High Temperature Superconducting DC Research Articles

Initial Study on Press Welding With Indium Applied to High-Temperature Superconducting DC Feeder Cables

High-temperature superconducting (HTS) DC feeder cable has been proposed for reducing energy loss in railway systems. The feeder cables are required to be installed during the night using a quickly-fabricable low-resistive joint. A soldering technique has been widely used for connecting HTS round-shaped cables in transmission or feeder cable systems. In this study, we proposed applying press welding with indium at room temperature (cold pressure welding) or with heat treatment at 90–120 °C as an alternative candidate for the cable-to-cable joint. First, we examined the necessity of the pre-joint process, polishing the HTS tape, and pickling indium foil with 10% HCl as well as the joining temperature. The examination was performed using tape-to-tape joints with copper-alloy-laminated Bi-2223 tapes. The joint resistance and mechanical strength for railway application were satisfied even by a joint fabricated at room temperature skipping the above two pre-joint processes. Then, we fabricated straight and spiral joints of the model cable made of copper-alloy-laminated Bi-2223 tapes with various joining processes and spiral pitches. The joint resistivity for the model cable joint was less influenced by the joining process and spiral pitch and it was comparable to that of a single tape-to-tape joint. As a conclusion, press welding with indium with a simplified process, joining at room temperature, and skipping some of pre-joint processes can be applied to quick and simple joint fabrication of HTS DC feeder cable.

Test operation of 500 m /100 MW Ishikari high-temperature superconducting DC transmission line to demonstrate the required cost reduction and robustness

This paper presents the results of the Ishikari Line 1 restart test conducted in 2021 to collect basic data for cost reduction, improved efficiency, and confirm the integrity of the equipment. The Ishikari Superconducting DC transmission (SCDC) demonstration test facility was constructed in 2013 in Ishikari, Hokkaido. It comprises the world’s longest class of 500 m (Line 1) and 1000 m (Line 2) SCDC systems. In 2015, Line 1 successfully demonstrated the world’s first power transmission from a solar power plant to an Internet data center. The restart test was conducted approximately six years after shutdown, and a strong earthquake occurred during the shutdown period. The restart test operation confirmed the integrity and robustness of the facility, which had experienced a major earthquake. Cooling tests and electrical tests were conducted, and useful data were obtained, including efficient cooling operation and the integrity and shunt current of the superconducting cables. Based on these results, we aim to promote the social implementation of SCDC by compiling a database for the quantitative evaluation of reliability, safety, and cost.

Optimization Design of a Saturated Iron Core Fault Current Limiter Using a GA and PSO Algorithms Coupled With Finite Element Method

A dc-bias high-temperature superconducting (HTS) coil is the main part of the Saturated Iron Core Fault Current Limiter (SIC-SFCL). Another relevant part of the SIC-SFCL is the iron core, which represents the main volume of the device. Therefore, optimizing the iron core volume decreases the equipment cost significantly. Also, it is important to find an optimized geometry for the dc HTS coil that allows extracting the maximum potential of the superconducting material. This article proposes a full design optimization of a SIC-SFCL, combining the GA and the particle swarm optimization (PSO) methods. For the optimization procedure, the authors consider the superconducting tape and the ferromagnetic material. The COMSOL software is used to implement the finite element model and the Python language coupled to the COMSOL software codes the optimization algorithms. The authors study different cognitive, social, and inertial parameters in PSO, in order to evaluate the impact of those parameters in the optimization. The results present the ability of the algorithm to provide adequate SIC-SFCL design parameters without any expert intervention. To validate the proposed methodology, the authors perform experimental tests and compare them with the simulation results. The steady-state regime presents an error less than 1% and the short-circuit regime shows a maximum error equal to 10.3%.

Fault Transient Study of a Meshed DC Grid With High-Temperature Superconducting DC Cables

This paper presents the DC fault transient study of a meshed DC grid with high-temperature superconducting (HTS) HVDC cables to integrate offshore wind power. A four-terminal meshed DC grid model with a ±100 kV DC voltage rating is developed. DC circuit breakers (DCCBs) are implemented in the DC grid to deal with DC cable faults. To conduct the fault transient study, the fault scenarios with different fault locations and protection delays are studied. The sensitivity study versus different DC fault locations reveals that the faults on the faulted cable will not cause the quenching of healthy cables. The sensitivity study considering different DC fault protection delays demonstrates that the HTS cables have a current-limiting effect and the long delay of DC fault protection does not result in large fault current and the quenching of healthy cables. Extensive simulations in PSCAD/EMTDC validate the founding.

