Low AC Loss Research Articles

Voltage Sag Compensation of Point of Common Coupling (PCC) Using Fault Current Limiter

In this paper, voltage sag compensation of point of common coupling (PCC) using a new structure of fault current limiter (FCL) is proposed. The proposed structure prevents voltage sag and phase-angle jump of the substation PCC after fault occurrence. This structure has a simple control method. Using the semiconductor switch (insulated-gate bipolar transistor or gate turnoff thyristor at dc current rout leads to fast operation of the proposed FCL and, consequently, dc reactor value is reduced. On the other hand, the proposed structure reduces the total harmonic distortion on load voltage and it has low ac losses in normal operation. As a result, other feeders, which are connected to the substation PCC, will have good power quality. Analytical analysis and simulation results using PSCAD/EMTDC software and experimental results are presented to validate the effectiveness of this structure.

Electric power grid application requirements for superconductors

Abstract

Effect of self-field on the current distribution in Roebel-assembled coated conductorcables

Roebel cables are a promising solution for high current, low AC loss cables made ofhigh-temperature superconductors in the form of coated conductors. High current createssignificant self-field, which influences the superconductor’s current-carrying capability. Inthis paper, we investigate the influence of the self-field on the cable’s critical currentand the current repartition among the different strands. In order to investigatethe cable’s critical current, we analysed the influence of flux creep on the cableproperties. Using the experimental material’s properties derived from measurementson a single conductor as input for our calculations, we were able to predict thecritical current of the cable in two limiting situations: good current sharing andcomplete electrical insulation among the strands. The results of our calculations showgood agreement with the measured critical current of three Roebel cable samples.

On the role of a tape’s aspect ratio in the hysteresis losses of round superconductingcables

Individual superconductors are made small to allow flexible manufacturing and low AClosses. When high currents are needed, several conductors are cabled together. These cablescan be used to wind magnets or to transmit huge currents in electric power lineswith very low losses. In DC use, the superconductor in these cables is lossless,but when the current or external magnetic field varies, AC losses arise in thecables. If a round cable is made of several flat tapes, the aspect ratio of the tape (α = width/thickness) plays an important role in the AC losses. Tapes with highα should be put as close to each other as possible in order to form asuperconducting cylinder to reduce the hysteresis losses. However, ifα is around 10, it is no longer evident what kind of configuration results in the lowest losses. Whenα approaches1, the minimum losses are reached when the tapes do not interact at all. YBCO coated conductors have highα, but Bi-basedconductors and MgB2 should not be forgotten. These can offer completely different values ofα and thus show different behaviours under AC conditions. In this paper we study how the hysteresislosses of round cables are influenced by the tapes’ aspect ratios. We investigate whether itis beneficial to pack the conductors tightly together or to let them operate individually, orwhether an intermediate state is preferred. According to the results, there is a break-evenα where the tight packing surpasses the individual operation, but the intermediatepositioning can result in even lower losses in some cases.

An Optimization of a NbTi Wire Cross Section to Attain Low AC-Losses and High Critical Current

Removing the heat generated by AC-losses in large-scale superconducting applications has high costs. This is also the case in semi-DC magnets such as the ones of large particle accelerators where magnets are ramped synchronously with the beam energy. Also, these magnets require large cables made of several individual strands. This makes even the AC-loss analysis of such magnets very difficult. However, to attain low-loss full-size magnets, it becomes necessary to use wire designs that provide low AC-losses and adequate stability. The design of individual wires includes at least the determination of filament sizes and locations and the selection of matrix material. At the same time a high critical current and good workability must be ensured. With numerical analysis it is possible to consider these factors and design new strands and cables having low AC-losses for specific applications. In this paper we consider how to employ a numerical tool for the optimization of the NbTi wire cross-section to achieve low AC-loss and high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Model Cable Tests for a 275 kV 3 kA HTS Power Cable

High-temperature superconducting (HTS) cables are considered the next generation transmission line because they are compact, lightweight, and demonstrate large capacity and low loss compared to conventional cables. In particular, since a coated conductor (YBCO wire) provides high critical current, high magnetic-field property, low AC loss, and low cost, it is expected to make the HTS cable more attractive than other superconducting wire. In Japan, 66/77 kV HTS cables have developed for about 20 years. We started developing 275 kV class HTS cables three years ago based on 66/77 kV HTS cables. The goal is a 275 kV 3 kA cable with a capacity of 1.5 GVA, the same capacity as a typical overhead transmission line, which serves as the backbone of Japanese power networks. The following technical developments will be carried out: high current and low AC loss cable conductors and high voltage insulation and low dielectric loss cables. Regarding high current and low AC loss cable conductors, 3-kA cables have been fabricated, and AC losses have been measured. We found that using thin YBCO wire reduced AC losses in experiments.

