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.     

Characteristics of critical current of superconducting solenoid wound with the stacked tape

The high-T_c superconducting (HTS) magnet is an important element for developing HTS power equipments such as the dc reactor of the inductive type superconducting fault current limiter (SFCL). In order to use the HTS magnet for the large-scale power system, its critical current needs to be high enough. Generally, the double pancake HTS magnet has the severe decrease in the critical current because of magnetic field perpendicular to the tape surface. To fabricate a high critical current magnet, we wound a solenoid with the stacked tape. In this paper, the characteristics of the critical current of the HTS solenoid wound with the stacked tape were investigated. The results of this research can be used as the background data for the design of the large-scale HTS magnet.     

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.     

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.     

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.     

Comparison of AC Losses of YBCO Circular Pancake Coils and Infinitely Long Stack Approximation

High-temperature superconducting (HTS) coils are key parts of many large-scale AC applications, such as transformers, superconducting magnetic energy storage, and motors. The estimation of AC losses of pancake coils is necessary for optimizing the design of HTS devices and cryogenic systems. To speed up the computation of AC losses, the numerical model of an infinitely long HTS stack is often utilized. An HTS stack is a good approximation of the circular coil only if the coil radius is sufficiently large, since AC losses will exhibit a stronger asymmetry along the radial direction for small values of coil radii. To assess the validation of an infinitely long stack approximation, the comparisons of AC losses between infinitely long stacks and circular coils with different radii are presented. The turn number varies from 10 to 80. We find that the AC losses of HTS circular coils will gradually increase to the same value as an infinitely long stack with increasing coil radii. A new parameter is proposed to quantitatively describe the correctness of infinitely long stack approximation. Finally, a method of AC loss estimation of HTS pancake coils is proposed.     

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.     

Thermal stability of high current density magnets

The stabilization theories hitherto proposed for superconducting (SC) magnets are not fully developed for application to high current density magnets such as pulsed dipole magnets for a synchrotron. Hence, thermal stability in such high current density magnets is studied by obtaining a minimum energy of thermal disturbances which barely leads a magnet to quench. To find the minimum energy by calculation a dynamic simulation of temperature distribution along a conductor is carried out following an application of the disturbances on the conductor. The minimum energy is found to depend largely on time duration and spatial length of the disturbances. The values of the minimum energy given by calculation agree almost with the experimental results obtained for a coil which simulates a pulsed dipole magnet from the viewpoint of cooling. Discussion is also made in relation to the minimum energy on the performance of a pancake type solenoid magnet which has the same cooling as in the simulating coil.     

An Experimental Investigation of the Transient Response of HTS Non-insulation Coil

A single pancake coil without turn-to-turn insulation was tested in this paper to investigate the transient responses under different situations. We performed charging and discharging test, AC current test, and regional quench emulation test on the non-insulated (NI) coil. The experimental test results show a significant time delay for charging and discharging characteristics of NI coil and can be validated by a simple proposed equivalent electrical circuit. Under the AC operating current, the NI coil can bypass nearly all the AC current from the coil spiral path to the radial path such that it is not possible for NI coil to store or be affected by the AC magnet field. Additionally, while carrying AC current, the AC loss dissipation of NI coil is inversely proportional to the frequency of the AC operating current. When a regional quench occurs, the NI coil can bypass the current in the regional quench zone to avoid further temperature accumulated and protect the NI coil itself.     

The fault current limiting characteristics of a flux-lock type high-Tc superconducting fault current limiter using series resonance

We analyzed the fault current limiting characteristics of a flux-lock type high-T_c superconducting fault current limiter (HTSC-FCL) using series resonance between capacitor for series resonance and magnetic field coil which was installed in coil 3. The capacitor for the series resonance in the flux-lock type HTSC-FCL was inserted in series with the magnetic field coil to apply enough magnetic field into HTSC element, which resulted in higher resistance of HTSC element.However, the impedance of the flux-lock type HTSC-FCL has started to decrease since the current of coil 3 exceeded one of coil 2 after a fault accident. The decrease in the impedance of the FCL causes the line current to increase and, if continues, the capacitor for the series resonance to be destructed. To avoid this operation, the flux-lock type HTSC-FCL requires an additional device such as fault current interrupter or control circuit for magnetic field.This paper investigated the parameter range where the operation as mentioned above for the designed flux-lock type HTSC-FCL using series resonance occurred from the experimental results. In the design of the flux-lock type HTSC-FCL, the some methods to avoid the continuous increase of the line current were suggested and confirmed by the experiments that the suggested methods were available to prevent the continuous increase of the line current after a fault happened.     

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.     

