High- T C Superconducting Cable Research Articles

Design and testing of a 10 kV/1 kA superconducting energy pipeline prototype for electric power and liquid natural gas transportation

High Tc superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the refrigeration system for HTS cable is indispensable but undesirable. Considering that LNG (liquid natural gas) is now one kind of clean energy that is used more and more widely, it is possible to integrate HTS cable and LNG transportation into a single pipeline, resulting in a new energy transportation system- a HTS energy pipeline. In this paper, the principle and structure of the energy pipeline are first discussed and then the basic physical properties of the HTS tapes and liquid insulation medium in the LNG temperature range are analyzed. On this basis, a 10 kV/1 kA energy pipeline is designed, fabricated and tested.

A single-stage Stirling-type pulse tube cryocooler achieving 1080 W at 77 K with four cold fingers driven by one linear compressor

This paper presents the design, optimization and experimental verification of a high-capacity single-stage Stirling-type pulse tube cryocooler (SPTC) with four cold fingers driven by only one linear compressor. The flow characteristics in both pulse tubes and regenerators are studied with the aid of CFD, and the effects of flow straighteners and operating parameters on the cooling performance analyzed in detail. The interactions and performance controlling among the four cold fingers are emphasized. The developed SPTC can experimentally achieve a cooling capacity of 1080 W at 77 K with a relative Carnot efficiency of 16%, which suggests a good alternative for cooling large-scale high-Tc superconducting (HTS) facilities such as HTS cables.

Analysis and measurement of thermal conductivity of polypropylene laminated paper impregnated with subcooled liquid nitrogen

We measured the thermal conductivity of polypropylene laminated paper (PPLP) impregnated with subcooled liquid nitrogen. PPLP is widely used for the electrical insulation of high-Tc superconducting (HTS) power transmission cables. Although the thermal conductivity of PPLP is an important factor in the design of HTS cables, there has been very limited work on its measurement in subcooled liquid nitrogen. We prepared PPLP samples and symmetrically stacked them on both sides of a heater. The stacked samples were immersed in liquid nitrogen in an open cryostat. A cryocooler mounted on the cryostat was used to maintain the subcooled temperature of the liquid nitrogen. The thermal conductivity of the stacked PPLPs was measured by the steady state method at a bath temperature of 65–75K and was found to be 0.23–0.26W/mK, which is about five times that measured in a vacuum as presented in available literature. We also discuss possible mechanisms for boosting the thermal conductivity of PPLP by liquid nitrogen impregnation.

超電導ケーブルの海外開発動向

High Tc superconducting (HTS) Cables are expected as a new technology to transmit large amount of electricity in a compact size with lower loss. There are many projects to demonstrate their performance and reliability in the grid around the world. Demonstrations of the cables include those with up to 275 kV in voltage and up to DC 10 kA in current. Maximum capacity is more than 1 GVA and length is in the km order. Therefore, their commercialization is expected to occur in the near future. This report shows the present development status of HTS cables outside Japan in the USA, Europe, Korea and China.

Development of a 10 kA HTS DC Power Cable

The new energy revolution which will be dominated by renewable energy will need to develop a corresponding new power grid. Based on the characteristics of renewable energy resources, and the stability problem of AC network, it was proposed that the DC-based transmission grid should be developed in the future. The high Tc Superconducting (HTS) power cable will be a competitive candidate for large-capacity power transmission of renewable energy. In this paper, the progress of the development of a 360 m/10 kA HTS DC power cable is presented, and the prospects of HTS cable is discussed.

Stability Analysis of HTS Power Cable With Fault Currents

We numerically calculated the transient temperature distribution of flowing subcooled liquid nitrogen in a high-Tc superconducting (HTS) model cable when faults occur. The coolant and cable core temperatures were calculated by numerically solving the heat equation using the finite difference method. In the calculation, we assume that the heat transfer coefficient between the flowing subcooled liquid nitrogen and the cable core surface is described by the Dittus-Boelter correlation. The calculation results reveal that the coolant temperature increases even after the fault has been removed and that it continues increasing until fresh coolant arrives from the inlet. The calculated temperature profile of the coolant agrees well with measured data obtained by conducting over-current tests on a model HTS cable. Using our computational code, we also evaluated the maximum HTS cable lengths that ensure that the coolant remains in the liquid phase for certain fault currents for an HTS model cable.

