Superconducting Transformer Research Articles

Survey on High-Temperature Superconducting Transformer Windings Design

Utilization of superconducting equipment increases every day due to availability of the HTS tapes with reasonable prices, commercially. Since 1997, many high-temperature superconducting (HTS) transformers have been designed and fabricated. In this paper, all the different developments of HTS transformer windings since 1997 will be reviewed. Additionally, the advantages and disadvantages of the application of the two main winding types (the pancake and the layer) in the HTS transformer will be studied.

The end part of cryogenic H. V. bushing insulation design in a 230/20 kV HTS transformer

Appealing advantages of high-temperature superconducting (HTS) transformers are very promising for the contemporary and future power delivery systems. Cryogenic insulation technology is one of the key technologies for their commercialization. In this paper insulation design of the end part of cryogenic high voltage (H. V.) bushing of a 230/20 kV HTS transformer is developed. Analytical relations and finite element method (FEM) modelling, through application of COMSOL Multiphysics software, are employed for determination of electric field distribution in this design process. Weibull 0.1% dielectric breakdown strength has been considered as criterion for insulation design. Different shape and dimension parameters have been tested for mitigation of electric field concentration. A stress cone has been employed to reduce the electric field concentration at the triple point interface of the materials. Additionally, a ring-shaped cut-outs is made on the GFRP-liquid nitrogen boundary to enhance the insulation characteristics.

Conducting a Survey of Research on High Temperature Superconducting Transformers

Given the unique electrical and magnetic properties of superconductors, high temperature superconducting (HTS) transformers will play a central role in the electricity industry in the future. Therefore, identification and analysis of this type of transformers seems necessary and indispensable. To date, many types of development plans for HTS transformers have been carried out by major power companies and important research institutes, and it is expected that this type of transformers will become one of the superconducting power equipments installed at the first stage of commercialization in the power system. However, a review of reports and papers published in this field shows that there is still much work to be done to improve the design and performance of these transformers. This article presented a review classified by subject of previous research to help orient future research into the design, analysis, and manufacturing of this type of transformers.

Superconducting Transformer for Superconducting Cable Testing up to 45 kA

A facility capable of testing superconducting cables with current of tens of kA is essential for the development of large superconducting magnets. A superconducting transformer (SCT) is a suitable choice as a high DC current source for testing superconducting cables. In this work, we will present our experimental results of a SCT up to output current of 45 kA. The properties of this SCT is characterized at 4.2 K. Its behaviors during quenches at different current levels are studied.

Early Detection of Normal Transitions in a High Temperature Superconducting Transformer Wound with a Plurality of HTS Tapes Using the Active Power Method

The authors have been developing an AC power source with a high temperature superconducting (HTS) transformer which is small in size and can supply large AC current. The HTS transformer needs a detection system of normal transitions for high reliability. The authors have reported the active power method as the detection system. However, the conventional method could not detect early the normal transition in a secondary coil of the transformer which was wound with a bundle of some HTS tapes since resistive voltage in the normal area is too small due to current sharing among the HTS tapes. In this paper, the authors propose a method to detect the normal transitions in the bundle conductor by improving the active power method. The early and correct detection can be achieved by measuring each active power of each HTS tape. In order to measure the active power, current leads are attached independently to all the HTS tapes. The current leads enable to measure the resistive voltage in each HTS tape, and each active power can be measured. The proposed method works well when even one of secondary tapes transits to the normal conducting state. Experimental results show the usefulness of the proposed method.

Multi-segment Winding Application for Axial Short Circuit Force Reduction Under Tap Changer Operation in HTS Transformers

High-temperature superconducting (HTS) transformers have remarkable appealing advantages over conventional ones. But higher brittleness of HTS windings with respect to copper windings makes HTS transformers more vulnerable in short circuit and inrush current situations. During tap changer operation, appreciable asymmetry and non-uniform distribution of ampere-turn along the windings causes high axial component of short circuit forces and makes the situation more severe. In this paper, multi-segment winding method is employed for reduction of axial short circuit forces. An analytical method is presented for calculation of axial component of short circuit forces under tap changer operation. Analytical method shows that with utilizing HLHLHLH asymmetrical multi-segment windings, during tap changer operation, the axial component of short circuit forces reduces significantly. The results of the analytical method are compared with the results of finite element method (FEM) model simulations, and the consistency is demonstrated.

