Analysis on the Fusion Technology of a Commercial Transformer and an SFCL

In this study, a new concept is proposed on a combination device with functions of a commercial transformer and a superconducting fault current limiter (SFCL). This device serves as a transformer by stepping the voltage up or down in normal condition. When a transient phenomenon occurs in the power system, it serves as an SFCL to limit the fault current. The device quickly detects the line current using a current transformer (CT), and is based on the high-speed, silicon-controlled rectifier (SCR) interrupter operation. This is done by identifying the fault using an SCR switching control system. The test results showed that the fault current was limited by the impedance of the superconducting element a half cycle after a fault occurred. An SCR that initially had a normally open contact was turned on within a half cycle. However, an SCR with a normally close contact was turned off after a half cycle because the current dropped below the holding current after a half cycle. The voltage of the superconducting element was low in the step-down turn ratio condition of the transformer. This was because the secondary and tertiary windings were connected in series due to the SCR-1 turn-off condition, and the sum of voltages on each winding appeared on the superconducting element. By combining the existing power device technology and an SFCL technology, it is expected that the existing problems of an SFCL can be addressed to construct a smart power system.     

Performance of TES X-ray Microcalorimeters with AC Bias Read-Out at MHz Frequencies

At SRON we are developing Frequency Domain Multiplexing for the read-out of superconducting transition edge sensor microcalorimeters for future X-ray astrophysical missions. We will report on the performance of Goddard Space Flight Center pixels under AC bias in the MHz frequency range. Superconducting flux transformers are used to improve the impedance matching between the low ohmic TESs and the SQUID. We connected 5 pixels to the LC filters with resonant frequencies ranging between 1 and 5 MHz. For X-ray photons of 6 keV we measured a best X-ray energy resolution of 3.6 eV at 1.4 MHz, consistent with the integrated Noise Equivalent Power. In addition, we improved the electrical circuit by optimizing the coupling ratio of the impedance matching transformer. In addition, we improved electrical circuit for impedance matching; modified transformer coupling ratio. As a result, we got the integrated noise equivalent power resolution of 2.7 eV at 2.5 MHz. A characterization of the detector response as a function of the AC bias voltage, bias frequency and the applied magnetic field is presented.     

DC reactor type transformer inrush current limiter

A new inrush current limiter (ICL) is presented to limit the inrush current of transformers. The proposed ICL consists of three similar sets. Each set includes a diode-bridge and a single DC reactor and is connected in series with the individual phases of transformer. The ICL has almost no effect on normal operation of transformer. It needs no control, measurement and gate driving system and has a simple power circuit topology. The equivalent instantaneous inductance of transformer is used for analysis of circuit operation. The theoretical analysis, design features, power losses and voltage distortion because of using ICL are presented. The proposed method has been tested by simulation and laboratory experiments. Both results show that the proposed ICL successfully limits the inrush current.     

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

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

Reduction of fault current peak in an inductive high-Tc superconducting fault current limiter

This paper dealt with current-limiting performances of an inductive high-T_c superconducting fault current limiter with an auxiliary coil. The fault current limiter mainly consists of the primary copper coil, secondary high-T_c superconducting rings, and auxiliary high-T_c superconducting coils, which are magnetically coupled through three-legged core. The superconducting fault current limiter as a series element in the power system is inserted to limit the fault current. The device presents fast variable-impedance features in the event of a fault condition. The fault current peak can become relatively large for certain ranges of the flux and the fault instant due to the core saturation. The auxiliary coil proposed in this paper was proven to increase the impedance of the SFCL up to more than 31% while preventing the core saturation.     

Effect of Current Polarity on BSCCO/Ag Ceramics Textured by Electrically Assisted Laser Floating Zone

Bi-2212/Ag superconducting ceramics were grown using the electrically assisted laser floating zone (EALFZ) technique. The main goal was improving the superconducting properties by applying an electrical current through the sample during the solidification process. The effect of a 100 mA current intensities in two configurations were studied: (i) direct current (connecting the seed rod to the positive pole and the feed to the negative one) and (ii) reverse current (positive pole connected to the feed rod and negative to the seed one). The resulting textured cylindrical bars were annealed in order to obtain the superconducting phases. A strong correlation between current polarity, grain alignment and superconducting properties was observed. The rods grown under direct current exhibit higher critical current density (～3000 A/cm²) than the ones grown under conventional method (～2300 A/cm²) or under reverse current condition (～35 A/cm²). The superior value obtained for the direct current results from a better grain alignment and fewer amount of second phases.     

