Search

This paper substantiates a new adaptive method for determining parameters of high-temperature superconducting power transformers with current limiting function. The main focus is the design of current-limiting superconducting windings in the light of new restrictions on current density, magnetic induction, critical current and critical temperature. The presented method considers the nature of alternating current (AC) losses in a superconductor under nominal operating conditions, features of the dielectric medium (liquid nitrogen), as well as the reduced values of the short-circuit voltage (0.5 to 1.5%). The main design features of high-temperature superconducting (HTS) transformers are specified, and a prototype of a three-phase HTS transformer of 63 kVA with a short-circuit current limiting function is developed. It is shown that HTS units have some advantages over conventional transformers: a 90 to 95% active losses reduction, short-circuit current limitation function, explosion and fire safety, a 60% reduction in weight and size, and increased efficiency (up to 99.8%). Experimental studies confirm that the short-circuit current limitation function is safe and efficient. It is demonstrated that during the short-circuit current limitation, significant heat flows occur on the windings, which should not exceed the critical value above which the superconductor could not return to the superconducting state by itself. [ABSTRACT FROM AUTHOR]

The superconducting inductive pulsed-power supply (IPPS) is considered to have broad application prospects in the field of electromagnetic launch because of its low electrical loss. In order to obtain a higher efficiency in continuous electromagnetic launch, a repetitive IPPS circuit topology based on a high-temperature superconducting pulse power transformer (HTSPPT) has been proposed in our previous studies. It can recover most of the remaining energy for generating continuous current pulses by using a bridge-type capacitive switching circuit. As a follow-up work, this paper presents a multimodule repetitive superconducting IPPS model for a railgun system. This multimodule model consists of 24 HTSPPT modules connected to an XRAM-like circuit topology. In order to verify the feasibility of the repetitive superconducting IPPS and study its driving characteristics, the dynamic launching process of a simple 4-m-long railgun system was simulated. The influence of the capacitance in the bridge-type capacitive switching circuit on the main performance of the system, such as the maximum output current, the maximum primary voltage, the launch efficiency, the maximum capacitive energy ratio, and the remaining energy recovery ratio, was analyzed. The simulation results show that the repetitive superconducting IPPS is suitable for driving the simple railgun. In addition, it also concluded that the system performances conflict with each other and the optimal capacitance value for each HTSPPT module should be determined by the tradeoff of system performances. [ABSTRACT FROM AUTHOR]

High temperature superconducting pulsed power transformer (HTSPPT) is an important device for pulsed power supplies. It consists of a superconducting primary and a normal conducting secondary, which is used for energy storage and current amplification. The critical current density, the energy storage, and the coupling coefficient are three main performance indexes. They are affected by the geometry parameters of HTSPPT modules, such as the height and the width of the superconducting coils. In addition, the hoop stress of the HTSPPT coils is limited by the maximum tensile strength of high temperature superconducting (HTS) tapes. In this paper, Bi-2223/Ag HTS tapes are selected as the wire of primary inductor and the toroidal structure model is selected for multiple HTSPPT modules. The relationships between the geometry parameters of HTSPPT modules and the electrical performance are studied. [ABSTRACT FROM AUTHOR]

Inductive energy storage for pulsed power supplies is considered to have great potential because its energy density is 1 order of magnitude higher than that of capacitive one. Associating with the superconducting technology and the STRETCH meat grinder circuit, which proposed by the Institute of Advanced Technology, a superconducting inductive pulsed power supply (SPPS) circuit has been proposed by using an air-core high-temperature superconducting pulsed power transformer (HTSPPT) in our previous study. It can not only recapture the energy in the leakage flux and slow down the current change in inductors by using a capacitor, but also reduce the coil loss and the power requirement of the primary source by using superconducting inductors. However, it can be found that the SPPS may generate a large residual current during the discharging phase, which results in an adverse influence on the working frequency and the energy transfer efficiency of the whole system. This paper proposes a modified circuit based on the bridge current switching circuit and HTSPPT. The main principle of the circuit is that the recaptured energy in the capacitor and the residual energy in the secondary are used to precharge the primary inductor for the next cycle, which shortens the charging time, generates continuous current pulses, and improves the energy transfer efficiency. The procedure of the repetitive SPPS is analyzed in detail, and simulations are carried out to explain how the circuit works clearly. [ABSTRACT FROM AUTHOR]

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A large cable-in-conduit-conductor (CICC) test facility has been designed and fabricated at the High Magnetic Field Laboratory of the Chinese Academy of Sciences (CHMFL) in order to meet the test requirement of the conductors which are applied to the future fusion reactor. The critical component of the test facility is an 80 kA superconducting transformer which consists of a multi-turn primary coil and a minor-turn secondary coil. As the current source of the conductor samples, the electromagnetic performance of the superconducting transformer determines the stability and safety of the test facility. In this paper, the key factors and parameters, which have much impact on the performance of the transformer, are analyzed in detail. The conceptual design and optimizing principles of the transformer are discussed. An Electromagnetic-Circuit coupled model built in ANSYS Multiphysics is successfully used to investigate the electromagnetic characterization of the transformer under the dynamic operation condition. [ABSTRACT FROM AUTHOR]

