YBCO Wire Research Articles

Experimental Study on Quench Protection of HTS Magnet Composed of Multiple Pancake-Coils by Use of Auxiliary Resistive Shunt Loop Method

In the authors' previous work, a new quench protection method to reduce hot-spot temperature and increase quench detection voltage was proposed to protect an HTS magnet composed of multiple pancake sub-coils from quench damages (Toriyama et al., 2019). In the method, a current of a quenching sub-pancake coil is transferred to the other sub-coils of the magnet forming auxiliary resistive shunt loop (ARSL) by resistively shorting the other sub-coils. In this work quench behaviors of the pancake sub-coils of a model magnet was investigated by a simulation experiment using small scale test pancake coils wound of YBCO wire. Current patterns of the sub-pancake coils of a model magnet at a quench event were calculated for the case that ARSL method was applied, and in the experiment, the same patterns of the calculated currents were applied to the quenching test coils by a controllable current supply. Experimental results show effectiveness of the new method.

Dynamic resistance measurement in a YBCO wire under perpendicular magnetic field at various operating temperatures

Dynamic resistance plays an important role in certain high-Tc superconducting (HTS) applications where an HTS coated conductor carries a DC current exposed to an AC magnetic field. Here, we report measurements of the dynamic resistance in a 4 mm-wide YBCO coated conductor under a perpendicular AC magnetic field at 77 K, 70 K, and 65 K. Dynamic resistance was measured at three different frequencies for the reduced current, i (It/Ic0), ranging from 0.04 to 0.9, where It is the DC current level and Ic0 is the self-field critical current of the conductor at each temperature. At all three temperatures, the threshold magnetic field (Bth) values increase with reducing DC current. These results show that, for a given set of applied conditions, dynamic resistance decreases with decreasing operating temperature, which we attribute to the temperature dependent increase in the critical current of the wire. We show that measured Bth values at all three temperatures agree well with the analytical values from nonlinear Mikitik and Brandt equation for i ≤ 0.2 and with a simple linear expression that assumes a current-independent penetration field for i > 0.2. We further show the measured Bth curves at different temperatures normalized by critical current density collapse into one common curve. The above result implies that dynamic resistance in coated conductors at different temperatures under perpendicular AC magnetic fields can be scaled simply using measured Ic0 values at those temperatures and analytical equations.

AC loss simulation in a HTS 3-Phase 1 MVA transformer using H formulation

One of critical issues for HTS transformers is achieving sufficiently low AC loss in the windings. Therefore, accurate prediction of AC loss is critical for the HTS transformer applications. In this work, we present AC loss simulation results employing the H-formulation for a 1 MVA 3-Phase HTS transformer. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm – wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) windings are 20 turn single-layer solenoid windings wound with 15/5 (15 strands of 5 mm width) Roebel cable per phase. The numerical method was first verified by comparing the numerical and experimental AC loss results for two coil assemblies composed of two and six double pancake coils (DPCs). The numerical AC loss calculated for the transformer was compared with the measured AC loss as well as the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current agree to within 20%. Further simulations were carried out to explore the dependence of the AC loss on the gap between the turns of the LV winding. The minimum AC loss at rated current in the 1 MVA HTS transformer appears when the gap between turns is approximately 2.1 mm turn gap in the LV winding. This is due to the change of relative heights between the HV and LV windings which results in optimal radial magnetic field cancellation. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers.

Transport AC Loss Measurements in Bifilar Stacks Composed of YBCO-Coated Conductors

Increasing the current-carrying capacity of bifilar coils is essential for the high-temperature superconducting (HTS) fault current limiter (FCL) application. We built three bifilar stacks with two, four, and six conductors using 4 mm wide stabilized SuperPower YBCO wires to investigate ac loss dependence of the bifilar stacks on the number of conductors. In the three basic configurations, the upper half conductors carry current in one direction, and the lower half conductors carry current in the opposite direction. We report transport ac loss results on these stacks measured at three different frequencies. The measured loss values were compared with those in stacks with the same conductor number and geometry but producing different degrees of field cancellation. We also explored the dependence of ac loss on a central gap in bifilar stacks composed of four and six conductors and the results were compared with the numerical results obtained from a two-dimensional FEM model. Results show that the bifilar stacks where each turn is stacked with multiple wires is a promising option which compromise both high current-carrying capacity and lower ac loss for HTS FCL application.

