No-insulation High Temperature Superconductor Research Articles

A design study on MVAr class superconductor synchronous condenser with no-insulation HTS field coil

With the expansion of distributed power systems and renewable energy worldwide, the use of synchronous condensers (SC) has been gradually increasing to maintain the power quality of an electric grid. Because SC generates fewer harmonics and has the advantage of strengthening system inertia, it is considered as a primary option in Korea over other options such as Static Var Compensator (SVC) and STATic synchronous COMpensator (STATCOM). If superconductor synchronous condenser (SSC) is applied instead of SC, it brings more advantages: (1) it can minimize the adverse impact on the power system due to lower synchronous reactance, and (2) the higher power density can lead to many advantages in a limited area of substation. With these backgrounds, we report a design study on 40 MVAr SSC with no-insulation high-temperature superconductor (NI-HTS) field coil. Firstly, design criteria and procedures for electromagnetic design were set. Secondly, with the design procedure, the electromagnetic design having the minimum HTS conductor usage was obtained among the designs satisfying design criteria. Based on the final design, key characteristics of SSC including no-load voltage, the maximum flux density under the rated-load condition, V-curve characteristic, synchronous reactance, zero sequence reactance, negative sequence reactance, critical current, and NI-HTS characteristics were calculated by using finite element method. Lastly, the design comparison between SSC and SC was presented.

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

The no-insulation (NI) winding technique is promising for applications in the persistent-current mode (PCM) operation of high-temperature superconducting (HTS) coils. To produce an NI PCM coil, it is essential to understand its demagnetization behavior (i.e. decay of persistent DC current) under an external AC field, which occurs in maglev trains, electric machines and other dynamic magnet systems. For this purpose, a 3D finite-element method (FEM) model, capturing the full electromagnetic properties of NI HTS coils is established. This work studied three kinds of AC fields, observing the impact of turn-to-turn contact resistivity on demagnetization rates, which is attributed to current distribution modulations. Under a transverse AC field, the lower contact resistivity attracts more transport current to flow in the radial pathway to bypass the ‘dynamic resistance’ generated in the superconductor, leading to slower demagnetization. Under an axial AC field, the demagnetization rate exhibits a non-monotonic relation with the contact resistivity: (1) the initial decrease in contact resistivity leads to a concentration of induced AC current on the outer turns, which accelerates the demagnetization; (2) the further decrease in contact resistivity makes the current smartly redistribute to avoid flowing through the loss-concentrated outer turns, thus slowing down the demagnetization. Under a rotating DC field, a hybrid of transverse and axial fields, the impact of contact resistivity on the demagnetization rate exhibits combined characteristics of the transverse and axial components. Additionally, quantitative prediction of the demagnetization rate of NI PCM coil under external AC field is instructive for practical designs and operations, which is tested by this 3D FEM model, and a comparison with experimental results is conducted.

Design, Construction, and Operation of Liquid Nitrogen Cooled MHD Miniature Ship with No-insulation High Temperature Superconductor Magnet

Design, Construction, and Operation of Liquid Nitrogen Cooled MHD Miniature Ship with No-insulation High Temperature Superconductor Magnet

Analysis of charging characteristics of a 500 MHz HTS-LTS series NMR magnet with an intra-layer no-insulation HTS layer-wound coil structure

The intra-layer no-insulation (LNI) layer-wound coil has the advantages of high spatial magnetic field homogeneity and a smaller number of joints, as well as the ability of self-protection during quench, which has great potential in making MRI or NMR superconducting magnet. In this paper, a 500 MHz NMR magnet connected in series by a high temperature superconducting (HTS) magnet and a low temperature superconducting (LTS) magnet was designed, fabricated, and tested at a low field for homogeneity, in which the HTS magnet is an LNI layer-wound coil structure. And a helical equivalent circuit model of the 500 MHz HTS-LTS NMR magnet coupled with the stacked homogenization T-A and mechanical model is established, which agrees with the experimental results. This paper provides a sufficient numerical analysis of the charging characteristics of the HTS-LTS series magnet, including magnetic field homogeneity, current distribution, loss distribution, screening current induced stress, etc. And this paper provides valuable insights into the ramping behavior of the HTS-LTS series magnet, which can be useful for practical charging experiments.

