Quench Initiation and Propagation in GdBCO Racetrack Pancake Coil for Large-Scale Rotating Machines

Quench tests of a GdBCO racetrack pancake coil were conducted in liquid nitrogen at 77?K. Minimum quench energy and two-dimensional normal zone propagation velocities of the coil are discussed. Longitudinal normal zone propagation velocities in the curved section were almost twice those in the straight portion due to stress effects, showing that protection of the straight portion is of greater importance than protection of the curved portion when using racetrack-type, GdBCO pancake coils in large-scale rotating machines.     

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.     

Quench current improvement through shape modification of racetrack coil

Superconducting racetrack coils have different shapes from those of ordinary solenoid coils which have the same curvature along the windings. So they have different or worse performance from the viewpoint of quench characteristics due to the different structures. Racetrack coils also have round curvatures along their end portions; however, the main difference of their structures from solenoid coils exists in the straight portions. It is considered that the worse performance is due to the straight portions. In the round portions, there exists strong continuous constraint along the windings against the movement toward the perpendicular direction during coil excitation. On the contrary, in the straight portions, there is almost no constraining. As a result, especially in the case of dry magnet, they show even worse characteristics than solenoid counterparts.For the compensation of this worse performance, we proposed one idea to improve quench characteristics of racetrack coils. We manufactured one racetrack coil with an ordinary shape and the other with the proposed idea. By experiments we made sure that the proposed one had a higher quench current. Moreover, after we had made an application of the proposed idea to the four field coils of an actual 30 kVA superconducting generator, we could get very good output voltage and current waveforms.     

Self-inductance of air-core circular coils with rectangular cross section

Using analytic integration, accurate self-inductance expressions (requiring only single numerical integration) for air-core circular coils with rectangular cross sections (and two special cases, namely thin-wall solenoids and disk (pancake) coils) are derived from the basic mutual-energy formula of two coaxial circular current loops. Based upon these expressions, with integration singularities analytically treated, the self-inductance of any air-core coil can be precisely calculated by use of a microcomputer. Values so computed agreed, to one part in 10⁴, with those reported in the literature.     

Two-dimensional normal zone propagation in BSCCO-2223 pancake coils

We present the results of an experimental and analytical study of two-dimensional normal zone propagation in pancake test coils, wound with silver-sheathed BSCCO-2223 tapes. Two test coils were studied in detail, one having three and the other eight layers. Each test coil was housed in an adiabatic environment whose temperature (20-70 K) was controlled and maintained by a two-stage G-M cryocooler and placed in a background field (0-6 T) generated by a Bitter magnet. With a test coil carrying a transport current (0-200 A), a local heat disturbance was applied by a heater attached to the outermost layer of the coil. The resulting electrical and thermal responses of the coil were recorded with voltage taps and thermometers attached to the coil. A normal zone propagation code was developed to accurately simulate the voltage and temperature responses of each coil for both quenching and recovering events. The code solves the nonlinear transient heat diffusion equation in two-dimensional cylindrical coordinates with a finite difference method. As an application of this code, a two-coil system, with each coil comprised of one double pancake wound with silver-sheathed BSCCO tape, was studied for its quench behaviour as one of the coils was driven normal locally. The simulation results indicate that the value of a shunt resistor connected across the terminals of each coil has a profound effect on the level of hot-spot temperature reached in the quench initiation spot.     

Morphologies of carbon micro-coils grown by chemical vapor deposition

The carbon micro-coils were obtained by the Ni-catalyzed pyrolysis of acetylene. The carbon micro-coils with various coiling morphology: regular double coils, coils built up by circular or flat fibers, super helix coils, single coils, etc. can be observed. The carbon coils with various coil diameters and coil pitches were obtained by controlling reaction conditions, such as reaction temperature, source gas flow rate of sulfur-impurity, acetylene or hydrogen.     

