2G High Temperature Superconducting Tapes Research Articles

The Effect of the Interfacial Resistance of the Superconducting-stabilizer Film on the Typical Sector Diffusion Pace for 2G HTS Tapes

High temperature superconducting (HTS) tapes of the second generation have been widely used as energy storage materials, such as in superconducting magnetic energy storage (SMES) devices. In order to enhance the current-carrying characteristics, these systems are typically run close to the critical currents of the coated conductors; as a result, hot spots may develop, which could cause the superconductor to become quenched. In order to prevent the start of hot spots and to reduce the amount of faulting, efforts have been made to raise the normal zone propagation velocity (NZPV) in this manuscript. The interfacial resistance between the superconductor and stabilizer layer, which can act as a current flow diverter during fault circumstances, has been shown to be the key to producing massive NZPV. The tape's architecture has been slightly modified by the addition of a high resistive layer between the superconducting and stabiliser layers, where various interfacial resistances have been utilised to forecast the temperature distribution between 10 cm lengths of HTS tape. A 2D numerical model was created using COMSOL to assess the NZPV and temperature distribution of the 2G superconducting tape. It has been concluded that larger NZPVs can be achieved by using substantial interfacial resistances to prevent superconducting tape quenching.

Comparative study of current carrying capacity between 1G and 2G HTS tape samples at different temperatures and magnetic fields

Two categories of high-temperature superconducting (HTS) tapes, the silver-sheathed BSCCO tape (1G HTS tape) and REBCO coated conductor (2G HTS tape), have been commercialized worldwide and widely used in applications. In this study, the 1G HTS tape produced by Sumitomo and 2G HTS tape produced by Superpower were selected for a systematic comparative study in critical current (I c) performances under various temperature and magnetic field conditions. The anisotropy and I c variation patterns of these two types of samples in the temperature range of 25–77 K, magnetic field from 0 to 7 T and field angle from 0 to 90° were compared and analyzed. The results of this study reveal certain comprehensive characteristic features of these two categories of HTS tapes, supplementing some new systematic information to the literature.

Experimental investigation of axial tensile and fatigue behaviors of HTS round strands

For the development of high-temperature superconducting (HTS) magnet systems of future fusion devices, a novel HTS round strand based on a stacking structure was designed and manufactured using second generation (2G) HTS tapes. Different mechanical loads during operation can result in irreversible degradation of the strand. The axial tension and fatigue loads need particular attention. Therefore, it is important to investigate the electromechanical behavior of the round strand under various axial tension and cyclic loads. In this paper, the axial tensile and fatigue tests were conducted at 77 K, self-field. Taking 95% critical current (Ic) retention as the criterion, the results of the tensile tests revealed that the average tensile stress and strain were as high as 344 MPa and 0.47%, respectively. Fatigue characteristics were also investigated as a function of axial tensile stress. No significant performance degradation was observed up to 100,000 loading cycles with stress amplitudes ranging from 20 MPa to 200 MPa. Ic degradation occurs after 16,000 loading cycles with 380 MPa as the maximum stress. Furthermore, the microscopic defects of the round strand samples due to fabrication imperfections and mechanical loading were investigated using metallographic microscope and scanning electron microscope. These results presented in this paper are useful for comprehending and improving the mechanical behaviors of the strand in high-field and large-scale fusion magnet systems.

Deterioration of the 2G HTS tapes by the Ne+ ions irradiation (250 keV)

We discuss changes in properties of Gd-based high-temperature superconducting (HTS) tapes induced by 250 keV Ne+ ions irradiation. HTS tapes are used in devices operating in extreme radiation conditions with the prospect of using them in space industry. Ionizing radiation, including heavy ion bombardment, can introduce defects (e.g. Schottky, Frenkel ones) to the microstructure of tapes, leading to deterioration of the superconducting parameters. Therefore, the superconducting layer of the tape was implanted with fluences from 1012 to 1014 Ne+/cm2 to determine the cosmic ray irradiation effect. Comprehensive studies of the microstructural, structural, magnetic, and electrical properties of the irradiated tapes prove the microscopic origin of their deterioration. It is mainly due to oxygen deficiency and microstructural as well as structural defects. The critical current densities are reduced by 33% – 60% (depending on the external magnetic field) compared to the reference sample.