Non-uniform current distribution in parallel-wound no-insulation high-temperature superconductor coil during ramping and fast discharging operations

A parallel-wound no-insulation (PWNI) high-temperature superconductor (HTS) coil is a kind of pancake-shaped no-insulation (NI) coil wound with parallel-stacked HTS tapes, which combines the characteristics of a NI coil and non-twisted stacked-tape cable. It shows a significant advantage in accelerating the ramping response compared with traditional NI HTS coils wound by a single tape, and is a promising alternative for large-scale high-field magnets. The stacked cable approach can lead to current redistribution between parallel tapes during ramping operations. It couples with the turn-to-turn current redistribution and leads to a much more complicated current redistribution inside the PWNI coil, the mechanism of which remains unclear so far. The aim of this work is to investigate electromagnetic behavior of a PWNI HTS coil in ramping and fast discharging process. A simulation model was developed by integrating an equivalent circuit network model and an improved T–A model. A three-tape PWNI coil and its insulated counterpart were wound and tested, and this model was validated by charging and discharging tests. Results show that there is a significant non-uniform current distribution on parallel tapes in the same turn during ramping operations and the maximum azimuthal current (transport current) can be 2.26 times the minimum one in the three-tape PWNI coil in this study. Meanwhile, the radial current shows a considerable accumulation in the tape near turn-to-turn contacts and the radial current through the turn-to-turn contacts can be 4.16 times of that the flow through tape-to-tape contacts (parallel tapes) in the same turn. During the fast discharging process, a significant coupling current is generated in the PWNI coil, leading to a large opposite transport current in local areas; the amplitude of variation of this can be 4.66 times the initial operating current. The radial current shows a similar distribution but opposite direction to that during ramping, and its amplitude is two orders of magnitude higher. These results provide practical guidelines for the design of large-scale high-field HTS magnets.

Electromagnetic Shielding Technique for No-Insulation Superconducting Rotor Windings in Electrical Aircraft Propulsion

No-insulation (NI) high temperature superconductor (HTS) machine is a kind of synchronous semi-superconducting machine with high power density and enhanced thermal stability, which has a great potential for electric aircraft propulsion systems. However, NI HTS rotor windings in this machine always suffer the problem of eddy currents and losses induced by ripple magnetic fields in the synchronous machine environment, which can significantly reduce the efficiency and safety of the HTS machine. In this paper, an electromagnetic shielding technique is developed to minimize this induced eddy current and loss in NI HTS rotor windings. A numerical model is developed to analyse the effectiveness of this shielding technique, and experiments are performed to validate the model. Then, the effect of this electromagnetic shielding on the induced eddy current and loss of NI coil is studied using this model. Results show that electromagnetic shielding technique based on copper discs can effectively reduce the eddy current and loss on NI HTS coil at the operating temperature range of HTS machines, 20-30 K, thus it is promising to solve the problem of eddy current and loss of NI HTS machine design in electrical aircraft propulsion.

Compensation of NI Behaviors in Synchronous Motors With NI HTS Field Winding Using Harmonic Current Injection

Focusing on the improved operation reliability of no-insulation (NI) high-temperature superconductor (HTS) coils, various attempts are being conducted to replace conventional superconductor coils with NI HTS coils for industrial and energy conversion purposed applications. When applying NI techniques, unique behaviors of NI HTS coils due to the removal of turn-to-turn insulation should be considered with given operating conditions, because it could affect the overall characteristics of the machines. In particular, in the case of synchronous motors to which NI HTS field winding is adopted, it has been reported that periodic turn-to-turn leak currents were identified even in steady-state operation due to harmonic components, which may result in torque and loss characteristics changes. Here we report a method to offset these additional NI behaviors that may appear when NI HTS field winding is applied to synchronous motors. A brief mechanism of the proposed method was presented, and the proposed method was applied to an example model through finite element analysis. As a result, it was shown that turn-to-turn contact loss could be reduced to 2.18% without adverse effect on torque characteristics.

Experimental and Equivalent Circuit Analysis Approach to Estimate Contact Resistances on Different Sections in a D-Shaped NI HTS Coil

With well-known self-protecting behaviors of no-insulation (NI) high temperature superconductor (HTS) coils, their performance was verified by numerous high field magnets. As one of the initial investigations of the NI HTS technique being applied to toroidal field (TF) coils, an equivalent circuit analysis is carried out on a D-shaped NI HTS coil to predict its electromagnetic behaviors regarding the contact resistance, one of the most important parameters for NI HTS coils. A charging/discharging test is performed at 77 K to obtain the experimental data for contact resistance estimation. Using the measured voltage and the pressure dependency of surface contact resistivity, a combination of contact resistance is obtained through the modified lumped circuit and the partial element equivalent circuit simulation by which the shape-dependent contact resistance can be estimated. The results are expected to provide a source for careful management in the design stage to consider the uneven contact resistance and possible uneven electromagnetic behavior among different sections of the TF coil.

