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.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

Proposal of DC shield reactor type superconducting fault current limiter

Saturated DC reactor type superconducting fault current limiter (SFCL) had been proposed two years ago. It was classified to rectifier type SFCL. The changing inductance value with the operating mode has superior characteristics to reduce voltage sag during step increase of the load current. But it has the disadvantage of its weight. In this paper, rectifier type SFCL with shielded reactor has been proposed. The reactor which has superconducting ring or tube inside its winding is substituted to the DC link of the rectifier. The configuration looks like an air core transformer with secondary short winding. When the current through the bulk shield-ring reaches to a certain level, the flux penetrates to the shield body and finite impedance appears in the primary winding. In other words, when the surface flux density exceeds its critical flux density, the flux penetrates into the bulk superconductor, and increases equivalent inductance. The equivalent transient resistance of the shield was represented as a function of exponential of the time. Using this equivalent transient resistance, the transient impedance was expressed. The transient wave analysis using EMTDC (electro-magnetic transients in DC systems) has been described. Simulated waveforms are shown considering the source inductance, the leakage inductance, the coupling coefficient and the forward voltage drop of the semiconductor. And voltage sag was also investigated with 50% step load increase.Preliminary design was also performed. The coil size and number of turns are designed to obtain adequate inductance for the current limitation, and the central magnetic field of the coils are calculated. There is optimal aspect ratio to minimize the magnetic field with restriction in outer diameter of the coil.     

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

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

Experimental study of a transformer with superconducting elements for fault current limitation and energy redistribution

Numerous proposed and developed superconducting fault current limiters and self-limiting transformers limit successfully fault currents but do not provide uninterrupted supplying of consumers. A design investigated in the work combines the functions of a conventional transformer with the functions of fast energy redistribution and fault protection. The device constitutes a transformer containing an additional high-temperature superconducting (HTS) coil short-circuited by a thin film HTS switching element. Fault current limitation and redistribution of the power flow to a standby line are achieved as a result of a fast transition of the superconducting switching element from the superconducting into the normal state. Transient and steady-state characteristics were experimentally investigated. A mathematical model of the device operation was proposed, and the calculated results were found to be in good agreement with the experimental data. The application field and basic requirements to such devices were discussed and it was shown that the proposed device meets these requirements.     

Experimental and numerical analysis of high resistive coated conductor for conceptual design of fault current limiter

Resistive superconducting fault current limiters (FCLs) using coated conductor (CC) have been developed to reduce fault current which exceeds ratings of circuit breaker in power grid. Our group has participated in the development of distribution level non-inductive winding type FCL using stainless steel-stabilized CC, as one of the 21st century Frontier R&D program. Recently, stabilizer-free CC with Hastelloy substrate was developed for FCL application. Since the CC has higher average resistivity than existing CCs with metallic stabilizer, required amount of wire can be reduced. Short-circuit tests were performed by increasing voltage applied to the small-scale FCL coil using stabilizer-free CC in sub-cooled liquid nitrogen of 65 K. Experimental results of the tests were compared with numerical analysis of current limiting characteristics of the CC by using finite element method (FEM). Conceptual design of the FCL was performed using test results and was compared with FCL using existing CC in regards to current limiting characteristics.     

Magnetization and rotation of MTG HTSC ring in magnetic field

The magnetization of a melt-texture growth (MTG) HTSC ring has been studied. It is shown that the magnetic field inside the ring is larger than the external field under a certain range of external magnetic fields. We have also investigated the magnetic field dependence of the response of a detective coil near a rotating superconducting ring. The responses of the MTG sample are different for different cooling methods.     

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.     

Irreversible change of surface impedance of high-Tc thin films due to D.C. magnetic field commutation

The magnetic field dependence of the surface impedance (resistance and reactance) of high-T _c thin films is found employing measurements of the quality factor and the frequency of the parallel plate resonator in a dc magnetic field up to 500 Oe at 10 GHz.c-Oriented YBa₂Cu₃O₇ thin films are examined. Enhancement of surface resistance and inductance with increase of magnetic field is observed. The effect of irreversible increase of surface impedance as compared to its initial values after a cycle of magnetic field commutation is found. A qualitative explanation of the observed effects based on the picture of magnetic vortex penetration and accumulation in the film due to strong pinning is presented.     

NMR detection with multiple solenoidal microcoils for continuous-flow capillary electrophoresis

Nuclear magnetic resonance (NMR) spectroscopy represents a promising on-line detector for capillary electrophoresis (CE). The inherent poor sensitivity of NMR mandates the use of NMR probes with the highest mass sensitivity, such as those containing solenoidal microcoils, for CE/NMR hyphenation. However, electrophoretic current degrades the resolution of NMR spectra obtained from solenoidal coils. A new method to avoid microcoil NMR spectral degradation during continuous-flow CE is demonstrated using a unique multiple solenoidal coil NMR probe. The electrophoretic flow from a single separation capillary is split into multiple outlets, each possessing its own NMR detection coil. While the CE electrophoretic flow is directed through one outlet, stopped-flow, high-resolution NMR spectra are obtained from the coil at the other outlet. The electrophoretic flow and NMR measurements are cycled between the outlets to allow a continuous CE separation with "stopped-flow" detection. As a new approach for improving multiple coil probe performance, the magnetic field homogeneity is automatically adjusted (via the shim coils of the magnet) for the active coil. The multiple microcoil CE/NMR coupling has been used to analyze a <3 nmole mixture of amines while obtaining between 1 and 2 Hz line width, demonstrating the ability to avoid electrophoretic current-induced line broadening.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

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

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

Critical current test results of 13 T-46 kA Nb3Al cable-in-conduit conductor

In the framework of ITER-EDA, a 13 T-46 kA Nb₃Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb₃Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb₃Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb₃Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb₃Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section.     