Superconducting pulsed-power supply (SPPS) provides an efficient method for both high-density inductive energy storage and high current pulse generation. An SPPS consisting of eight high-temperature superconducting pulsed-power transformer (HTSPPT) modules with XRAM methodology was designed and simulated in preliminary studies. It recycled the leakage flux energy and reduced the voltage across the opening switch with a capacitor in primary. However, the SPPS with XRAM methodology has two problems. The first one is that it can provide high pulse amplitude, but cannot provide enough width. The other one is that the ratio of the maximum capacitive energy over the total inductive energy is relatively large, which cuts down the overall system energy density. This paper proposes a modified topology of SPPS with capacitor reuse methodology. Multiple HTSPPT modules are divided into several groups. Each group shares the same capacitor. The procedure of the modified SPPS is presented in detail. A 270-kJ SPPS with capacitor reuse methodology is designed and simulated to illustrate the modified topology. With the load parameters of a small railgun, the peak value of the load current is 207.15 kA and the pulse half-height width is 5.56 ms, the ratio of the maximum capacitive energy over the total inductive energy is 11%. The characteristics of the modified topology are also described in detail in this paper. [ABSTRACT FROM PUBLISHER]

In Japan, superconducting cables for the direct current railway electrification systems have been progressively developed. When superconducting cables are used as a feeder of the railway electrification system, the voltage drop could be significantly reduced. However, fault current due to short-circuit and the like should be enhanced since superconducting cables have no resistance. In this study, it is proposed that superconducting transformers with current limiting function are applied as rectifier transformers so that the fault current can be suppressed. Although superconducting feeding system has many effects, the rated secondary current of the rectifier transformers is very large. So the current capacity of the secondary windings of the rectifier transformers should also be enhanced up to several kA. To realize that, transposed parallel conductors composed of HTS tapes are expected to be adopted. For low ac loss and uniform current sharing among the tapes, the constituent tapes of parallel conductors need to be transposed so that their inductances balance each other. In this paper, the optimum transposition method in the case where a 32-tape parallel conductor is wound into a single-layer solenoid is investigated by the numerical simulation of the current sharing among the constituent tapes and the validity is verified experimentally. [ABSTRACT FROM PUBLISHER]

Superconducting current-limiting transformers offer an effective current limitation and recovery-under-load characteristics; they are more compact and have a lightweight design and a reduced short-circuit impedance. Several designs for superconducting current-limiting transformers have been developed and proposed by researchers over the last two decades. However, less research work was devoted to effective current limitation of superconducting transformers in combination with recovery under full or under partial load. In this paper, we present the process of manufacturing a superconducting transformer demonstrator with current limitation and recovery-under-load capabilities. The demonstrator consists of a laminated iron core and a normal conducting 20-kV primary winding, both operated at the room temperature. 2G REBCO tapes are used for the secondary winding, which is operated at 1-kV nominal voltage. In order to keep the secondary winding at cryogenic operation conditions at 77 K, a vacuum insulated fiber reinforced plastics cryostat was designed and manufactured. [ABSTRACT FROM PUBLISHER]

This paper focuses on the comparison of the characteristics among three different types of high temperature superconducting (HTS) transformers. The iron core structures of the three types of HTS transformers are air core, partial core, and full core. The iron cores are made of oriented silicon steel sheets. The primary windings are four layers connected in series, and the secondary windings are nine double pancake coils connected in parallel. Fundamental tests, such as critical test, no-load test and load test, are performed in the environment of liquid nitrogen. The current nonuniformity of HTS partial core and full core transformers in secondary windings are also confirmed by the load test. Finally, we also investigate the characteristic of fault current limiting by sudden short-circuit tests. The experiment results show that the structure of the iron core have significant effects on the characteristics of HTS transformers. The results of this paper could support with useful data for the design and selection of HTS transformers. [ABSTRACT FROM PUBLISHER]

An 11 T superconducting magnet has been manufactured for a 45 T hybrid magnet, the superconducting magnet has a large room temperature bore of 800 mm, so it can provide an 11 T background field for performance test of cable in conduit conductors (CICC). A test facility for superconducting CICCs has been designed as an insert combined with the superconducting magnet, wherein an 80 kA superconducting transformer has been designed to provide high operating current for the CICC samples. The CICC sample was designed to be a bifilar non-inductive solenoid coil. This paper will describe the principle design of the test facility and the overview of the 80 kA superconducting transformer. [ABSTRACT FROM PUBLISHER]