Eddy Current and Loss of Graded-Resistance No-Insulation Coils in HTS Synchronous Machines of Electrical Aircraft

The No-insulation (NI) high temperature superconductor (HTS) machine is a promising propulsion technique for future large-scale electrical aircraft because of its high power density and enhanced thermal stability. However, in the machine environment, the ripple magnetic fields can induce significant eddy currents and losses on the NI coil. A grading turn-to-turn resistivity technique has been proposed to solve this problem, in which higher turn-to-turn resistivity is applied on the outermost turns of the NI coil, and lower turn-to-turn resistivity is kept on middle turns. The purpose of this research is to study the effects of grading turn-to-turn resistivity technique on the eddy current and loss of both single and multiple NI HTS coils exposed to ripple fields. To investigate the electromagnetic behavior and loss mechanism of NI HTS coils, a simulation model is proposed that couples the equivalent circuit model and finite element model. Results show that the grading turn-to-turn resistivity can effectively reduce the induced eddy current and lead to a uniform distribution of eddy current among turns of both single and multiple windings. It reduces magnetization loss in superconducting layers but increases the turn-to-turn loss on the radial contacts. These results are useful for the development of NI HTS machines with high thermal stability.

A Study of the Dynamic Characteristics of an Electrical Rotating Machine Applying No-Insulation HTS Field Coils

This paper presents a dynamic characteristic analysis of an electrical rotating machine that applies no-insulation (NI) high-temperature superconductor (HTS) field coils. The HTS electrical rotating machine applying HTS field coils has high power density compared to conventional electrical rotating machines that utilize normal conductor field coils, meaning that it has great potential for next-generation mobility applications such as electric ships and aircraft. Recently, several studies have reported the possibility of solving the quench protection problem of the HTS field coil, which is one of the most pressing issues in HTS electrical rotating machines, and the key technology is the no-insulation (NI) winding technique. The NI winding technique enhances the electrical and thermal stability of the HTS field coil because the over-current can be automatically bypassed through the turn-to-turn contacts. Although there have been many studies of the electrical and thermal stability of NI HTS field coils, there are still issues related to NI HTS field coil applications to electrical rotating machines. In particular, there are few studies of the behavior of electrical rotating machines with a NI HTS field coil under dynamic conditions, such as active shortcircuit and speed drop conditions. In this study, we designed an electrical rotating machine that utilizes NI HTS field coils and analyzed the dynamic characteristics of the designed machine. The results demonstrate there is a possibility of larger torque fluctuations and armature current fluctuations in the NI HTS machine than in the insulated HTS machine.

Analysis of Nonuniform Contact Resistivity Distribution on No-Insulation HTS Racetrack Coil

This paper presents a numerical analysis and experiment to investigate a contact resistivity distribution of a no-insulation (NI) high-temperature superconductor (HTS) racetrack coil. Since the racetrack coil consists of straight and curved sections with continuously varing curvatures, the local inhomogeneity of contact resistivity can occur. We adopt a partial element equivalent circuit (PEEC) method to investigate this inhomogeneity. A 50-turn NI HTS single pancake test coil is fabricated to figure out the discrepancy of contact resistivity, and the coil is charged up to 100 A in a liquid nitrogen bath. Voltage profiles between inserted voltage taps and the magnetic fields at the coil center are measured and compared with two simulation cases: 1) uniform contact resistivity; 2) nonuniform contact resistivity distribution. The results indicated that nonuniform distribution of contact resistivity may exist. Lastly, the current and Joule heat distribution during the ramping process is analyzed by the PEEC model.

A Study on the Critical Current Measurement of No-Insulation HTS Coils and the Effect of Screening Current-Induced Voltage

This paper presents a study on the critical current measurement of the no-insulation (NI) high-temperature superconducting (HTS) coils and the effect of the screening current-induced voltage. During the analysis of the critical current measurement data, we observed the unexpected “resistive-like” slope in the I-V curve of the NI rare-earth barium copper oxide (REBCO) double-pancake (DP) coils. Because the coil is confirmed superconducting and has no internal joints, therefore we estimate the unexpected resistive-like slope is the result of the screening current-induced voltage. The circuit analysis using the one-dimensional simple equivalent circuit could not explain the screening current-induced voltage adequately. However, using the three-dimensional advanced PEEC model, we were able to successfully reproduce the resistive-like slope.

