No-insulation High-temperature Superconducting Coils Research Articles

2D axisymmetric electromagnetic modeling of HTS coils based on T-A formulation with modified Newman boundary conditions

The T-A formulation based on thin film approximation has been widely used in electromagnetic modeling of high temperature superconducting (HTS) coated conductors (CCs). However, with the emergence of no-insulation (NI) HTS coils and its variant HTS coils, the electrical connection of HTS coils has become increasingly complex, and the traditional T-A formulation is challenging to handle the problems of conductors with non-negligible thickness and current sharing. This paper firstly describes the Neumann boundary condition of the T-A formulation under 2D axisymmetric symmetry in detail, as well as the conversion of different boundary conditions. And additional voltage variable is added to correct the Newman boundary condition from the perspective of circuit. Then, considering HTS CCs series or parallel stacking to carry current, the effectiveness of this method is verified by comparing with benchmark model. Finally, we extend the application range of the T-A formulation with modified Newman boundary conditions to simulate thick superconductors, and naturally process current sharing of azimuthal and radial current in circular NI HTS coils.

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

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

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

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

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

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

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

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

Influence of Copper Rings and Plates on Eddy Losses of No-Insulation High Temperature Superconductor Coil in Synchronous Machines

No-insulation (NI) high temperature superconducting (HTS) machine is a semi-superconducting machine with improved thermal-stability and power density, in which the rotor windings are replaced with NI HTS coils. However, the AC background field from stator windings leads to significant eddy current in NI HTS windings. The induced eddy current and loss affect the operating efficiency and safety of HTS machine. Traditionally, copper discs and rings are used for conduction cooling and mechanical support of NI HTS coil. In this paper, the influence of copper discs and rings on reducing the eddy current and loss of double pancake NI HTS coil is studied. Three structures of copper discs, copper ring, and both copper discs and ring are analyzed, by both simulation and experiment. The results show that all these three structures of copper shielding can effectively reduce the eddy loss on NI HTS coil within the HTS machine operating temperature of 20–30 K. The combination of copper discs and ring has the best shielding performance.

Thermal Conductivity of NI HTS Coil Fabricated by Diffusion Bonding Technique

The no-insulation (NI) winding technique shows superior thermal protection performance when quenching of a high- temperature superconducting (HTS) coil occurs compared to the insulation winding technique. However, the thermal conductivity of an NI HTS coil are not clearly known because the HTS tape has various surface conditions such as roughness and oxidation. In this study, the diffusion bonding technique was applied to predict the thermal conductivity of an NI HTS coil, and the core of this technique is to remove the interface between turns by applying Ag diffusion bonding. To investigate the thermal conductivity of an NI HTS coil, an NI HTS coil was fabricated using a diffusion bonding technique, and a stainless-steel 304 block was fabricated to verify the experimental results. The thermal conductivity of the NI coil was measured using a conduction cooling experimental apparatus by applying heat loads to the outside of the coil. The measured thermal conductivity was compared to the values calculated using the thermal resistance circuit at temperatures from 20 K to 77 K.

Analysis of a No-Insulation HTS Pancake Coil Including Multiple Resistive Joints

One of the technical issues in the application of High Temperature Superconducting (HTS) tapes to magnet technology is the need to wind long tape sections with uniform electric properties along the length. It has been reported that a No-Insulation (NI) coil can properly work even in the presence of defective super-conductive regions, with a minimal drop of performance as com-pared to its “defect-free” counterpart. This could open up the possibility of manufacturing coils by jointing together several tape segments of limited length, lowering the conductor cost. In this work, a single pancake NI HTS coil is wound using several tape segments cut from the same lot, jointed together. The electrical resistance of each joint is set independently, realizing either high or low resistance joints. Thus, the coil is designed to include multiple defective sections at specific locations. The coil is refrigerated by conduction-cooling, and tested at different temperatures and charging rates. The coil instrumentation allows measuring the voltage over the whole winding, as well as the magnetic field in the central bore. The measurements are used to study the defect-irrelevant behavior of the coil. A simple equivalent lumped parameter circuit is applied to derive the coil effective parameters and analyze its electromagnetic behaviour.

