Large-scale High-temperature Superconducting Research Articles

Fast modeling approach of large-scale non-inductive HTS coils under different current supply

This paper presents a fast-integral modeling approach for the design and characterization of large-scale non-inductive high temperature superconducting (HTS) coils made of first-generation HTS tape. The integral modeling is combined with a strategy of far tape approximation allowing to model a reduced set of consecutive tapes instead of the entire coil. It makes it possible to estimate the losses and the current density distribution in the coil, under different current supply. This leads to a considerable reduction in the computation time. The calculated AC losses show a good agreement with the Norris ellipse formula and measurements, highlighting the importance of considering the elliptical section of the superconductor in the tape, in the modeling. The developed modeling approach is in particular useful in the AC + DC mode, where Norris’s formulas cannot be used to determine the losses. Besides, some interesting phenomena have been observed, related to the current distribution in the tapes at different AC and DC current values.

A homogenised anisotropic J-model for accelerating computations of screening current profile in large-scale HTS magnets

Owing to the induced excessive strain, plastic deformation, and distortion of the magnetic field in high-temperature superconducting (HTS) tapes, there is a rapidly growing research interest in the screening current profile in high-field superconducting magnets. The J-model was developed to calculate the current density distributions in HTS stacks and cables, which improves the calculation efficiency because no air domain is considered in the calculation regions. However, for a large-scale high-field magnet wound with hundreds and thousands of layers of HTS tapes, there is still a big challenge for J-model simulation involving prohibitive time consumption and memory requirements. This study proposes an improved J-model approach based on a homogenised anisotropic (HA) scheme for large-scale HTS magnet systems to decrease the number of elements and enhance calculation efficiency. The reliability of the improved HA J-model was verified by solving 2000-tape HTS stacks, and a significant improvement in computational performance was achieved. Furthermore, the HA J-model was applied to a large-scale scenario of a 32 T hybrid all-superconducting magnet at the National High Magnetic Field Laboratory to numerically capture the electromagnetic responses. The magnetic flux and current densities in the HTS insert coils were obtained completely, and the screen current induced field, which is the most concerning, showed a good agreement with the literature. Meanwhile, the time consumption by the HA J-model is approximately half that of the traditional J-model and even less than 1–2 orders of magnitude of the others, such as the traditional T-A model and H model, with acceptable accuracy. The improved HA J-model may provide an appropriate prior design with rapid and real-time calculations for large-scale magnet systems.

Effect of winding methods: transport AC losses in CORC coils

AC losses undoubtedly increase the burden on the cooling system and affect the overall performance of high-temperature superconducting (HTS) cables. Therefore, AC losses in HTS cables are important design factors for large-scale HTS equipments. Extensive research has been conducted on straight, densely wound conductors. However, AC losses of bending/circular HTS cable conductors have not been sufficiently studied. In this article, the transport AC losses in a circular HTS conductor coil bended by a conductor on round core (CORC) type cable were investigated using experiments and numerical simulations. The effect of different winding methods was also studied. First, the modeling method of the three-dimensional simulation and the experimental procedures were presented. The simulation results of the straight densely wound conductor (SDWC) and circular densely wound coil (CDWC) were compared with the corresponding experimental results. Finally, the winding method for the CDWC was optimized using both the simulation and experimental approaches. Both the experiments and simulations showed that reducing the tape-to-tape frontal area was able to decrease the transport AC loss, and the results show that the optimum winding angle of the HTS CORC coil was 24.15°. Overall, the modeling and experiments provide useful optimization strategies for designing winding structures of high-performance CORC coils/magnets.

Calculation of CORC Cable Loss Using a Coupled Electromagnetic-Thermal T-A Formulation Model

High-temperature superconducting (HTS) potential solution of solving the urban demand of large load and high-density power. However, the ac loss within the cable body is one of the primary factors limiting the large-scale HTS cable application. Due to the complex spiral structure of CORC (conductor on round core) cable and the high aspect ratio of conductors, modeling and calculation of CORC cable are pretty challenging. To better assess the cable loss, this article illustrates an improved model based on T-A formulation. Combined with a low-dimensional thermal model, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cBT</sub> (B,T) characteristics of HTS tape are applied in simulation. With a global parameter control equation, the current redistribution of the tape can be analyzed by the model. The loss of a three-phase coaxial (tri-axial) HTS cable under different electric and thermal conditions has been calculated. This article provides a method based on T-A formulation to study the current redistribution and quench process.