Design and Comparative Analysis of MgB2 and YBCO Wire-Based-Superconducting Wind Power Generators

This paper presented a comparison study result on the design of direct-driven-type-superconducting wind power generators using MgB2 and YBCO wires. Detailed features of MgB2 and YBCO wires were investigated and 4-MW superconducting generators were designed using MgB2 and YBCO wires. The magnetic field distributions of the generators were analysed using a 3-D finite-element method program. The total length of the wires, the volume, and weight of the designed generators were compared. As a result, the weight and volume of the generator using the YBCO wire were lighter and smaller than the generator using the MgB2 wire. The required length of the MgB2 was shorter than the YBCO wire which is much more expensive than MgB2 wire. It is therefore necessary to select the appropriate superconducting wires considering cost, weight, and volume issues when designing superconducting generators.

Study on the Persistent Current Switch in HTS Coils Wound with 2G Wire for Compact NMR Magnets

High temperature superconducting (HTS) magnets wound with REBCO wires are used in many applications, and the superconducting magnet which is operated in persistent current mode has many advantages. Therefore, we have been developing compact NMR relaxometry devices using HTS coils operated at liquid nitrogen temperature. The required strength and homogeneity of magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm3 respectively. The proposed HTS magnet for NMR relaxometry device consists with stacked HTS double pancake coils wound with REBCO wires and operated by persistent current mode (PCM) using superconducting joint between REBCO wires. In PCM operation, the ability of persistent current switch (PCS) is very important, so, in this study, the thermal properties of YBCO wire against the various thermal inputs by heater were investigated experimentally to design the PCS for PCM HTS magnets. The thermal behaviors of the YBCO wires were measured as functions of amount of heat input using two types of epoxy resin. The current bypassing properties on the YBCO loop coil with developed PCS were studied experimentally.

Study on Hot-Spot Temperature Limits for YBCO Epoxy-Impregnated Coil to be Safe From Damages Caused by Quenches

Hot-spot temperature limits for YBCO epoxy-impregnated coil to be safe from damages caused by quenches are experimentally investigated using small-scale test coils. In the authors' previous study, it is found that the hot-spot temperature limits for YBCO paraffin-impregnated coils are around 500 K where the critical current of YBCO wire is degraded thermally. However, in the cases of the epoxy-impregnated coils, it is shown that the hot-spot temperature limits are around 300 K, much lower than those of the paraffin-impregnated coils. In this paper, a reason for those lower hot-spot temperature limits of the epoxy-impregnated coils is discussed.

Winding and Testing of YBCO Coils Using Different HTS Wires

YBCO coils with epoxy resin impregnation tend to experience degraded performance, which is due to the mismatch of the thermal contraction coefficients among former, epoxy, and HTS conductors which will generate thermal stresses that induce delamination of the superconducting layer. Several coils have been fabricated and tested, using the YBCO wires from SuperPower Inc. (SP), and Shanghai Superconductor Technology Corporation (SSTC). No degradation of the coil performance was detected for small SP coils, even with epoxy resin impregnation. However, for SP coils and SSCT coils using more than 100 m of YBCO wires, they all exhibited degraded performance after the epoxy resin impregnation. A possible solution to this problem is proposed and a 400 m SSCT coil has been successfully fabricated and tested.

AC Loss Characteristics of HTS Wires Carrying Currents With Different Waveforms

When applying high-temperature superconductor (HTS) devices in power systems, ac loss is a significant obstacle from an economic perspective. Many studies have been performed to investigate this issue, but the ac losses for different current waveforms have rarely been reported. However, in power systems, many nonlinear devices can distort current waveforms. For example, currents of converters have ripples, no-load currents of transformers are peaked waveforms, and currents of motors are normally nonsinusoidal. To investigate the influence of current waveforms on ac losses, three types of currents with different waveforms, namely, sine waves, peaked waves, and flat-topped waves, are studied using three types of HTS wires, namely, Bi2223 wire, YBCO wire with nonmagnetic substrate (Hastelloy C276), and YBCO wire whose substrate is ferromagnetic (Ni-5at.%W). The experimental results show that ac losses of HTS wires are independent of frequency when carrying sine currents and that they are dependent on frequency when carrying peaked and flat-topped wave currents. Moreover, for all types of measured wires, particularly YBCO wire with ferromagnetic substrate, measured ac losses are highly dependent on current waveforms. In the end, the authors provide a discussion for the experiment phenomenon from the point of magnetic flux creep.

Design and Development of a Cryogen-Free Superconducting Prototype Generator With YBCO Field Windings

A prototype machine has been developed to study large-capacity wind turbine generators using high-temperature superconducting technology. The superconducting field windings, using 2G YBCO wires, are used in the rotor, whereas the conventional technology is employed in the stator. The racetrack superconducting magnets are cooled directly by two cryo-refrigerators and the operating temperature is 30 K. The cold heads of the refrigerators are installed on the rotor, which is driven by a drive motor, rotating synchronously with a vacuum chamber. A rotary gas seal is employed as the gas slip ring connecting the rotary cold head to a stationary compressor. This paper also presents the electromagnetic characteristics of the prototype machine and the feasibility of the cooling system. Experimental results of back electromotive force are shown at field current from 10 to 100 A and rotating speed from 10 to 30 r/min. The electromagnetic design, based on the finite-element method (FEM), is available in this paper, and the FEM data are coincident with the experimental results very well.