Ramping Loss Analysis of No-Insulation HTS Coil Under External Field Using an Improved Equivalent Circuit Model

This article is to study the ramping loss of no-insulation (NI) high temperature superconducting (HTS) racetrack coil under different external magnetic fields during the charging process. An improved equivalent circuit electromagnetic coupling model for the HTS racetrack coil is introduced with good computational speed and accuracy. The simulation results show that the turn-to-turn loss and the change of the lowest critical current position are obviously influenced by the external field that have not been mentioned from our knowledge base. During the rising time of the power supply current, there is little difference in current distribution under different external fields. But after that, steady state of charging process is deeply influenced by the external field. A larger external field will lead to a lower charging current and the external bypass phenomenon. Another result is that the relationship between direction of external field and orientation of coils will influence the lowest critical current amount and position. The relationship between turn-to-turn resistivity of co-winding materials and ramping loss is also discussed in the paper. The distributed unevenness and value of the turn-to-turn loss change with the increase of turn-to-turn resistivity.

Design and Characteristic Analysis of a Homopolar Synchronous Machine Using a NI HTS Field Coil

This paper deals with a homopolar synchronous machine (HSM) applying high-temperature superconducting (HTS) field coils. Superconductors, especially high-temperature superconductors, have high potential as advanced materials for next-generation electrical machines due to their high critical current density and excellent mechanical strength. However, coils made with high-temperature superconductors have a high risk of being damaged in the event of a quench due to the intrinsic low normal zone propagation velocity (NZPV). Therefore, the coil protection issue has been regarded as one of the most important research fields in HTS coil applications. Currently, the most actively studied method for quench protection of the HTS coils is the no-insulation (NI) winding technique. The NI winding technique is a method of winding an HTS coil without inserting an insulating material between turns. This method can automatically bypass the current to the adjacent turn when a local quench occurs inside the HTS coil, greatly improving the operating stability of the HTS coils. Accordingly, many institutions are conducting research to develop advanced electrical machines using NI HTS coils. However, the NI HTS coil has its intrinsic charge/discharge delay problem, which makes it difficult to successfully develop electrical machines using the NI HTS coil. In this study, we investigated how this charging/discharging problem appeared when the NI HTS coil was used in an HTS homopolar synchronous machine (HSM) which is one of the electrical machines with a high possibility of applying the HTS coil in the future because it has a stationary field coil structure. For this, the characteristic resistances of HTS coils were experimentally obtained and applied to the simulation model.

Study on the Current Bypassing and Mechanical Properties of No-Insulation HTS Coil With Protection Ring

We have been studying no-insulation (NI) high temperature superconducting (HTS) coils that do not use turn-to-turn electrical insulation. The no-insulation winding technique can provide high stability of the HTS coils against the quenching, but it cannot prevent mechanical deterioration of the HTS coil due to electromagnetic force and multiple cooling. To solve these problems, we propose the installation of metal protection ring on the outermost turn of NI HTS coil to improve thermal, mechanical, and electrical stability. In this study, the current bypass characteristics in the transverse direction within an NI test coil wound with 37 turns of REBCO wire with/without copper protection ring were experimentally investigated for various amounts of heating by a heater and various heater positions. Eight repeated cooling experiments with liquid nitrogen proved that the proposed copper protection ring was effective against shape deformation of the NI test coil and deterioration of contact resistance between turns due to thermal stress.

Temperature Distribution Observation of No-Insulation HTS Coils Wound by Optical-fiber-encapsulated Tapes

Effective quench detection technologies are extremely important for protecting high temperature superconducting (HTS) applications. In recent years, an optical fiber encapsulated HTS tape (OF-tape) is proposed for quench detection by using distributed temperature sensing (DTS) technology. In this paper, an HTS coil (OF-coil) wound with a 39 m long OF-tape was prepared and energized by an AC overcurrent of 700 A to verify the feasibility of temperature observation during quench events. Furthermore, anti-stokes intensity was also measured to understand dependence of temporal resolutions on response times of coil quenches. The main results show that temperature variations of the OF-coil can be correctly measured after quench and the maximum temperature is shown at the middle turn of the OF-coil. Particularly, for the method of detecting anti-stokes intensity, quench response time of temporal resolution (TR.) with 30 ms is about 500 ms faster than that with 300 ms. However, when the TR. is 30 ms, more noise signal is observed. Accordingly, a trade-off should be considered between response time and noise signal for efficient quench detection.