Pushing Coated Conductor Critical Currents Beyond 1 kA per cm Width: Stacks of YBCO Layers

For a number of superconducting power applications, a high value of the engineering critical current density (Je) for the wire is crucial. The superconducting layer in the coated conductor is typically a small portion of the overall cross-section, so increasing the superconductor fraction will directly result in an increase of Je . However, as the thickness of the superconductor is increased, Jc eventually drops. We describe a way to increase Je by making a stack of superconducting layers using sequential Ion-Beam Assisted Deposition (IBAD)/Superconductor deposition. Reactive Co-Evaporation by Cyclic Deposition and Reaction (RCE-CDR) is used for superconductor. An IBAD-textured layer resets the crystalline structure after each superconducting layer and we use IBAD-MgO for this purpose. However, IBAD-MgO texturing requires an extremely smooth starting surface (about 1 nm root mean square roughness), whereas the YBCO layer is typically 10-100 times rougher. We employ the Solution Deposition Planarization (SDP) process to planarize the rough surface of YBCO. The SDP layer is insulating and it provides for an easy way to separate the superconducting layers electrically. We discuss unique features of the stacking structure that allow for high Ic, low ac-losses in applied fields, as well as high Je .

Joint Design and Test for the SCH

The NHMFL Series Connected Hybrid (SCH) magnet will provide an energy-efficient 36 T to the DC user facility by employing a 20 kA superconducting outsert coil in series with a resistive insert. The magnet outsert consists of three concentric layer-wound sub-coils using three different grades of Nb3Sn Cable-in-Conduit Conductors (CICC). The electrical joints in the superconducting outsert require low DC resistance to minimize the refrigeration requirement at the operational 4.5 K temperatures and low AC losses to ensure good stability against ramping operation required by the users. There are four internal splice joints in the outsert, which are Nb3Sn to Nb3Sn joints with the same design configuration. There are another two terminal joints between the Nb3Sn outsert and the two NbTi buslines, which connect the outsert terminals to the two current leads. The two Nb3Sn to NbTi terminal joints are of identical configurations. All of the joints will be praying-hands configuration with an operation current of 20 kA. The R&D for the joins has been carried out at the NHMFL. The joints design and test results are discussed in this article.

Progress in Reducing AC Losses of Bi2223 Tapes with Interfilamentary Resistive Barriers

This paper presents our recent progress for the development of low-AC loss Bi2223 tapes with interfilamentary oxide barriers. For the compatibility with Bi2223 phase formation during sintering, SrZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was selected as barrier materials. Moreover, small amount of Bi2212 was mixed with SrZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> to improve its ductility for cold working. Although some breakages of barrier layers still existed, the effective transverse resistivity was approximately 10 times higher than a tape with pure Ag matrix. By controlling barrier thickness, reducing a tape width below 3 mm and twisting the filaments with its length below 5 mm , coupling frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> attained to 260 Hz in an AC perpendicular transverse field. Critical current densities J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of our twisted barrier tapes were ranged in 12-15 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K and self-field, which was 25-30 % lower than non-twisted one (=18 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). In our knowledge, this is the first report to achieve both J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> >; 12 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> 250 Hz simultaneously in a single Bi2223 tapes. Our barrier tapes showed 60-70% lower perpendicular field losses than a conventional 4 mm-width tape with fully coupled filaments at 50 mT and 50 Hz. These results are promising for remarkable improvement in AC performance for Bi2223 tapes in future.

Conceptual Design of a 10 HP Homopolar Motor with Superconducting Windings

Superconducting motor has a lot of benefits from high power density for ship propulsions, so a number of research project are in progress worldwide. Despite of all the benefits, there is always a difficulty of cryo-moving part for conventional air-core superconducting synchronous motors. In order to get rid of this moving cryogenic part, we propose a homopolar superconducting synchronous motor, which has high temperature superconducting armature and field coils. The rotor is supposed to be made of iron only and excited by the stationary HTS field coils. The stationary field coils make the cooling system simple and easy to realize because there is no cryo-moving part. A design result of a 10 hp homopolar synchronous motor is presented in this paper. The self and mutual inductance of the motor having the size of air gap as variable parameter are calculated by a 3-dimemsional finite element method. The value of design variables such as the dimension of a motor and the number of turns, etc. is decided by performing the coordinate transformation of the calculated inductance. The operating frequency is supposed to be below 5 Hz for low rotating speed which is needed for a purpose of ship propulsion. Low frequency also has the benefit of low AC losses.

