Second-generation HTS Research Articles

Nonequilibrium states in the second-generation HTS composites under overcritical pulsed current impact

This paper presents the results of studying the processes of epy HTS composites switching from the superconducting to the resistive state at microsecond current pulses. Two modes of pulsed current load were used: with an amplitude of ~1.1Ic (the so-called "soft" mode) and with an amplitude of ~3Ic ("hard" mode). The possibility of passing supercritical currents through the tape without superconductor characteristics degradation is shown. To explain the processes occurring in the tape during the current pulse, 2D FEA (finite element analyzes) was developed, with the help of which the dynamic resistance of the HTS composite superconducting layer was calculated in the "hard" load mode and nonstationary processes of current redistribution between the different tape layers were demonstrated. Keywords: HTS composites, superconducting switch, nonequilibrium states, stable switching, irreversible switching.

The Magnetization of HTSC Tapes Stack in the Flux Pump Regime

In the present paper, the possibility of magnetization of HTSC tape by means of pumping by a local magnetic field is considered. Within the framework of the critical state model, the calculation of the density of the induced currents and the magnetization of a square (12 × 12 mm) fragment of a second-generation HTS tape (YBCO) was made. The model considers real dependence of the critical current density on the magnetic field and has been successfully tested previously. In the sample, the currents were induced by the impact of a local magnetic field. The size of the localized area of the external magnetic field to be considered was much smaller than the size of the HTS sample. It is showed that the repeated cyclical impact of the local outer field leads to the accumulation of the total magnetic moment in the sample up to a magnitude of 94% of the theoretical magnetization limit for this sample. The results obtained make it possible to optimize the mode of magnetization of a square fragment of a tape (or a stack of tapes) when implementing a magnetic flux pump. The experimental studies showed results comprisable well with theoretical ones.

Analysis of Internal Stress in a Tape Subsrate Made of the AISI 310S Stainless Steel for the Second-Generation HTS Wires Using Neutron Stress Diffractometry

Neutron stress diffractometry is used to study internal (residual) stress in a tape made of the AISI 310S stainless steel with a thickness of 100 μm and a width of 4 mm in three directions (along, across, and perpendicularly to the rolling plane). The residual macrostresses averaged over the tape length are determined at a segment of 40 cm. The macrostress distribution indicates a crescent-type deformation of the tape (bending in the rolling plane along the rolling direction). The correlation between macrostresses and internal microstresses is established (the stronger macrostresses, the higher concentration of the microstresses).

Modeling and verification of the equivalent circuit for high temperature superconducting partial-core transformer with ReBCO-coated conductors

PurposeThe purpose of this study is to propose a modeling method of the equivalent circuit for a new type of high-temperature superconducting partial-core transformer (HTS-PCT) made of ReBCO-coated conductors.Design/methodology/approachThe modeling process is based on the “Steinmetz” equivalent circuit. The impedance components in the circuit are obtained by the calculations of the core losses and AC losses of the HTS windings by using theoretical methods. An iterative computation is also used to decide the equivalent resistances of the AC losses of the primary and secondary HTS windings. The reactance components in the circuit are calculated from the energy stored in the magnetic fields by finite element method. The validation of the modeling method is verified by experimental resultsFindingsThe modeling method of the equivalent circuit of HTS-PCT is valid, and an equivalent circuit for HTS-PCT is presented.Practical implicationsThe equivalent circuit of HTS-PCT could be obtained by the suggested modeling method. Then, it is easy to analyze the characteristics of the HTS-PCT by its equivalent circuit. Moreover, the modeling method could also be useful for the design of a specific HTS-PCT.Originality/valueThe study proposes a modeling method of the HTS-PCT made of the second-generation HTS tapes, i.e. ReBCO-coated conductors.

Effect of picosecond laser exposure on the transport characteristics of second-generation HTS tapes

The main purpose of the work is to improve the magnetic and transport characteristics of 2G HTS tapes by creating an ordered array of artificial pinning centers. Using a picosecond laser exposure, a local modification of the HTS film were performed. The pulse energy varied from 300 to 4000 nJ, the pulse duration was 25 ps. A triangular array of micron size defects with a period of 50 μm was created. The dependences of the critical current and n-value change on the pulse energy were obtained. The parameters of the exposure resulting in an increase of the HTS tape critical current and tune of the I-V characteristic slope were found.

