Unbalanced Current Distribution Research Articles

Investigation of the Relationship Between Unbalanced Current Distribution and Twist Pitches in Cable-in-Conduit Conductors

Investigation of the Relationship Between Unbalanced Current Distribution and Twist Pitches in Cable-in-Conduit Conductors

AC Resistance and Leakage Inductance Estimation for Planar Transformers With Parallel-Connected Windings

Regarding the ever-increasing switching frequency of power converters, there is a lack of tools to perform a quick and effective optimization of the high-frequency transformers embedded in isolated power converters. Indeed, at high operating frequency, the geometry of the core and windings deeply impacts the overall transformer performance. Specifically, assessing accurately the current distribution in parallel-connected windings is a challenging and key issue. Winding parallelization, frequently implemented in low-voltage and high-current applications so as to decrease Joule losses, may be an ineffective solution because of skin and proximity effects. They create an unbalanced current distribution in the parallel-connected windings, which greatly impacts the ac resistance and leakage inductance of the transformer. This article proposes an innovative frequency analytical model permitting to simply and accurately compute the current distribution in the transformer windings, the ac resistance, and the leakage inductance. The proposed approach is comprehensively presented, validated using both measurements and two-dimensional finite element analysis and analyzed giving guidelines to engineers designing high-frequency transformers.

A fast forecasting model of electrothermal distribution for parallelled SiC MOSFETs

Parameter dispersion of SiC MOSFETs causes unbalanced current distribution in parallelled devices, which could increase voltage and current stresses of power devices, and lead to thermal imbalance between the devices, and thus affecting the working life and the safe operation of the converter. In order to accurately forecast the thermal balance of SiC MOSFETs, this paper investigates the problem of predicting their electrothermal models. Firstly, the operating mode of the two parallelled MOSFETs is analysed, and the influencing factors affecting the current balance are derived. Then the losses of the power devices in parallel are analysed, and the forecasting model of the electrothermal distribution is derived. Finally, the electrothermal model is verified on the experimental platform. The experimental results show that the thermal distribution are basically similar to the prediction results of the mathematical model, which largely solves the inaccuracy of forecasting model of the electrothermal distribution.

Unbalanced distribution of electric current in underwater electrical wire array explosion

The uniformity of electric current distribution in a wire array and its unstable behavior during the process of underwater electrical explosion have been investigated. Two exploding wires in parallel were used in the experiments and the current waveforms flowing through each wire were obtained using two self-integrating coils. Significant differences in the current waveforms of the two wires were observed near the melting point, which was attributed to the non-simultaneity of heating and phase transition. Unbalanced current distribution caused by the deviations of wire dimensions was analyzed based on a magneto-hydrodynamic model, and the simulation results show that thermodynamic state difference between two wires is present throughout the entire electrical explosion process. It is also found that the initial stored energy of pulse generator will affect the thermodynamic state evolution of exploding wires, resulting in different behaviors of current distribution after the explosion time. The slightly different heating rate caused by unbalanced current distribution in a wire array can break the symmetry of converging shock waves and lower the pressure peak in the vicinity of implosion axis, which was discussed based on the two-dimensional hydrodynamic simulations.

A Study on the Electrochemical Impedance Analysis of PEMFC Under the Low Humidity Condition