Field test of 70 MW class superconducting generator

Super-GM and the Kansai Electric Power Co., tested a 70 MW class model superconducting generator in actual power system. The model machine, operated as a rotary condenser, was connected to a 77 kV commercial power grid through 77/6.6 kV transformers. It supplied 40 MVar of leading reactive power to the electric power system. The model machine showed an ability to operate steadily through all the tests and various load changes in the commercial electric power system. The test results proved that the small synchronous reactance of the superconducting generator enhances voltage stabilization and their leading reactive capacity is larger, compared with conventional generators.     

Development of a Superconducting Magnet System with Zero Liquid Helium Boil-off

A homogeneous magnetic field superconducting magnet with a cold bore of 250 mm and a central field of 4.3 T has been designed, manufactured, and tested with zero liquid helium boil-off. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils: one main coil and two compensation coils. All coils are connected in series and can be charged with a single power supply. The magnetic field homogeneity is about ±3.0 % from ?200 mm to 200 mm in axial direction with 86 mm in diameter. The magnet can be operated in persistent mode with a superconducting switch. A two-stage GM cryocooler with a capacity of 1.5 W at 4.2 K was used to cool the superconducting magnet. The cryocooler prevents the liquid helium from boiling off and leads to zero helium loss during static operation. The magnet can be operated in liquid helium circumstance by cooling the gas helium with the cryocooler without additional supply of liquid helium. Under this condition, the magnet is successfully operated up to 4 T without quench. The magnet system can be generating 0.25 L/h liquid helium with the cryocooler by supplying the gas helium without loading the magnet. In this paper, the magnet design, manufacture, mechanical behavior analysis, and the performance test results of the magnet are presented.     

Dc bias system of a 35 kV/90 MVA saturated iron core SFCL

Saturated iron core superconducting fault current limiter (SIC-SFCL) is a promising fault current limiting device for high or extra-high voltage power grids. It has low impedance in normal power transmission and turns high impedance when a short-circuit takes place. The dc bias system of a saturated iron core superconducting fault current limiter plays a key role in realizing these features. Our 35 kV/90 MVA SIC-SFCL has been running live-grid since January 2008 at Puji substation in Yunnan, China. In this paper, the working principle of the dc bias system will be introduced and results of artificially imposed short-circuit tests will be provided, which verifies the validity of this system.     

A Method for Driving an Oscillator at a Quasi-Uniform Velocity

We describe a simple way to drive an actuator, comprising a superconducting coil moving in a static magnetic field, at a quasi-uniform velocity. The main objective is to avoid oscillations in this system with low damping, as they undermine the uniformity of the velocity. The method consists in calculating the force that should be exerted on the coil from the equation of motion and programming a waveform generator to produce the corresponding current through the coil. The method was tested on a device towing a grid through a closely fitted channel filled with superfluid ⁴He at temperatures below 100 mK. The motion of the grid over the distance of 4.3 cm at 10?cm/s resulted in oscillations of less than 50 μm in amplitude (or less than 1 mm/s in terms of velocity). The method can be applied to any oscillator.     

Development of a superconducting fault current limiter using various high-speed circuit breakers

The authors constructed and tested a model superconducting fault current limiter (SFCL) using a high-temperature superconducting film according to a design that includes a vacuum interrupter with an electromagnetic repulsion mechanism. The superconductor and the vacuum interrupter are connected in parallel with a bypass coil. If a fault occurs and current flows through the system, the superconductor is quenched and the current is transferred to the parallel coil because of the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter. On opening the vacuum interrupter, the current in the superconductor is interrupted. This model is expected to exhibit very low-energy consumption by the superconductor. The authors succeeded in interrupting the current flowing in the superconductor within a half-cycle using a prototype SFCL. An improved SFCL with higher voltage and current ranges was used to carry out current-limiting tests and to investigate the possibility of adapting our SFCL in a power system. The authors also carried out a currentlimiting test using a conventional high-speed vacuum circuit breaker (HSVCB) as a new method for realising our concept.     