Persistent Current Mode Operation of A 2G HTS Coil With A Flux Pump

A flux pump is a kind of power source that energizes superconducting magnets usually embedded in the cryogenic system. Therefore, heat loss through current leads is eliminated, allowing for more efficient cooling design. A winding technique, the so-called “wind-and-flip,” had been proposed by the authors to realize a jointless HTS coil for a perfect resistanceless closed loop, which may enable to induce a persistent current in an HTS coil wound with second-generation HTS tape. In this paper, we investigated charging/discharging characteristics and a possibility of persistent current operation of a jointless HTS coil with a flux pump. Results are discussed with regard to current capacity and temporal stability with a prototype jointless HTS coil.

Charge–Discharge and Thermal–Electrical Characteristics of GdBCO Coils Wound With Various Types of Grease as an Insulation Material

This paper reports the charge-discharge and thermal-electrical characteristics of GdBCO single-pancake coils employing various types of grease, such as N-grease (N coil), Si-based grease (Si coil), and vacuum grease (V coil), as an insulation material to ameliorate the charging-discharging delay observed in no-insulation (NI) coils. The charge-discharge test results confirmed that the use of grease as an insulation material can effectively reduce leakage current because the turn-to-turn grease layers serve as an effective insulation material. In sudden discharge tests, the time constant values of coils wound with N-grease, Si-based grease, and vacuum grease were drastically lower than those of the NI coil. In the over-current test at 1.5 Ic, the maximum voltages of N and Si coils were 3.8 times lower than that of the V coil, indicating that the thermal and electrical stabilities of N and Si coils were considerably enhanced. Moreover, the heat dissipating test results indicated that the peak temperature of the Si coil was lower than those of other coils because the Si-based grease acted as the most effective heat dissipater in the event of local hot spot generation in the coil. In conclusion, among the various types of grease tested in this study, Si-based grease may be the most effective turn-to-turn insulator for the development of highly stable second-generation (2G) HTS magnets with ameliorated charging-discharging delay of NI coil.

Transport ac losses of a second-generation HTS tape with a ferromagnetic substrate and conducting stabilizer

The current–voltage curve and transport ac loss of a second-generation HTS tape with a ferromagnetic NiW substrate and brass stabilizer are measured. It is found that the ac loss is up to two orders of magnitude larger than what is expected by the power-law E(J) determined by the current–voltage curve and increases with increasing frequency. Modeling results show that the overly large ac loss is contributed by the ac loss in the HTS strip enhanced by the NiW substrate and the magnetic hysteresis loss in the substrate, and the frequency-dependent loss occurs in the brass layer covering the substrate but not in the ferromagnetic substrate itself as assumed previously. The ac loss in the brass layer is associated with transport currents but not eddy currents, and it has some features similar to ordinary eddy-current loss with significant differences.

사고전류 인가 시 초전도선재의 상전도-초전도 접합부가 통전전류와 ?치저항에 미치는 영향

The second-generation HTS wire its YBCO coated conductor is widely used in the superconducting power apparatus. The YBCO coated conductor uses the normal-superconducting junction to increase the transport capacity of superconducting power apparatus when it is applied. The normal-superconducting junction can be a cause of reducing the stability of the superconducting power apparatus when a fault current is applied. Thus, in this study we have conducted the effect analysing normal-superconducting junction for the fault current using transport current and quench resistance. From the experimental results when a fault current is applied, the effect on the normal-superconducting junction is reduced the larger the amplitude of the fault current and is helpful to maintain the thermal stability of the HTS wire.

Modeling of Second Generation HTS Cables for Grid Fault Analysis Applied to Power System Simulation

HTS power cable systems are an emerging technology aimed at competing with XLPE cable systems. Knowledge on the thermal operating conditions of HTS power devices is needed to estimate their availability when connected to a power system, because the HTS material must remain below its critical temperature to transport current. In this work, a simple finite difference method is used to assess the temperature distribution at certain cross-section of a second-generation coaxial HTS cable. This method has been implemented in MATLAB and its proper functioning has been verified with the software package FLUX. This method is a tool to establish temperature distributions among HTS cable layers under normal operating conditions. Additionally, the aim of this work is to serve as basis for future simulations including heat generation changes within the cable layers typically caused by grid fault events.

