Multilayer Conductor Research Articles

Unifying calculation of vortex-induced vibrations of overhead conductors

This paper deals with a unified way for calculating vortex-induced vibrations (Aeolian vibrations in transmission line parlance) of undamped single overhead conductors. The main objective of the paper is to identify reduced parameters which would unify the predicted vibration response to the largest possible extent. This is actually done by means of a simple mathematical transformation resulting, for a given terrain (associated to a given wind turbulence intensity), into a single, unified response curve that is applicable to any single multi-layered aluminium conductor. In order to further validate the above process, the predicted, unified response curve is compared with measured response curves drawn from tests run on a full-scale test line using several aluminium-conductor-steel-reinforced (ACSR), all-alloy-aluminium-conductor (AAAC) and aluminium-conductor-alloy-reinforced (ACAR) conductors strung at different tensions. On account of the expected scatter in the results from such field tests, the agreement is shown to be good. The final results are expressed by means of only four different curves pertaining to four different terrain characteristics. These curves may then be used to assess the vibration response of any undamped single, multi-layer aluminium conductor of any diameter, strung at any practical tension.

New scalar potential method for computation of electromagnetic variables in multilayer slab conductors

We present a new scalar potential method for computing the electromagnetic variables in multilayer slab conductors, excited by waves of arbitrary shape. In this method a large number of layers can be modeled separately, and it is possible to consider the arbitrary shape exciters in between the conductor layers. We use the scalar potential method to solve the Helmholtz equation in three dimensions. For the solution of this problem, two-dimensional fast Fourier transform and exponential functions are used with scalar potential variables. We verify the method by comparing the results of an example problem solved both by the new method and the finite-element method.

Computing electromagnetic field distribution due to an arbitrary shape exciter near multilayer slab conductors

We present a semianalytical method for calculating the electromagnetic field distribution in a multilayer slab conductor. This is a very advanced and comprehensive method because there is no limitation for the shape of the exciter and the number of layers (i.e., an exciter of any arbitrary shape and at any distance near the surface of a multilayer conductor with a large number of layers can be modeled). We solve the Helmholtz equation in three dimensions by separation of variables. We develop a two-dimensional fast Fourier transform (FFT) integral model and use exponential functions in the third dimension. We solve this integral model analytically and numerically by using the custom matrix form AX = B and a system of algebraic equations, respectively. To verify this method, we solve a simple example problem both by the new method and a finite-element method (FEM) program. The results are in good agreement with each other.

Analytical solution of electric potential produced by a direct current point sou rce located in a multilayered spherical volume conductor

A general analytical solution is derived for an electric potential produced by a direct current point source located at any position in a multilayered spher ical volume conductor. Starting from Poisson's equation, the electric potential whose coefficients are rapidly got in the form of matrix expressions is obtained according to the boundary conditions. Simulation for a four-layered spherical volume conductor denoting the scalp, skull, cerebrospinal fluid and brain tissue proves that electric potential in this paper converges more rapidly in the whol e region than that expressed only in spherical harmonics expansion form, and ite rative times decrease. Electric potential contour and current streamline figur es show that it is important to consider the effect of the low conductivity of s kull on electric potential and current flow. The distribution of electric potent ial on the surface of sphere is discussed, which will be useful in electroenceph alography inverse problems, and also in analyzing and interpreting the electric potential measurement.

Design Study of Model Cable Conductor by Using HoBCO Thin Film Tape

As a fundamental development of RE-123 cable conductor, the electromagnetic and mechanical properties of multi-layer spiral cable conductors using HoBCO thin film tapes were studied. The critical current of the HoBCO tape shows 95% performance at the 180 MPa tensile stress. Self magnetic field simulation of a 4-layer spiral cable conductor was carried out. AC loss calculation of the conductor with current capacity of 3 kA was also performed. The AC loss was calculated less than 0.5 W/m, which shows the possibility of low AC loss of HoBCO cable conductor. In the calculation, it was found that the gap between thin film tapes in the cable conductor makes an important role in reducing AC losses. Based on these design studies, fabrication of model cable conductor was performed. Several kinds of 1 m multi-layer spiral conductors and a 7 m 4-layer spiral conductor were made by using HoBCO tape and dummy tape. The stranding machine was especially applied to the 7 m 4-layer dummy cable conductor.

Numerical Analysis of AC Loss and Current Distribution Characteristics of Co-Axial Multi-Layer High Temperature Superconducting Cable

A numerical model to calculate the inter-layer current distributions and the AC losses in co-axial multi-layer cable conductors was developed. The inter-layer current distributions and the AC losses in 4-layer cables were numerically calculated for various combinations of twist pitches and twist directions of the cables. With the numerical model, the twist pitch combinations to equalize the inter-layer current distribution and to reduce the AC loss were also investigated. The analytical results showed the remarkable dependence of the losses and the current distributions on the twist pitches and the directions of the cables. The optimized cable structures were discussed by the numerical results.