Design and Property Analysis of a Performance Evaluation System for HTS Wind Power Generators

Large-scale High Temperature Superconducting (HTS) wind generators produce strong electromagnetic force and high torque due to the high magnetic load of the machines, which is produced by HTS coils with high excitation current. Therefore, in order to properly maintain the mechanical strength of the HTS coil part, the torque and Lorentz force of the HTS wind generator must be scrutinized and reflected in the design. The authors proposed a method for assessing mechanical and electrical stability of a large-scale HTS wind generator based on a Performance Evaluation System (PES) that has the same characteristics as some of the pole-pairs of the HTS generator. The generator and the PES were analyzed using 3D finite element method. The performance of the HTS coil was evaluated by comparing the tangential, radial force and magnetic field distribution of the 10 MW HTS generator with the PES. The tangential and radial forces of the 10 MW HTS generator were 73 kN and 11 kN, respectively. The tangential, radial forces and magnetic flux density of the HTS coils in the PES were equal to one pole in the 10 MW HTS generator. Therefore, the proposed PES-based generator characteristic evaluation method can be used to estimate the mechanical stability of the generator under a high magnetic field and high torque before manufacturing the HTS wind turbine.

Screening Currents and Hysteresis Losses in the REBCO Insert of the 32 T All-Superconducting Magnet Using T-A Homogenous Model

The 32 T all-superconducting magnet of the National High Magnetic Field Laboratory (NHMFL) was successfully tested in December 2017 and it is expected to be soon available for users. This all-superconducting magnet, comprised of a high-temperature superconducting (HTS) insert and a low-temperature superconducting (LTS) outsert, is the first superconducting magnet reaching more than 30 T. One of the challenges facing this new magnet technology is the estimation of the screening currents, and the corresponding hysteresis losses in the two HTS coils. These coils are made of more than 20,000 turns of insulated REBCO conductor connected in series. The modelling of such system represents a significant challenge due to the huge computational load imposed by the size of the system. Up to now, only medium size magnets (made of units of thousands of turns/tapes) have been successfully modelled with methods based on the well-known H formulation of the Maxwell's equations. In the present work, a new model based on the T-A formulation and a homogeneous technique is proposed. This new approach greatly reduces the computational load and allows performing real-time simulations of large-scale HTS magnets on personal computers.

Design of a 1 MJ/100 kW high temperature superconducting magnet for energy storage

With significant progress in the manufacturing of second-generation (2G) high temperature superconducting (HTS) tape, applications such as superconducting magnetic energy storage (SMES) have become promising for implementation in the electricity grid. Compared to Li-ion batteries, SMES can provide higher power levels at a lower capital cost (down to $200/kW), they have a longer lifetime (over 20 years) and a comparable cycle efficiency of over 95%. Nevertheless, SMES technology is still underdeveloped, which is reflected in the lack of large-scale 2G HTS SMES units. One of the main challenges is designing an optimal magnet that can persistently store energy while withstanding the forces arising from the magnetic field and maintaining a temperature below the critical transition value. This paper outlines a methodology of designing a 2G HTS SMES, using Yttrium-Barium-Copper-Oxide (YBCO) tapes operating at 22 K. The target storage capacity is set at 1 MJ, with a maximum output power of 100 kW. The magnet consists of a stack of double pancake coils designed for maximum storage capacity, using the minimum tape length. The properties of a commercial YBCO tape published in the literature are used to derive the equations for scaling the critical current with magnetic field intensity and direction, as well as operating temperature. The inductance of the resulting coil configuration is calculated analytically, and is used for estimating the total storage capacity of the magnet. Finally, the properties of the superconducting magnet are summarised in a table.

Design and Feasibility Study of a Performance Evaluation System for a Large-Scale HTS Generator Under Short-Circuit Conditions

Generally, in a high temperature superconducting (HTS) generator, a large magnetic air gap exists because of needed space for a cryostat or non-magnetic cores in the HTS generators. Consequently, the inductance becomes lower than conventional generators. As a result, the short circuit current becomes as high as more than 10 times the rated current. The design of the mechanical structure of a generator is very challenging to endure a high force and torque at the HTS field coils. Therefore, a performance evaluation system (PES) is required to test the HTS field coil and support structures of the generator. In this paper, the design and feasibility study of a PES for and HTS field coil of large-scale HTS generator are presented. The 10 MW class HTS generator and PES was designed based on the electromagnetic analysis results obtained using the finite element method. The torque and force of the HTS field coil were analyzed. As a result, the HTS field coil can be practically tested using the proposed PES whether the structure of the coil can withstand the high torque and force of the generator. The proposed PES design and short-circuit analysis results can be effectively used for the development of large-scale HTS wind power generation systems.