We developed a method to combine parallel superconducting HTS tapes to a current carrying element with high current and uniform current distribution among strands at ac mode. The current carrying elements are made of several parallel HTS tapes placed side-by-side on a round mandrel. The test of the small model made of five parallel 2G HTS tapes confirmed the idea. The method was implemented for 10/0.4 kV, 1 MVA power transformer. For both low and high voltage windings, we used 12 mm Amperium tapes from American Superconductor Co. For low-voltage winding 19 parallel tapes were wound on four mandrels and connected in series. High-voltage winding was wound with a single 12 mm tape. The assembled windings of each phase were tested at liquid nitrogen temperature without iron yoke. AC losses were measured in low-voltage winding at two modes: when high voltage winding was either shortened or open. In this paper, we present the details of current carrying element development and test, 1 MVA transformers winding designs and test results. [ABSTRACT FROM PUBLISHER]

Mixed refrigerant process refrigerators are ideal for use in superconducting transformers, fault current limiters, etc. placed in a liquid nitrogen bath. Traditional mixed refrigerant processes used above 70 K cannot be used in these applications. The performance of two mixed refrigerant processes suitable for the above applications has been studied, the results of which are presented in this paper. [ABSTRACT FROM AUTHOR]

The superconducting inductive pulsed-power supply (SPPS) circuit consists of a high-temperature superconducting pulsed-power transformer (HTSPPT) and an auxiliary capacitor. The electrical losses and the power of the primary source can be cut down with the superconducting inductive energy storage. Although its energy density is slightly less than a purely superconducting inductive system, it can reduce the maximum voltage of the opening switch by the capacitor and achieve a high degree of current amplification factor. However, during the discharge process, the magnetic leakage flux energy stored in the primary of the HTSPPT requires the capacitor to have a certain capacity. The higher the ratio of the capacitive stored energy over the total inductive stored energy, the more the dominant position of inductive energy storage reduces. The current amplification factor ( \gamma ), the maximum voltage of the opening switch ( V_{S\max } ), and the ratio of capacitive stored energy ( \eta ) are the three main properties of the SPPS circuit, which depends on several intertwined critical parameters, such as the inductance ratio n = L2 /( L2+LL ) and the capacitance ( $C_{{\mathbf {1}}}$ ). This paper studies the influence of the critical parameters on the main circuit performance with a 1-kJ SPPS system. The detailed selection method of the critical parameters for the specific system is also discussed with a coupling coefficient of 0.95. Such a system is easy to be utilized for the basic experimental study of a small railgun. [ABSTRACT FROM PUBLISHER]

Transformers represent one of the oldest and most mature elements in a power transmission and distribution network. The new superconducting transformers are smaller and lighter than conventional ones and they have lower power losses, too. Also, the new 2G superconducting tapes with high resistivity in the normal state allow to build transformers with high short-circuit strength. The short-circuit current limiting feature of the superconducting transformer, which is the most important benefit of replacing conventional windings by superconducting ones, provides protection and significantly reduces the wear and tear of circuit breakers and other substation power equipment. This paper describes the design and experimental investigations results of a model of a 1-phase, 8.8 kVA superconducting transformer with windings made of 2G HTS tape. A special regard is given to the ability of the device's superconducting winding to limit the short-circuit current, in particular its equivalent resistance in normal state at a temperature of 77 K (i.e. resistance of the resistive layers of the HTS tape just after transition to the non-superconducting state). [ABSTRACT FROM AUTHOR]

The main coils of the new European DIPOle (EDIPO) test facility, which is able to provide a background magnetic field up to 12.35 T, were commissioned at the Center for Research in Plasma Physics in July 2013. A field homogeneity of +/−1% was achieved over a 900-mm length, as compared to SULTAN's 10.9 T over 425 mm. To complete the commissioning of the test facility, the new superconducting transformer has been operated in 2015, and a high-current NbTi sample, which was earlier tested in the SULTAN test facility, has been tested in EDIPO under the same conditions as in SULTAN to benchmark the results. During the initial commissioning of the 100-kA transformer in 2013, the primary coil was found to be insufficiently cooled due to the debonding of the primary winding from its former that also served as the liquid helium cryostat. The NbTi primary coil was therefore modified to be in direct contact with liquid helium. At the next attempt of commissioning, in March 2014, a mechanical failure due to the imperfect alignment of the primary and secondary windings caused major damage. A replacement primary coil, which was wound in Summer 2014, was used in the third commissioning campaign in November 2014, but it failed due to the poor impregnation of the NbTi winding. Eventually, the original primary winding was repaired and commissioned in April 2015. The commissioning included a calibration of the current meter for the secondary winding and the high-current test, i.e., up to 95 kA at various background fields, of the “Trasek” conductor sample, which is a rectangular cable-in-conduit conductor made of 324 SnAg-coated NbTi strands. An excellent agreement was found between the results obtained from the two facilities. The sample holder unit is also equipped with a high-temperature superconducting (HTS) adapter and a counterflow heat exchanger to allow testing in EDIPO of HTS high-current samples over a broad range of operating temperatures, from 4.5 up to 50 K. [ABSTRACT FROM PUBLISHER]