Dependence of current carrying capacity and AC loss on current distribution in coaxial multi-layer HTS conductor

We had developed a simulation method of current distribution in coaxial multi-layer HTS conductor and investigated influence of the nonlinear voltage-current characteristic of HTS tape on current distribution. It had been reported that homogeneous current distribution, especially the same layer current, is effective in terms of reducing AC loss. There are, however, many sets of cable parameters to achieve homogeneous current distribution in such the coaxial multi-layer cables. Therefore, using our developed evaluation method, we numerically investigated the relationship between AC loss and the cable parameters such as twisting pitch, radius, and direction in coaxial three- and four-layer conductors. We evaluated both hysteresis loss and flux flow loss as AC loss using the Norris's model and V-I characteristic of HTS tape, respectively. The critical current of whole cable and current density of each tape are key parameters in terms of reducing AC loss. The larger twisting pitch is better for increasing the critical current of cable due to the greater number of usable tapes and the shorter tape-length per unit length of cable in longitudinal direction. Alternate twisting pitch, however, is ineffective for increasing the critical current due to small twisting pitch and small number of tapes for realizing homogeneous current distribution. There is no effect of the degradation of the critical current caused by magnetic field generated by the other layers on AC loss in the cable with the current carrying capacity of the order of at least 1 kA.

A new winding method to reduce AC losses in stable LTS pulse coils

A new winding method was proposed to reduce ac losses in stable LTS pulse coils. The suitable conductor for this winding method is a multi-layer type conductor composed of stacked Rutherford type cables with low resistive contact between strands. In this winding method, the twist angle around axis of the conductor is controlled during winding process to adjust the direction of edge-on orientation to stacked cables to direction of local magnetic fields applied to the conductor in winding areas. Inter-strand coupling losses in this coil are expected to be small in spite of low resistive contact between strands in the wound conductor. In order to clarify the effect of this winding method, firstly, a test conductor was fabricated and ac losses in short samples of the conductor were measured. This was an aluminum stabilized conductor, and a Rutherford cable composed of 8 Cu/Nb-Ti strands was used as the core of this conductor. The loss is measured with the transverse magnetic fields applied either perpendicular or parallel to the broad cable face, the "face-on" (FO) and "edge-on" (EO) orientations, respectively. From observed data, it is found that EO loss is 0.12 times FO loss . Secondly, ac losses in test coils wound with this conductor were calculated. The result showed that this winding method was very effective.

Influence of current distribution on conductor performance in coaxial multi-layer HTS conductor

We have developed a simulation method based on magnetic flux conservation between two filaments of adjacent layers to estimate current distribution in coaxial multi-layer HTS conductor. Using this method, we have demonstrated homogeneous current distribution and verified that current distribution was controllable directly by the conductor parameters of layer radius, twisting pitch and twisting direction. Although it has been considered that homogeneous current distribution is effective for reducing AC loss, the most suitable conductor parameters and operating condition have not been investigated sufficiently yet. Therefore, we improved our developing method to estimate current distribution more rigorously considering the nonlinear voltage-current characteristic of HTS tape. To verify the validity of the simulation method, we measured current distribution using coaxial two-layer conductors. Agreement of current distribution between the experiment and the analysis was good.

Highly Transparent Tin Oxide Films Prepared by DC Magnetron Sputtering and Its Liquid Crystal Display Application

A thin film of tin oxide (SnO2) was prepared on a plastic substrate by DC magnetron roll sputtering. The deposition thickness of SnO2 rapidly decreased with the increase of O2 gas flow rate, even though the refractive index remained almost constant at the level of 2.0 after the flow rate exceeded 40 sccm. The SnO2 thin film was applied as the oxide layer of the inner side of the multilayer conductive electrode having the sandwich structure of the oxide/metal/oxide system for flat panel display (FPD). The SnO2 layer in the sandwich structure (SnO2/Ag/ In2O3–SnO2; ITO) serves as a nucleation modification layer of the metal layer while preventing the diffusion of materials from the metal layer (Ag) to the barrier layer (SiO2). In particular, the transmittance in the 600–700 nm range is about 10% higher than that measured for the ITO/Ag/ITO structured electrode. This multilayered conductor also has approximately 5–8 Ω/square electrical resistance and 85% optical transmission at 550 nm.

Development of 66 kV-class high- Tc superconducting power transmission cable – remarkable decrease in AC losses and production of prototype cable

TEPCO and Furukawa have been developing a compact 66 kV-class high- T c superconducting (HTS) cable that employs two new important technologies. One technology reduces AC losses of HTS cable. In our studies, we found that using twisted filaments in the HTS tapes in the multi-layer conductor and adjusting winding pitches of layers dramatically reduces AC losses. Another technology is related to the mass production of HTS cable. To facilitate mass production, we improved the cable design. A three-phase prototype HTS cable was then successfully fabricated with machines at Furukawa Electric. The results of the performance testing of the cable demonstrated that the proposed design and the production method were appropriate.

The effect of interlayer insulation on the transport AC loss of the high- Tc superconducting multilayer conductor

Two high- T c superconducting conductors are fabricated with and without interlayer insulation. The transport properties of the conductors are compared. The AC loss of the conductor with interlayer insulation is much less than that of the other. A result of the calculation of AC properties of the conductor shows that the current distribution of each layer of the conductor is not uniform and the magnetic fields are remained between the layers. In the case of the conductor without interlayer insulation, it is speculated that the magnetic fields cause the coupling currents between Ag substrates. Moreover, if each layer has a distribution of the critical current on a direction of the length, a part of the transport current in each layer is passing through between layers because of a balance of the impedance of each layer. The coupling currents and/or the passing currents increase dramatically the AC loss of the conductor.