Analytical solutions to eddy current in carbon fiber-reinforced composites induced by line current

This paper presents analytical solutions to eddy current distribution in carbon fiber-reinforced plastics induced by line current. Derived solutions show that eddy current distribution in unidirectional carbon fiber composite is dependent only on electrical conductivity in the drive current direction when the drive current is directed in the fiber direction or the transverse direction. Moreover, according to the derived analytical solution, skin depth of eddy current depends not only on frequency of drive current but on lift-off and width of the drive current unlike general expression of skin depth. Finally, we discuss improvement of sensitivity of eddy current conductivity sensor using analytical solutions. It is found that analytical solutions can clearly divide effects of magnetic field from drive current and that from eddy current. Sensitivity of eddy current sensor can be improved by 10 times placing pickup coil at region where the effect of magnetic field from eddy current is large.     

Losses in a multifilamentary superconducting wire caused by a simultaneous sweep of current and magnetic field

A theoretical discussion is presented on the energy loss in a multi-filamentary superconducting wire when an applied transport current and an external transverse magnetic field are varied simultaneously with a repeating pulsive wave form. In the present calculation, the effects of the ‘uniforming time constant’ which has been introduced by the authors as a characteristic time constant for the change in the transport-current distribution inside the wire is taken into account, together with the field dependence of the critical current density of superconducting filaments.Thus the present analytic expression for the energy loss of multi-filamentary wire is available to the whole range of the external magnetic field. It is shown that the contribution of the dynamic resistance loss to the total loss is strongly dependent on the position of the wire inside a coil.     

溶胶-凝胶法制备Bi0.975La0.025Fe0.975Ni0.025O3铁电薄膜的结构及物理性能

采用溶胶-凝胶法在Pt/Ti/SiO_2/Si(111)衬底上制备了Bi_(0.975)La_(0.025)Fe_(0.975)Ni_(0.025)O_3(BLFNO)铁电薄膜。利用X射线衍射(XRD)、原子力显微镜(AFM)及其压电模式(PFM)对薄膜的晶体结构、表面形貌以及铁电畴结构进行了研究。研究发现,BLFNO为结晶良好的钙钛矿结构多晶薄膜,且薄膜表面颗粒生长均匀。PFM测试图显示铁电薄膜在自发极化下的铁电畴结构清晰,铁电电容器具有良好的铁电性能。应用铁电测试仪对Pt/BLFNO/Pt电容器进行测量,得到了饱和性良好的电滞回线。在828kV/cm的外加电场下,Pt/BLFNO/Pt电容器的剩余极化强度为74.3μC/cm~2,表明La、Ni的共掺杂没有明显抑制铁电电容器的剩余极化强度,铁电电容器具有良好的铁电性能。漏电流研究结果表明,La、Ni元素的共掺杂有效降低了薄膜的漏电流密度,在277.8kV/cm外加电场下漏电流密度在10-4 A/cm2量级,明显小于纯BFO薄膜的漏电流密度。正半支漏电流曲线满足SCLC导电机制,对于负半支曲线,当电场强度大于22.2kV/cm时,同样遵循SCLC导电机制;但是,当电场强度小于22.2kV/cm时,曲线斜率约为4.8,表明参与导电贡献的电子数较多,归因于极浅陷阱俘获的电子在外加电场作用下参与了导电行为。室温下磁滞回线测试结果表明BLFNO薄膜具有反铁磁性质。     

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.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

Impedance transition and series resonance between bulk plasma and sheath in biased inductively coupled plasma

The impedance transition and electron series resonance at an RF bias substrate were observed in 13.56 MHz inductively coupled plasma (ICP). As ICP coil power increased, the impedance of the RF bias transitioned from a capacitive to an inductive load. When bias voltages and discharge impedances reached minimums, bias voltages and currents were in-phase during the transition. The transition can be understood as a series LC resonance between the sheaths (capacitor) and plasma bulks (inductance due to electron inertia). This corresponds to the electron series resonance (ESR) observed in very high-frequency capacitive discharges, and a new ESR frequency which considers sheath resistances is presented.     

Fe基纳米晶粉芯磁致频移特性的研究

测量了Fe基纳米晶粉芯线圈与电容并联成LC回路的共振频率 (fr) ,研究了fr 随外加磁场变化的特性 ,发现fr 随外加磁场有显著变化。在 0— 5 6kA m范围内有很好的线性和灵敏度 ,可以用于磁场测量