For electromagnetic emission applications that need a very high amplitude and an appropriate pulsewidth, a pulsed power supply based on multiple high-temperature superconducting pulsed power transformer (HTSPPT) modules is verified in this paper. First, the working processes of multiple HTSPPT modules are analyzed and two small experimental HTSPPTs are described in detail. Then, simulation is carried out to show the major pulse characteristics of two different discharge methods. To verify the feasibility of two discharge methods, high-current testing is carried out with the two HTSPPTs. The results show that a higher amplitude of current pulse can be achieved using synchronous parallel discharge and a larger pulsewidth of current pulse can be achieved using asynchronous parallel discharge. [ABSTRACT FROM AUTHOR]

A three-phase 1250-kVA/10.5-kV/0.4-kV high-temperature superconducting (HTS) transformer has been designed and fabricated. The windings of the HTS transformer are wound by copper-alloy-reinforced multifilamentary Bi2223/Ag tapes, and the structures of the primary and secondary windings are solenoid and double pancake, respectively. The three-phase three-limb core is made of domain-refined high-induction-type grain-oriented silicon sheet steel, and the cryostat is made of glass-fiber-reinforced plastics. According to the tests, the no-load loss is 2319.2 W at an exciting current of 0.254% of the rated level, the short-circuit impedance of the transformer is 5.6% of the rated value, and the average load loss is 249.6 W. In addition, the leakage current between the primary and secondary windings is about 12.6 mA under the condition of 35 kV/1 min/50 Hz, and the negative full-wave lighting impulse test with a peak voltage of 75\ \mboxkV/1.0\ \mu\mboxs/50\ \mu\mboxs from the primary winding is carried out without breakdown. The experimental results show that the developed HTS transformer meets the requirements of the Chinese standards, and the demonstration of the HTS transformer at the superconducting power substation since September 9, 2014 has been reliable, showing its potential for long-term operation in the grid. [ABSTRACT FROM AUTHOR]

In recent years, using high-temperature superconductor (HTS) transformers in transportation and electrical power systems has attracted much interest. The major advantages of HTS transformers compared with conventional ones are much smaller size and volume and lighter weight in confined spaces of urban areas or transportation devices. One of the most critical parameters to design, fabricate, and commercialize HTS transformers is hysteresis loss in HTS tapes. Using numerical methods, the accurate value of hysteresis loss and efficiency can be calculated or estimated in HTS transformers. In this paper, two innovative techniques to reduce the leakage flux and thereby hysteresis loss in HTS transformers have been proposed. In the first technique, the concentric windings of HTS transformer are apportioned into a certain number of sub-windings. In the second one, with optimizing the distributive ratios of sub-windings, the windings become unsymmetrical. In order to demonstrate the effectiveness of proposed techniques, an accurate and advanced numerical method based on finite element analysis has been used to design four symmetrical and unsymmetrical primary (P)-secondary (S)-P-S-P and P-S-P-S-P-S-P winding schemes. The simulation results prove the effectiveness of both techniques in reducing the radial and axial leakage flux magnitudes and thereby hysteresis loss in HTS windings. [ABSTRACT FROM AUTHOR]

Because high-temperature superconducting (HTS) coils possess low-resistivity, large-current-carrying, and state transition characteristics, the design of HTS transformers is much different from that of conventional ones. The general design of 10.5-kV/1250-kVA HTS transformer is given first. In the electromagnetic design of the 1250-kVA HTS transformer windings via field-circuit coupling simulation, the influence of leakage magnetic field on the critical current of tapes and the circulating current of parallel branches was considered. Moreover, the transient stability of transformer windings combined with superconducting fault current limiter in the HTS power substation is discussed. Based on electromagnetic analysis, the primary and the secondary windings are constructed, and a special transposed method for double pancakes wound with four parallel tapes is proposed. Finally, the current-carrying ability of the primary and the secondary windings is tested in 77 K. Their critical currents are higher than 200 and 3200 A, respectively, which is enough in rated operating condition. [ABSTRACT FROM PUBLISHER]

Because high-temperature superconducting (HTS) coils possess low-resistivity, large-current-carrying, and state transition characteristics, the design of HTS transformers is much different from that of conventional ones. The general design of 10.5-kV/1250-kVA HTS transformer is given first. In the electromagnetic design of the 1250-kVA HTS transformer windings via field-circuit coupling simulation, the influence of leakage magnetic field on the critical current of tapes and the circulating current of parallel branches was considered. Moreover, the transient stability of transformer windings combined with superconducting fault current limiter in the HTS power substation is discussed. Based on electromagnetic analysis, the primary and the secondary windings are constructed, and a special transposed method for double pancakes wound with four parallel tapes is proposed. Finally, the current-carrying ability of the primary and the secondary windings is tested in 77 K. Their critical currents are higher than 200 and 3200 A, respectively, which is enough in rated operating condition. [ABSTRACT FROM PUBLISHER]

Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3. [ABSTRACT FROM AUTHOR]

Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3. [ABSTRACT FROM AUTHOR]

We describe a novel superconducting transformer design for the efficient transfer of magnetic flux from a macroscopic pickup coil to a low-inductance SQUID. A large number of highly-efficient one-to-one transformer elements are wired in series and in parallel, for the primary and secondary of the transformer, respectively. This transformer coupling allows us to isolate the SQUID from the microwave resonances inherent in high-inductance coils. [ABSTRACT FROM AUTHOR]

