Noboru Fujiwara

Model Cable Tests for a 275 kV 3 kA HTS Power Cable

High-temperature superconducting (HTS) cables are considered the next generation transmission line because they are compact, lightweight, and demonstrate large capacity and low loss compared to conventional cables. In particular, since a coated conductor (YBCO wire) provides high critical current, high magnetic-field property, low AC loss, and low cost, it is expected to make the HTS cable more attractive than other superconducting wire. In Japan, 66/77 kV HTS cables have developed for about 20 years. We started developing 275 kV class HTS cables three years ago based on 66/77 kV HTS cables. The goal is a 275 kV 3 kA cable with a capacity of 1.5 GVA, the same capacity as a typical overhead transmission line, which serves as the backbone of Japanese power networks. The following technical developments will be carried out: high current and low AC loss cable conductors and high voltage insulation and low dielectric loss cables. Regarding high current and low AC loss cable conductors, 3-kA cables have been fabricated, and AC losses have been measured. We found that using thin YBCO wire reduced AC losses in experiments.

Hysteretic ac losses in power transmission cables with superconducting tapes: effect of tape shape

Hysteretic ac losses in monolayer power transmission cables with superconducting tapes are theoretically investigated. The ac losses depend on the tape shape and the cable configuration. The ac losses in two types of ideal cables are compared: the ac loss Qrnd in a round cable with curved superconducting tapes conforming to a cylindrical former, and the ac loss Qagl in an angular cable with flat superconducting tapes. The Qagl is larger than Qrnd because of the polygonal cross section of the angular cable, and the tape-width dependence of Qagl is very different from that of Qrnd. (Some figures in this article are in colour only in the electronic version)

AC loss characteristics of superconducting power transmission cables: gap effect and Jc distribution effect

Four groups of superconducting power transmission cables composed of coated conductors with 4.0xa0mm width and 2xa0µm superconductor thickness have been designed, including mono-layer, two-layer, four-layer and six-layer cables. In each group, three types of cables have been constructed with gaps of different size between adjacent coated conductors, which are classified into small gap, medium gap and large gap. Moreover, different lateral critical current density (Jc) distributions, specifically a uniform distribution and a trapezoidal distribution with a sloping shoulder, have been assumed while calculating the AC losses of these cables numerically by using a one-dimensional FEM model. Numerical results show that AC losses in mono-layer cables are significantly influenced by gaps between coated conductors as well as lateral Jc distribution, while cables with many layers are hardly affected by them. This proves that small gaps between coated conductors are not absolutely essential to reduce AC losses in multi-layer cables, and a sloping shoulder of the Jc distribution is more allowable in multi-layer cables than in mono-layer cables. The AC loss distributions among layers in six-layer cables are presented and the reasons for different influences of the gap as well as the lateral Jc distribution in mono-layer and multi-layer cables are discussed.

Ac losses in two-layer superconducting power transmission cables consisting of coated conductors with a magnetic substrate

Electromagnetic field analyses were made in cross-sections of two-layer superconducting power transmission cables consisting of coated conductors to calculate their ac losses, neglecting the spiral arrangement of the coated conductor and assuming each coated conductor was parallel to the cable axis. A substrate of each coated conductor was placed inside when assembling the coated conductors into a cable. Based on the electromagnetic field analyses, ac losses in the superconductor layer in the coated conductor as well as those in the magnetic substrate were calculated. The magnetism of the substrate increases ac losses in the outer-layer coated conductors and in the inner-layer coated conductors in different ways. In outer-layer coated conductors, the hysteretic loss in the magnetic substrate itself dominates their ac losses, because their magnetic substrates are directly exposed to the magnetic field produced by the inner-layer current. In inner-layer coated conductors, ac loss in the superconductor layer increases, because the magnetic substrate of the outer-layer coated conductor attracts magnetic flux to increase the magnetic field component normal to the superconductor layer of the inner-layer coated conductor. The influence of the space between the two layers as well as the width of the coated conductor on the ac loss characteristics is also discussed.

Effects of Lateral-Tailoring of Coated Conductor for Ac Loss Reduction of Superconducting Power Transmission Cables

The effects of removing low-Jc edges of coated conductors by a laser for ac loss reduction were studied in two-layer superconducting power transmission cables. By removing the low-Jc edges, narrower coated conductor with more uniform Jc distribution can be obtained. The original 5 mm-wide coated conductors as well as the edge-removed 4 mm-wide and 3 mm-wide coated conductors were assembled spirally around cyrindrical formers in two layers to form a cable. The measured ac losses were compared with the ac losses calculated using a numerical model where the spiral structure is neglected. For ac loss calculations, we used the lateral Jc distribution of the coated conductor measured by the magnetic knife method or those which are assumed based on the losses of the critical current by the edge removal.