Development of Multiscale Model in Large-Scale HTS Coils With Improved Coupling

Due to the nonlinear properties of superconductors, modeling of high-temperature superconducting (HTS) devices is difficult, especially for the ones in large scale. In this paper, we develop the multiscale model from a two-dimensional (2-D) planar geometry to a 2-D axisymmetric geometry to successfully calculate ac loss of large-scale HTS coils. By applying appropriate boundary conditions, the infinite-turn coil is built in the 2-D-axisymmetric model to produce approximated current density for an HTS coil. Besides, the multiscale model is improved in terms of coupling, so that submodels can be connected in one file and the external data processing is not needed. Validation on accuracy and efficiency is demonstrated through a double-pancake coil. Results are compared with a conventional model that considers the superconducting characteristics on each turn of the coil in actual size. This paper shows the development and improvement of multiscale model in the 2-D axisymmetric geometry is feasible. Due to its high accuracy and efficiency, the developed multiscale model can be a useful approach to analyze electromagnetic characteristics and calculate ac losses in large-scale HTS magnets.

Design and Comparative Analysis of Performance Evaluation Systems for a Large-Scale HTS Generator

Generally, in a high temperature superconducting (HTS) generator, the electromagnetic force can cause problems due to high current density and magnetic field. HTS field coils must therefore be structurally designed and analyzed to ensure the stability of the generator before the HTS generator is built. As part of the stability assessment, this paper designs a performance evaluation system (PES) for large-scale HTS generators and discusses comparative analysis with a generator model. The PES for a 10 MW class HTS generator was designed based on the electromagnetic analysis results obtained using the finite-element method. The detailed structures of the swing and fixed-type PESs were investigated taking into account the HTS coil, support, and stator components. The torque and force of the HTS field coil were analyzed and the results were compared with the generator model considering the amplitude and direction of the force of the HTS field coil to confirm the effectiveness of the PESs. As a result, the output torque and tangential force of the PESs were the same as those of the HTS generator. Although both types of PES can evaluate the torque and force of an HTS generator, the fixed-type PES is more suitable for the characteristic test of HTS field coil of large-scale HTS generators.

A Performance Evaluation System of an HTS Pole for Large-Scale HTS Wind Power Generators

Currently, the bottleneck in the development of large-scale high temperature superconducting (HTS) wind power generators is that high torque causes problems in HTS coils. Therefore, it is necessary to confirm the applicability to full generators by evaluating the performances of HTS coils in high torque condition. This paper deals with a performance evaluation system (PES) design for a 10-MW HTS wind power generator and a comparative analysis of the PES with the full generator. The magnetic field distribution and output power of the full generator were presented. The rated output torque and the force of the 10-MW HTS generator were calculated, and the PES for the full generator consisted of three HTS poles and the 1/20 times stator of the full generator. The modeling verification process of the PES for the generator was defined. The stator winding and circuit diagram of the PES were illustrated. Finally, the torque and force of the full generator and PES were analyzed and compared. As a result, the electromagnetic forces of one HTS pole were calculated considering the stator interaction, and the performance of the HTS pole using the PES was proven similar to the HTS pole in the full generator. The PES was confirmed suitable for testing HTS coils for large-scale HTS generators.

Real-time simulation of large-scale HTS systems: multi-scale and homogeneous models using the T–A formulation

The emergence of second-generation high temperature superconducting (HTS) tapes has favored the development of large-scale superconductor systems. The mathematical models capable of estimating electromagnetic quantities in superconductors have evolved from simple analytical models to complex numerical models. The available analytical models are limited to the analysis of single wires or infinite arrays that, in general, do not represent actual devices in real applications. The numerical models based on the finite element method using the H formulation of Maxwell’s equations are useful for the analysis of medium-size systems, but their application in large-scale systems is problematic due to the excessive computational cost in terms of memory and computation time. Therefore it is necessary to devise new strategies to make the computation more efficient. The homogenization and the multi-scale methods have successfully simplified the description of the systems allowing the study of large-scale systems. Also, efficient calculations have been recently achieved using the T–A formulation. In the present work, we propose a series of adaptations to the multi-scale and homogenization methods so that they can be efficiently used in conjunction with the T–A formulation to compute the distribution of current density and hysteresis losses in the superconducting layer of superconducting tapes. The computation time and the amount of memory are substantially reduced up to a point that it is possible to achieve real-time simulations of large-scale HTS systems under slow ramping cycles of practical importance on personal computers.