An ATLAS Barrel Toroid conductor was tested in the Saclay High Current Test Facility. The conductor is a Nb–Ti Rutherford cable imbedded in a high purity aluminum stabilizer. The conductor's width was reduced from 57 mm to 30 mm in order to be able to use an existing sample holder. We tried to measure the critical current in background fields of up to 3 T. The field was produced by a 0.8 m long superconducting dipole magnet. The test station was equipped with a superconducting transformer transferring maximum primary and secondary currents of respectively 174 A and 80 kA. The secondary current was measured with flux coils and with a superconducting Direct Control Current Transducer (DCCT), a modified version of the “Macc+” 600 A commercial DCCT from Hitec, which was operated at currents of up to 57 kA. This paper reports on the performance of the test station, on the results of the quench current measurements performed on the stabilized ATLAS conductor and on the difficulties to measure the critical current of an aluminum stabilizer conductor. [ABSTRACT FROM PUBLISHER]

For a power distribution system with distributed generation (DG) units, its fault current and induced overvoltage under abnormal conditions should be taken into account seriously. In consideration that applying superconducting fault current limiter (SFCL) may be a feasible solution, in this paper, the effects of a voltage compensation type active SFCL on them are studied through theoretical derivation and simulation. The active SFCL is composed of an air-core superconducting transformer and a PWM converter. The magnetic field in the air-core can be controlled by adjusting the converters output current, and then the active SFCLs equivalent impedance can be regulated for current-limitation and possible overvoltage suppression. During the study process, in view of the changes in the locations of the DG units connected to the system, the DG units injection capacities and the fault positions, the active SFCLs current-limiting and overvoltage-suppressing characteristics are both simulated in MATLAB. The simulation results show that the active SFCL can play an obvious role in restraining the fault current and overvoltage, and it can contribute to avoiding damage on the relevant distribution equipment and improve the systems safety and reliability. [ABSTRACT FROM PUBLISHER]

We study microwave radiation emitted by a small voltage-biased Josephson junction connected to a superconducting transmission line. An input–output formalism for the radiation field is established, using a perturbation expansion in the junction's critical current. Using output field operators solved up to the second order, we estimate the spectral density and the second-order coherence of the emitted field. For typical transmission line impedances and at frequencies below the main emission peak at the Josephson frequency, radiation occurs predominantly due to two-photon emission. This emission is characterized by a high degree of photon bunching if detected symmetrically around half of the Josephson frequency. Strong phase fluctuations in the transmission line make related nonclassical phase-dependent amplitude correlations short lived, and there is no steady-state two-mode squeezing. However, the radiation is shown to violate the classical Cauchy–Schwarz inequality of intensity cross-correlations, demonstrating the nonclassicality of the photon pair production in this region. [ABSTRACT FROM AUTHOR]

The urgency of the discussed issue is caused by the widespread use prospect of superconductivity phenomenon in power devices including transformer for losses reduction. The main aim of the study is to investigate the influence of superconducting transformers on electromagnetic transients, to define the current limiting possibility by the superconducting transformers, to reveal the features of current limitation using superconducting transformers. The methods used in the study: calculations using the software package MatLab, ATP EMTP, the use of the superconductivity theory, mathematical modeling in power. The results: the mathematical model of electromagnetic and thermal transients during short-circuit current limiting is developed; the processes of superconducting transformer transition in the normal (nonsuperconducting) state and recovery to the superconducting state after the fault current clearance is simulated; the criterion of superconducting transformer recovery in the superconducting state after fault clearance under load is defined; the possibility of short-circuit current limiting from the viewpoint of providing the required resistance is determined.

Abstract: The efficiency of power transformers should be always taken in account when design and operation conditions parameters are chosen. In high temperature superconducting transformers (HTS transformers), normally cooled by liquid nitrogen at 77 K, cores are usually kept at room temperature in order to minimize total magnetic losses, losing the possibility to use the cooling liquid to minimize electric risks and to cool the core, and increasing the complexity of the cryostats that, in these cases, must only embrace the superconducting windings. This work try to evaluate the magnetic core losses increasing at 77 K, for different magnetic materials, and the possibility of reducing these losses under some specific manipulation of magnetic materials. For this purpose, several low temperature measurements are presented to characterize the magnetic behavior of four electrical steels usually used in transformer cores. The chosen magnetic materials are three crystalline materials, two grain-oriented and one non oriented steel, and an amorphous elaectrical steel. The most significant results show that grain oriented steels have lower losses increasing at cryogenic temperature, comparing with the other two magnetic materials, and that above a certain value of magnetic induction, B, total magnetic losses at 77 K became smaller than room temperature losses. Results interpretation is presented and some suggestions are made concerning production of magnetic materials for applications at 77 K. [Copyright &y& Elsevier]

The ITER Central Solenoid has 36 interpancake joints, 12 bus joints, and 12 feeder joints in the magnet. The joints are required to have resistance below 4 nOhm at 45 kA at 4.5 K. The US ITER Project Office developed two different types of interpancake joints with some variations in details in order to find a better design, qualify the joints, and establish a fabrication process. [ABSTRACT FROM PUBLISHER]