Multi-Objective Optimization for Improving Weight and Fault Characteristics of a DC HTS Cable in Cryo-Electric Aircraft

The aim of the presented study is to optimize the different classes of high-temperature superconducting (HTS) DC cables for improving their performances in a cryo-electric aircraft considering their weight, peak temperature during faults, and the ratio of current passing through each tape to the critical current of HTS tapes. These terms were interpreted into three objective functions, and a multi-objective optimization algorithm known as non-dominated sorting genetic algorithm II was used to find the optimal solution clusters. The cable optimization was conducted for different former materials by changing the former thickness and radius. Results showed that the DC HTS cables with aluminum former have the lowest weight while cables with copper formers have the best thermal performance against faults.

Magnetic Field Distribution and Ripple Loss Analysis of an Optimized Self-Shielding DC HTS Cable

Compared with AC HTS cable, DC high-temperature superconducting (HTS) cable has prominent advantages of low transmission loss, large current capacity and high controllability. The current directions in adjacent layers of “sandwich” cable with self-shielding characteristic are opposite, which can greatly reduce the magnetic field influence and the critical current degradation. In order to homogenize the magnetic field distribution of the cable, an optimized self-shielding DC HTS cable is proposed. Compared with the previous structure, the optimized self-shielding DC HTS cable has a larger critical current and smaller ripple loss. In this paper, the magnetic field distribution and the ripple loss of three different DC HTS cables with ripple current are studied by a two-dimensional (2D) finite element method (FEM) based on T-A formulation. The 2D simulation model offers a valuable reference for the design and operation of DC HTS cable with low voltage and large current.

Study on Surface Discharge Characteristics of GO-Doped Epoxy Resin-LN2 Composite Insulation.

Superconducting power lead equipment for epoxy insulation, such as high-temperature superconducting DC power or liquefied natural gas energy pipelines, as well as high-temperature superconducting cables, has long been used in extreme environments, from liquid nitrogen temperatures to normal temperatures. It is easy to induce surface discharge and flashover under the action of strong electric field, which accelerates the insulation failure of current leads. In this paper, two-dimensional nano-material GO was used to control the electrical properties of epoxy resins. The DC surface discharge and flashover characteristics of the prepared epoxy resin–GO composite insulation materials were tested at room temperature with liquid nitrogen. The surface discharge mechanism of the epoxy resin–GO composite insulation materials was analyzed. The experimental results show that the insulation properties of epoxy composites doped with GO changed. Among them, the surface flashover voltage of 0.05 wt% material is the best, which can inhibit the discharge phenomenon and improve its insulation properties in extreme environments, from room temperature to liquid nitrogen temperature. It is found that the development process of surface discharge of composite insulating materials under liquid nitrogen is quite different from that under room temperature. Before critical flashover, the repetition rate and amplitude of surface discharge remain at a low level until critical flashover. Furthermore, the voltage of the first flashover is significantly higher than that of the subsequent flashover under the action of the desorption gas on the surface of the composite insulating material and the gasification layer produced by the discharge. Given that the surface flashover voltage of 0.05 wt% epoxy composite is the best, the research and analysis of 0.05 wt% composite is emphasized. In the future design of superconducting power lead insulation, the modification method of adding GO to epoxy resin can be considered in order to improve its insulation performance.

DC Fault Study of a Point-to-Point HVDC System Integrating Offshore Wind Farm Using High-Temperature Superconductor DC Cables

This paper presents a feasibility study of an offshore wind farm (OWF) HVDC integration system using high-temperature superconductor (HTS) DC cables. A representative ±100 kV/2 GW point-to-point OWF HVDC system is proposed including HTS DC cables and two converter stations using modular multilevel converters (MMCs). To be compatible with the high current rating of the HTS cables, each of the offshore and onshore converter stations consists of three MMCs in parallel. To study the interaction between HTS DC cables and MMCs, a multiple lumped π-section model of a HTS DC cable considering electrical and thermal functionality is developed. This paper provides a critical assessment of the proposed HVDC-HTS system, with emphasis on the performance under fast DC fault transients. Detailed simulations presented in this paper reveal that the HVDC-HTS system provides effective current limiting against DC cable short circuit faults.

Effect of barrier thickness of Polytetrafluoroethylene on the Direct Current breakdown characteristics of liquid nitrogen

When high-temperature superconducting (HTS) tapes in high voltage DC HTS devices are quenched, amounts of thermal bubbles are generated in liquid nitrogen (LN2) that will reduce the insulation strength greatly. Especially when resistive-type superconducting fault current limiters meet a short-circuit fault, the insulation strength of the LN2 decreased significantly due to the thermal bubbles and electrothermal pressure. Solid insulating barriers can improve the insulation strength of the LN2. The objective of this study is to obtain the effect of barrier thickness on the DC dielectric breakdown characteristics of LN2. Insulating barrier Polytetrafluoroethylene with three different thicknesses were tested. Three types of electrodes such as rod-plane, needle-plane, and plane-plane were applied. The results showed that the barriers can increase the negative and positive breakdown voltage both with and without thermal bubbles in a slightly non-uniform field (SNF) and non-uniform field (NF). For the SNF, the DC breakdown voltage is higher when the barrier is thinner. The effect of the barrier thickness in the NF is lower than that in the SNF field. For the NF, the positive breakdown voltage is higher when the barrier is thicker.

