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As air traffic has been increasing in recent years, the environmental impact of aviation is more obvious, forcing governments to impose stringent regulations on emissions. In order to meet these regulations, and reduce the carbon footprint, research has been directed towards the all-electric and hybrid aircraft, where the use of cryogenic HTS machines and cables has been proposed to reduce the overall size of the aircraft. With the cryogenic system already in place, research has been exploring the use of power electronics at lower temperatures in order to obtain systems with higher power densities and lower losses. In this paper several power semiconductor devices are tested at room and cryogenic temperature in order to evaluate their performance at lower temperature. One of the tested devices, a 'CoolMOS' superjunction MOSFET, is used in a single voltage source phase-leg which is experimented with at room and cryogenic temperature to evaluate its efficiency as a primary indication of its usefulness in the All-Electric Aircraft.

The second-generation high-temperature superconductor REBCO is one of the most valuable candidates that meet the requirements of future magnetic confinement fusion device. Quench behavior of an improved high-current REBCO composite conductor is simulated by a 1-D finite element method. Indium-tin alloy foil placed in the gap between stabilizer, jacket, and superconducting tape stacks can significantly reduce thermal contact resistance, which leads to an increase on the minimum quench energy of composite conductor. Short perturbations have a better response to this improvement than long perturbations, due to the low normal zone propagation velocity (NZPV) of REBCO conductor. For short perturbation (perturbation length ( $l_{p}$ = 0.1 m), the peak temperature in cable and helium drops 12.76 and 17.99%, respectively, after soldered. Due to the change of heat transfer in a composite conductor caused by helium wave propagation, the development and maximum length of normal zone significantly decrease from 13.06 to 4.78 m under long perturbation ( $l_{p}$ = 10 m) circumstance. High current REBCO composite conductor with proper indium-tin alloy soldered can enhance the stability and safety of composite conductors under severe working conditions, but it also slows down the NZPV and hinders the quench detection of high-temperature superconducting magnet.

This research investigates the economic aspects of using superconducting magnetic energy storage (SMES) systems and high-temperature superconducting (HTS) transformers as reported by utilities and other projects. The focus is on producing a preliminary set of information that could form a guideline in estimating cost-benefit-related numbers for these technologies. In this paper, a literature review of studies using SMES systems and HTS transformers is presented and an effort is made to highlight major cost factors and drivers based on available information. In conclusion, it is evident that more utility scale studies need to be undertaken to have a comprehensive understanding of the economic benefits of SMES and HTS-based systems in addition to a thorough comparative assessment to prevalent and conventional options.

The magnetism of substrate has significant influence on the transport loss in high temperature superconducting (HTS) coated conductors (CCs). Taking three types of substrates (i.e., nonmagnetic, weakly magnetic, and strongly magnetic) into account, a study on the frequency dependence (in the range of 10–1000 Hz) of transport loss in HTS CCs with these substrates is performed numerically and experimentally. Different loss components and magnetic field profile are calculated individually to clarify the effect of substrate magnetism. Results show that HTS CCs with magnetic substrate exhibit higher transport loss than CCs with nonmagnetic substrate. Both hysteresis loss in HTS layer and eddy current loss in conventional metal increase with the enhancement of substrate magnetism.

Growing electricity demand from a range of sources and the changes of the power grid structures open the possibility of more frequent and much higher fault current. Traditional solutions to the fault are difficult to satisfy the requirement of the new power grid due to many factors, such as high cost, additional impact to power grid in normal loading condition, which leads to the research for an efficient alternative solution of interest to both academia and industry: Superconducting fault current limiter (SFCL). In this paper, two types of low-inductance solenoidal coils, the braid type and the nonintersecting type, are built with 2G high-temperature superconductors. The current limiting performance, the recovery characteristics, and the ac losses of both types of coils are tested and compared in detail. Experimental results show that both types of coils can effectively limit fault current and recover in several seconds. Furthermore, comparison between the ac losses of both types of SFCLs shows that the ac loss of the braid type coil is lower than that of the single tape by about an order of magnitude in low-current regions.

This paper investigates how the current distribution and ac losses of second-generation high-temperature superconducting (2G HTS) pancake coils are affected by the material anisotropy. The 2G HTS tape has a strong anisotropy, which means that its critical current densities are different at different magnetic field orientations. 2G HTS pancake coils with different anisotropies have different ac losses. We studies this phenomenon by both experimental and numerical methods. Three 2G HTS coils with different turns are fabricated and tested in liquid nitrogen. AC losses with different frequencies are measured and compared with simulation. We use the critical-state model to calculate the current distribution and ac losses. Our modeling results are well validated by experimental measurements. We apply this numerical model to pancake coils with different anisotropies and calculate the current distribution, magnetic field, and ac loss. We find that the anisotropy of 2G HTS affects the ac loss and current distribution. It is possible to decrease the ac loss by an advanced pinning technique.