Seed layers on RABiTS tapes for Second-Generation HTS wires

Epitaxial films of CeO2 and Y2O3 on RABiTS metal tapes have been obtained by pulsed laser deposition. The dependence of the film crystal orientation on the temperature of synthesis has been studied. It is established that Y2O3 films grow with 100% (100) orientation, whereas a parasitic orientation is present in CeO2 films obtained in the entire range of growth temperatures. The texture sharpness in the substrate plane amounted to 8°. Electron-microscopic examination showed that an increase in the temperature of synthesis leads to the appearance of cracks on the surface of CeO2 films, while the surface of Y2O3 films remains continuous in the entire range of growth temperatures. Thus, Y2O3 films most fully obey the requirements to seed layers in (i) being epitaxial with 100% (100) crystal orientation, (ii) inheriting the substrate texture, and (iii) having a smooth crack-free surface.

Progress in Performance Improvement and New Research Areas for Cost Reduction of 2G HTS Wires

Second-generation (2G) HTS wires are now being produced routinely in kilometer lengths using Metal Organic Chemical Vapor Deposition (MOCVD) process with critical currents of 300 A/cm. While this achievement is enabling several prototype devices, in order to reach a substantial commercial market, the cost-performance metrics of 2G HTS wires need to be significantly improved in device operating conditions. Zr-doping has been found to be an effective approach to improve in-field critical current performance of MOCVD-based HTS wires. In this work, we have explored modifications to the Zr-doped precursor compositions to achieve three and two-fold increase in deposition rate in research and production MOCVD systems respectively. Production wires made with modified Zr-doped compositions exhibit a self-field critical current density of 50 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm MA/cm}^{2}$</tex> </formula> at 4.2 K and a 55 to 65% higher performance than our previous wires with Zr-doping, over magnetic field range of 0 to 30 T. We have also developed an alternate, low-cost technique, namely electrodeposition, to deposit silver overlayer on superconducting film. Wires made with electrodeposited silver are able to sustain the same level of overcurrent as sputtered silver layers. This process has been successfully scaled up to 100 m lengths.

C-axis critical current density of second-generation YBCO tapes

We report on measurements of the temperature and field dependence of thec-axis criticalcurrent density (Jcc) obtained on mesa structures that were patterned into the YBCO layer of second-generation HTS tapes. Wefind the Jcc—values of ∼ 4 kA cm − 2 at 77 K and self-field, corresponding to an unexpectedly high anisotropy in the critical current densityJcab/Jcc of500–600. C-axis current flow is expected to arise in applications such as the helically wound wires inHTS cables. A simple estimate is given of the fraction of tape width for such ac-axis flow; while in our samples this fraction is approximately 5% for a typical geometry,the fraction will grow linearly with increasing current density anisotropy and could affectthe current-carrying ability of the tape.

