Shinichi Mukoyama

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.

50-m long HTS conductor for power cable

The long conductor fabrication is one of key technologies to realize the High-T/sub c/ superconducting power cable. A 50-m long conductor was fabricated by helically winding the High-T/sub c/ superconducting tape onto a former with a winding machine. The conductor consisted of ten layers of Ag-sheathed Bi-2223 tape which had a high critical current density of 10,000 A/cm/sup 2/ (at 77 K). AC losses and layer-by-layer current distribution were measured, feeding AC current of 100 to 2,000 Arms to the conductor cooled by LN2. The results showed that most of the current flowed in the outer layers where the impedance was low, and that AC losses were remarkably reduced by making the current distribution uniform.

Manufacturing and installation of the world's longest HTS cable in the Super-ACE project

The 500 m high temperature superconducting cable (HTS cable) is 77 kV 1 kA single-core cable with LN2-impregnated paper insulation. Demonstration and verification test of 500 m HTS cable has been started from March 2004 and many useful results can be obtained in the test for future practical uses. Furukawa Electric has mainly taken charge of designing, manufacturing and installation of the 500 m cable. In the manufacturing process, the cable could be fabricated without Ic degradation in Ag/Bi-2223 tapes. Moreover, various factory tests were carried out for the 500 m cable. The result of tests showed that the cable has sufficiently satisfied the quality requirement. In the installation, the cable was successfully pulled into a cable duct of 170 m long like actual underground cable installations.

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.

Development of 66 kV and 275 kV Class REBCO HTS Power Cables

A Japanese national project called “Materials & Power Applications of Coated Conductors (M-PACC)” started in FY2008. In this project, we are developing a 66 kV/5 kA large-current high-temperature superconducting (HTS) cable and 275 kV/3 kA high-voltage HTS cable, using rare-earth barium copper oxide (REBCO) tapes. These HTS cables are expected to offer a compact cable with a large capacity and low power transmission loss. After the cable design has been studied and elemental technologies for each component of the cable system, such as ac loss reduction, protection against over-current, and high-voltage electrical insulation have been developed, two cable systems will be constructed and verified to meet the required specifications in FY2012. This paper describes the progress and status of these HTS cable developments in the M-PACC project.

Experimental Results of 275-kV 3-kA REBCO HTS Power Cable

A 30-m-long 275-kV 3-kA high-temperature superconducting (HTS) cable had been developed in a national project of the Materials and Power Applications of Coated Conductors project in Japan. The design of the cable was based on the design values obtained from ac loss properties, thermal behavior under short-circuit tests, and electrical properties, such as partial discharge properties, impulse withstand properties, and dielectric properties. Through the development, the material of the cable insulation was determined and designed on the basis of its design stresses and test conditions based on the IEC, JEC (Japan electrical standards), and other HTS demonstrations. This cable was also designed to withstand a short-circuit test of 63 kA for 0.6 s and to have low losses of 0.8 W/m at 3 kA, 275 kV, including ac loss and dielectric loss. Based on these designs, a 50-m cable was manufactured and tested. The short samples obtained from 50 m were confirmed to have the designed characteristics. Furukawa Electric constructed a demonstration system of a 30-m cable with two terminations and a cable joint. The demonstration had started since November 2012 at Shenyang in China. In this demonstration, a 30-day long-term test was conducted and monitored at a current of 3 kA and at a test voltage selected to verify a 30-year operational lifetime. Removal tests revealed the superior reliability of the 275-kV HTS cable system.

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.

Over-Current Characteristics of a 20-m-Long YBCO Model Cable

To achieve large current capacity and mechanical flexibility, high-temperature superconductor (HTS) power transmission cables consist of a number of YBCO coated conductors, which are assembled and wound spirally on a Cu former. In practical applications, superconducting cables might be subjected to short-circuit fault currents that are 10 to 30 times the operating current. Therefore, in order to ensure the stability and feasibility of HTS power cables and protect them from fault currents, it is important to estimate the redistribution of the transport current and electromagnetic coupling among the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 20-m-long YBCO model cable, which was composed of two jointed 10-m-long YBCO model cables. Over-current with a peak of 31.8 kArms and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a newly developed computer program based on the 3D finite element method (FEM) in order to clarify the electromagnetic and thermal behaviors of the YBCO model cable in the presence of an over-current.

Transient Stability Characteristics of Parallel-Connected YBCO Coated Conductors for Power Transmission Cables

YBCO coated conductors are expected to be used in future high-temperature superconductor applications. For the application to power transmission cables, a number of YBCO coated conductors would be connected in parallel and they might be subjected to short-circuit fault currents. Therefore, in order to examine the stability and feasibility of YBCO power transmission cables, it is important to investigate the normal-zone initiation and propagation characteristics of the YBCO tapes and demonstrate the redistribution of transport current and the electromagnetic coupling between YBCO tapes during a short-circuit fault current. In this study, over-current pulses were applied to the two parallel-connected YBCO sample tapes in a liquid nitrogen bath, and the characteristics of the normal-zone initiation and propagation were examined experimentally. Additionally, we also show the experimental results of the redistribution of the transport current between parallel-connected YBCO sample tapes.

Design and production of high-Tc superconducting power transmission cable

The design and production of high temperature superconducting (HTS) power transmission cables was studied. In the production of HTS cable, difficulties are mainly caused by the poor mechanical properties of HTS tapes, because critical currents of the HTS tapes deteriorate due to the strains applied during cable production and usage. Therefore, two basic characteristics of HTS cables were experimentally analyzed to improve HTS cable design and production: (1) the mechanical-electrical properties of the HTS cable; and (2) the properties of electrical insulation. The analysis results indicate that the most important technology is the control of the strains applied to the tape in the cable. Based on the results, the design of the HTS cable was then improved, and the machines at Furukawa Electric fabricated a three-phase prototype HTS cable of 30 m in length. The results of the performance test of the cable demonstrated the proposed design and the production method are appropriate.