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

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

Analysis and Methods for HTS DC Cables

High-temperature superconducting (HTS) DC cables with the merits of light weight, large capacity and low loss provide a novel power transmission method for future utility grid. In this paper, the electromagnetic characteristics of unipolar and bipolar coaxial HTS DC cables are compared by finite element simulation. Besides, the 3D magnetic flux density distribution characteristics of HTS DC cable are obtained. The method to investigate the influence of winding angle on HTS DC cable magnetic flux density is also applied to 3D HTS DC analysis.

Effect of Harmonic Voltage On Partial Discharge Properties of LN2/PPLP Insulation for HTS DC Cable

In this paper, the effect of harmonic voltage on partial discharge properties of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /PPLP (Liquid nitrogen/ Polypropylene-Laminated-Paper) insulation system for high-temperature superconducting (HTS) DC cable is studied. The harmonic components with a frequency from 50 to 350 Hz are superimposed on the DC voltage to analyze the mechanism of the harmonic amplitude and order on partial discharge (PD). The experimental results show that when the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> of the composite voltage remains consistent, the increase in the proportion of the harmonic amplitude will cause the enhancement of partial discharge amplitude, discharge counts and pulse repetition rate, especially as the amplitude of harmonic component exceeds that of DC voltage. The increase of the harmonic order results in variation of the derivative of the applied voltage with time (dv/dt) along the phase, which further causes the pulse repetition rate to increase. It is concluded that the harmonic components superimposed on DC voltage promote the occurrence of PD, which poses a significant threat to the insulation of superconducting DC cable.

Electromagnetic Analysis of HTS DC Cables Based on Critical State Model

High-temperature superconducting (HTS) DC cables have the advantages of low transmission loss, high current density and large transmission capacity, which have gained more and more attentions in recent years. The typical structures of HTS DC cables include unipolar and bipolar coaxial that doubles the cable's transmission capacity. Based on the critical state model, this article analyses and contrasts the critical current density distributions and degradations due to magnetic field in the HTS tapes of the unipolar and coaxial bipolar DC cables. The influences of the bipolar cables with different voltage levels are also compared. The results show that the critical current density degradations due to magnetic field are almost the same in both unipolar and coaxial bipolar structures, which are mainly caused by the self-field. For the bipolar cables, the current density distribution in the outer layer HTS tapes changes with the voltage level. To evaluate the influence of the axial magnetic field, a 3-D model of the unipolar cable is established with a helical wound pitch of the HTS tapes. The results show that the axial magnetic field is relatively low and the critical current density would not degrade if the helical angle of the HTS tapes is reasonably small.

Influence of Electrical Conductivity on Heating Power of Metal Billets in HTS DC Induction Heater

A novel high-temperature superconducting (HTS) direct current induction heater exhibits great advantages such as high efficiency and superior heating quality for non-magnetic metal billet heating process in industrial applications. An HTS induction heater of 1 MW is used in the Al press industry. The HTS induction heater injects high resistive heating power through the driving motor. The higher the resistive heating power, the shorter the heating time. The heating power is a significant parameter used to analyze the heating time and improve the production efficiency. In this study, three types of non-magnetic metal billets were analyzed, including Al, TC4 (Ti-6AL-4V), and Cu, which are widely used in the heating process in industrial applications. The experimental results of the heating of TC4 and Al billets (Φ = 220 mm × L = 1,250 mm) using an HTS induction heater were obtained. The changing trend of the heating power during heating was different. The heating power of TC4 was different from those of Al and Cu. The heating power of TC4 was much lower than those of Al and Cu. A numerical 3-D finite element method model was built to analyze the heating power. The results of the simulation and experiments were consistent with each other. The electrical conductivity of the billet is a significant material property affecting the heating power. Finally, the influence of the billet diameter on heating behavior was analyzed for the three different metal billets. Understanding the effect of electrical conductivity of important industrial metals such as Al, Ti, and Cu alloys on heating power is useful for improving metal processing technologies.

Thermal and Electromagnetic Design of DC HTS Cables for the Future French Railway Network

The use of High Temperature Superconducting (HTS) cables as a technology is considered as key to increasing the power of the electrical grid while reducing the volume of installation. In addition, when transmission currents exceed a few kA, DC HTS cables significantly reduce power losses, rights-of-way and total system mass. This article describes the various studies that need to be carried out in order to correctly size HTS DC cables for the new railway network envisaged by the French company SNCF, which must consider ultra-urban needs. The process used to design DC cables for operating powers of 10, 20 and 30 MW at a nominal voltage of 1.5 kV using commercial (RE)BCO tapes is presented. In this sizing process, the critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (B, θ, T) dependence of the superconducting tapes, the thermal properties of the materials and the different cooling modes are taken into account. Several solutions are suggested to ensure that the cable can withstand fault currents and reduce recovery time in the initial state. Finally, with the help of analytical models and industrial data, once the cables have been designed, the various system losses are reported as well as the power or size of the various auxiliary components.