Research and development of superconducting cable in Super-ACE project

Research and development of fundamental technologies for superconducting AC power equipments (called Super-ACE) project started as a national project in 2000 fiscal year by Ministry of Economy, Trade and Industry and New Energy and Industrial Technology Development Organization. This project is to research and develop the basic technology of high temperature superconducting cable (HTS cable), fault current limiter, and transformer. Main subjects of the cable in this project are to study the 3 kA superconducting cable conductor, cooling technology of 500 m HTS cable, and power system analysis of cable and AC equipments. The present paper describes the background of this project, target of each research subject, schedule of research and development, and major results of research activity in 2001.     

Development,Updating and Long-Term Operations of a 10.5 kV HTS Fault Current Limiter

There are many irreplaceable advantages of high temperature superconducting (HTS) fault current limiter, applying in electric utilities. It is expected to be able to solve excessive fault current problems and to enhance safety and stability of power systems. In this paper, the R and D of an improved bridge-type 10.5 kV three-phase superconducting fault current limiter (SFCL) was present. Each phase adopted a HTS coil with inductance of 6.2 mH. The three coils wound with 8,571 m Bi2223/Ag tapes totally. After installation at a 110 kV/10.5 kV substation in Hunan Province of China, the performances of the whole SFCL were tested, including a three-phase-to-ground short circuit experiment at the pre-setup short circuit point. And then, the SFCL was put into operations in the 10.5 kV power grids for more than 11000 h. About 3 years later, the SFCL was moved, reinstalled, and put into operation again since February 16 of 2011 at a 10.5-kV superconducting power substation located in Baiyin, Gansu Province of China. In this paper, the redesigned and manufactured cryostats with nonmagnetic stainless steel, tests, and long-term operations of the SFCL in 10.5 kV power grids were also described in detail.     

Design,fabrication techniques and test results of 1.2 kV/80 A inductive fault current limiter by using conduction-cooled system

An inductive superconducting fault current limiter protects power system by limiting the amplitude of fault current by the inductance of its dc reactor. Therefore, it is very important to design the dc reactor of high critical current prior to fabrication. At first, the optimal design parameters were calculated by using finite element method and then the superconducting dc reactor for 1.2 kV/80 A_rms inductive superconducting fault current limiter was designed by considering the conduction-cooling characteristics. Moreover, the design, fabrication and conduction-cooling method of the superconducting dc reactor were introduced. Actually, the superconducting dc reactor was fabricated and cooled down to 20 K by using GM cryocooler. Finally, the short-circuit test was performed and the experimental results were discussed.     

Current Sensitivity Enhancement of a Quasi-One-Junction SQUID Comparator as an Input Circuit of SFQ Readout Circuit for a Superconducting Detector

We evaluated the current sensitivity of a quasi-one-junction SQUID (QOS) comparator with an input transformer at 4.2 K. A comparator based on a QOS is promising for constructing the single flux quantum (SFQ) readout circuits of an array system of multiple superconducting detectors.The QOS comparator is made of three Nb/AlO \(_x\) /Nb Josephson junctions, senses an output signal of a superconductor detector, and generates the SFQ pulses.There are strong demands for enhancing the current sensitivity of the QOS comparator because an output current from superconducting detector typically remains at the magnitude of a few \(\upmu \) A. We fabricated the QOS comparator with an input transformer using AIST Standard Process 2, where the critical current density of the Josephson junctions is chosen as high as 2.5 kA/cm \(^{2}\) . We designed the input transformer to enhance the current sensitivity under the conditions of 200  \(\upmu \) m  \(\times \)  200  \(\upmu \) m in size and 20:1 in turn ratio. Consequently, we succeeded in reducing a gray zone width of the comparator, and achieved the current sensitivity of 400 nA at 4.2 K in the low frequency range.     

Resistance varying characteristics of DC superconducting fault current limiter in MTDC system

Recent years, voltage source converter-based multi-terminal high voltage DC power transmission (MTDC) is widely developed in the world. However, it is difficult for the existing DC breaker to cut off the fault transmission line with large short-circuit fault current. Then, it would be helpful to develop DC fault current limiter for the MTDC system. In this paper, DC superconducting fault current limiter (DCSFCL) is proposed to limit fault current. In order to study the resistance-time performance of the DCSFCL under the rapid change of fault current, a simulation model of Zhoushan MTDC system with DCSFCL is established, and the current-limiting performance of the DCSFCL at different location of the grid is studied. The simulation results show that DCSFCL can effectively limit short-circuit current and improve the operation reliability of MTDC system.     