Thermal Characteristics of 275 kV/3 kA Class YBCO Power Cable

In Japan, the development of a 275 kV/3 kA class YBa2Cu3O7 (YBCO) power cable was started in 2008. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, an electrical insulation layer, HTS shield layers, and copper shield layers. 275 kV/3 kA class YBCO power cables may be subjected to a fault current of 63 kArms for 0.6 s. Therefore, in order to ensure stability and feasibility of superconducting power cables, we need to investigate the thermal characteristics and current distribution in the cable under fault conditions. In this study, we performed numerical simulations on a YBCO model cable under fault conditions by using a computer program we developed on the basis of 3D finite element method (FEM) and electrical circuit model.

Over-Current Characteristics of 66-kV RE123 HTS Power Cable

In Japan, the development of the 66-kV-class superconducting power cable was begun in 2008 as a national project. A high-temperature superconducting (HTS) power cable typically consists of a copper former, HTS conductor layers, electrical insulation layers, HTS shield layers, and copper shield layers. 66-kV-class superconducting power cables may be subjected to a fault current of 31.5 kArms for 2 s. Therefore, in order to ensure the stability and feasibility of superconducting power cables, we need to investigate these cables with respect to their thermal characteristics and current distribution under fault conditions. In this study, we carried out over-current experiments on a 2-m-long HTS model cable. We also performed numerical simulations on the model cable by using a computer program developed by us on the basis of a 3D finite element method (FEM) and an electrical circuit model.

Thermal Conductivity of YBCO Coated Conductors Reinforced by Metal Tape

We have measured the thermal conductivity, κ(T), of YBCO coated conductors (YCCs) reinforced by Cu or CuNi tape. κ(T) of YCC reinforced by Cu tape with 100-300 μm in thickness, YCC-Cu, was 250-400 W m-1K-1 at 77 K, which depended on the thickness of Cu tape and was roughly one order of magnitude larger than that of the bare YCC. On the other hand, κ(T) of YCC reinforced by CuNi tape of 300 μm thickness, YCC-CuNi, was comparable to that of the bare YCC except for the low temperatures. The contribution of each reinforcing material to the thermal transport was estimated by analyzing the measured κ(T) using an equivalent heat current circuit. The applied heat flows mainly through the Cu tape for the YCC-Cu tapes and through both CuNi tape and stabilizing Ag layer for the YCC-CuNi tapes, respectively. Phonons cannot be ignored as thermal carriers in YCC-CuNi tapes.

Effects of Unevenly Distributed Critical Currents and Damaged Coated Conductors to AC Losses of Superconducting Power Transmission Cables

Two groups of superconducting power transmission cables composed of two layers of coated conductors with 4 mm width and 2 μm superconductor-thickness have been designed. In one group, four styles of unevenly distributed critical currents (<i>I</i>_c) are applied, while in the other group there is one damaged coated conductor in each cable. Trapezoidal critical current density (<i>J</i>_c) distribution with a sloping shoulder of 0.3 mm is assumed while calculating the AC losses of these cables numerically by using a one dimensional FEM model. Numerical results show that unevenly distributed <i>I</i>_c increase AC losses along with the increasing variance ratio which defines the difference between individual <i>I</i>_c and the average <i>I</i>_c. Even only one damaged coated conductor can dramatically increase AC losses no matter which layer it locates at. The detailed AC loss distributions among coated conductors are presented and the reasons for different influences of unevenly distributed <i>I</i>_c as well as damaged coated conductor are discussed.

Over-Current Characteristics of 275-kV Class YBCO Power Cable

In Japan, a project for the development of a 275-kV class YBCO power cable started in 2008. High-temperature superconducting (HTS) power cables typically consist of a copper former, an HTS conductor layer, an electrical insulation layer, an HTS shield layer, and a copper shield layer. In practical applications, the 275-kV class transmission line may be subjected to short-circuit fault currents such as 63 kArms for a duration of 0.6 s. Therefore, in order to ensure the stability and demonstrate the feasibility of the cable, it is important to estimate the current distribution and temperature increase under the fault condition. We designed the copper former, copper shield layer, and copper plating of the YBCO coated conductor carefully so as to fulfill the requirements of practical applications. In this study, we carried out over-current experiments on a 2-m long YBCO model cable and performed numerical simulations by a computer program developed using the finite element method (FEM) and an electric circuit model. We investigated the electromagnetic and thermal behaviors of the cable under fault conditions from the experimental and simulation results.