Thermal Analysis of Leakage Currents of No-Insulation HTS Racetrack Coils Using a Three-Dimensional Turn-Distributed Equivalent Circuit FEA Model

In promoting novel products with superconducting magnets to transportation and science industries, the most important factor is reliability. Recently, it was reported that a no-insulation (NI) superconducting magnet could be damaged with quenches caused by a thermal runaway, significant temperature deviation, extremely high characteristic resistance, etc. To resolve this issue, numerical analysis of high-temperature superconducting (HTS) racetrack coils with NI winding technology should be conducted. The authors proposed a homogenization method to analyze operating characteristics of the NI HTS coils by simplifying the finite element analysis (FEA) model and decreasing the solution time. In order to estimate their thermal stabilities, the proposed FEA simulation is one of the efficient methods for solving the electrical and thermal factors simultaneously. In this paper, the thermal analysis of the leakage currents of NI HTS racetrack coils is presented using a three-dimensional (3D) turn-distributed equivalent circuit FEA model. The charging and discharging delay phenomena were modeled by considering the contact resistances by turns. With the equivalent circuit model of the NI HTS magnets, the 3D racetrack coils were designed by considering a single turn and stacked turns, including metal insulation layers. Their material properties according to temperature variation were applied in the transient analysis. The critical currents by turns were estimated by considering the magnetic field, its angle, and temperature with a given current simultaneously. The simulation results were compared with a fabricated NI HTS magnet and are presented in this paper.

Experiment and Analysis on Temperature-Dependent Electric Contact Resistivity of an NI HTS Coil

This paper reports experimental and numerical analysis results of a no-insulation (NI) high-temperature superconductor (HTS) coil in terms of temperature-dependent electric contact resistivity. A test coil was designed, constructed, and operated. The coil is divided into four sections according to the radial locations of inserted voltage taps to measure local voltages. A preliminary experiment was performed to evaluate the electromagnetic properties of the coil in liquid nitrogen. Then, the temperature-dependent contact resistance of the coil was evaluated from 10K to 80K at every 10K increment in a cooling conduction facility. To investigate the impact of the contact resistivity on the charge/discharge dynamics of the coil, we selected two arbitrary current ramp rates values, namely fast (0.5) and slow (0.01) charge/discharge. In this study, the experimental results were analyzed with equivalent circuit and finite element models. The analysis results suggest the following conclusions. First, the contact resistivity of the test coil calculated from the experiment at the coil terminals level increases by a factor 2 with respect to the temperature, ranging between 1.16 to 2.59. Second, although a constant winding tension was applied during the coil fabrication, the contact resistivity was found to increase within different sections (inner to outer radius). Third, the impact of the ramp rate on temperature dependency is negligible.

Investigation on nonuniform current density and shape deformation affecting the magnetic field performance of a saddle-shaped no-insulation HTS cosine–theta dipole magnet

High magnetic fields are desirable for discovering new particles in particle accelerators. Dipole magnets using superconductors have played a key role in creating the required field intensity and uniformity. In contrast, high temperature superconductor (HTS) dipole magnets have recently been spotlit because of their ability to generate higher magnetic fields compared to their low temperature superconductor counterpart. Similar needs have emerged in other fields using magnets, and no-insulation (NI) technology is considered a feasible option to reach high magnetic fields by overcoming the disadvantages of HTS magnets. However, research has rarely been carried out on the utilization of NI HTS magnet technology for dipole magnets in high-field accelerators. Here we show the design, fabrication, and test results of an NI HTS dipole magnet with numerical analysis results. This paper aims to investigate the effect of nonuniform current density and undesirable shape deformation on the magnetic field distribution of a saddle-shaped NI HTS dipole magnet. The magnet is designed and constructed considering the ‘constant perimeter winding’ technique and tested in liquid nitrogen. The field mapping process is also performed along a designated mapping trajectory to obtain the magnetic field distribution. A T-A formulation-based simulation model, named the ‘sequential simulation model,’ is suggested to reproduce the measurements and employed considering the current distribution and shape deformation. As a result of quantitative analysis of the transverse direction measurements, the magnetic field error decreased by 0.02 percent point (pp) when the nonuniform current density is considered. It decreased by 0.13 pp when the shape deformation is considered. Moreover, the critical current calculated through an additional numerical analysis shows an error of up to 10%. In conclusion, the saddle-shaped NI HTS dipole magnet can produce a sufficient magnetic field level for particle accelerator research, even though the field distribution shows a uniformity of 0.37% within this study.