Development and Test of a Cooling System for a 154 kV Superconducting Fault Current Limiter

The superconducting fault current limiter (SFCL) is an electric power device that limits the fault current immediately in a power grid. Korea Electric Power Corporation (KEPCO) has been developing a 154 kV, 2 kA SFCL since 2011 to protect power grids from increasing fault current and improve the stability and quality of electric power. This SFCL adopts 2G YBCO wires and operates at 71 K and 5 bars. In this paper, a cooling system for the 154 kV SFCL and its cooling test results are reported. This cooling system uses a Stirling-type cooler to make sub-cooled liquid nitrogen (<TEX>$LN_2$</TEX>), which cools the superconductor modules of the SFCL. The <TEX>$LN_2$</TEX> is circulated between the cooler and the cryostat that contains superconductor modules. The <TEX>$LN_2$</TEX> also plays the role of a high voltage insulator between the modules and the cryostat, so the pressure was maintained at 5 bars for high insulation performance. After installation in a test site, the cooling characteristics of the system were tested. In this operation test, some important data were measured such as temperature distribution in <TEX>$LN_2$</TEX>, pressure change, performance of the heat exchanger, and cooling capacity of the total system. Consequently, the results indicate that the cooling system operates well as designed.

Experimental study of thrusts of a cylindrical linear synchronous motor with an HTS coil magnet as the excitation system

The thrusts of a cylindrical linear synchronous motor with an HTS coil magnet as the excitation system were measured the first time and will be presented in this paper. The HTS coil magnet is made of second generation YBCO wire. The coil is a double pancake coil consisting of 34 turns wire. The inner diameter, outer diameter and height of the coil are 20, 30 and 13mm, respectively. The armature of the motor is three phases, and the inner diameter is 40mm. It is made of copper windings. With a direct current of 40A for the HTS coil magnet and a RMS current of 10A for the armature, a peak thrust of 3.8N was measured at the temperature of 77K and a radial gap of 5mm between the armature and the excitation system. Effects of armature current, coil current, running time, magnetizers and seams between magnetizers were also studied. In the experiments, the peak thrusts of different types of HTS coil magnets were about from 1.3 times to 8 times as strong as the peak thrust of the coreless coil magnet under the same conditions.

Development of a YBCO Racetrack Coil for HTS Machine Applications

A YBCO racetrack coil was developed for HTS machine applications. The coil was constructed as step-type to fit in the limited space of the machine rotor. Two double-pancake (DB) windings were fabricated for the whole coil that has used 388 m of YBCO wires purchased from American Superconductor Corporation (AMSC). A finite-element method (FEM) software was used to model the coil and the critical currents for each DB winding were estimated. It was found out during the winding process that by using AMSC wires the coils can be wound in a rather simple way. The coils show no degradation even after a few accidents occurred during the winding process.

Influence of Current Direction on AC Losses of a Multi-Wire YBCO-NiW Conductor With Horizontal Arrangement

Multi-wire conductors consisting of YBCO wires with magnetic substrate are usually applied in the high temperature superconductor (HTS) applications. In these applications, ac loss is a critical factor to determine the capacity of a refrigerator. And the ac losses of YBCO-NiW wires will be affected by the magnetic substrate. In this work, the self-field loss of the YBCO wire with the magnetic substrate is firstly measured, and compared with the numerical data. Moreover, the ac losses of a multi-wire YBCO-NiW conductor consisting of three wires are investigated for three transport modes. The main results show that the measured self-field loss of the wire is identical with the numerical ones. Accordingly, the reliability of the experimental and numerical methods is verified. Also, the ac losses of the conductor are in good agreement with the numerically calculated losses. As for the multi-wire conductor's total losses, the change of the direction of current in wires will not have an apparent influence on the total ac losses.

Control and Operation of a High Temperature Superconducting Synchronous Motor

We have built a four-pole high temperature superconducting (HTS) permanent magnet synchronous motor (PMSM) in our lab. At this stage, the HTS PMSM uses two 2G HTS racetrack coils, which are YBCO wires, type 344 from AMSC, and four conventional copper coils as stator windings. 75 YBCO bulks are mounted on the surface of the rotor. After the pulsed field magnetization system had been developed and tested in our lab in 2011, the rotor can trap a four-pole magnetic field. This makes HTS bulks possible for motor application, other than HTS coils. The HTS PMSM can successfully run at a low speed of around 150 rpm for an initial test. This paper states theoretical and practical works on the HTS PMSM's operation including HTS motor drive development and its application.