Dependence of dynamic resistances in high-temperature semiconducting quasi-isotropic strands on magnitude and orientation of AC magnetic field

The dynamic resistance of a superconductor appears if it carries a direct current (DC) current and is simultaneously exposed to alternative current (AC) magnetic fields if its amplitude is higher than its full penetrated field. Due to its strong anisotropy, the dynamic resistance of high-temperature superconducting (HTS) tape is not only affected by the magnitude of AC magnetic fields but also their orientations. In particular, the dynamic resistance has considerable effect on the current distribution in HTS cables (except for Roebel cables) and even cable-in-conduit conductors made from those HTS cables, except for dissipative loss. This paper presents the behavior of dynamic resistance in an HTS quasi-isotropic strand (Q-IS), which consists of four sub-stacked 2G HTS tapes proposed by our group at North China Electric Power University, while it carries a DC current and simultaneously experiences an AC magnetic field with different magnitudes and orientations. An angular dependence of dynamic resistance in a single 2G HTS tape in an AC magnetic field with various orientations is first performed in simulations and experiments, respectively. The experimental results are in good agreement with the simulated ones, which shows that the dynamic resistance of a Q-IS also exhibits quasi-isotropic characteristics and has an important effect on the distribution of DC transport current. The related results are promising for the design and application of other DC HTS cables, feeders and magnets.

Numerical Studies of Dynamic Characteristics of a Stack-Type HTS Maglev System Based on H-Formulation

High temperature superconducting (HTS) magnetic levitation (maglev) is a promising technology for future high-speed transit systems. Superconducting magnetic bearings are made of a permanent magnet guideway and HTS bulks (bulk-type) or stacks of 2G HTS tapes (stack-type). If the dynamic behavior of bulk-type HTS maglev systems has been well reported, there is little literature about the dynamics of stack-type HTS maglev system. To fill the gap, we built here an electromagnetic-mechanical model of a stack-type bearing. We used a segregated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> -formulation finite element model to account for the nonlinear resistivity. The multi-layer structure of the HTS tape is either homogenized or modeled in detail. We studied the bearing dynamic response when subjected to different time-varying external magnetic fields mimicking the guideway irregularity and the vehicle speed. The results show that the induced current in the copper layer negatively impacts the stability. By extension, since it cannot account for the copper layer, the homogenized model is not suitable for such analysis. This work contributes to finding viable measures for improving the stability of stack-type HTS maglev systems.

Intrinsic Influence of Dynamic Resistance on Current Distribution of Two HTS Parallel Branches Without Terminal Contact Resistance

If a superconductor carries DC transport current and exposes to an alternating current (AC) magnetic field whose amplitude is more than its threshold field, a DC resistance so called dynamic resistance occurs on the superconductor because of the dissipative interaction between the DC transport current and the external magnetic field, which possibly appears in many kinds of high temperature superconductor (HTS) apparatus, such as linear synchronous motors. Occurrence of the dynamic resistance significantly affects the current distribution of the HTS components connected in parallel such as cables and conductors except for increasing the AC loses. In this paper, we employ a partially slit second-generation (2G) HTS tape with two branches to study the influence of dynamic resistance on current distribution such that the terminal contact resistances can be avoided. In experiment, DC transport current is applied to the whole 2G HTS tape and branch 1# is only exposed to an applied AC magnetic field. In addition, we use an analytical formulation of the dynamic resistance of branch 1# based on Bean critical state model and voltage calculation of branch 2# relies on E-I curve to build analytical model by using Newton Iteration. It is shown that DC current value of branch 1# decreases and branch 2# increases with the increasing of the dynamic resistance of branch 1#. The results are important for HTS DC devices with carrying ripple current or exposed to AC magnetic field.

Preliminary design and analysis of 20 K helium cooled MgB2 based superconducting current feeder system for Tokamak application