There are a number of AC power devices using high temperature superconductor (HTS) such as Superconducting Fault Current Limiter (SFCL), Superconducting Transformer, AC Superconducting cable and so on. However, the operating current of HTS power devices at alternating current (AC) circumstances is almost decided by critical current which is measured in direct current (DC) circumstances. Applying DC critical current criterion to AC power devices cannot be ideal. So, for AC power devices, a new AC critical current criterion must be needed. Commercial AC frequencies of current grid are 50 and 60 Hz. Therefore, in AC circumstance, the voltage induced by inductance is not negligible. [ABSTRACT FROM PUBLISHER]

EDIPO is the new 12.5 T superconducting test facility that is being installed at CRPP in Villigen, Switzerland. It will be placed beside the existing test facility SULTAN. The main purpose of both facilities is testing Nb3Sn samples for the ITER project. EDIPO will provide a uniform high field region of \sim1 m, i.e. longer than SULTAN's (\sim45 cm). EDIPO includes the main magnet and a superconducting transformer to power it with a current up to 100 kA in the test sample. The energy stored in the dipole is 16 MJ and in the undesired event of a quench this energy must be safely extracted. A Quench Detection System (QDS) is required to trigger the quench protection system for both the dipole and the transformer. This paper describes the overall QDS from the conceptual design, components, manufacture and commissioning as well as interactions with related systems. [ABSTRACT FROM PUBLISHER]

Selection of the applicable 2G superconducting tapes for transformer windings allows the use of a superconducting transformer to limit short-circuit currents in a power network and significantly reduce wear and tear of circuit breakers and other substation power equipment. [ABSTRACT FROM PUBLISHER]

Recently developed 2G superconducting tapes with high resistivity in the normal state at a temperature of 77 K allow us to build transformers with high short-circuit strength. High resistance of the HTS windings in the normal state causes the limitation of short-circuit current to the level depend on the value of that resistance. Rapid limitation of the short-circuit current can be achieved and the steady-state fault current can be limited to a value resulting mainly from the parameters of HTS tapes used. The required reduction of prospective fault current in a power grid can be achieved by installing in it a superconducting transformer with a winding made of 2G HTS tape only if the tape provides proper resistance-per-length in normal state at a temperature of 77 K. This article presents the results of an experimental investigation of a 1-phase model of a superconducting transformer, with special regard to the possibility of limiting the short-circuit current by the superconducting winding, according to the parameters of the HTS tapes used, in particular their resistivity in normal state at a temperature of 77 K. [ABSTRACT FROM PUBLISHER]

The amount of hysteresis losses is a key parameter for the design of high-temperature superconducting (HTS) transformers. Leakage field, particularly its radial component, decreases critical current and increases hysteresis losses in HTS tapes. Prior art proposed the adoption of two auxiliary windings to an HTS transformer for minimizing the leakage flux densities by generating a field that opposes the leakage field, thereby reducing the hysteresis loss in an exemplary HTS transformer. In this paper, a proposed design will become prominent by obtaining a variant of distributive ratios via an intelligent choice of the secondary–primary–secondary–primary turn ratios in the neighborhood of the auxiliary windings to produce magnetic poles canceling or diminishing the corresponding leakage fields to the maximum effect. The mentioned schemes require accurate, yet efficient, advanced numerical techniques using 3-D solver of FLUX software for a precise calculation of the leakage flux density and hysteresis losses in an exemplary HTS transformer as a precious guide. [ABSTRACT FROM AUTHOR]

To meet the performance test of the International Thermonuclear Experimental Reactor correction coil conductors, a 50-kA superconducting transformer has been manufactured, which allows rating of test samples under different operating conditions at the Institute of Plasma Physics of the Chinese Academy of Sciences (ASIPP). This transformer system is made up of two concentric layer-wound superconducting solenoids with the primary inside the secondary. The 50-kA secondary winding consists of 4-turn rectangular cable-in-conduit conductor type. This paper describes the design and manufacture techniques of the 50-kA transformer, including a secondary conductor. The secondary terminal design and manufacture are also discussed. Finally, the first test results of the 50-kA transformer are presented. [ABSTRACT FROM AUTHOR]

This paper presents the design process and test results of a high-temperature superconducting pulsed-power transformer (HTSPPT). The primary winding of the HTSPPT is constructed out of several double-pancake-type coils connected in series. Every double-pancake-type coil is made of Bi2223/Ag tapes. The secondary winding is constructed out of several single-pancake-type coils made of normal conductors. The configuration of the HTSPPT is designed such that the primary and secondary coils are arranged in interleaving. The testing experiments show that a high-current impulse could be achieved with a small current in the primary, which proves that the designed test circuit and the configuration of HTSPPT are feasible. [ABSTRACT FROM AUTHOR]