Detection of Series Faults in High-Temperature Superconducting DC Power Cables Using Machine Learning

A novel method to detect series faults in multi-strand high-temperature superconducting (HTS) power cables, based on monitoring the transmission characteristics as an indirect way to detect the magnetic signature, is reported. The efficacy of the non-destructive detection method was studied using finite element analysis (FEA) and measurements on a model cable with a varying number of disconnected strands. An S-parameter model block was implemented, and the changes in the magnetic signature resulting from the failed superconducting strands in the cable were investigated. The transmission characteristics of the cable were experimentally analyzed with various types of series faults. The S-parameters obtained experimentally on the model cable were analyzed, and the output of the implemented linear time-invariant (LTI) system was evaluated with the experimental data. A machine learning tool was developed to predict the type of series fault with a regression algorithm based on the Scikit-learn library. The reported method has the potential of enhancing HTS power system reliability by providing early indications of series faults in HTS cables.

Electrical Insulation Characteristics of LN2/CF4 Mixture at Cryogenic Temperatures

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.

Simulation and experiment on superconducting DC energy pipeline cooled by LNG

Superconducting DC energy pipeline is an energy channel that simultaneously transmits electrical energy as well as the liquid fuel. It improves the efficiency of energy transmission and reduces the overall cost by cooling high-temperature superconducting DC cables with the low-temperature liquid fuel, such as liquid hydrogen, liquefied shale gas, and liquified natural gas (LNG). In order to properly design this, it is very important to know the temperature distribution along the pipeline, which is yet to be carried out by the researchers. In this paper, a 10 m energy pipeline with LNG cooling and having a concentric nested structure is simulated and tested experimentally. The simulation results of the temperature rise along the pipeline were in good agreement with the experimental results. The effect of various parameters on the pipeline temperature were also obtained. Finally, based on the theoretical equations and simulation results, an empirical formula is proposed to calculate the temperature at the outlet of the energy pipeline.

A Wireless Rectifier for Inductively Energizing High Direct-Current High-Temperature Superconducting Magnets

High-temperature superconducting (HTS) magnets have been widely used in various applications due to their excellent performance. One long-lasting problem, however, is that they have to be powered by electronic power supplies via a pair of thick current leads, which go through room temperature environment into a cryogenic environment. The considerable heat load generated by these resistive current leads at a cryogenic temperature substantially limits the operating current and the energy density of the magnet. In this article, we report a novel mechanism of inductively energizing closed-loop HTS dc magnets. This exploits a newly discovered effect, which appears within a superconducting loop when a global screening current interacts with local screening current. This results in a dc voltage across the superconductor, which enables an alternating superconducting current to be rectified in order to energize a superconducting magnet. Based on this principle, a superconducting transformer–rectifier prototype is realized and demonstrated. Test results show that the prototype can output a dc voltage of up to 25 mV and a maximum direct current over 500 A. It is envisaged that this work will enable future HTS dc magnets to be operated in a closed cryogenic environment, eliminating the need for electronic power supplies and bulky current leads. This would greatly reduce the footprint and power demand of HTS magnet systems and unlock many new opportunities for the applications of this technology.

Practical Estimation of HTS Dynamo Losses

In High Temperature Superconducting (HTS) DC magnet applications, the current leads contribute a significant fraction of the cooling load. Higher current operation requires larger current leads and a larger thermal bridge into the cryogenic superconducting components. Removal of current leads into the cryogenic environment would lead to a significant reduction in cooling load and hence reduce ongoing operation costs. Superconducting dynamos are a non-contact method of energising HTS coils, capable of producing significant currents in the connected superconducting circuits that exceed usual current lead arrangements. The use of superconducting dynamos has the potential to reduce the size and mass of a complete magnet system by replacing the power source, current leads, and cryocooler. Previous work on superconducting dynamos has focused on driving larger currents, and understanding the underlying physical principles of operation, but has not investigated the AC losses associated with operation. In this work the losses of a kA class multi-stator squirrel-cage superconducting dynamo are investigated experimentally, and compared to the losses expected in the conventional copper current lead based energisation method. The thermal load associated with conduction cooled copper leads prevents practical application of kilo-amp class dc HTS magnets, however we demonstrated the feasibility of using this superconducting dynamo as an energisation method to enable feasible portable high current HTS magnet systems.