Current feeders system (CFS) is one of the most critical part of a large-scale superconducting (SC) magnets based machines like a tokamak or accelerator. One end of the SC feeder is connected to magnets and another end is connected to the power supply via optimised current leads (CL) for energizing the magnets. So far, low temperature superconductor (LTS) is the popular choice for such applications. For high temperature superconductor (HTS) applications, cuprate oxide based coated conductors or magnesium diboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) wires are the options. MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> despite of its superconducting transition temperature of 39 K,is now gaining wide attention for SC feeders system due to its lower raw material cost and ease of availability than 2G HTS tapes. To assess its suitability, we report 1-D thermo-hydraulic study of 10 kA rated MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> current feeders system cooled with 20 K helium for SST-1 tokamak as a case study. For varying mass flow rate, temperature margin along the feeder are calculated considering helium at 20 K, 4 bar (a) with cooling inlet from magnet side. Such a feeder yields a temperature margin of 6 K–16 K across its entire length depending up on mass flow rate of helium. There is also a possibility of cooling the binary current lead (HTS + MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) from cold helium coming out from the feeder. From our study, MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> current feeder system provides benefits of higher temperature margin, lower mass flow requirement and cryogenic savings by use of 20 K helium as coolant compared to LTS based feeders. In future, it could provide a safe, reliable and cryogenic operational cost saving solution for SC current feeders for Tokamak applications.

Thermal stability of quasi-isotropic strands with different cross-sections and with/without metal sheaths

This paper mainly focuses on the thermal stability performance, such as minimum quench energy (MQE) and normal zone propagation velocity (NZPV), of a quasi-isotropic strand (Q-IS) with three different configurations made from 2G high-temperature superconducting (HTS) tapes in the adiabatic approximation. Due to the anisotropic performance of thermal conductivity in 2G HTS tapes, the tensor form of the heat conduction equation was used instead of conventional isotropic forms. We first measured the MQE and NZPV of three strands in the adiabatic approximation. Subsequently, the experimental results were verified by finite element analysis and compared with the values of MQE and NZPV in a liquid nitrogen (LN2) bath. Finally, the effect of a metal sheath on the current distribution and generation of Joule heat during the quench process were analyzed through simulation. The results show that the arc-shaped aluminum filler brings slightly higher thermal stability of Q-IS while the copper sheath largely improves stability. Moreover, compared with the thermal stability of the strand fully exposed to the LN2 bath, for the strand in the adiabatic approximation, the MQE decreases a lot while the maximum of NZPV is increased by 2–3 times.

Development and Testing of 2G High Temperature Superconducting (HTS) Field Coils for HTS Synchronous Machines

High temperature superconducting (HTS) synchronous machines are compact and light weight machines with higher torque to weight ratio, lower noise and better dynamic response for strategic and critical industrial applications. The development of HTS field coil is one of the main technological challenges in realizing the HTS synchronous machines, as it involves the selection of HTS tape, winding technology, handling of the HTS tape while winding on a winding machine and testing of the developed HTS coil at desired cryogenic temperatures. With advancements in HTS tape technology, recent 2G HTS tapes are offered with higher engineering current densities with respect to 1G HTS tapes even at higher magnetic fields. Hence, the replacement of 1G HTS field coils of previously designed 200 kVA HTS synchronous machine with 2G HTS field coils at the same operating cryo-temperature shows a potential to enhance its capacity to 350 kVA (more than 75% increase in machine output). In the present work the details of design, development and testing of a double pancake type 2G HTS field coil are presented for capacity enhancement of earlier 1G based 200kVA HTS synchronous machine.

Characterizations of a Novel Structure of Fault-Tolerant HTS Cable

This paper presents a type of high temperature superconducting (HTS) cable, which is fabricated by winding the second generation (2G) HTS tapes on round former consisting of stranded stainless steel (SS) wires. It has the advantages of strong flexibility, high mechanical strength, and high engineering current density. With taking the resistivity of SS much larger than that of copper or aluminum, the cable has higher resistance and strength than the conventional HTS cable with former made from copper strands at condition of fault accident. After the configuration of the cable is designed in detail, the quenching resistance and fault current limiting are numerically analyzed. We fabricate a model cable and carry out the experiment of fault current. The experimental results show that the quenching resistance of this cable is much higher than the resistance of conventional underground XLPE cable and overhead conductor, which indicates that the proposed cable has a strong ability of fault current limiting and can be applicable in HTS devices with functions of fault current limiting such HTS cable, transformer, motor/generator, and current limiter.

Transient Test and AC Loss Study of a Cryogenic Propulsion Unit for All Electric Aircraft

This paper reports a pioneering demonstration platform of a cryogenic propulsion unit at liquid nitrogen temperature. A high temperature superconducting (HTS) machine is connected with a cryogenic power rectifier in a generator mode to prove the feasibility of a cryogenic propulsion unit for electric aircraft propulsion applications. Machine operation was carried out with a special focus on the total heat dissipation inside the HTS AC windings. Different types of 2G HTS tapes were tested to provided data for stator coils’ design. The transient operation was carried out to represent a short-circuit failure in one of the power electronic devices. The test shows AC loss performance of the HTS windings using calorimetric method during the short circuit event, indicating the importance of developing protection schemes for the cryogenic propulsion units to prevent damage to the HTS components. The paper provides initial insights into the interaction between superconducting machines and cryogenic power electronics within a cryogenic propulsion unit. It is an important first step to understand and further develop cryogenic propulsion technology for future electric aircraft.