Polymer electrolyte membrane fuel cells (PEMFC) have received much attention as a next generation energy conversion system due to the pollutant-free byproduct (water), and high efficiency. PEMFC fundamentally consists of many components such as membrane electrode assembly (MEA) gas diffusion layers (GDLs), and bipolar plates (GDLs). Among them, MEA is directly correlated to the performance and durability of PEMFC since the MEA components such as catalyst materials, supporting carbon, and an ionomer (binder) can be degraded under corrosive environment during operating Hence, it is important to develop the meticulous analysis method to diagnose the degraded condition of PEMFC; thus, fault detection have to be properly identified before severe deterioration of MEA happens. Among an in situ diagnosis tool to investigate the degraded components of MEA, the electrochemical impedance spectroscopy (EIS) technique is well-known electrochemical method to classify each resistance such as high frequency resistance (HFR, Ohmic resistance), charge transfer resistance, and Warburg impedance, separately; thus, it makes it possible to trace the degraded components of MEA. Among them, the performance of PEMFC is closely related to the activity of catalyst that is reflected as a charge transfer resistance, and therefore, usual impedance studies are mostly focused on the catalyst [1]. In fact, however, the degradation behavior of a fuel cell system is very complicated since it originated from a complex chain and combination of corrosion issues [2], such as ionomer degradation, membrane thinning, corrosion of the supporting carbon, and Pt corrosion, which are the crucial degradation factors that affect both performance and durability of PEMFC. Hence, to elaborate the impedance measurement, it is necessary to focus on the inherent properties of all components of MEA.In this context, we performed the impedance measurements under low humidity (RH 30 %) condition to emphasize the ionic resistance during PEMFC operation. By deliberately limiting the ionic conductivity, the dehydrated ionomer accentuated the influence of ionic resistance among various resistance components inside the MEA, and thus the degraded MEA can be analyzed with respect to the ionomer surrounding catalyst layer (CL), as shown in Figure 1. Moreover, the complex capacitance adopting transmission line model (TLM) circuit under non-faradaic process facilitates to investigate the double layer capacitance (EDLC) of degrading CL [3, 4] as shown in Figure 2. To verify the feasibility of this approach, we studied degradation mechanism of MEA under dehydration and cathode flooding condition, respectively, since these problems related to water management are typical issues to be solved in systemic point of view [5].As a result, under the dehydration degradation, the increase of gas permeation induced the mixed potential formation at the anode, causing the cathode CL deterioration with surface variation of cathode CL. It could be monitored though the volume fraction of supporting carbon and ionomer. In addition, under the flooding degradation, the unbalanced current distribution caused the irregular anode deterioration, and phase differences between transported protons were reflected at horizontal aspect at low frequency region in Warburg impedance spectra. The detailed mechanism based on the aforementioned EIS measurement will be discussed in this presentation.

The "Loopole" Antenna: A Hybrid Coil Combining Loop and Electric Dipole Properties for Ultra-High-Field MRI.

Purpose.To revisit the “loopole,” an unusual coil topology whose unbalanced current distribution captures both loop and electric dipole properties, which can be advantageous in ultra-high-field MRI.Methods.Loopole coils were built by deliberately breaking the capacitor symmetry of traditional loop coils. The corresponding current distribution, transmit efficiency, and signal-to-noise ratio (SNR) were evaluated in simulation and experiments in comparison to those of loops and electric dipoles at 7 T (297 MHz).Results.The loopole coil exhibited a hybrid current pattern, comprising features of both loops and electric dipole current patterns. Depending on the orientation relative to B0, the loopole demonstrated significant performance boost in either the transmit efficiency or SNR at the center of a dielectric sample when compared to a traditional loop. Modest improvements were observed when compared to an electric dipole.Conclusion.The loopole can achieve high performance by supporting both divergence-free and curl-free current patterns, which are both significant contributors to the ultimate intrinsic performance at ultra-high field. While electric dipoles exhibit similar hybrid properties, loopoles maintain the engineering advantages of loops, such as geometric decoupling and reduced resonance frequency dependence on sample loading.

1200-V 4H-SiC Merged p-i-n Schottky Diodes With High Avalanche Capability

In this article, the avalanche capability of the 1.2-kV 4H-SiC junction barrier Schottky (JBS) and merged p-i-n Schottky (MPS) diodes is investigated through simulation and experiments. For MPS diodes, the width of the wide P+ region (W) is found to have great effects on the device avalanche capability. MPS diodes with varied W-values (3-20 μm) and JBS diode are designed and fabricated, and their avalanche energy/current capability is tested with unclamped inductive switching tests. The experimental results show that the MPS diode has an optimized avalanche capability at W = 8-μm design. Simulation study reveals that the avalanche current is mainly distributed at the edges of the P+ regions in the JBS/MPS diodes as a result of the curvature-effect-induced electric field crowding. The localized current crowding and unbalanced current distribution in the avalanche mode contribute to a reduced effective power dissipation area and a weaker avalanche capability. The current nonuniformity coefficient (k) is used to characterize the severity of the current unbalance, and it is found that kdeclines as Wincreases when W = 3-8 μm and then gets increased when W exceeds 8 μm. Both the experimental and simulation results indicate that the MPS diode is superior to the JBS diode in avalanche capability, and the width of the wide P+ region in the MPS diode has an optimal design (8 μm in this article) which corresponds to the alleviated current crowding issue and 9%-19% improvement of the avalanche capability.