Rapid changes and developments are being witnessed in the transformer design technologies. The phenomenal growth of power systems has put tremendous responsibilities on the industry to supply reliable and cost-effective transformers. The advent of high-temperature superconductor (HTS) materials has increased interest in research and development of superconducting transformers with major projects being carried out worldwide. The major challenges in the design and development of HTS transformers are the modeling of short-circuit and inrush currents the transformer can withstand. Even though HTS technology is claimed to be more efficient, reliable, and eco-friendly, use of HTS transformers must be appropriately verified through the proper modeling of power system network. [ABSTRACT FROM AUTHOR]

The ITER Central Solenoid has 36 interpancake joints and 12 bus joints in the magnet. The joints are required to have resistance below 4 nOhm at 45 kA at 4.5 K. The interpancake joints will have to withstand a significant cyclic tensile strain during ITER operation. The joints are located at the OD of the magnet embedded in the winding pack and therefore need to be compliant with the rest of the winding pack to avoid excessive stress. [ABSTRACT FROM AUTHOR]

We present design details for the planned construction of a 3-phase 1 MVA 11 kV/415 V transformer using YBCO Roebel cable (YBCO—Yttrium Barium Copper Oxide second generation high temperature superconducting coated conductor). The YBCO Roebel cable is a promising technology for carrying high currents. It simplifies manufacture of the winding while managing AC loss. We present the transformer winding design which follows a simple layout with the low-voltage windings utilizing a 15 strand \times 5 mm (15/5) Roebel cable in a single-layer 20 turn solenoid. The target rated current capacity of the cable is 1500 A rms at 77 K. The high-voltage winding will be in the form of a stack of double pancake coils arranged on a composite former outside the low-voltage windings. The coils will be immersed in sub-cooled liquid nitrogen with a target maximum operating temperature of 70 K. The development methodology is described along with the results of experiments and modeling to validate the performance characteristics of the windings. [ABSTRACT FROM AUTHOR]

The HTS transformer is one of the promising HTS power applications that are expected to be commercialized in the near future. Among the auxiliary functions of a transformer, the voltage regulation function is known to be the most important one. For the voltage regulation of HTS transformers, they are normally equipped with the on-load tap changers (OLTC). It is also required that, in order to develop a HTS transformer as a complete system, not only the basic technologies for all the components including OLTC and bushing, but also the technologies for the insulation of these components at a cryogenic temperature should be well established as applicable. Taking such facts into account, the breakdown characteristics of the insulation components have been evaluated for such application areas as turn-to-turn insulation, layer-to-layer insulation, winding-to-winding insulation, drive shaft-to-drive shaft insulation, tap connecting terminal insulation and others. In overall, this paper discusses the electric insulation design and experimental results with regard to HTS transformer, OLTC and bushing. [ABSTRACT FROM AUTHOR]

High temperature superconducting transformers offer many economic, operational, and environmental benefits over conventional power transformers for utility applications. To establish the technical and economic feasibility and benefits of HTS transformers of medium-to-large (>10 MVA) ratings, a team that includes Waukesha Electric Systems (transformer manufacturer), SuperPower, Inc. (HTS systems manufacturer), Oak Ridge National Lab, and Energy East has designed, built and tested a prototype 5/10 MVA superconducting transformer. The transformer's 4.5-ton cold mass has been successfully maintained at temperatures of 30-50 K for several months, without full-lime operator attendance. The transformer has reached its full three-phase operating current, and has been tested to 1.4 limes operating current (limited by available power supplies) in single-phase mode. It is now undergoing long-term tests at various current levels, high-voltage tests, and transient overcurrent tests at the Waukesha site. This paper summarizes the manufacturing, cooldown, and the test results achieved to date. [ABSTRACT FROM AUTHOR]

The interest in superconducting systems stems from their promise to be more efficient, smaller, and lighter than those made from conventional conductors. The types of applications in which superconductivity has the potential to be effective in an electric power system can be separated into two general classes. The first type includes those technologies in which superconductivity is simply a replacement of existing resistive materials, for example, cables, motors, generators, and transformers. The second type includes technologies that will be enabled by superconductivity and that have little or, at most, limited capability if conventional resistive or other materials are used. Examples are superconducting magnetic energy storage (SMES) and large fault current limiters (FCL). Before looking at the applications under development the article discusses the discovery and development of superconductivity [ABSTRACT FROM PUBLISHER]

Practical applications of high-temperature superconductors will be governed by their current transport and mechanical characteristics. With careful control of the processing parameters high-quality long-length mono- and multifilament Bi-2223 conductors have been fabricated by the powder-in-tube technique. A critical current density (J/sub c/) of up to 1.2/spl times/10/sup 4/ A cm/sup -2/ has been observed at 77 K in a 1260-m-long multifilament conductor containing 37 monocore filaments. A high-temperature superconducting magnet and a prototype transformer were fabricated and characterized from such long-length conductors. Extensive studies on the in-situ strain characteristics of the mono- and multifilament conductors have been conducted. Multilayer silver/superconductor composite tapes, fabricated by a novel chemical etching technique, were also observed to exhibit improved strain tolerance characteristics. To further improve the current characteristics of the Bi-2223 tapes the wire-in-tube technique was adopted. A J/sub c/ value >10/sup 5/ A cm/sup -2/ at 77 K and in self field have been obtained in a Bi-2223 tape fabricated by the wire-in-tube method. [ABSTRACT FROM PUBLISHER]