Structural Analysis of 2G HTS Tapes under Different Loading Conditions for HTS Power Cable using Finite Element Modeling

In High Temperature Superconducting (HTS) power cable, HTS tapes are wound helically around a circular former and are subjected to different types of mechanical loads during manufacturing, transportation and installation of cable. The strain developed within the HTS tape during these processes may affect the superconducting properties of the tape. In the present work, 3D structural finite element (FE) models using ANSYS® are developed to obtain the strain behaviour of the tape under different loading conditions, considering the layered sandwich composite structure of 2G HTS tapes. The analysis is carried out with axial tensile and twisting loads on the tape, during helical winding of tape around the former and a combined effect of helical winding with external tensile and bending loads. The result obtained from FE simulation due to applied tensile load is compared with the established experimental result, which shows a good agreement. Further, the strain developed within the tape affects the critical current. Therefore, the strain is coupled with an empirical relation to obtain the critical current degradation of the tape. The FE analysis is carried out for five different winding pitches and four different bending radii with YBCO layer in the downward position and the corresponding critical currents are estimated. Further, a minimum winding pitch is obtained, below which the rapid degradation of critical current is initiated. The results show that the HTS tape with YBCO in downward position can withstand more amount of external load without affecting the critical axial strain of the tape.

Measurements of the Effects of Compressive Loading on the Narrow Side of the HTS TAPE

Brookhaven National Laboratory (BNL) is involved in building high field magnets wound using Second Generation (2G) High Temperature Superconductor (HTS) tape. Certain magnet designs demand a load on the narrow edge of the tape approaching 170 MPa. While significant experimental data is available on the performance of the 2G HTS tape under radial and azimuthal stresses [1] , limited experimental data is available for the load on the narrow edge of the tape. To validate the design of a 25 Tesla, 100 mm cold bore Non-Insulated (NI) HTS solenoid, a series of compressive tests were performed on coils wound using 12 mm 2G HTS tape from SuperPower with an Hastelloy substrate thickness of 50 µm and varying copper stabilizer layer thicknesses of 65 μm, 40 μm, and 20 μm. This paper summarizes the initial design, several iterations, and results in the development of this test to simulate the conditions of the coils in a solenoid energized to 25 Tesla.

AC Losses Analysis in stack of 2G HTS tapes in a coil

The model coil with the racetrack geometry based on second generation (2G) High Temperature Superconducting (HTS) tapes has been developed for an electrical machine where a winding pack is a stack of 2G HTS tapes. It is important to evaluate transport current AC losses and possible methods to reduce them in a winding. In a device made of 2G HTS tapes, the main the AC losses are the hysteresis ones. Only numerical simulation permits to predict them in full. The FEM model for calculation of the hysteresis losses developed before for 2G HTS power cables was modified for a stack of the 2G HTS tapes in a coil. In this paper the methods to increase a computational speed are presented. Possible ways to reduce AC losses are discussed also. The details of the model and comparisons of calculations with measurements of the AC loss are presented.

Optimization of HTS Superconducting Solenoid Magnet Dimensions for Maximum Energy Density

Superconducting coil provides enormous amount of stored energy inside its magnetic field. Such a pure inductive superconducting (SC) coil can be designed for high power density or high energy density depending on coil dimensions and inductance based on the prerequisite of application. In this paper, a design procedure is developed to optimize dimensions of a high-temperature superconducting (HTS) coil to store maximum energy for a given length and critical properties of HTS tape. A minimum volume constraint (MVC) is developed to confine the variation of aspect ratios to keep the coil volume constant, thereby limiting the material, cryogenic and vacuum costs of HTS coil. A global optimum inner diameter is established by MVC, with which the optimum dimensions can be reached directly. Multiple constraints such as coil volume, stacked pancake arrangement, HTS tape length and mechanical properties are considered. The electrical, magnetic and mechanical parameters for different dimensions are then found by finite element method (FEM). The proposed method is applied to different lengths of 1G and 2G HTS tapes. The optimum dimensions of maximum stored energy are decided which gives a solenoid coil of maximum energy density.