Design of a Combined Screening and Damping Layer for a 10-MW-Class Wind Turbine HTS Synchronous Generator

This paper is focused on the design of a single protection layer to cover both the screening and damping characteristics of a large-scale high-temperature superconducting (HTS) wind turbine synchronous generator under sudden short-circuit conditions. Due to small synchronous reactance of HTS generators, short-circuit condition causes substantial fault current in the stator windings, which produces high unbalance variable magnetic field as well as substantial mechanical fluctuations issues. The former one can lead to unstable condition for HTS field windings, due to induced ac loss and field overcurrent, and the latter one can produce large fluctuations on the generator rotational speed and rotor torque angle. The proposed protection layer is designed in a way that not only screening capability is provided for the HTS windings by preventing unbalance magnetic field penetration, but desirable damping ratio is also achieved so that the fluctuations are suppressed in a proper transient time. A 10-MW-class HTS synchronous generator is modeled and simulated using finite element method under different short-circuit conditions. The temperature variation study and field overcurrent analysis are provided to show the compatibility of the proposed model. In addition, magnetic force distribution and static eccentricity fault detection analyses are provided, which guarantee the proper dynamic performance of the studied generator using the proposed protection layer. Simulation results confirm that with the proposed protection layer, both the screening and damping characteristics of the 10-MW-class HTS generator are obtained and proper transient dynamic responses are deduced for wind power applications.

Structural design and heat load analysis of a flux pump-based HTS module coil for a large-scale wind power generator

Superconducting generators typically require a power supply, current lead (CL) and slip ring to deliver DC current to a high-temperature superconducting (HTS) coil, which causes a conduction heat load. On the other hand, a flux pump (FP) is possible to supply DC current to the HTS coils of the generator without the heat transfer loads. This paper deals with a structural design and heat load analysis of an FP-based HTS module coil for a 12 MW wind power generator. The structures such as HTS coil bobbins, coil supports, and the connection components between the FP and the HTS coils were designed. The conduction and radiation heat loads of the FP-based HTS module coils were analysed using a 3D finite element method program. The results of the HTS module coil of the generator were compared with a conventional CL-based HTS module coil. As a result, the total heat loads of the FP-based HTS module coil were lower than the conventional CL-based HTS module coil. The structural design and heat load analysis results of the FP-based HTS module coil can effectively be utilized to develop a large-scale HTS wind power generator.

Fast Algorithm for Evaluating Critical Current of High-Temperature Superconducting Pancake Coil

Second-generation (2G) high-temperature superconducting (HTS) tapes are now capable of carrying very high transport current and promising for a wide range of applications. The critical current of HTS coils is important for applications, such as superconducting electric machines, superconducting magnetic energy storage, and superconducting magnets. Therefore, precisely and quickly calculating critical current of HTS coils is very important for designing HTS devices. This paper provides a fast algorithm for evaluating critical current of HTS pancake coil. The fast algorithm is realized through a stationary model, which is based on finite element method (FEM) software. The stationary model means that the model is solved by stationary study instead of time-dependent study. To validate this method, a pancake HTS coil was wound and its critical current was measured. Meanwhile, an axial symmetric stationary model was built according to the geometry of the measured HTS coil. By comparing measured and calculated results, the effectiveness of the stationary model was demonstrated. Moreover, the stationary model is compared with H formulation model. The calculated results by the two models are nearly the same. However, by using stationary calculation, the stationary model can remarkably speed up the computational process. Due to the advantage of calculating speed, the stationary model can be used to characterize and design large-scale HTS applications.

Structural Design of a Module Coil for a 12-MW Class HTS Generator for Wind Turbine

Recently, high-temperature superconducting (HTS) generators are being studied to reduce the weight and size of large-scale gearless generators. However, the large-scale HTS generators have problems relating to high electromagnetic force because of their high current density and magnetic field. Therefore, the forces acting on HTS field coils should be considered as well as their support structures. This paper discusses the structural design of a module coil for a 12-MW class HTS generator for wind turbine. The 12-MW HTS generator module was designed based on electromagnetic analysis results obtained using the 3-D finite-element method. The detailed structure of the module coil was investigated considering the HTS coil bobbin, support, and cooling system. For secure design of the module coil, stress and deformation analyses were performed. Structural analysis results were compared with safety factors and critical yield stresses for each material. As the results, electromagnetic force and maximum stress of the module coil were calculated, and pole-type supports were found to be the most suitable for the 12-MW class HTS generator.