Influence of Twist Pitch in CICC on Contact Condition Between Individual Superconducting Strands and a Copper Sleeve in a Lap Joint

One of the reasons for degradation of the critical current in a Cable-in-conduit conductor is unbalanced current distribution due to inhomogeneous resistance distribution in a lap joint. As a way of solving this problem, a sleeve longer than the highest order sub-cable twist pitch (cable twist pitch) and soldering a joint are actually effective to reduce the inhomogeneity in the resistance distribution between individual strands and the copper sleeve. However, the influence of individual sub-cable twist pitch on contact condition between individual strands and the copper sleeve in a CICC has not been investigated. In this study, the influence of the individual sub-cable twist pitch on contact condition between individual strands and the copper sleeve in a CICC lap joint was investigated using calculated strand paths. Our calculated results show that contact condition between strands and the copper sleeve in a CICC lap joint are improved regardless of the cable configuration when the n-th order sub-cable twist pitch is a multiple of all the previous sub-cables twist pitch. Moreover, it is shown that the difference in the characteristic associated with the contact condition and the combination of sub-cable twist pitch is caused by the cable outline.

Design and Tests of Prototype Hybrid Superconducting Fault Current Limiter With Fast Switch

This paper deals with design and operating tests of prototype hybrid superconducting fault current limiter (SFCL) which is series connected non-inductive solenoid resistive coil and asymmetric non-inductively wound coils. The role of resistive SFCL is for reducing the fault current in the first swing. Moreover, resistive SFCL can reduce a load of dielectric strength in hybrid SFCL. Two kinds of high temperature superconductor (HTS) wires were used to fabricate asymmetric HTS coil. These HTS wires had different quench characteristics such as different critical current and n-value. Unbalanced current distribution between two HTS wires in the asymmetric HTS coil during the fault should occur. Thus, the asymmetric HTS coil generates impedance and repulsive force from magnetic flux. The finite element method was performed to calculate maximum repulsive force. From experimental tests and FEM simulation results, proper combination of HTS wires and configuration of asymmetric HTS coil with fast switch were determined to increase maximum repulsive force. Also, the prototype hybrid SFCL with fast switch was fabricated and performed.

Optimal Pitch Analysis Governing Contact Strand Number and Lengths With Cu Sleeves at CIC Joints

It is observed that measured critical current of Cable-in-Conduit-Conductor (CICC) for ITER TF coil are lower than expected. This is partly explained an imbalance of the contact resistance at the joint between double pancakes which causes an unbalanced current distribution in a cable and hence during a slow temperature increase some strands reach the critical current earlier than other. In order to estimate the contact resistances, we identify the three-dimensional positions of all strands inside the CICC, and then evaluate the contact parameters such as number and length of the strands which appear on the cable surface and have contact with a copper sleeve. It is observed that many strands do not appear on the surface of the cable, and can lead to unbalanced current distribution. We developed a numerical code to analyze all strand positions in the CICC. We evaluated the contact parameters by using the numerical code, and then compared them with those evaluated from the measured strand positions. It is found that since both results are in good agreement, the numerical code is available for evaluating the contact parameters. By using the code, we can optimize the contact parameters by varying the twist pitches of all staged subcables. The results show that all strands appear at the surface of the cable and have contact with the copper sleeve, and moreover the contact parameters have been improved.

Analysis of Contact Lengths of Strands with Cu Sleeves in CICC Joints

Cable-in-Conduit-Conductor (CICC) is used for the international thermonuclear fusion experimental reactor (ITER) toroidal field (TF) coils. But the critical current of the CICC is measured lower than expected one. This is partly explained by unbalanced current distribution caused by inhomogeneous contact resistances between strands and copper sleeves at joints. Current density in some strands reaches the critical under unbalanced current, and the quench is occurred under smaller transport current than expected one. In order to investigate the contact resistances, we measure the three-dimensional positions of all strands inside the CICC for Large Helical Device (LHD) poloidal field (PF) outer vertical (OV) coil, and evaluate contact parameters such as number and lengths of strands which contact with a copper sleeve. Then, we simulate the strand positions in the CICC using the numerical code which we developed, and compare the contact parameters which evaluated from the measured strand positions and the simulated ones. It is found that the both results are in good agreement, and the developed numerical model is useful for evaluation of the contact parameters. We apply the code to various CIC conductor joints to obtain optimal joint parameters.

Analysis of Contact Length Distribution of Superconducting Strands with Copper Sleeves at Cable-in-conduit Conductor Joints

It is observed that the measured critical currents of cable-in-conduit (CIC) conductors for ITER TF coils become lower than expected due to the unbalanced current distribution that is caused by contact resistance between the strands and the Copper (Cu) sleeves in CIC conductor joints. In order to evaluate the contact length, we identify the three-dimensional positions of all strands in the CIC conductor, and then measure the contact number and lengths of strands that appear on the surface of the cable to contact with the Cu sleeves. It is found that some strands do not appear on the surface of the cable, and the contact lengths are widely distributed with a large standard deviation. We develop a numerical code that simulates strand positions in the CIC, and then compare the analyzed contact strand number and contact length with measured ones. It is found that the results are in agreement, and hence the code can be used to evaluate the contact parameters. After varying the twist pitches of the sub-cables, we show that all strands appear on the cable surface and have contact lengths with small standard deviation. It is found that the twist pitches are a key parameter for optimization of the contact resistance.