This paper proposes a novel use of superconducting magnetic energy storage (SMES) hybridized with the battery into the electric bus (EB) with the benefit of extending battery lifetime. A new power control algorithm, which integrates a power grading strategy with the filtration control method, is introduced in this paper, achieving further improvement of battery lifetime. To demonstrate the performance of the SMES/battery hybrid energy storage system (HESS), a dynamic EB system is described with the advantage of considering more factors into the driving patterns. Simulation results show that the proposed HESS has successfully combined the SMES with the battery forming an optimal system that has the advantages of primary energy storage systems while complementing the disadvantages of each system. This paper also does the quantitative analysis of battery lifetime extension in the battery-only system, the filtration-based HESS, and the novel control-based HESS.

Growing electricity demand from a range of sources with higher loads and the industry structural changes open the possibility of more frequent and larger fault currents producible in liver power grids. Traditional solutions to the fault have difficulties in satisfying the requirement of the new power grid requirement due to many factors, such as high cost and additional impact to power grid in normal loading condition, which leads to the research for an efficient alternative solution of interest to both academia and industry: Superconducting fault current limiter (SFCL). Critical current Ic is used to describe the maximum current that a superconductor can transport and is one of the most important parameters to be considered while designing a SFCL. Guaranteeing the homogeneous distribution of the critical current density flowing in a superconductor is not possible due to manufacturing process limitations. In this paper, the impact of critical current inhomogeneity of coated high temperature superconductors (HTSs) during the quench process is studied experimentally. The results show that both the amount of Ic degradation and the size of Ic degraded segments have a great impact on the maximum temperature generated in the quench process of HTSs when the prospective fault current is low ( 1.13Ic to 1.51Ic. Higher amount of localized Ic degradation allows higher maximum temperatures under low voltage fault. Additionally, smaller Ic degraded segment allows higher maximum temperatures.

This paper demonstrates an efficient method to calculate the ac losses in multipancake coils. We use the front track line to analyze stacked pancakes by assuming that the current fronts are straight lines and using the critical state model. The current distribution is solved by minimization of total magnetic energy. The calculated ac loss results are compared with the well-accepted H-formula in COMSOL. We further apply this model to the design of a SMES with 2 kJ, which is under construction. The object of the design is to maximize the stored energy and decrease the ac loss. We discuss the detailed geometry parameters and operation temperature with the validated numerical model. Our model provides a fast calculation method of ac loss in stacked (Re)BCO pancakes and is useful to HTS applications in high field magnets, energy storage devices, and electric machines.

This paper focuses on the 2G HTS armature winding design for fully HTS machines. The main challenge for HTS armature windings is the ac loss calculation. This paper proposes an effective way to estimate 2G HTS armature efficiency. The originality of ac loss calculation is effectively coupling a machine model with a HTS model using boundary conditions to simulate machine environment while considering material anisotropy. The paper then uses H formulation to estimation the total armature ac loss for a 1-MW propulsion motor. The ac loss is calculated for different electrical loadings and magnetic loadings. A new double-layer armature winding is proposed to reduce the spatial harmonics of HTS armature.

The AC superconducting power transmission cable projects are being carried out around the world, while the DC superconducting cable is also proven to be very competitive for power transmission in electrical grid due to the negligible AC losses. The critical current and current distributions are two important factors to improve the performance of DC HTS cable for large transmission capacity. In this paper, we proposed an experimental method to investigate the behavior of critical current affected by various gap distances between the individual HTS tape and transverse tape width. The results show that the critical current of the HTS tape in the cable field can be increased by about 10% if the gap distance is less than 1 mm. Finite element method (FEM) is used to calculate the magnetic field distribution to verify the experimental results. A short HTS cable has been constructed to investigate the influence of contact resistance on the current distribution among the HTS tapes. The results show that the contact resistances have significant influence on the current sharing of each HTS wire in the cable structure.

Concept design and optimization for a high-temperature superconducting (HTS) rotor of a 100-kW generator is studied in this paper. The rotor has eight HTS magnets constructed by air-cored racetrack coils. The HTS magnets with a unique circular cross-sectional shape can produce a gap field over 2.0 T. The present design is based on a new graphical method developed to estimate the maximum operating current of an HTS magnet that takes both I c -B characteristics and angular dependence into consideration. The optimized stepped shape design saved more than 8% of the HTS material compared with the rectangular one without sacrificing the gap field.