Low resistance splices for HTS devices and applications

This paper discusses the preparation methodology and performance evaluation of low resistance splices made of the second generation (2G) high-temperature superconductor (HTS). These splices are required in a broad spectrum of HTS devices including a large aperture, high-field solenoid built in the laboratory to demonstrate a superconducting magnetic energy storage (SMES) device. Several pancake coils are assembled in the form of a nested solenoid, and each coil requires a hundred meters or more of 2G (RE)BCO tape. However, commercial availability of this superconductor with a very uniform physical properties is currently limited to shorter piece lengths. This necessitates us having splices to inter-connect the tape pieces within a pancake coil, between adjacent pancake coils, and to attach HTS current leads to the magnet assembly. As a part of the optimization and qualification of splicing process, a systematic study was undertaken to analyze the electrical performance of splices in two different configurations suitable for this magnet assembly: lap joint and spiral joint. The electrical performance is quantified in terms of the resistance of splices estimated from the current-voltage characteristics. It has been demonstrated that a careful application of this splicing technique can generate lap joints with resistance less than 1 nΩ at 77 K.     

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.     

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.     

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.     

4C code analysis of thermal–hydraulic transients in the KSTAR PF1 superconducting coil

The KSTAR tokamak, in operation since 2008 at the National Fusion Research Institute in Korea, is equipped with a full superconducting magnet system including the central solenoid (CS), which is made of four symmetric pairs of coils PF1L/U–PF4L/U. Each of the CS coils is pancake wound using Nb₃Sn cable-in-conduit conductors with a square Incoloy jacket. The coils are cooled with supercritical He in forced circulation at nominal 4.5 K and 5.5 bar inlet conditions. During different test campaigns the measured temperature increase due to AC losses turned out to be higher than expected, which motivates the present study.The 4C code, already validated against and applied to different types of thermal–hydraulic transients in different superconducting coils, is applied here to the thermal–hydraulic analysis of a full set of trapezoidal current pulses in the PF1 coils, with different ramp rates. We find the value of the coupling time constant nτ that best fits, at each current ramp rate, the temperature increase up to the end of the heating at the coil outlet. The agreement between computed results and the whole set of measured data, including temperatures, pressures and mass flow rates, is then shown to be very good both at the inlet and at the outlet of the coil. The nτ values needed to explain the experimental results decrease at increasing current ramp rates, consistently with the results found in the literature.     

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.     

Quench back in thin superconducting solenoid magnets

Superconducting magnets with well coupled, low resistance, secondary circuits have been observed to become fully normal faster than quench propagation in the coil would permit. This process is referred to as ‘quench back’. Quench back observed at the Lawrence Berkeley Laboratory (LBL) was caused by heating the secondary circuit from the current induced from the primary circuit as normal region in the superconducting coil propagated. This paper develops the theory for thermal quench back in thin solenoid magnets and compares this theory with measurements made in two one-meter diameter superconducting solenoid magnets.     

AC loss measurements of twisted and untwisted BSCCO multifilamentary tapes

AC losses in twisted and untwisted BSCCO multifilamentary superconducting tapes with Ag matrix developed in DAPAS program were measured by an electrical method. Magnetization and transport losses were measured by a pick-up coil and by a voltage taps. Total AC loss during simultaneous application of AC transport current and an AC transverse magnetic field was given by the sum of the magnetization and transport losses measured during this simultaneous application. The magnetization loss without transport current of untwisted and twisted tapes was measured first to evaluate the effect of twisting to decouple filaments. Then, the total AC loss of the twisted tape was measured in transverse magnetic fields with various amplitudes and orientations, while the amplitude of the transport current was fixed. The measured total AC loss in a parallel transverse magnetic field was compared with some theoretical models to study the detailed characteristics of the measured total AC loss of the sample.     

Analysis on the thermal and electrical characteristics of impregnating materials for the bifilar winding-type superconducting fault current limiter

The resistive type high temperature superconducting fault current limiter (HTSFCL) limits the fault current with the resistance that generated by fault current. The generated resistance by fault current makes large pulse power which makes the operation of HTSFCL unstable. So, the cryogenic cooling system of the resistive type HTSFCL must diffuse and eliminate the pulse energy very quickly. Although the best way is to make wide direct contact area between HTS winding and coolant as much as possible, HTS winding also need the impregnation layer which fixes and protects it from electromagnetic force. This paper deals with thermal conductivity and dielectric strength of some epoxy compounds for the impregnation of high temperature superconducting (HTS) winding at 77 K. The measured data can be used in the optimal design of impregnation for HTS winding. Aluminar filling increased the thermal conductivity of epoxy compounds. Hardener also affected the thermal conductivity and the dielectric strength of epoxy compounds.