Empirical Modeling of Cryogenic System for Hybrid SFCL Using Support Vector Regression

The hybrid superconducting fault current limiter (SFCL) is now at the stage of practical use in a power grid in Korea. A cryogenic cooling system was designed, fabricated, and successfully tested for a prototype of 22.9 kV/630 A SFCL. The operation scheme of cryogenic system has been investigated in preparation for temporary loss of cryocooler power in hybrid SFCL (in Kim et al., IEEE Trans. Appl. Supercond. 21(3):1284–1287, 2011). In this paper, we investigated the empirical modeling of cryogenic cooling system for SFCL using principal components and auto-associative support vector regression (PCSVR) for the prediction and fault detection of the cryogenic cooling system. For empirical model, data were acquired during a blackout test of cryogenic cooling system. Blackout times of the test were 1 hour and 4 hours at two operation current levels. Three set of data were used for training and optimization of the model and the rest set of data was used for verification. Signals for the model are temperatures measured at copper band and cold head of cryocooler, system pressure and liquid temperatures measured at two locations in liquid-nitrogen pool. For optimization of the SVR parameters, the response surface method (RSM) and particle swarm optimization (PSO) were adopted in this paper. After developing the empirical model we analyzed the accuracy of the model. Also, these results were compared with that of auto-associative neural networks (AANN). RSM and PSO gave almost the same optimum point. PCSVR showed much better performance than AANN in accuracy aspects. Moreover, this model can be used for the prognosis of cryogenic cooling system for SFCL.     

Structure and magnetic properties of Fe films with pyramid-like nanostructures deposited by oblique target direct current magnetron sputtering

Fe thin films were deposited by oblique target direct current magnetron sputtering on Si (100) and (111) substrates. The structure, surface morphology and magnetic properties of the thin films were characterized using X-ray diffraction, field emission scanning electron microscopy, and superconducting quantum interference device magnetometer, respectively. The results reveal that the structure of the as-deposited Fe thin films is body-centered cubic with the preferential [110] crystalline orientation. A pyramid-like nanostructure with sharp tip was formed on the surfaces of Fe thin films under appropriate sputtering power. Formation of the pyramid-like nanostructure is mainly owed to the enhancement of atomic mobility and the bombardment effect with increasing of sputtering power. Meanwhile, the crystalline orientation of Si substrate and the intrinsic stress in the films are expected to have little contribution to the formation of the pyramid-like nanostructure. The magnetic anisotropy was found in the as-deposited Fe thin films, and varies with the thickness of the Fe thin films. As the film thickness increases from 604 to 1,786 nm, the magnetic anisotropy field and the uniaxial anisotropy constant increase from 3.8 to 5.6 kOe, and from 0.4 × 10⁶ to 1.1 × 10⁶ erg/cm³, respectively, which indicates that besides magnetocrystalline anisotropy, stress induced anisotropy and shape anisotropy also exist in the as-deposited Fe thin films.     

Comparative Study of Resistive and Inductive Superconducting Fault Current Limiters SFCL for Power System Transient Stability Improvement

This paper presents a comparative study of resistive and inductive superconducting fault current limiter (SFCL) for power systems transient stability improvement. Two applications of transient stability assessment are presented in this paper: The first shows the efficiency of the resistive and inductive SFCL in series with a generator, the second uses SFCL installed in series with a transmission line. SFCL can just be operated during the period from the fault occurrence to the fault clearing; the modeling and the effect of SFCL has been investigated to have higher benefits for the power system. In the present work, modification of the admittance matrix method is used for modeling of SFCL; Critical Clearing Time (CCT) has been used as an index for evaluated transient stability. The transient stability is assessed by the criterion of relative rotor angles, using the Runge–Kutta method. The effectiveness of the proposed method is tested on the WSCC3 nine-bus system applied to the case of three-phase short circuit fault in one transmission line. A simulation and comparison are presented in this document.     

Production and superconducting properties of laboratory scale NbTi and NbTiTa multifilamentary cables

A comparison of high field, low temperature superconducting properties between a Nb47^w/o Ti and two NbTiTa alloys as bases of laboratory scale cables was carried out Optimization cycles needed to obtain fair superconducting properties imply intermediate thermal treatments in the range 380–400°C during deformation at cold-work of 70%. The ternary alloys exhibit excellent ductile properties as do NbTi alloys. After an adequate optimization cycle the alloy Nb38^w/o Ti34^w/o Ta showed the best high field superconducting properties with values of more than 600 A mm^?2 at 12 Tesla and 2.15 K.