Superconducting Power Cable Application in DC Electric Railway Systems

A superconducting power cable is a prospective application of high-temperature superconductors. Although power cables using Bi2223 or YBCO wires have been developed in the world so far mainly for utility grid application, we focus attention on railway application of a superconducting power cable. We have studied how to introduce superconducting power cables effectively and economically to a dc electric railway. Numerical analysis models based on the MATLAB-Simulink are constructed for a dc electric feeder system of singe- and double-track railways. Tractive force and train resistance characteristics are modeled. Energy saving by the superconducting power cable is investigated with the influences of the number of substations, the train operation interval, etc., taken into account. The results show that the superconducting power cables can improve regeneration rate and system energy saving, and reduce the substation capacity very effectively. It will be also an advantage that if the transportation capacity of a railway line needs to be increased, the introduction of superconducting cables could achieve it without changing the existing substations along the line.

Loss Test for a 5-m YBCO Cable Sample of the 22.9-kV KEPCO System Under Grid Operation

We have operated the 22.9-kV high temperature superconductor cable system installed at Icheon Substation of Korea Electric Power Corporation (KEPCO), which consists of a conductor and a shield using YBCO-NiW wires. We prepared the same 5-m cable sample as the Icheon Substation in order to understand its ac loss characteristic. Especially, the contact resistance between copper current leads and YBCO wires was minimized through the increased soldering surface between them. Besides, four voltage leads are attached to both the conductor and the shield with an equal angle of 90°. Under ac transport current, which was supplied to the conductor and shield in opposite direction, losses of the 5-m sample were tested for various contact positions of voltage leads, frequencies, and transport periods. The result shows that the measured ac losses for various contact positions in the conductor and shield are the same. However, their loss exponents are very different. More details will be presented in this work.

Superconducting Levitation Using Coated Conductors

The second-generation (2G) YBCO wire has been used for several applications, such as electric power transmission and distribution cables, fault current limiters, MagLev vehicles, electric machines, high field magnets, superconducting magnetic energy storage, etc. The main advantages of 2G wires are: high critical current density values, large scale production, lower manufacturing cost than first generation, and superior performance under high magnetic fields. This work presents a preliminary study of superconducting magnetic bearings (SMB) using 2G wires as passive levitators. A superconducting block using stacked 2G wires was built to evaluate the behavior of coated conductors in the SMB application. The levitation and lateral force between the 2G block and a permanent magnet guideway was measured and compared with the results for an YBCO bulk with approximately the same dimensions. The results indicate that 2G wires have potential for applications with superconducting levitation, but its economic feasibility still depends on the coated conductors price decreasing.

Development of a High-Efficiency Conduction Cooling Technology for SMES Coils

YBCO(YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> ) wires have superior characteristics such as high energy densities, thus allowing the fabrication of large-scale, high power-output superconducting equipment applications. A high voltage is desired to achieve a large power output from a superconducting coil, and is realized by improving the insulating properties of the coil with the introduction of solid insulation. However, because high temperature superconducting wires have high critical temperatures and thermal tolerance compared to metal-based superconducting wires, the benefits offered allow the insulated superconducting coil to be directly cryocooled. In a helium circulation cryocooling system, the aluminum-fabricated transmission heat shields are coated around the surface of the coils to cryocool equally, and are embedded into the transmission heat shields with the conducting pipe allowing helium gas to circulate. We manufactured a conduction cooled model coil of 600 mm class diameter, and have verified the ability to realize a cooling of more than 3 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for coil surface heat flux at an operating temperature of 20 K using the demonstration cooling system. In addition, we have also evaluated the electric insulation performance, which is in contradiction to the cooling performance. The conduction-cooled YBCO coil withstood up to DC 13 kV voltages at 20 K.

Development and Performance Analysis of a 22.9 kV/50 MVA Tri-axial HTS Power Cable Core

Tri-axial high-temperature superconducting (HTS) power cables are being developed to maximize their advantages such as reduced amount of HTS wires, a low-leakage magnetic field, and compactness compared with the different types of HTS power cables. The authors designed a 22.9 kV/50 MVA-class tri-axial HTS power cable core to apply to the distribution system in Korea. The inherent imbalance in the three-phase currents of the tri-axial HTS power cable core was calculated using the impedance matching program. A tri-axial HTS power cable core was fabricated using second-generation YBCO wires and the cable core was tested under 77 K liquid nitrogen to verify the performance of the cable core through obtaining its electrical characteristics data. This paper describes the results of the design, fabrication, and evaluation of the 22.9 kV/50 MVA-class tri-axial HTS power cable core. They will be used to develop a tri-axial HTS power cable for the distribution system.