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance is generated in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for the time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time that the NI magnet has been exposed to the AC field, showing several qualitatively different waveforms (e.g. some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyze/explain these experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) the azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

Transient Electrical Characteristics of No-Insulation REBCO Coil Impregnated With GaInSn Liquid Metal

Due to higher critical current density and better self-protection, no-insulation (NI) high-temperature superconducting (HTS) coils wound by REBCO coated conductors have been widely studied and used in various superconducting magnets. When HTS coils are applied in rotating machines and high filed magnets, epoxy resin impregnation is indispensable to improve their mechanical stability. However, the use of epoxy resin may cause delamination problems and increase the turn-to-turn characteristic resistance when the epoxy penetrates through the winding turns. Although the solder impregnation method has been demonstrated to enhance the mechanical robust of NI coils, the solder needs to be heated and melted into a liquid before it can be used, which makes the impregnation process extremely inconvenient. The gallium−indium−tin (GaInSn) alloy is liquid at room temperature with a melting temperature of 10.5 °C. Therefore, GaInSn liquid metal alloy can be applied as impregnation material, which makes the impregnation process more convenient. In this article, NI HTS coils with and without GaInSn impregnation are fabricated and tested for comparison. The transient electrical characteristics including charging and sudden-discharging tests are conducted at 77 K. The critical current experiment shows that the conductor of the impregnated coil can be reused. These results can provide useful data for the potential application of GaInSn impregnated NI HTS coils.

Multi-physics quench simulation of no-insulation HTS insert coils with high background magnetic field

Compared with traditional insulated High Temperature Superconducting (HTS) coil, no-insulation (NI) HTS coil has higher thermal stability and self-protection capability, so that is considered as an important way to generate high steady-state magnetic field. This paper focuses on the experimental phenomena that mechanical damage always occurs in NI coils during quench with high background field. A multi-physics quench simulation model is established for NI insert coils, which contains a 2-d axisymmetric partial element equivalent circuit (PEEC) model for electromagnetic calculation and two axisymmetric finite element models for thermal and mechanical analysis. Based on these models, we analyzed the electromagnetic, thermal and mechanical phenomena of insert coils during quench propagation and recovery. The simulation results show that mechanical damage may occur in the outer superconducting zone near the normal zone in no-insulation coils during quench, and thermal strain caused by temperature rise during quench may exacerbate mechanical damage. Meanwhile, quench at different single-pancake coils has different effects on the mechanical performance of the entire insert coils. Based on the electromagnetic simulation results, we also analyzed the time delay of the traditional terminal voltage quench detection method and its causes. This study analyzed the multi-physics quench phenomenon of no-insulation HTS insert coils. And the results imply that hoop strain will increase to the critical strain due to high magnetic field and current redistribution caused by radial current shunt during quench, resulting in mechanical damage. Additionally, radial current shunt and induced voltage will retard quench propagation velocity, resulting in the time delay in terminal voltage quench detection.

Maximum DC operating current degradation and magnetization loss of no-insulation (RE)Ba2Cu3O x coil under AC axial background magnetic fields

No-insulation (NI) high-temperature superconductor (HTS) coils show a great advantage on enhanced thermal stability during quenches. It is inevitably exposed to ripple AC magnetic fields in some applications, such as synchronous machines, tokamak magnets and maglev trains. The AC applied fields can induce an eddy current in NI coils due to the absence of turn-to-turn insulation. This eddy current may cause considerable maximum DC operating current degradation and additional magnetization loss in NI coils, which are still unclear. In this paper we study this issue using both experiments and simulations. An experimental platform is built to measure the maximum operating current of HTS coils exposed to AC axial applied fields, and the results show that the axial AC applied fields can lead to a significant maximum operating current degradation (22.9% in this study) on the NI HTS coil due to the eddy current induced even though the field is parallel to tape’s ab-plane and has a very low amplitude and frequency (26.88 mT/50 Hz). Meanwhile, this low applied field has little effect on the critical current of insulated HTS coils. A numerical model is applied to elucidate the underlying physical mechanism of this phenomenon, and the magnetization loss induced by an additional transport current is analyzed using this model. The influence of graded turn-to-turn resistivity technique is also investigated, and the results show that this technique can effectively prevent the maximum operating current degradation and reduce the magnetization loss of NI HTS coils exposed to AC axial applied fields.

Test and Analysis of Laboratory-Scale D-Shaped Co-Wound No-Insulation HTS Single Pancake Coil for TF Coil Application

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.