AC loss analysis of a HTS coil with parallel superconducting tapes of unbalanced current distribution

Considering the current capacities of the HTS tapes recently being developed, it is inevitable to use these HTS tapes in parallel in order to apply them to power transformers. But non-uniform distributions of current and large AC losses are major problems in a HTS coil with parallel superconducting tapes. Many parallel HTS tapes have been fabricated and tested so far. The results showed that we had to transpose each tape in order to reduce AC losses and have uniform distributions of current. But the measurement of the current distribution process has been quite difficult not only in the case of the parallel HTS tapes but also in the case of the coil with parallel HTS tapes.In our study, we investigated the effect of current distribution on the AC losses in parallel HTS tapes and coils with parallel superconducting tapes. The current distribution in parallel HTS tapes could be measured with small Hall sensors, the dependence of AC losses and critical current on the current distribution was analyzed. We fabricated a parallel HTS tape and HTS coil with the transposed parallel HTS tapes for an application of a power transformer. The performance of non-uniform current distribution in the parallel HTS wires is worse than that of a uniform one.

Assessment of the Effect of Current Non-Uniformity on the ITER ${\rm Nb}_{3}{\rm Sn}$ Good Joint Short Sample DC Performance

The DC performance of the so-called good joint Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn short sample conductor, measured in the SULTAN facility in 1999 with a progressively reduced (chopped) joint length, became worse as the joint became shorter. In this paper we present the analysis of those tests using the THELMA code. The results confirm quantitatively that the loss of performance was due to increasingly unbalanced current distribution. Once the unbalance due to the joint is properly modeled, the performance of the conductor can be fitted using the same extra longitudinal strain in the critical current scaling, proportional to the electromagnetic load but independent of the joint length.

JORDI, a Test Facility for Measurement of Contact Resistance Distribution in Full-Size Conductor Terminations

A new test facility, dedicated to the measurement of contact resistance distribution in full-size conductor termination, has been designed and manufactured at CRPP Fusion Technology in Villigen, Switzerland. The facility will be used to characterize and qualify the ITER joints (both prototype and industrial). The basic components of the new facility are: a medium size, liquid helium cryostat to host the termination sample, a cryogenic, parallel resistor array for providing balanced current injection in up to 100 channels, a 10 kA DC power supply (100 channels of each 100 A), a pair of conventional current leads rated at 10 kA and a data acquisition system with 200 analogue input channels (100 voltage and 100 current readouts). As opposed to the earlier attempts where the contact resistance was measured under an unbalanced current distribution (only one strand was connected to the current source) in the new facility the contact resistance is measured under imposed balanced current distribution among all current carrying elements and the voltage drops are sensed simultaneously.

Effect of self-field and current non-uniformity on the voltage–temperature characteristic of the ITER central solenoid insert coil by numerical calculations