Silver-sheathed BiPbSrCaCuO 2223 superconducting wires with long length and high Jc of over 10/sup 4/ A/cm/sup 2/ were developed by using the powder-in-tube method. Future possibilities to obtain much higher Jc's are discussed in relation to crystal alignment, connectivity between grains and flux pinning. High amperage wires and high strength wires were also developed for large scale magnet application. High I/sub c/ of over 300 A at 77 K were obtained for the wire with large cross sectional area. Good stress and strain tolerant characteristics were also obtained for silver alloy sheathed wires. In order to apply the HTS wires for AC use, it is necessary to reduce AC loss. So, the AC loss of the wires are also discussed. As a progress of wire technology, we have fabricated many types of magnets, such as pancake magnets and solenoidal magnets. In the case of 77 K application, it is promising to apply for AC use because of large heat capacity of HTS wires and liquid nitrogen. So, we are developing the 500 kVA transformer and pulse magnet for SMES. A refrigerator cooled magnet operated near 20K was fabricated. This magnet was operated at 21 K and generated 3 T inside a /spl phi/40 mm room temperature bore, stably generated 2.5 T continuously for over 150 hours and could be operated at high ramp rate of over 12 T/min. As for the 4.2 K operation, high field insert magnet for 1 GHz NMR application are developed. Highest magnetic field of 24 T was achieved using the hybrid magnet at MIT and persistent current mode operation was done by using the layer wound coil with persistent current switch. [ABSTRACT FROM PUBLISHER]

The use of high T/sub c/ SQUID (HTSQUID) magnetometers for magnetocardiography (MCG) and nondestructive testing (NDT) requires effective shielding from magnetic noise fields. The authors have investigated the shielding characteristics of high-T/sub c/ superconducting (HTS) material and developed a compact, portable system consisting of several HTS components. A process for fabricating these components out of Bi-Pb-Sr-Ca-Cu-O has been developed. Tubes and disks of various sizes have been formed by cold isostatic pressing (CIP) and sintered at reduced oxygen pressures. The HTS tubes exhibit shielding factors of up to 10/sup 3/ at low frequencies. A novel technique for fabricating superconducting flux transformers (specifically, first-order gradiometers) has also been developed. The flux transfer efficiencies of these first-order gradiometers have been evaluated. The tubes, gradiometers, and flux focusers have been incorporated into an HTSQUID setup. The authors find that the low-frequency noise rejection obtained in their setup is very promising for potential MCG and NDT applications in an unshielded environment. [ABSTRACT FROM PUBLISHER]

The effect of layout modifications of large-area single-layer YBa/sub 2/Cu/sub 3/O/sub 7/ flux transformers onto the effective area and system noise has been investigated systematically and the excellent performance of sputtered large-area flux transformers is demonstrated. First, the gain of sensitivity and noise contribution of the transformer (coupled to a 8/spl times/8 mm/sup 2/ washer rf-SQUID) was determined as a function of the line width of the YBa/sub 2/Cu/sub 3/O/sub 7/ pickup loop w/sub p/ ranging between 1 and 8.2 mm for 1" transformer (O=25.4 mm) and 1 and 19 mm for 2" transformer (O=50.8 mm), respectively. Leaving the diameter of the pickup coil (b/sub 1/=23 mm, b/sub 2/=46 mm) and the coupling coil (d=6 mm, width=1.2 mm) unchanged, the gain in sensitivity increases from 2.1 for w/sub p/=1 mm to 2.7 for w/sub p//spl ges/6 mm in the case of 1" transformers and from 3.3 for w/sub p/=1 mm to 4.6 for w/sub p/=19 mm for 2" transformers. No additional noise contribution of the transformer could be observed, i.e. a flux noise of 70 /spl mu//spl Phi//sub 0//spl radic/(Hz) was determined above 5 Hz and at 77 K. The best values for the field-to-flux conversion efficiency and the effective area were 0.74 nT//spl Phi//sub 0/ and 2.98 mm/sup 2/ for 1" and 0.43 nT//spl Phi//sub 0/ and 4.94 mm/sup 2/ for 2" transformers, respectively. The comparison of the experimental data with the theory indicates, that the expressions derived for low-T/sub c/ superconducting planar devices have to be modified in the case of high-T/sub c/ devices. Second, the performance of the sputter deposited large-area devices is demonstrated for the case of an axial gradiometer. The white noise level of the whole gradiometer system in an unshielded environment was <100 fT//spl radic/(Hz) for frequencies above 5 Hz and the common mode rejection of the homogeneous field was measured to be better than 10/sup 4/. Real-time magnetocardiographic signals obtained via this gradiometer in a magnetically unshielded environment show a large signal-to-noise ratio. [ABSTRACT FROM PUBLISHER]