Investigation on Key Parameters of NI HTS Field Coils for High Power Density Synchronous Motors

As global environment regulations are gradually strengthened, active research has been conducted on electric propulsion applications. High power density is one of the major research interests, as well as a machine's efficiency for a high-performance propulsion system. Among various electric propulsion technologies under development, a motor incorporating the no-insulation (NI) high-temperature superconducting (HTS) winding has the advantages of high current density and robustness to external disturbances, which are suitable for high specific power. However, still, there is little research on the motor with NI HTS coil. Therefore, as basic research, we investigated the potential application of the NI HTS motor, then derived required machine parameters and NI HTS coil designs in this paper. For that, first, basic parameters of the existing propulsion motors were investigated, then the target application and performance of the NI HTS motor were determined. Based on the target performance, conceptual designs of a motor with NI HTS coil were obtained by changing key design parameters including rated power, main dimensions, operating temperature, and width of air-gap. Calculated parameters were numerically compared, and expected design requirements and challenges were suggested.

Charging Characteristics of Series Connected Insulation and No-Insulation HTS Coils by Rotary HTS Flux Pump

High-temperature superconducting (HTS) coil wounds without turn-to-turn insulation (NI coils) present excellent electrical and thermal performance compared to HTS coil wounds with Kapton tape (INS coils). However, NI HTS coils are physically connected to power supplies at room temperature, resulting in increased operating costs due to complex cooling systems and constant heat loads. To overcome this issue, recent studies have investigated alternative charging methods such as flux pump, a power source without any physical contact with the HTS coil at cryogenic temperatures. Even though the NI coil is charged using a flux pump, the disadvantageously slow charge and discharge time remain. In this study, we conducted experiments to compare the charging characteristics of INS and NI coils using a rotary HTS flux pump. A superconducting circuit comprising two coils with equal inductance magnitude is connected in series. From the experiments, we compare the charging characteristics of charging time constant and magnetic field when the same current is applied to the two coils with a power supply and a rotary HTS flux pump. In the NI coil rotary HTS flux pump case, the charging time constant converged to about 6 s in the alternating magnetic field (AMF) above 20 Hz, and the value is similar to the power supply. At 60 Hz and higher AMF, the maximum magnetic field difference from the power supply converged to less than 1%, which appears to be a difference in current and voltage source.

Performance Study on the No-Insulation HTS Coil Wound With Narrow-Stacked Wire

As a novel high-temperature superconducting (HTS) conductor structure composed of the stacked 1-mm wide REBCO tapes, Narrow-Stacked (NS) wire has been demonstrated to have a significant reduction effect of AC loss and screening current induced field (SCIF) in the insulated HTS coils. Thus, NS wire is a promising HTS conductor design for practical HTS applications. Meanwhile, compared with traditional insulated HTS coil, no-insulation (NI) HTS coil has obvious advantages in engineering current density, thermal stability and self-protecting ability, and these features benefit for HTS magnets. Although NS wire was regarded as an improvement for insulated HTS coil, the NS wire effects on NI coil are not confirmed yet due to the special characteristics of the NI coil. In this paper, NI pancake coils were wound with NS wire and conventional HTS tape respectively. To evaluate the performance of NI coil wound with NS wire, its characteristic resistance and charging delay were analyzed at the liquid nitrogen temperature of 77 K. Besides, the over-current test results show that NI NS coil also has the satisfied self-protecting feature. This paper intends to provide a possible reference for NS wire application on NI HTS magnets.

Thermal Stability Study of a Solder-Impregnated No-Insulation HTS Coil Via a Raman-Based Distributed Optical Fiber Sensor System

Mechanical stability of no-insulation (NI) high-temperature superconducting (HTS) coils will be greatly enhanced after using a solder impregnation (SIM) method. For superconducting high-field magnet applications, besides the mechanical stability, the thermal stability of SIM coils is also a crucial parameter which needs to be investigated. Due to its high insulation, convenient installation, distributed temperature measurement, and insensitive to stress, a Raman-based optical distributed temperature sensor (RmDTS) system is an ideal option for monitoring the temperature of SIM coils. In this study, an NI HTS coil and an optical fiber were successfully impregnated together by solder. The critical current of SIM coil determined by the central magnetic field is 69 A. To create quench process, a dc current of 240 A was injected into an SIM coil, and the temperature of the coil was monitored by the RmDTS simultaneously. The test results show that the temperature of the SIM coil was unchanged although a four times Ic was applied.