Using a high-temperature superconducting (HTS) coil as the transmitting coil is an optional way to increase the efficiency and the distance of a wireless power transfer (WPT) system. In practical cases, receiving coils might be not only one, or not convenient to get a low-temperature environment, or not in the same sizes with the transmitting coil. Thus, we constructed a WPT system from one HTS coil to two copper coils, and one of the copper receiving coils had a different size from the HTS transmitting coil. In addition, we placed one HTS relaying coil between the transmitting coil and the receiving coils to increase the efficiency and the distance. Based on this system, we conducted several experiments and analyzed the results. We found that a denser coil could obtain more power in an HTS WPT system and that WPT from one HTS coil to two copper coils was more efficient than WPT from the same HTS coil to one of the two copper coils. We also demonstrated that an appropriate layout of the HTS relaying coil can increase the HTS WPT efficiency.

HTS cable is an emerging technology for electricity power transmission. It is very important to set up an accurate and easy-to-build measurement system to characterize HTS cables. In this paper, a novel measurement system has been developed to measure the ac losses for a prototype HTS cable using cancelation coil technique. This system is based on the National Instrument Data Acquisition card. It can also be used to measure the critical current and current distribution in the prototype cable. The prototype cable has been innovatively designed using an improved particle swarm optimization algorithm which is more computationally efficient than conventional methods. The measurement system will be improved further by introducing an elaborate pre-amplifier prior to data collection process and the result will be compared with theoretical calculations for validation.

A heat-shrink tube insulation for second-generation high-temperature superconducting (2G HTS) coils is studied experimentally in this paper. The insulation can be used to prevent the delamination of 2G HTS due to epoxy impregnation. According to the experiment results, the insulated coil shows no degradation before and after the epoxy impregnation. To validate the performance of 2G HTS coils with the heat-shrink insulation, we measured the transport loss, magnetization loss and total loss of a 2G HTS coil. The study demonstrates the feasibility of the heat-shrink tube insulation method for electrical applications.

As we known, the high temperature (77 K) superconducting (HTS) motor is considered as a competitive electrical machine by more and more people. There have been various of designs for HTS motor in the world. However, most of them focus on HTS tapes rather than bulks. Therefore, in order to investigate possibility of HTS bulks on motor application, a HTS magnet synchronous motor which has 75 pieces of YBCO bulks surface mounted on the rotor has been designed and developed in Cambridge University. After pulsed field magnetization (PFM) process, the rotor can trap a 4 poles magnetic field of 375 mT. The magnetized rotor can provide a maximum torque of 49.5 Nm and a maximum power of 7.8 kW at 1500 rpm.

High-temperature superconductors have created the opportunity for a step change in the technology of power applications. Racetrack superconducting coils made from YBCO coated conductors have been used in several engineering applications including SMES, rotor or stator windings of electric machines. AC loss is one of the most important factors that determine the design and performance of superconducting devices. In this paper, a numerical model is developed to calculate the AC losses in superconducting racetrack coils in different magnetic conditions. This paper first discusses the AC losses of the coils in self-field or external field only. It then goes to investigate the AC losses of the coils being exposed to AC ripple field and a DC background field. Finally, the AC losses of the coils carrying DC current and being exposed to AC field are calculated. These two scenarios correspond to using superconducting coils as the rotor field winding of an electric machine.

YBaCuO-coated conductors offer great potential in terms of performance and cost-saving for superconducting fault current limiter (SFCL). A resistive SFCL based on coated conductors can be made from several tapes connected in parallel or in series. Ideally, the current and voltage are shared uniformly by the tapes when quench occurs. However, due to the non-uniformity of property of the tapes and the relative positions of the tapes, the currents and the voltages of the tapes are different. In this paper, a numerical model is developed to investigate the current and voltage sharing problem for the resistive SFCL. This model is able to simulate the dynamic response of YBCO tapes in normal and quench conditions. Firstly, four tapes with different Jc's and n values in E-J power law are connected in parallel to carry the fault current. The model demonstrates how the currents are distributed among the four tapes. These four tapes are then connected in series to withstand the line voltage. In this case, the model investigates the voltage sharing between the tapes. Several factors that would affect the process of quenches are discussed including the field dependency of Jc, the magnetic coupling between the tapes and the relative positions of the tapes.