Conductor loss in thin-film transmission lines

By means of the surface impedance applicable to a finite film thickness, the conductor loss of a transmission line is investigated. It is found that there exists an optimum conductor thickness at which the conductor loss becomes a minimum. This thickness is predicted to be π/2 of the skin depth. Next, by using the thin-film microstrip line with varying conductor thickness and width, the measurement of transmission loss is carried out. It is confirmed that the optimum conductor thickness is near π/2 times the skin depth. In the process of investigation, the relationship with the surface current density in the incremental inductance method, often used as a simple computation method of conductor loss, is presented. The calculation of the conductor loss of a multilayered conductor is made possible. Previously, it has been considered that the conductor loss is sufficiently small if the conductor thickness is more than three times the skin depth. It is shown in the present study that the conductor loss becomes small with a conductor thickness about one-half as great. In addition to the increased performance of a transmission line, a contribution to reduction of both the consumption of valuable interconnect materials (such as gold) and of fabrication cost is expected. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(10): 83–91, 1999

Uniform current distribution conductor of HTS power cable with variable tape-winding pitches

In the multilayer conductor, the inner layers have higher impedance than the outer layers. As a result, the current concentrates in the outer layers. Our early study showed that its AC losses were reduced to one-tenth by making the current of each layer uniform. From such a point of view, a trial to realize the uniform current distribution was made by adjusting the winding pitches of high-temperature superconducting (HTS) tapes layer by layer. A 1 m long conductor was fabricated, where the inner layer had longer winding pitch than the outer layer. Experimental results showed that the currents flowing in individual layers were almost the same and that this conductor had lower AC losses than the nonuniform current distribution conductor.

Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi2Sr2Ca1Cu2O x /Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJ c. We have found that the partial melting in oxygen atmosphere is effective to have large transportJ c with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJ c. TheJ c of 5×105A/cm2 at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb3Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheI c was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJ c of the coil at the temperatures below 30 K.

Optimization of Multilayered Bi-2223/Ag-Au Conductors for Current Lead Applications

Multilayered Bi-2223 composite conductors fabricated by means of the "Accordion folding" method appear to be suitable for applications as current leads (CL). In this work, we report on the results of electrical characterization of silver-gold alloy sheathed Bi-2223 conductors up to 400 mm long, having a DC overall critical current density J c eng exceeding 3000 A/cm2 at 77 K. The I c evolution from the first to the third thermal treatment as well as the uniformity along the specimen lenght has been monitored. Lastly, calculations of the magnetic field profile across stacked multilayered Bi-2223 conductors are reported and discussed.

The transport current redistribution between the core layers on the models of HTS cables

The multilayer conductors with insulated and non-insulated layers are analyzed. The space–time current redistributions between the layers of the conductor are investigated for the next parameters: values of electric resistance at the current leads, transverse contact electric resistance between the layers, the model length, time, the conductor dimension, the current—its peak value, the change law and the ramp of the total current input. Most of the published test results for short (from 1 to 10 m) conductor models do not reflect the true current distribution of the real long length cable conductors. The criterion of experimental model constructions and, in particular, the calculation of its minimum length are presented. The calculations demonstrate the electromagnetic processes in real cable conductors.

Influence of the multilayer HTS-cable conductor design on the current distribution

The work presents the basic principles of the multilayer cable conductor design to achieve the maximum current-carrying capacity and the minimum losses in a superconductor and constructive cable elements. The multilayer conductors of two to ten layers are analyzed. The results show that the traditional core design with alternative winding directions from layer to layer is useful only for two-layer conductor. The conductor with more layers must have either the layers wound in one direction but with different pitch lengths or two layer groups wound with different pitch lengths. Only for these cases, the balanced design can be realized and current distribution will be uniform. In such balanced design, the interlayer electrical voltage and as a result, the coupling losses, are absent and interlayer electrical insulation is not needed. The recommendations to achieve the maximum critical current as a function of conductor dimensions are derived.

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.

円筒多層構造の超電導導体の交流損失測定法

Self-field AC loss of cylindrical multi-layered conductor, which was an assembly of Bi-2223 Ag-sheathed superconducting wires, was investigated. The self-field loss of the conductor derived from the electric field measured by the 4-probe method could be explained theoretically by the critical-state model. It was also explained that the self-field loss could be measured by the pick-up coil which formed a loop surrounding all the superconducting wires in the conductor. To compare these methods, a cylindrical 2-layer conductor with two types of taps to detect the voltage was fabricated: one was a common voltage tap by solder and the other a pick-up coil surrounding all the superconducting part of the conductor. AC losses obtained by the two methods mentioned above were nearly identical when the transmission current was smaller than the critical current. Thus, we suggest that a pick-up coil is a useful tool for AC loss measurement of multi-layer conductors where no soldering is required.