Numerical Simulation and Experiment of AC Loss in Quasi-Isotropic Superconducting Strand

With the development of REBCO coated conductor (CC), the high temperature superconducting (HTS) strand with high current capacity and quasi-isotropic performance is expected to have a wide application. However, the alternating current (AC) loss is one of the major obstacles. This paper aims to study the AC loss of the quasi-isotropic strand stacked by the second generation (2G) HTS tapes and copper tapes at 77 K. In order to fully understand AC loss characteristics of the stacked-tape conductor, an efficient 3D finite element method (FEM) model based on T-A formulation was adopted in this simulation. The transport AC loss was measured by four-probe method using SR-830 lock-in amplifier at liquid nitrogen (LN2) bath. Moreover, the amplitude and frequency of transport current were also varied in this experiment. The simulation results are consistent with the experiment which is helpful for studying the AC loss of the quasi-isotropic strand.

Superconducting state stability in second generation high-temperature superconducting wires with varying copper stabilizer thickness under AC overcurrent conditions

Superconducting state stability in high-temperature superconducting (HTS) wires is of great importance for the reliable operation of superconducting electric power equipment. To this end, a study was made of the steady state operation and transients under AC current overloading in second generation (2G) HTS tape wires with a copper stabilizing layer of varying thickness. The existence of a threshold stabilizer thickness has been found, such that determines the HTS tape I-V curve and transients when an overcritical current flows. If the current exceeds the critical value, primary instability takes place in a wire with no stabilizer layer as well as with a layer of any thickness. In a wire with the stabilizer thickness less than the threshold level, an irreversible superconducting-to-normal transition occurs. However, if the copper layer thickness exceeds the threshold thickness, the HTS wire stable overloaded state recovers after some transients that have an irregular oscillation shape. The recovery of the stable thermal state under overcurrent conditions in HTS wires with an over-threshold stabilizer thickness is applicable in a variety of superconducting electric power devices, such as transformers and current limiters with a high capability to withstand faults. The measurements also show that the loss of superconductivity under AC overcritical current is followed by the phase angle changing between voltage and current. This phenomenon could be used for fast response normal zone detection in current-carrying conductors of superconducting equipment.

Effects of Mechanical Force on Critical Current of Soldered-Stacked-Square (3S) HTS Wires

Narrowing high-temperature superconducting (HTS) tapes can effectively enhance the ability of antielectromagnetic interference. In addition, different structures of HTS wires are necessary for HTS applications. Thereby, a kind of soldered-stacked-square (3S) HTS Wires has been proposed. This paper presents a process of fabricating several 3S HTS wires. A normal second-generation (2G) HTS tape having 4-mm width was narrowed into four 1-mm-width narrow HTS tapes. The main propose of this paper is to study different effects of mechanical force on the critical current of 3S HTS wires. Therefore, we measured the critical current variation of these wires from the torsion, bending, and tension effects points of view, respectively. Compared with their initial critical current values, if the reduction of the influenced critical current is smaller than 5%, it can be considered that these 3S HTS wires are workable. Based on the results of the experiment, the limitations of mechanical force for these wire applications are: minimal twisting pitch of 64 mm, minimal bending diameter of 54 mm, and maximum tensile force of 374 N.

An effective way to reduce AC loss of second-generation high temperature superconductors

Second-generation (2G) high temperature superconducting (HTS) devices have high power density and efficiency for energy applications. However, in alternative current conditions, the vortex movement in 2G HTS leads to energy dissipation at low temperatures, which significantly affects efficiency and adds an extra burden to cooling systems. This energy dissipation is identified as AC loss. Reducing AC loss to improve efficiencies of 2G HTS devices, e.g. HTS machines, cables and fault current limiters, has become a key research focus. We report here on an effective way to reduce AC losses by using a new 2G HTS wire. The principle is to stack narrow 2G HTS tapes into a wire structure, which is called a soldered-stacked-square (3S) wire. Our experiments on solenoid coils have proved that the 3S wire helps to reduce the AC loss by 80%. Moreover, the critical current of the 3S wire can be easily adjusted for various applications. It is envisioned that the 3S wire will become a powerful technology enabling the development of 2G HTS power devices with high efficiency.