Electromagnetic Analysis of YBCO Superconducting Cables With High Current Transporting for Electric Devices

Most high-temperature superconducting (HTS) electric devices have been built by employing a superconducting composite cable. These HTS devices generally have large current-carrying capacities. Therefore, the electromagnetic characteristic of a composite superconducting cable with high current density becomes a key point for the application of these HTS devices. A helical superconducting cable and a Roebel cable consisted of YBCO-coated conductors with a critical current of 500 A @ 77 K in the self-field are both proposed in this paper. The helical superconducting cable that has two conductor layers and the structure parameters of the helical cable are given. The Roebel cable has four layers with a critical current of 500 A. Two 3-D models are built for a helical cable and a Roebel cable with variable geometries. Their magnetic flux distributions along the axis are presented based on the finite-element analysis method, and their electromagnetic characteristics are discussed as well. The analysis model can be further applied for the large-scale HTS electric device design with high current-carrying ability.

Design and Heat Load Analysis of a 12 MW HTS Wind Power Generator Module Employing a Brushless HTS Exciter

Brushless high-temperature superconducting (HTS) flux pump exciters, which enable large currents to be injected into a superconducting circuit without requiring a power supply, slip ring, and current leads, are promising candidates for HTS rotating machine application. This paper outlines the design and heat load analysis of a 12-MW HTS wind power generator module employing a brushless HTS exciter. The 12-MW HTS generator module and the HTS exciter were simulated using the 3-D finite element method. The module design of the generator was focused on reducing the heat load and inductance per rotor pole for application of an HTS exciter. A highly permeable ferromagnetic material was used to increase the magnetic flux density incident on the HTS stator wire of the exciter, even with a large radial gap between the rotor and the stator, and hence increase the injected current. Based on the electromagnetic simulations, the design of the module was confirmed, and the iron loss of the exciter was calculated. Then, the conduction and radiation heat loads were simulated. The induced dc current value and ramping time of the DC current at the HTS stator wire of the exciter were calculated. The detailed results of the module with the HTS exciter were discussed, and the results obtained in this paper are useful in designing large-scale HTS generators.

Simulation and Experimental Demonstration of a Large-Scale HTS AC Induction Furnace for Practical Design

Preheating for nonferrous metals in industry is realized through induction furnaces with a large capacity at commercial frequency. The energy efficiency of this process is about 50%-60% due to enormous energy loss through the copper coils. An ac induction furnace applying 2G high-temperature superconducting (HTS) magnets has been suggested in order to achieve higher efficiency. The most important factor in this technology is the ability to predict and minimize the ac loss, which is generated in HTS magnets during operation. In this paper, we demonstrate the simulation and experimental results of a large-scale HTS ac induction furnace for practical design. The HTS ac induction furnace was designed and simulated through a finite-element method (FEM) tool and an equivalent circuit model. The inductors were designed by using 2G HTS magnets, with consideration given to the ac loss generated in the magnet. To predict the ac loss of the magnet, the FEM was used, and the results were compared with those of the fabricated HTS magnets. The simulation and experimental results will be applied to fabricate a full-scale HTS ac induction furnace.

Heat loss analysis-based design of a 12 MW wind power generator module having an HTS flux pump exciter

The development of an effective high-temperature superconducting (HTS) generator is currently a research focus; however, the reduction of heat loss of a large-scale HTS generator is a challenge.This study deals with a heat loss analysis-based design of a 12MW wind power generator module having an HTS flux pump exciter. The generator module consists of an HTS rotor of the generator and an HTS flux pump exciter. The specifications of the module were described, and the detailed configuration of the module was illustrated. For the heat loss analysis of the module, the excitation loss of the flux pump exciter, eddy current loss of all of the structures in the module, radiation loss, and conduction loss of an HTS coil supporter were assessed using a 3D finite elements method program. In the case of the conduction loss, different types of the supporters were compared to find out the supporter of the lowest conduction loss in the module. The heat loss analysis results of the module were reflected in the design of the generator module and discussed in detail. The results will be applied to the design of large-scale superconducting generators for wind turbines including a cooling system.