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB2 single crystals, unsubstituted and carbon substituted with the composition of Mg(B0.94C0.06)2. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.

Proposal of new structure of MgB2 wires with low AC loss for stator windings of fully superconducting motors located in iron core slots

The new structure of MgB2 monofilamentary wires for stator windings of fully superconducting motors is proposed to reduce their AC losses in iron core slots for the application of an alternating transport current. In order to validate the proposed structure of wire for loss reduction, numerical calculations are carried out by means of a finite element method using edge elements formulated with a self-field due to currents induced in an analysis region. It is assumed that the voltage–current characteristics of the MgB2 superconductor are given by Bean’s critical state model, in which the critical current density is independent of the local magnetic field. The influences of wire structures on the AC losses are discussed quantitatively toward the optimum design of stator windings in fully superconducting motors with the MgB2 wires.

Investigation of the effect of striated strands on the AC losses of 2G Roebel cables

The assembly of meander shaped coated conductor tapes by the Roebel technique is apromising way to manufacture high current cables with low ac losses. The application oflongitudinal striations to the single strands can be an option to create a filament structurefor further possible reduction of the ac losses. Due to the complex Roebel strandgeometry, it was important to identify a reliable technique to produce such structuresusing a picosecond-infrared (IR) laser for the groove etching process. We analyzedthe effects of the filament structure on the magnetization ac loss behavior bycomparing the losses of a cable with striated strands with those of a reference one withnon-striated strands. The ac loss reduction in the Roebel cable with striated strandswas confirmed. The measured magnetization loss of the 125 mm striated singlestrand is five times lower than that of the non-striated one. In the case of thecable sample the loss reduced by a factor of three, but not in the whole intervalof amplitudes of the applied magnetic field. We also compared the results withthose for a cable with insulated striated strands: they seem to indicate that thecoupling currents occur mostly between the filaments, not between the strands.

Application of high temperature superconductors for fusion

The use of High Temperature Superconductor (HTS) materials in future fusion machines can increase the efficiency drastically. For ITER, W7-X and JT-60SA the economic benefit of HTS current leads was recognized after a 70 kA HTS current lead demonstrator was designed, fabricated and successfully tested by Karlsruhe Institute of Technology (KIT, which is a merge of former Forschungszentrum Karlsruhe and University of Karlsruhe). For ITER, the Chinese Domestic Agency will provide the current leads as a part of the superconducting feeder system. KIT is in charge of design, construction and test of HTS current leads for W7-X and JT-60SA. For W7-X 14 current leads with a maximum current of 18.2 kA are required that are oriented with the room temperature end at the bottom. JT60-SA will need 26 current leads (20 leads @ 20 kA and 6 leads @ 25.7 kA) which are mounted in vertical, normal position. These current leads are based on BiSCCO HTS superconductors, demonstrating that HTS material is now state of the art for highly efficient current leads. With respect to future fusion reactors, it would be very promising to use HTS material not only in current leads but also in coils. This would allow a large increase of efficiency if the coils could be operated at temperatures ≥65 K. With such a high temperature it would be possible to omit the radiation shield of the coils, resulting in a less complex cryostat and a size reduction of the machine. In addition less refrigeration power is needed saving investment and operating costs. However, to come to an HTS fusion coil it is necessary to develop low ac loss HTS cables for currents well above 20 kA at high fields well above 10 T. The high field rules BiSCCO superconductors out at temperatures above 50 K, but RE-123 superconductors are promising. The development of a high current, high field RE-123 HTS fusion cable will not be targeted outside fusion community and has to be in the frame of a long term development programme for DEMO. KIT has already demonstrated a scalable concept using RE-123 HTS tapes that are assembled to Roebel type conductors. This concept can be expanded to form Rutherford cables as starting point for a development of a high current fusion cable. The status and prospect of using HTS conductors for fusion is discussed.

Current Status of Coated Conductor Development in M-PACC Project

This is a review of the current status of RD degradation, higher in-field Ic, lower AC losses, higher mechanical strength, and higher Je and lower cost. During the three-year development, the interim targets were almost satisfied and the activities performed were at the top technical level in the world. We are now in the final stages of development, aiming to achieve the final targets, which are based on the specifications for marketing the cables, transformers and SMES.