The Influence of Flux Diverter Structures on the AC Loss of HTS Transformer Windings

Compared to conventional transformers, high temperature superconducting (HTS) transformers have higher efficiency and fewer hazards, whose main loss comes from the ac loss of high temperature superconducting windings carrying ac current. Their radial leakage magnetic fields are perpendicular of HTS winding tapes, which is the main parameter affecting the ac loss. In the study of superconducting magnets, the flux diverter has been used to reduce the radial leakage magnetic field. The effect of the flux diverter installed at the top of the transformer windings on the coil ac loss was explored in this paper. The influence of different structure, slotting, and the number of slots on the transformer was also discussed, which laid a theoretical foundation for the optimization of flux diverter on the 6.6 MVA HTS transformers. Besides, in this paper, we choose the homogenization treatment method to reduce the computing time and memory when modeling.

First magnetic resonance imaging under three tesla magnetic field [News From Japan

Superconducting (SC) technologies have long histories of research, and some of them are already in a stage for practical applications. For example, a SC current lead, a SC transformer, a SC cable in grid operation, and a SC flywheel for magnetic energy storage have been introduced in this “News From Japan” column as recent Japanese typical examples of SC applications [1]–[4].

HTS Transformer Windings Design Using Distributive Ratios for Minimization of Short Circuit Forces

High-temperature superconducting (HTS) transformers have a promising feature in reduction of total weight, total size, and the losses of large-scale distribution transformers. However, the lower leakage reactance of HTS transformers results in a higher short-circuit fault currents and electromagnetic forces. Therefore, optimization of short-circuit electromagnetic forces is one of the crucial aspects in the design of HTS transformers. In this paper, a novel analytical method is proposed for determination of optimum distributive ratios resulting in minimization of these forces for asymmetrical multi-segment windings of an HTS transformer. Employing these distributive ratios, radial and axial components of short-circuit electromagnetic forces in an HTS transformer are significantly reduced. Two- and three-dimensional (2D and 3D) finite element method (FEM) simulations are employed to verify the analytical method results.

Study on a Magnetic Flux Detection Coil for Detection of Normal Transitions in a Hybrid Single-phase Bi2223 Superconducting Transformer by the Active Power Method

The authors have been developing a small and light power source with a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer. The transformer consists of a primary copper coil and a secondary Bi2223 superconducting coil and can output current of 500 A. For safe operation of the transformer, it is important to detect normal transitions in the transformer and to protect it from excessive heating in the normal area. The authors have presented a protection system based on the active power method and magnetic flux detection coil. However, in the conventional method, the active power does not become zero in a superconducting state due to AC loss and therefore incorrect recognition may be caused. In this paper, the authors propose a method to reduce the loss signals by configuration and mounting position of the magnetic flux detection coil. Through experimental results for a hybrid single-phase Bi2223 superconducting transformer, the authors show that the proposed method can detect the normal transitions more accurately than the conventional one.

Preliminary Investigation on Economic Aspects of Superconducting Magnetic Energy Storage (SMES) Systems and High-Temperature Superconducting (HTS) Transformers

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.

Modeling and Simulation of Railgun System Driven by Multiple HTSPPT Modules

In the rail-type electromagnetic launching system, the equivalent load of pulsed-power supplies (PSs) varies with the launching process, while it has a certain influence on the discharge of pulsed PS. However, in the analysis and design of pulsed PS, researchers usually use the fixed resistance and inductance to simulate the load, which is impossible to accurately study the characteristics of the pulsed PS. The electromagnetic launching needs high-amplitude current pulse. Therefore, in this paper, a mathematical model of electromagnetic launching system driven by multiple superconducting inductive pulsed PS modules is established. In this model, the fixed resistance and inductance load are replaced by the dynamic load model of the railgun; the effects of dynamic load and fixed load on the discharge characteristics are analyzed; the parallel discharge characteristics of multiple high-temperature superconducting pulsed-power transformer (HTSPPT) modules are studied; and the characteristics of the simple rail-type electromagnetic launching driven by multiple HTSPPT modules are analyzed. The simulation results show that a high amplitude of current pulse can be produced using synchronous parallel discharge of multiple HTSPPT modules. The simulation results show that when the resistance and inductance of load are fixed at 2 $\text{m}\Omega$ and 0.5 $\mu \text{H}$ , respectively, the current amplification factor is 15.666, and the maximum voltage of the auxiliary capacitor is 1079 V. In the dynamic load model, the current amplification factor is 17.174, and the error is 9.6%. The maximum voltage of the auxiliary capacitor is 983 V, and the error is 8.9%. Therefore, the discharge characteristics of pulsed PS can be more accurately studied using dynamic models.