Analysis of a flux-coupling type superconductor fault current limiter with pancake coils

The characteristics of a flux-coupling type superconductor fault current limiter (SFCL) with pancake coils are investigated in this paper. The conventional double-wound non-inductive pancake coil used in AC power systems has an inevitable defect in Voltage Sourced Converter Based High Voltage DC (VSC-HVDC) power systems. Due to its special structure, flashover would occur easily during the fault in high voltage environment. Considering the shortcomings of conventional resistive SFCLs with non-inductive coils, a novel flux-coupling type SFCL with pancake coils is carried out. The module connections of pancake coils are performed. The electromagnetic field and force analysis of the module are contrasted under different parameters. To ensure proper operation of the module, the impedance of the module under representative operating conditions is calculated. Finally, the feasibility of the flux-coupling type SFCL in VSC-HVDC power systems is discussed.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Surface flashover characteristics in liquid nitrogen for application of superconducting pancake coils

For the development of superconducting power apparatus, it is necessary to establish the dielectric technology in coolants like LN₂. Among the dielectric technology, surface flashover characteristics are studied with several simplified spacers at the structural aspects. Double pancake coil can apply to transformer and fault current limiter, etc. To design dielectric system of high temperature superconducting transformer consisting of double pancake coils, this study discusses an effective insulator composition. Circular shape insulator divided into two parts should be inserted between coils and the insulator should cover electric stresses concentrated at the circumference of the coils which are in the same section of double pancake coils facing each other.     

Integrated optical ring resonators made by silver ion-exchange in glass

The first (to our knowledge) integrated optical ring resonators to be fabricated using silver ion-exchanged waveguides are reported. Both circular and racetrack shaped resonators have been made, both types being capable of high finesse (>15) and efficiency (>90%). The circular resonators are much more difficult to make, however, requiring a double-diffusion process and precise control of the ion-exchange. For this reason, the racetrack resonators have been the more successful and have behaved exactly as expected from the previous work on losses and directional couplers.     

Inductance Calculations for Noncoaxial Coils Using Bessel Functions

A relatively simple and general method for calculating the mutual inductance and self-inductance of both coaxial and noncoaxial cylindrical coils is given. For combinations of cylindrical coils, thin solenoids, pancake coils, and simple circular loops, the mutual inductance can be reduced to a one-dimensional integral of closed form expressions involving Bessel and related functions. Coaxial and noncoaxial cases differ only by the presence of an extra Bessel factor J ₀(sp) in the noncoaxial integral, where p is the perpendicular distance separating the coil axes and s is the variable of integration. The method is related to a recently given noncoaxial generalization of Ruby's formula for a nuclear radiation source and detector system, the analogy being close but not exact. In many cases, the Bessel function integral for the inductance can be easily evaluated directly using Maple or Mathematica. In other cases, it is better to transform the integral to a more numerically friendly form. A general analytical solution is presented for the inductance of two circular loops which lie in the same plane     

Low resistance splices for HTS devices and applications

This paper discusses the preparation methodology and performance evaluation of low resistance splices made of the second generation (2G) high-temperature superconductor (HTS). These splices are required in a broad spectrum of HTS devices including a large aperture, high-field solenoid built in the laboratory to demonstrate a superconducting magnetic energy storage (SMES) device. Several pancake coils are assembled in the form of a nested solenoid, and each coil requires a hundred meters or more of 2G (RE)BCO tape. However, commercial availability of this superconductor with a very uniform physical properties is currently limited to shorter piece lengths. This necessitates us having splices to inter-connect the tape pieces within a pancake coil, between adjacent pancake coils, and to attach HTS current leads to the magnet assembly. As a part of the optimization and qualification of splicing process, a systematic study was undertaken to analyze the electrical performance of splices in two different configurations suitable for this magnet assembly: lap joint and spiral joint. The electrical performance is quantified in terms of the resistance of splices estimated from the current-voltage characteristics. It has been demonstrated that a careful application of this splicing technique can generate lap joints with resistance less than 1 nΩ at 77 K.     

Interactions between drive coils and ferrite sheets in bubble memory packages

Sheets of ferrite or other permeable materials situated very close to drive coils are expected to affect the field distributions within the coils and the inductances of the coils. A theoretical study has been made of a system consisting of a flat rectangular coil with sheets in contact with its flat faces, including the effects of the shapes of the sheets and their permeability and of an angle of tilt between the coil axis and the sheets. The results are compared with experimental results.     

Experimental analysis of AC loss and recovery characteristics for proposal of effective thickness of spacers in pancake type

For the design of superconducting fault current limiter, the research about recovery time and AC loss is essential issue because this characteristic is closely related to stability and efficiency. In general, superconducting fault current limiter (SFCL) modules can be made into pancake or solenoid type coil. While the pancake type coil has smaller AC loss, it has longer recovery time compared with the solenoid type coil. In this paper, a new pancake type coil was proposed to decrease recovery time by improving their cryogenic condition. A new pancake type coil is made with spacers between adjacent superconducting tapes. However, as the thickness of the spacer was increased, the AC loss was increased unlike the recovery characteristic. From the experimental results, our group confirmed that the pancake type coil with the spacer has trade-off relationship between AC loss and recovery characteristics with the spacer thickness.The proposed results in this paper would be utilized to design superconducting fault current limiters.     