Simulation of ac loss in Roebel coated conductor cables

Roebel cables are a promising concept for high-current coated conductor cables with low acloss. In order to optimize their design, simulations are necessary. In this paper wecompute the field and current distribution and the ac loss due to either a transportcurrent or an applied magnetic field. We distinguish between the coupled anduncoupled situations. These situations correspond to the limits of low and highresistance between strands in one transposition length, respectively. We perform thesimulations with two different methods. One method assumes the critical-state modelfor the superconductor and calculates the current distribution by minimizingthe magnetic energy variation; the other describes the superconductor with apower-law resistivity and uses the finite-element technique for solving the problem. Wefound that the results from both models agree with each other. Moreover, thecoupling strongly influences the current distribution for all cases, as well as themagnetization loss. However, the effect on the transport loss is small. In conclusion,simulations are necessary in order to predict the loss reduction caused by the strandtransposition in a Roebel cable. Our simulation methods are useful for these predictions.

Design of HTS Modular Magnets for a 2.5 MJ Toroidal SMES: ReBCO vs. BSCCO

A 2.5 MJ HTS SMES is being developed as a national program in Korea. This paper presents design results of modular HTS magnets for the 2.5 MJ toroidal SMES. We considered a choice of BSCCO-2223 or ReBCO tapes as the conductors for the HTS magnets of the SMES. Operating temperature was decided to be 20 K which could be accomplished by conduction cooling with a couple of GM cryo-coolers. Key design factors may include minimization of total conductor requirement and low AC loss.

Improvement of Electromagnetic Properties of ${\hbox{MgB}}_{2}$ Filaments Due to Deformation to Tape Shape

We have proposed MgB2 multifilamentary tapes with high critical current densities and low ac losses under external magnetic fields which is parallel to the tape face. The purpose of this proposal is to develop the MgB2 new conductors composed of plural tapes as windings of superconducting coils with both large current capacities and low losses for fusion reactors, superconducting magnetic energy storage systems, and so on. Our MgB2 multifilamentary tapes are fabricated by rolling process from round wires. The tapes are fabricated by using in-situ method. The purpose of this paper is to clarify the effects of deformation to tape shape on the electromagnetic properties of MgB2 filaments in multifilamentary wires. For the purpose, measurements of both critical currents and magnetizations have been carried out on MgB2 fine single filament wires which have simulated a filament in multifilamentary wires. Measurements on wires with large diameter have been also carried out, in order to clarify the effect of diameters of original round wires. We prepared pure and SiC-added MgB2/Nb/Cu composite wires as the original round wires before rolling process. The diameters of round wires have been varied and then the aspect ratios of tapes have been varied. The observed critical current densities of the tapes are about 7 times larger than that of the original round wire at a maximum. The ratios of Jc of wires to tapes don't have dependence of the diameter of the original round wire. In addition, the results of measurements of magnetizations have shown that magnetization losses in MgB2 tapes decrease to 1/3 compared with original round wires.

A new numerical approach to find current distribution and AC losses in coaxial assembly of twisted HTS tapes in single layer arrangement

This paper presents a novel technique for evaluating AC losses and current distribution in single layer assemblies of coaxially wound thin conductors, such as YBCO coated conductors. The proposed approach takes into account the twisted geometry of the individual superconducting tapes by considering the integral relation between the magnetic vector potential and the current density in the tapes (Biot-Savart formula). The integrals are solved numerically and semi-analytically, and the results are used to generate a discretized system of equations based on the magnetic flux diffusion equation (eddy current problem). The latter is solved using an efficient time transient solver (DASPK). It is assumed that, due to the helical symmetry of the problem, it is sufficient to solve for the current distribution in half of a single tape cross-section, even if many tapes are present, which allows a drastic reduction of the 3-D problem to a simple 1-D domain. The method was used to evaluate the AC losses of a HTS cable made of coated conductors, and it was observed that for a given radius of the former and number of tapes, twisted tapes with smaller pitch have lower AC losses.

Fault Current Analysis in a Tri-Axial HTS Cable

Tri-axial cable has been rapidly developed recently because of the advantages such as more compact structure, small leakage field, and low heat and AC losses. One the other hand, it causes an inherent imbalanced three-phase currents distribution due to the different radii of concentric layers. In our previous research, it is demonstrated that a proposed tri-axial cable composed of two longitudinal sections with adjusted twist pitches can transmit balanced three-phase currents. However, before the cable can be installed in a real grid, the heat reliability of the cable in steady-state operation and fault condition should be ensured. The heat balance calculation of the tri-axial cable in steady state under the balanced current distribution allows us to estimate thermal parameters of the cable such as inlet and outlet temperatures, pressure drop and operation temperature. When a phase current suddenly changes under fault conditions, the remaining phase currents become large because of mutual inductances between fault and sound phases. Simulation of the typical fault suggests the copper stabilizer amount on the HTS tape for stability and safety. It is found that copper of 2 mm in thickness keeps the cable temperature under 110 K.