A Study on a Single-Phase HTS Transformer with a Cylindrical Central Iron Core

During the past decade, a number of high temperature superconducting (HTS) transformer prototypes have been designed, and the majority of them are full-core transformers and air-core transformers. However, in this study, we proposed a new type of HTS transformer, where the iron core is different in that the limbs and connecting yokes are absent. To investigate the performance of this type of HTS transformer, we design and fabricate a single-phase HTS partial core transformer prototype using the secondary generation (2G) HTS GdBCO wires. The iron core is at room temperature, and the windings are immersed in liquid Nitrogen. The structures of primary and secondary windings are three layers connected in series and nine double pancakes connected in parallel, respectively. Fundamental characteristics are obtained by standard short-circuit, no-load and load tests in liquid Nitrogen temperature of 77 K. In addition, an equivalent circuit of the HTS transformer is proposed to analyze the characteristic of the transformer. The calculated values based on the equivalent circuit are consistent with that of experiment. The detailed results about the design of the HTS partial-core transformer, the experiment, and the equivalent circuit are presented and discussed in this study.

A Zero-Current Opening Circuit for Inductive Pulsed-Power Supply Based on High-Temperature Superconducting Pulsed-Power Transformer

The opening switch is one of the critical technologies in the development of inductive pulsed-power supply. The power requirement is very high when the opening switch is desired to commutate a large current. This paper presents a zero-current opening circuit for inductive pulsed-power supply based on a high-temperature superconducting pulsed-power transformer. First, there are two types of current flowing through the opening switch, one comes from the secondary side of the pulse transformer, and the other one comes from superconducting energy storage inductance. The orientation of the two current is opposite. Then, when the current flowing through the opening switch closes to zero, the switch is opened and this restrains the over-voltage pulse of the opening switch. The simulation results indicate that this zero-current opening circuit is feasible and has potential applications for high-energy systems in the future electromagnetic launcher.

Stability and superconducting properties of GaH5 at high pressure

Using genetic algorithm (GA) method combined with first-principles calculations, the structures, dynamical and thermodynamic stabilities of GaH5 were studied. The calculated results suggested that at the pressure range 150–400GPa, the P21/m phase of GaH5 is the most favorable phase and dynamically stable, but thermodynamically it is unstable and can decompose into GaH3 and H2. The superconducting property of GaH5 was further calculated, and the predicted superconducting transformation temperature Tc of GaH5 P21/m phase is about 35.63K at 250GPa. Besides, we compared the GaH5 and GaH3 superconducting properties, and found that GaH3-Pm-3n structure has a larger DOS near Fermi level than GaH5-P21/m structure, which may be the main reason causing higher Tc of GaH3 than GaH5.

Detection of normal transitions in a hybrid single-phase Bi2223 high temperature superconducting transformer by using the active power method and a magnetic flux detection coil

The authors have been developing a hybrid single-phase Bi2223 high temperature superconducting (HTS) transformer used in the AC current source with rated current of over 500A. Its primary coil is a copper coil and secondary coil is a Bi2223 HTS coil. In this paper, the authors propose a new detection method of normal transitions in the secondary coil by using the active power method and a magnetic flux detection coil attached on the inside of the secondary coil. In the proposed method, the normal transitions are detected by measuring active power dissipated in the secondary coil, and induced voltage of the magnetic flux detection coil by a primary and leakage flux of the secondary coil enables to calculate the active power dissipated in only the secondary coil. As experimental results for a hybrid single-phase Bi2223 HTS transformer, it was found that the proposed method enabled to detect the normal transitions in its secondary superconducting coil.