Oxidation characteristics of the graphite micro-coils, and growth mechanism of the carbon coils

The oxidation characteristics of the graphite coils obtained by high-temperature heat treatment of the vapor grown carbon micro-coils were examined, and the growth mechanism of the carbon micro-coils is discussed. The ruptured cross section of the graphite coils with a circular cross section (circular graphite coils) exposed in air or an Ar atmosphere at 800–1400°C have generally negative or positive trigonal cone-forms. On the other hand, that of the graphite coils with flat or rectangular cross sections (flat graphite coils) have negative or positive rectangular cone or roof-like forms. The edge between two graphite layers was preferentially oxidized to form three deep striations that extended in the direction of the fiber axis, and then formed six coils from the double circular graphite coils. It is reasonably considered that eight thin coils are formed from the double flat graphite coils. These observations strongly supported the growth mechanism based on the catalytic anisotropy between the catalyst crystal faces.     

Recovery estimation for over-current test of non-inductive fault current limiters using numerical analysis

When an HTS coated conductor (CC) is used as a conductor of a superconducting fault current limiter (SFCL), the CC is expected to be exposed to the over-current and temperature of the CC is expected to be increased rapidly by electrical joule heating. Because the CC is a composite tape, thermal and electrical properties of composite materials could affects over-current limiting capacity and recovery time of SFCL. This paper presents experimental and numerical results of over-current test and recovery time measurement test on four bifilar wound SFCL modules. The temperature transitions of the samples were estimated from total electrical resistance of the coils. We fabricated one bifilar solenoid coil and three bifilar pancake coils whose cryogenic conditions were different from the other coils. An numerical model was also fabricated to simulate the temperature transition and the numerical results were compared with experimental results.     

A target-field method to design circular biplanar coils for asymmetric shim and gradient fields

The paper presents a method for designing circular, shielded biplanar coils that can generate any desired field. A particular feature of these coils is that the target field may be located asymmetrically within the coil. A transverse component of the magnetic field produced by the coil is made to match a prescribed target field over the surfaces of two concentric spheres (the diameter of spherical volume) that define the target field location. The paper shows winding patterns and fields for several gradient and shim coils. It examines the effect that the finite coil size has on the winding patterns, using a Fourier-transform calculation for comparison.     

The electric characteristics of HTS double-pancake coil with AC pulse over currents

A double pancake coil was designed and manufactured with a 36-m long Bi-2223/Ag tape. The tape was insulated by 25 μm thick Kapton tapes, which can stand a voltage of 400 V_rms in liquid nitrogen. The whole double pancake was impregnated with epoxy resin. AC over-current experiments of the coil were performed by applying constant AC voltages to the two terminals of the coil and lasted for 3 s. The experiment began first at a lower voltage of 33.6 V_rms, and then the voltage stepped up till the coil was burned out at the pulse voltage of 202.7 V_rms. All of the experiments were carried out with the coil dipped in liquid nitrogen. The current waveforms were measured. The impedance and resistance characters of the HTS coil with its over pulse currents were analyzed from the experiment results. At the end of this paper, some conclusions derived from the experiment results and their analyses are given, which are helpful for the safety operating of the HTS coils in power applications.     

Design and manufacture of a toroidal-type SMES for combination with real-time digital simulator (RTDS)

The authors designed and manufactured a toroidal-type superconducting magnetic energy storage (SMES) system. The toroidal-type SMES was designed using a 3D CAD program. The toroidal-type magnet consists of 30 double pancake coils (DPCs). The single pancake coils (SPCs), which constitute the double pancake coils, are arranged at an angle of 6° from each other, based on the central axis of the toroidal-type magnet. The cooling method used for the toroidal-type SMES is the conduction cooling type. When the cooling method for the toroidal-type SMES was designed, the two-stage Gifford–McMahon (GM) refrigerator was considered. The Bi-2223 HTS wire, which was made by soldering brass on both sides of the superconductor, is used for the magnet winding. Finally, the authors connected the toroidal-type SMES to a real-time digital simulator (RSCAD/RTDS) to simulate voltage sag compensation in a power utility.     

Magnetic field enhancement of high-T c superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.