This paper begins with introducing the winding techniques of two superconducting double-pancake coils wound using 2G coated conductors. These winding techniques are able to guarantee a good performance for the superconducting coils. Then the coil critical currents were measured and compared with a simulation model. The results were consistent. Finally the coil AC losses were measured using an experimental circuit including a compensation coil. The simulation results are close to the experiment results.

AC loss can be a significant problem for any applications that utilize or produce an AC current or magnetic field, such as an electric machine. The authors are currently investigating the electromagnetic properties of high temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. In this paper, a 2D finite element model based on the H formulation is introduced. The model is then used to calculate the trans port AC loss using both a bulk approximation and modeling the individual turns in a racetrack-shaped coil. The coil model is based on the superconducting stator coils used in the University of Cam bridge EPEC Superconductivity Group's superconducting permanent magnet synchronous motor design. The transport AC loss of a stator coil is measured using an electrical method based on inductive compensation using a variable mutual inductance. The simulated results are compared with the experimental results, verifying the validity of the model, and ways to improve the accuracy of the model are discussed.

In order to design a High Temperature Superconducting (HTS) machine that is able to operate safely and reliably, studies on the characterization of Second Generation (2G) HTS tapes are of paramount importance. This paper presents an experimental setup to measure critical current of 2G HTS tapes in high DC magnetic fields (up to 5 Tesla) with an AC current ripple superimposed, as well as various temperatures ranging from 25 K to 77 K. The 2G tape measured is the SGS12050 coated conductor made by SuperPower. The critical current is measured by a flux vector with reference to the widest sample face from 0 to 90 degrees in 10 degree steps. Smaller steps are required close to 0°. A Variable Temperature Insert (VTI) is utilized to control temperature change.

High temperature superconductors, such as melt-processed YBCO bulks, have great advantages on trapping strong magnetic fields in liquid nitrogen. To enable them to function well, there are some traditional ways of magnetizing them, in which the YBCO bulks are magnetized instantly under a very strong source of magnetic field. These ways would consume great amounts of power to make the superconductors trap as much field as possible. Thermally Actuated Magnetization (TAM) Flux pump has been proved a perfect substitution for these expensive methods by using a relatively small magnet as the source. In this way, the field is developed gradually over many pulses. Unlike conventional flux pumping ways, the TAM does not drive the superconductor normal during the process of magnetization. In former experiments for the flux pump, some fundamental tests were done. In this paper, the experiment system is advanced to a new level with better temperature control to the thermal waves moving in the Gadolinium and with less air gap for the flux lines sweeping through the superconductor. This experiment system leads to a stronger accumulation of the magnetic field trapped in the YBCO bulk. We also tried different ways of sending the thermal waves and found out that the pumping effect is closely related to the power of the heaters and the on and off time.

It is particularly attractive for High Temperature Superconductors (HTS), such as the melt-processed YBCO single-grain bulks, to trap strong magnetic fields at cryogenic temperatures. With the flux density orders of magnitude much greater than the rare earth magnets, the YBCO bulks can be formed as small and as compact as them. As a result, YBCO bulks are used as magnets in magnetic bearings, MRI scanners and motors, etc. The trapped field by the YBCO magnet is decided by the magnetization, which normally includes three different ways. However, the traditional ways to magnetize the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it. In this paper, we will describe a technique which facilitates the creation of the high magnetic field and utilize a normal permanent magnet in stead. By using this rare earth permanent magnet which is not as strong, this technique involves the superconducting flux pump concept, which means that a much larger field being trapped by the superconductor bulk is caused by a small field repeatedly applied to it. To achieve such pumping effect, an intermedia material is necessary and in the related experiments, Gadolinium is used as such an important thermally actuated material. Traveling magnetic field through it will magnetize the YBCO in such a way that the flux density will be accumulated step by step on the surface on the bulk.

High-current high temperature superconducting (HTS) cables have been developed for use in HTS power devices. This paper presented the structures of HTS cables, including Conductor on Round Core (CORC) cable, Twisted Stacked-tape Conductor (TSTC) cable, and Double coaxial cable. Subsequently, three-dimensional finite element method numerical models were built to analyze the electromagnetic characteristics of the cables, and the critical current of the cables is about 380 Ampere 77 K, self-field. Using the T-A formulation, the numerical model assumed a sheet approximation for conductors, which shortened computational time. The T-A formulation were verified by experiments on a superconducting tape. Then HTS cables with different configurations were made, as functions of different transport current and background magnetic field, and different pitches of Double Coaxial Cable inner conducting layer. According to the results, the ac losses of Double coaxial cable and CORC cable decreased 40% than the TSTC cable with different transport current, and the Double coaxial cable ac loss decreased 20% than the CORC cable when background magnetic field was in the range of 20-60 mT. Conclusions obtained from this study will be helpful for understanding the ac loss properties of HTS cables and useful in design of HTS power devices (such as HTS transformer), using HTS cables.

