposed, and some prototypes were developed. Alternating current (ac) losses arise in HTS tapes in ac operations, and those losses are an important consideration for applications since they influence the stability and operating cost. Conventionally, a twisting technique is used to effectively reduce the ac losses. This paper presents the twisting properties and tensile properties of a quasi-isotropic (QI) strand made from 2G tapes. The strand consists of 2G tapes, aluminum foil, and a metal sheath. A twisting apparatus was designed and demonstrated to research the characteristics of the strands in twisting situations. The critical currents and strain of a single CC were tested for different twist pitches in self-field and at 77 K. The results show that the critical twist pitch of the QI strand is higher than that of a single tape. The critical current of the QI strand was also measured at 77 K after the application of different tensile stresses at room temperature.

Electrical Characterization of ENEA High Temperature Superconducting Cable

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values I c at 77 K (liquid N 2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance(R inter ) has also been measured and the observed dependence of R inter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA.

Device for Measuring the Thermal Cycling Degradation in 2G Tapes for Electrical Power Applications

The standard electrical engineering applications of high-temperature superconductors (HTSs) as coils (SMESs), motors, power cables, fault current limiters (SFCLs), etc., usually involve operating conditions under which the critical current of HTS tapes might be degraded. The dependence of such magnitude on mechanical properties, frequency variations, or thermal changes is a key concern for the accurate design of those devices. Two of the parameters affecting the critical current when the superconductor operates in broad temperature ranges are the number and the speed of transitions from the superconducting to the normal state. In order to determine how these parameters affect the critical current, we have designed an automated dipping setup in liquid nitrogen to reproduce heating and cooling cycles on superconducting tapes. In this paper, this device is described, and several tests on a short sample of YBCO coated conductor are reported and analyzed.

Investigation of Electro-physical and Physical-mechanical Properties of HTS 2G Tapes

High temperature superconducting tapes of 2nd generation (HTS 2G) are being manufactured by several companies worldwide, which use different methods of fabrication of superconducting layers that affect the behaviour of HTS 2G tapes in actual practice. This paper presents the results of the study of HTS 2G tapes delamination process, the influence of alternating temperature impacts on critical current and the behaviour of current carrying capacity of HTS 2G tapes in external magnetic fields. The abovementioned problems can arise in almost any case of designing devices based on HTS 2G tapes. Furthermore, the developed measuring techniques are very important for controlling of output parameters of HTS 2G tapes that are to be manufactured in NRC “Kurchatov institute”.

Analysis of critical current reduction in self-field in stacked twisted 2G HTS tapes

Twisted stacked-tape cable (TSTC) is one of the methods to produce flexible current carrying elements (CCE) with high current density made of 2G coated conductors. The critical current of CCE in self-field is less than the sum of critical currents of all tapes used to make CCE. This critical current reduction is attributed to self-field influence of critical currents of tapes. To describe the current reduction accurately in TSTC it is necessary to take into account current and field distribution in a stack cross-section using dependencies of critical current on magnetic field for each tape. In other words one has to make self-consistent calculations of current and magnetic field distribution in a stack. In this paper we present the self-consistent analysis of magnetic field and current distribution in TSTC with different number of tapes using the generalized Kim model. We also measured current degradation in TSTC made of from two to ten 2G tapes and compared it with calculations. We found good coincidence of calculated and measured current degradation that confirm the validity of our approach.

Flux pinning distribution and E-J characteristics of 2G YBCO Tapes

E-J characteristics of SuperPower YBCO 2G tapes have been measured in the temperature range 15K-80K. It was found that the E-J characteristics deviate significantly from the standard power-law behaviour with apparent power exponent decreasing continuously at high voltage with increasing current. The deviation of E-J characteristic from the standard power-law was found to be consistent with a Weibull distribution of critical current. The identical scaling of E-J characteristics above 40K suggests a common critical current distribution. At lower temperature the critical current distribution becomes narrower to give higher apparent power exponent at lower temperature. With the critical current distribution model it is shown that the dissipation can be correlated directly to the flux flow resistance of Bardeen. The distribution of the critical current can be associated with a distribution of pinning potential of collective pinning and the headline pinning range is obtained as function of temperature.

Preparation and Experimental Investigation of Heavy-Current Transposed HTS Conductors

Various high-temperature superconductor (HTS) devices (transformers, electrical machines, accelerator magnets, power transmission lines, current leads, etc.) need operating currents that many times exceed single HTS tapes current capacity. Conductors made of many transposed HTS tapes wound in parallel onto several mm-diameter flexible formers can be made of modern 2G tapes. Such conductors combine large operating currents, low power losses, and flexibility. Our electrical and mechanical properties measurements of various manufacturers' tapes showed the possibility to make such conductors. The HTS tapes (up to 16) were wound helically onto several mm-diameter flexible formers made of twisted bundle consisted of 3000 stainless steel wires of 0.1 mm diameter. The V-I curves at dc in external magnetic field up to 0.6 T and the V-I curves and ac losses at 50 Hz in self field and in the synchronous background field were measured at different cooling conditions. For all cable samples the current capacities were proportional to the number of tapes.

Comparison Between the Fault Current Limiting Performance of Bi-2212 Bifilar Components and 2G YBCO Coils

Short-circuit tests with commercial Bi-2212 and with a YBCO 2G tape components were performed. The Bi-2212 bulk coil has been recently improved by the manufacturer, exhibiting higher homogeneity and better transport properties than previous ones. Such bulk components consist of a shunted bifilar coil (Ic = 400 A at 77K). The YBCO 2G tape component consist of two antiparallel 2G shunted coils (Ic = 1200 A at 77 K). Fault current limiting tests were performed under asymmetrical prospective currents varying from 1 kArms (2.7 kApeak) to 30 kArms kArms (80 kApeak). The source voltage (Vo) was also varied in order to evaluate the influence of Vo on the limited current (138-400 Vrms). For the same Vo value (200 Vrms), the Bi-2212 components limited prospective currents as high as 80 kApeak to about 6.5 kApeak (first peak) and less than 1.5 kArms , whereas the YBCO 2G tape component limited the same prospective currents to less than 5 kApeak (first peak) and less than 2.5 kArms. The YBCO 2G component presents a faster actuation than the Bi-2212 component.