Design and test of current limiting modules using YBCO-coated conductors

Within the cooperation between American Superconductor Corporation (AMSC) andSiemens Corporate Technology we have investigated the fault current limiting performanceof YBCO-coated conductors (also called second-generation or 2G HTS wires) stabilizedwith stainless steel laminates. Design rules for the length and width of the wiredepending on utility grid requirements have been established. Bifilar coils have beenmanufactured and tested with a typical limitation period of 50 ms under stepwiseincreasing voltage loads to determine the maximum temperature the wires canwithstand without degradation. Coils have been assembled into limiter modulesdemonstrating uniform tripping of the individual coils and recovery within seconds. Atpresent this cooperation is proceeding within a joint project funded by the USDepartment of Energy (DOE) that encompasses the design, construction and testingof a 115 kV FCL for power transmission within a time frame of 4–5 years, andadditional partners. Besides AMSC and Siemens, Nexans contributes the highvoltage terminations and Los Alamos National Lab investigates the ac losses.Installation and testing are planned for a Southern California Edison substation. Themodule planned for the transmission voltage application consists of 63 horizontallyarranged coils connected in parallel and series to account for a rated current of 1.2 kArms and voltageof 31 kVrms plus margins. The rated voltage of the module is considerably lower than the line to groundvoltage in the 115 kV grid owing to our shunted limiter concept. The shunt reactorconnected in parallel to the module outside the cryostat allows for adjustment of thelimited current and reduces voltage drop across the module in case of a fault. Thefault current reduction ratio is 42% for our present design. A subscale modulecomprising six full-size coils has been assembled and tested recently to validate the coilperformance and coil winding technique. The module had a critical current of 425 ADC and a nominal power of 2.52 MV A at 77 K. A complete series of tests with applied voltage up to 8.4 kVrms, prospective short circuit current up to 26.6 kArms and variation of phase angle at initiation of the fault has been performed. After more than40 switching tests the critical current of the module remained unchanged, indicating thatno degradation of the wire occurred.

Second-Generation HTS Conductor Design and Engineering for Electrical Power Applications

Besides critical current and conductor length, thermal stability, mechanical properties and ac loss characteristics of second-generation (2G) HTS wire are very important performance parameters to electric power device applications. Each application sets specific technical requirements on conductors. The properties of the practical 2G HTS wire manufactured at SuperPower, Inc. have enabled satisfactory performance in a number of demonstration devices including cable, high-field magnetic coil and fault current limiter. Yet, effort is being made to develop new conductor structures and configurations toward further enhanced performance in order to extend the potential of 2G HTS wire over a wider application range. This paper describes a variety of newly developed configurations: striation, narrow width, low-resistance joints and high quality insulation. The performance of wire with new configurations will promote the wide use of 2G HTS wire.

High Performance 2G Wires: From R&amp;D to Pilot-Scale Manufacturing

Tremendous progress has been accomplished in 2008 in key metrics of 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">x</sub> (YBCO) second-generation (2G) HTS wires at SuperPower. Using improved precursor chemistry in our metal organic chemical vapor deposition (MOCVD) process, critical currents (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) as high as 813 A/cm were reached over meter-long lengths, representing the highest reported I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> in meter long 2G HTS. By use of Zr doping of (Gd,Y)Ba <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">x</sub> , strong enhancements were achieved in in-field performance over a wide angular range. A I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 229 A/cm was achieved at 77 K and 1 T perpendicular to the wire which is about a factor of 2.5 times better than that of standard MOCVD-derived films. At 65 K and 3 T, a I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 340 A/cm was demonstrated perpendicular to the wire. Kilometer lengths of 2G HTS have been demonstrated for the first time. Over 35 tapes 1,000 to 1,500 m in length with a complete 5-layer buffer stack have been routinely produced in our pilot-scale manufacturing facilities. An excellent in-plane texture of 6 to 7 degrees with a uniformity of about 2% was reproducibly achieved in the kilometer-long fully-buffered tapes. Using these high-quality buffers, the longest 2G wire length to date of 1,311 m was produced with a minimum Ic of 153 A/cm corresponding to a I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> times length value of 200,580 m. Also, an 1,030 m long complete 2G HTS wire has been demonstrated with a minimum Ic of 227 A/cm corresponding to a record I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> times length value of 233,810 A-m. Further, a 630 m long segment of the wire sustained a minimum I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 302 A/cm, the longest 2G wire reported to date with a I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> over 300 A/cm. Also, a I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 337 A/cm was achieved over a 540 m long segment. All these values represent a 2 to 3 fold improvement in length and I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 2G wires compared to our achievements in 2007. In 2008, a 30 m long 2G cable made with nearly 10,000 m of 2G wire previously delivered by SuperPower was successfully installed and energized in the grid of National Grid in downtown Albany, marking the world's first of 2G HTS device in the power grid.