Application of the second generation High Temperature Superconducting (HTS) YBCO tapes has been increasingly popular since the low-cost superconducting materials were discovered. This paper mainly presents the properties of two types of 2G YBCO tapes from American Superconductor Corporation (AMSC) and SuperPower, Inc, respectively. All superconducting materials are cooled by liquid nitrogen (77 K). At first, we will introduce the basic design strategy for synchronous motor and characteristics of superconducting materials. Then, the basic principle and algorithm for calculation and measurement of Ic and AC transport loss will be explained. Secondly, the novel measurement system using a high-precision digital lock-in amplifier is presented. Finally, we present and analyze simulation and experimental results for two types of YBCO tapes in bending condition.

This paper describes in detail the magnetization process of a synchronous motor, constructed with High Temperature Superconducting (HTS) elements on the rotor. The Pulse Field Magnetization (PFM) method is considered to magnetize the HTS rotor as a four-pole trapped flux magnet. A 2D model is built for the magnetization process. During the machine operation, intolerable torque ripples occur, which are mainly caused by the irregular field distribution from the magnetized HTS rotor. In order to suppress the large torque ripples, an iterative learning control method is used in addition to the traditional field-oriented control method.

When a bulk superconducting material is used as the magnets of an electric motor, the magnetized superconducting pucks are subjected to a varying magnetic field which has a detrimental effect on the trapped field. This may lead to a long term decay of the magnetization of the bulk superconductors. In this paper, we analyze numerically the demagnetization effect on bulk superconductors subjected to a magnetic field applied orthogonally to the initial direction of the trapped flux. The condition of a superconducting sample being exposed to a transverse field with varying magnitude is simulated and is very close to the operating environment in a synchronous machine. The decay of magnetization is calculated. The mechanism is explained in terms of the distortion of current distribution on the cross section of the sample. The long term effect of the application of a transverse field is predicted by the model.

A coaxial cable made out of helically wound 2G HTS tapes is being widely implemented based on its low critical bending radius, ease of manufacturing and installation. Though the design technique is mature, the cable still lacks standard Ic definition. With increased usage of such cables, robustness in its operation is expected. HTS tapes are highly sensitive to externally applied mechanical stress and strain values. These limitations force the coaxial cables to be operated at 50-75% of its rated capacity. One of the major problems in coaxial cable design is the contact resistance of the tapes with the terminal, which affects the current distribution among different tapes in the same layer. Further current redistribution at constant intervals helps the applied current in bypassing the localized faults in the individual HTS tapes, thus making efficient use of the major length of the tape. This paper proposes the use of voltage shunts between HTS layers at constant intervals for regular current redistributions along the length of the cable. This method significantly helps to sustain the Ic value of the cable, even if it has got several minor defects in the tapes.

A cold dielectric high-temperature superconducting (HTS) cable has multilayer transmission conductors to carry a large current. When the conductor layer number increases, the unevenly distributed currents caused by the skin effect in each conductor layer become more and more nonuniform, and would lead to the increased amount of loss and poorer transmission performance. In order to fully exploit the performance of conductors in cables, it is important to understand the current sharing mechanism between layers. An optimal design model for minimizing current difference in each conductor layer was proposed using an improved particle swarm optimization algorithm. A 1 km, 110 kV/3 kA cold dielectric HTS cable consisted of YBCO tapes and with current evenly sharing was optimally designed based on an equivalent circuit model. After optimization, we find that the maximum unbalanced rate of the current in conductor layers is 4.1% and there is a uniform current distribution among the conductors in shield layers. The current calculation result of the designed HTS cable shows the current is evenly shared in different layers and thus this optimal design method is validated.

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

This paper presents the design of an AC loss experiment using nitrogen boil-off method. This experiment is aimed at exploring the AC loss of HTS double race-track coils which will be installed on the rotor of a wind turbine generator. The operating environment is simulated by designing a cryostat with rotating magnetic field windings. Apart from the fact that the alternating magnetic field causes most of AC loss on the HTS coils, we also believe that the DC background field would be another important factor causing AC loss if the HTS coil is experiencing by both alternating magnetic field in the perpendicular direction and DC background field in the parallel direction. In order to perform the boil-off measurement, we present the method to estimate the heat leakage in the cryostat which might cause errors to the measurement .