For the optimisation of a magnet design with cable-in-conduit conductor (CICC) technology it is essential to comprehend the scaling of the critical current from the separate strand characteristics to the finally assembled cable performance in a coil. Several model coils have been tested in the framework of research for the International Thermonuclear Experimental Reactor (ITER). At present, the scaling of the critical current from the strand to the full cable performance and the apparent decrease of the n-index from strand to cable in the voltage–current curves is not understood. It is important to recognize the mechanisms behind this phenomenon in relation to the cost of the superconducting strand, which is significant in the manufacture of the magnets. Therefore, basic phenomena like the cable conductor self-field, the current unbalance introduced by the non-uniformity of the joints and a possible reversible or irreversible degradation of the voltage current characteristic of a strand during cable manufacture or electromagnetic loading of the magnet have to be considered. The voltage–current characteristic of the strand is extensively explored for the relevant range of magnetic field, temperature and axial strain space. Accordingly a numerical six-element network model is developed to simulate the conditions and behaviour of the last stage cable elements of a full-size ITER conductor. The experimental data, mainly in terms of voltage–current ( VI) or -temperature ( VT) characteristics, are obtained on the central solenoid insert coil (CSIC) experiment performed in Naka (Japan) in the framework of the research for ITER. The numerical model, which is briefly introduced, is used to study the cable performance by using experimentally obtained cable parameters like inter-strand (and bundle) contact resistance, strand critical current data as a function of magnetic field, temperature and applied axial strain, and external cable self-field measurements by Hall sensors for reconstruction of the current non-uniformity. The effect of a current redistribution due to the cable self-field on the voltage–temperature curve is calculated in correlation with the transverse resistance between the strands and last cabling stage bundles (petals). A realistic unbalanced current distribution is established by introducing non-uniform joints at the extremities of the CS-insert cable. It appears that the cable self-field effect hardly gives any change in the shape of the VT curve but merely a shift towards lower temperature giving a reduction of the current sharing temperature T cs (10 μV/m) of <0.1 K. For typical CICCs with Cr-coated Nb 3Sn strands, there is practically no current redistribution due to the cable self-field, because of the high inter-strand contact resistance. An unbalanced current distribution also gives an earlier voltage rise in the VT curve, mainly at low levels of the electric field. At a 10 μV/m criterion practically no reduction of the T cs (<0.1 K) is found by the numerical simulation. However, in the CSIC the experimentally obtained overall reduction in T cs from strand to cable is 0.7 K for an operating current of 40 kA at 12.5 T background field. According to the results of the numerical simulation, the cable self-field effect and the non-uniform current distribution, which is unavoidably caused by the joints, cannot explain the early voltage rise and low n-index in the VT curve of the CS-insert coil. It is very likely that electromagnetic forces play a role in causing reversible degradation in critical current or even irreversible due to strand (filament) damage. Neither can it be excluded that strand deformation during cabling has an impact on the final conductor performance as well. Therefore additional effort is required in detailed 3D modeling of the possible strand deformations inside a cable and the impact it has on the strand performance by experimental verification on strand level.

Analysis of the current distribution in the ITER CS-insert model coil conductor by self field measurements

Non-uniformities in the paths of the currents in a cable-in-conduit conductor (CICC), resistive or inductive, will result into an unbalanced current distribution. The current nonuniformity may affect the performance of a magnet system and it is therefore essential to evaluate this phenomenon. The current distribution in the Central Solenoid Insert Coil (CSIC) from the Central Solenoid Model Coil (CSMC) experiment is reconstructed from the four Hall-sensor voltages at the top and bottom joint of the CSIC. Four Hall sensors are used, near both joints at the extremities of the cable, to measure the self-field of the conductor. The inverse identification problem is solved in order to find the currents that match as close as possible to the set of measured data. Solutions are found for the current amplitudes in the six petals of the cable from the set of measurements of the tangential field component. Differences in the petal currents are found which rate up to a factor of two.

Current measurements of the individual strands in CICC magnet

Non-uniform current distribution in CICC (cable-in-conduit conductor) is one of the major concerns for developing a successful magnet system. The three-strand CICC magnet has been constructed in KAIST (Korea Advanced Institute of Science and Technology) for the study of current nonuniformity. The magnet is equipped with a forced flow supercritical helium system at variable temperature to simulate the various thermohydraulic conditions of superconducting Tokamak. Each strand of the magnet has its own shunt resistor to measure the current it carries during the DC and AC operations. This paper describes an experimental apparatus and some current measurement results of the individual CICC strands. The unbalanced current distribution has been obtained directly from the shunt resistor voltage data. The surface contact resistance of the superconducting strands was small enough to allow current redistribution inside the conduit rather than at the terminal joints of the magnet.

スピニングローター真空計の渦電流と温度による誤差の解析

This paper deals with an analysis on the offset pressure change of a Spinning Rotor Gauge (SRG), which caused by unbalanced eddy current distribution and thermal change in a stainless steel rotor. Themal and electromagnetic analysis method (physical models, equations, simulation algorism) are recognized to have practicability and validity in this analysis that are usefully applied to design a new construction and to operate with carefully treatment.

Ｂｉ系銀シース線を用いた超電導導体の交流通電特性

The characteristics of a multi-layered conductor, which was an assembly of Bi-2223 Ag-sheathed superconducting wires, were investigated. We confirmed that experimental value of the self-field loss in the multi-layer conductor was nearly identical with a theoretical value based on a critical-state model. In detail, the experimental result was well explained by the power law model assuming that the current-voltage characteristic of the high-Tc superconductor is well approximated as V∝In. We also investigated the AC current distribution of each superconducting layer using 50m/4-layer conductor. As measured with a Rogowski coil, it was found that an unbalanced current distribution occurred in the multi-layer conductor. It is possible to reduce the AC loss by suppressing unbalanced current distribution. As a solution, we suggested to adjust the pitch of the conductor in order to equalize the impedance of all the layers. For this we estimated the condition for a balanced AC current to each layer by calculation.