Progress in second-generation HTS wire development and manufacturing

2007 has marked yet another year of continued rapid progress in developing and manufacturing high-performance, long-length second-generation (2G) HTS wires at high speeds. Using ion beam assisted deposition (IBAD) MgO and associated buffer sputtering processes, SuperPower has now exceeded piece lengths of 1000 m of fully buffered tape reproducibly with excellent in-plane texture of 6–7 degrees and uniformity of about 2%. These kilometer lengths are produced at high speeds of about 350 m/h of 4 mm wide tape. In combination with metal organic chemical vapor deposition (MOCVD), 2G wires up to single piece lengths to 790 m with a minimum critical current value of 190 A/cm corresponding to a Critical current × Length performance of 150,100 Am have been achieved. Tape speeds up to 180 m/h have been reached MOCVD while maintaining critical currents above 200 A/cm in 100+ m lengths. Thick film MOCVD technology has been transitioned to Pilot manufacturing system where a minimum critical current of 320 A/cm has been demonstrated over a length of 155 m processed at a speed of 70 m/h in 4 mm width. Finally, nearly 10,000 m of 2G wire has been produced, exhaustively tested, and delivered to the Albany Cable project. The average minimum critical current of the wire delivered in 225 segments of 43–44 m is 70 A in 4 mm widths. A 30 m cable has been fabricated with this wire by Sumitomo Electric and has been installed in the power grid of National Grid in downtown Albany and is the world’s first 2G device installed in the grid.

Enhanced flux pinning by BaZrO 3 nanoparticles in metal-organic deposited YBCO second-generation HTS wire

We have formed BaZrO 3 nanoparticles in YBa 2Cu 3O 7− δ second-generation HTS wires through modification of the chemical solution precursor for metal-organic deposition. Transmission-electron microscopy shows the particles to be 10–25 nm in diameter, approximately spherical, and well dispersed through the film. The in-field critical current density is enhanced over a wide angular range compared to undoped wires.

Recent Progress in Second-Generation HTS Conductor Scale-Up at SuperPower

YBa2Cu3Ox (YBCO) second-generation (2G) HTS conductors have been produced in lengths over 300 m in pilot scale facilities established at SuperPower. High throughput pilot-scale manufacturing has been demonstrated with tape speeds at or above 30 m/h of 12 mm wide tape (corresponding to 90 m/h of 4 mm wide conductor) in all steps. A 322 m long conductor with a minimum critical current value of 219 A/cm has been produced, which corresponds to a critical current x length value of 70,520 A-m. A 270 m long, 4 mm wide conductor with an end-to-end critical value of 100 A has also been demonstrated. In a campaign to manufacture 2G conductor for the Albany cable project, SuperPower has produced 12,470 m of conductor that meets or exceeds the specification for piece length (42.4 m) and critical current (100 A/cm). In fact, more than 55% of the conductor produced is at least 100 m in piece length and more than 27% greater than 200 m in piece length. In addition to scaling up 2G conductors to high-throughput pilot-scale manufacturing, we have demonstrated high critical currents in short samples produced by Metal Organic Chemical Vapor Deposition (MOCVD). Critical current values of 557 A/cm have been achieved in 2.1 micrometer thick films in 12 mm wide, 10 cm long tapes. These samples exhibit a critical current value of 116 A/cm at 1 T and 76 K, in the orientation of field parallel to the c-axis. We have also constructed a 4-pancake coil that generated a magnetic field of 1.1 T at 77 K and 2.4 T at 64 K. A Fault Current Limiter (FCL) assembly has been successfully constructed and tested at high power levels. A prospective current of 90 kA was successfully limited to 32 kA within 1 ms without any HTS element failure.

Enhanced Flux Pinning in MOD Second Generation HTS Wires by Silver- and Copper-Ion Irradiation

We have studied the effect of medium-energy ion irradiation on flux pinning in second-generation HTS wires. Irradiation with 74 MeV silver ions and 50 MeV copper ions have produced up to 60% enhancements in critical current. The field-angle dependence shows enhancements over a wide angle range, suggesting that the irradiation defects are point-like defects rather than continuous columns. Such isotropic enhancements are generally preferable over narrow-angle anisotropic effects for wire applications. Ion irradiation can then be used to create a model population of defects to study concentration dependences. We present transport, magnetization, and TEM results to probe the nature of the defects formed by this irradiation.