Experimental Study on Quench Detection of a No-Insulation HTS Coil Based on Raman-Scattering Technology in Optical Fiber

In the high-temperature superconducting (HTS) magnets applications, fast quench detection is very important to protect the magnets from burning out. Usually, the method of voltage measurement is widely applied to detect quench. However, this method is not satisfied for large HTS magnets intrinsically. Besides, a novel quench detection, which is measured by Raman-scattering-based distributed temperature sensors (DTS) using flexible optical fibers, is suggested to be a potential candidate. To verify the feasibility of this method, we prepared a DTS system based on interrogating Raman-scattering signals and a no-insulation (NI) HTS coil mounted by a 2-m length optical fiber as a test sample. The main results show that critical current of the NI coil is 111 A. In the continued overcurrent-induced quench state under coil current of 160 A, the temperature rise was observed obviously at the position of 1.2 m in the optical fiber, and the temperature was up to 94 K in the near middle region of top surface in the NI coil.

Current Bypassing and Transient Stability in a Partially Insulated HTS Coil

The no-insulation (NI) winding technique to improve the transient stability of high-temperature superconducting (HTS) magnets has been studied and a cowinding method using a metallic tape was tested. Normally, the operating current in NI HTS coils is bypassed into the transverse direction due to the normal region generated within the coils. Therefore, a method that controls such current bypassing into the turn-to-turn direction of NI HTS coils was investigated as a means to regulate the effective number of turns. In this study, we experimentally investigated the characteristics of current bypassing in the transverse direction within a test coil wound with a GdBCO tape. First, to find an optimized heater location, a 20-turn HTS pancake with two partial insulation shapes was prepared and tested experimentally. The current bypassing characteristics in the normal region created by heaters (strain gauges) were studied while varying the number of heaters, heater positions, and the amount of heating at liquid nitrogen temperature. Using both short and long heating periods, we found that we could effectively control the number of turns (coil inductance) via long-duration heating.

Feasibility Study of the Impregnation of a No-Insulation HTS Coil Using Solder

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using solder. NI coils are widely studied and used in many situations. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. Epoxy impregnation is a common method to protect the HTS field from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability. However, traditional epoxy resins are electrical insulating materials which might cause the turn-to-turn contact resistance of NI coils to increase. Therefore, we proposed a new kind of impregnation method using the electrically conductive material, solder. Two coils were fabricated and tested for comparison. One is the NI coil without impregnation and the other is the NI coil impregnated with solder. The charge-discharge and sudden-discharge tests were performed in liquid nitrogen at 77 K. The experimental results were studied with equivalent circuit analyses. The results of this paper could provide useful data for the application of solder impregnated NI HTS coils.

Feasibility Study of the Impregnation of a No-Insulation HTS Coil Using an Electrically Conductive Epoxy

This paper reports the feasibility of the impregnation of no-insulation (NI) high-temperature superconducting (HTS) coils using an electrically conductive epoxy resin. Recently, several studies of HTS coils without turn-to-turn insulation have been reported for field coils used in rotating machines such as motors and generators. The NI winding technique enhances the thermal stability of the HTS coil without requiring complicated protection techniques because the quench current is automatically bypassed through the turn-to-turn contacts within the HTS coil. Nevertheless, there is still a question as to whether the NI technique can be applied to rotating machines. To utilize an HTS coil under high mechanical loads such as field coils for rotating machines, the HTS tapes must be stabilized mechanically. For HTS field coils intended for use in rotating machines, epoxy impregnation is generally necessary to protect the HTS field coil from mechanical disturbances caused by the magnetic field and rotational vibration of the rotor to enhance the mechanical stability [1] , [2] . However, the NI HTS coil cannot be fabricated by wet winding using epoxy resin because epoxy resins such as Stycast 2850 FT and CTD 521 are electrically insulating materials. This study examines the electrical stability of an NI HTS coil impregnated with an epoxy resin containing electrically conductive particles. The results are likely to present useful data for the application of electrically conductive epoxy impregnated NI HTS coils to rotating machines.