Basic study for a large AC current supply with a single phase air-core Bi2223 high temperature superconducting transformer

The authors have been developing a compact power supply with a single-phase Bi2223 high temperature superconducting (HTS) transformer. The conventional transformer has an iron-core for enhancing magnetic coupling between its primary coil and secondary one. However, the iron-core has great majority of size and weight of the transformer and therefore it is desirable to be removed for a smaller and lighter transformer. In this paper, the authors propose an air-core HTS transformer for a more compact power supply than the conventional one. As experimental results, it is shown that appropriate design of the air-core transformer has a possibility to decrease the weight and volume of the large AC current supply.

Characteristic Tests of GdBCO Superconducting Transformer With Different Iron Core Structures

This paper focuses on the comparison of the characteristics among three different types of high temperature superconducting (HTS) transformers. The iron core structures of the three types of HTS transformers are air core, partial core, and full core. The iron cores are made of oriented silicon steel sheets. The primary windings are four layers connected in series, and the secondary windings are nine double pancake coils connected in parallel. Fundamental tests, such as critical test, no-load test and load test, are performed in the environment of liquid nitrogen. The current nonuniformity of HTS partial core and full core transformers in secondary windings are also confirmed by the load test. Finally, we also investigate the characteristic of fault current limiting by sudden short-circuit tests. The experiment results show that the structure of the iron core have significant effects on the characteristics of HTS transformers. The results of this paper could support with useful data for the design and selection of HTS transformers.

On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi2Sr2CaCu2Ox conductors

Precursor powder composition is known to strongly affect the critical current density (Jc) of Ag/Bi2Sr2CaCu2Ox (Bi-2212) wires. However, reasons for such Jc dependence have not yet been fully understood, compromising our ability to achieve further optimization. We systematically examined superconducting properties, microstructural evolution and phase transformation, and grain boundaries of Bi-2212 conductors fabricated from precursor powders with a range of compositions using a combination of transport-current measurements, a quench technique to freeze microstructures at high temperatures during heat treatment, and scanning transmission electron microscopy (STEM). Samples include both dip-coated tapes and round wires, among which a commercial round wire carries a high Jc of 7600 A mm−2 at 4.2 K, self-field and 2600 A mm−2 at 4.2 K, 20 T, respectively. In the melt, this high-Jc conductor, made using a composition of Bi2.17Sr1.94Ca0.89Cu2Ox, contains a uniform dispersion of fine alkaline-earth cuprate (AEC) and copper-free solid phases, whereas several low-Jc conductors contain large AEC particles. Such significant differences in the phase morphologies in the melt are accompanied by a drastic difference in the formation kinetics of Bi-2212 during recrystallization cooling. STEM studies show that Bi-2212 grain colonies in the high-Jc conductors have a high density of Bi2Sr2CuOy (Bi-2201) intergrowths, whereas a low-Jc conductor, made using Bi2.14Sr1.66Ca1.24Cu1.96Ox, is nearly free of them. STEM investigation shows grain boundaries in low-Jc conductors are often insulated with a Bi-rich amorphous phase. High-Jc conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.

Analysis of leakage field and current non-uniformity for HTS transformer windings using three dimension coupled field-circuit approach

PurposeThe purpose of this paper is to analyze the influence of different types of winding configurations on the distribution of leakage field and branch currents for a 330-kVA high-temperature superconducting (HTS) transformer winding.Design/methodology/approachA three-dimensional electromagnetic model coupled with a circuit model validated by an experiment is developed to calculate the leakage field and current uniformity under four different types of secondary winding configurations. The four types of gaps between secondary windings are uniform gap, arithmetic progression (AP) gap, six sections with three different gaps and eight sections with four different gaps. A coefficient named as uneven degree is used to define the current nonuniformity.FindingsThe simulation results show that the currents and leakage field of double pancakes (DPs) on both sides are larger than those of the other DPs, and the currents of several middle DPs are smaller than the average rated current with an ISOB gap and larger than the average rated current with an IBOS gap. For any one of the four types of winding configurations, the type with the ISOB gap can prohibit the current nonuniformity more effectively, whereas the IBOS gap can decrease the leakage field more. The AP with the ISOB gap is a wise choice for decreasing the uneven degree and leakage field.Practical implicationsThere is an optimal winding configuration for decreasing the leakage field and uneven degree of branch currents. The results and numerical model are very useful for the design of a HTS transformer.Originality/valueThe leakage field distribution and branch currents nonuniformity for 25 parallel DPs are investigated and optimized.