Development of a 10 m long 1 kA 66/77 kV YBCO HTS cable with low AC loss and a jointwith low electrical resistance

We have developed high-Tc superconducting (HTS) cables using YBCO tapes. A 10 mm-wide YBCOtape was divided into five strips using a YAG laser to reduce AC loss. A0.3 m conductor and a 1 m conductor were fabricated achieving AC losses at1 kArms and 50 Hzof 0.054 W m−1 and of 0.048 W m−1, respectively. Based on the successful AC loss reduction in the 0.3 and 1 m conductors, wefabricated a 10 m HTS cable with a three-layer HTS conductor, electrical insulation,and a one-layer HTS shield. The AC loss in this 10 m HTS conductor was 0.090 W m−1 at1 kArms and 50 Hz. An HTS cable joint was designed and fabricated that included athree-layer HTS conductor and a one-layer HTS shield. Joint resistance of 5.2 nΩ for the HTS conductorand that of 10 nΩ for the HTS shield were achieved. The HTS cable joint was compact and had sufficientwithstand voltage properties for 66/77 kV. In addition, we also confirmed the efficiency ofjoint construction. Moreover, a 20 m long HTS cable was successfully constructed andtested in January 2008. This HTS cable consisted of two 10 m HTS cables and the HTScable joint. One 10 m HTS cable using HoBCO was made by Sumitomo, and the HTScable joint and the other 10 m cable using IBAD YBCO were made by Furukawaand Chubu. The electrical resistance of the HTS cable joint was less than 10 nΩ, and the 20 m HTS cable including the joint withstood an over-current of 31.5 kArms whose duration was 2 s.

Numerical study on self-field losses of 30 m BSCCO HTS transmission cable

The self-field losses of the one phase of high- T C superconducting (HTS) transmission cable are calculated by the electric circuit (EC) model. The one phase of HTS cable is constructed by the former of fine-strands copper rod, HTS conductor with four superconducting layers, the insulation made by polypropylene laminated paper, and HTS shielding with two superconducting layers, which was fabricated by Sumitomo Electric Industries (SEI). The length of the cable is 30 m. Each HTS layer comprises BSCCO tapes. The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse. The calculated losses are compared with the experimental results measured by 4-terminal method by SEI. The calculation of alternating current (AC) losses, a summation of the self-field losses in HTS layers and the eddy-current losses in the former, is almost equal to the measurement at wide transport-current range below the lowest value of the layer critical current. This result indicates that the numerical calculation by EC model is quite reliable. The minimum AC loss is also calculated by obtaining the optimum helical-pitch lengths of HTS layers at transporting 1 kA rms. The minimum loss is 36% lower than the loss of HTS cable designed by SEI at the transport current value. In HTS cable with the optimum helical-pitch lengths, the calculation of the layer currents are not uniform in HTS conductor but are almost uniform in HTS shielding, which is contradict to SEI’s one. It is considered that the numerical calculation by EC model is useful to obtain the optimum helical-pitch lengths in HTS cable with the minimum AC loss.

Overview of High-temperature Superconducting Power Cable for Practical Use

A High Tc Superconducting (HTS) Cable, which has a conductor with superconducting materials discovered in 1986, has been developed since early 1990s. After 10 and several years, the HTS cable has reached to the stage of evaluation at the real grid. Improvement of the performance and progress on the productivity of HTS wires are enhancing opportunities for commercialization of the HTS cable as well as other HTS applications. Especially in the USA, several demonstration projects of the HTS cable are undergoing in order to strengthen the fragile electric power network. In the Albany project, which is one of them, the 3-in-One type HTS cable was installed in 2006 and the world first operation has been conducted with an actual configuration in the long underground duct.In Japan, a new HTS cable project supported by NEDO has just started in order to study operation, maintenance method and related technologies for operating in the actual grid.This paper describes the position of HTS cable in accord with the history of large capacity power cables, summarizes its prospects and technical problems and introduces various projects in the world.

Calculation of the total AC loss of high-TC superconducting transmission cable

We have calculated the AC properties in a high-TC superconducting (HTS) cables fabricated by Sumitomo Electric Industries Ltd. and Furukawa Electric Industries Ltd., respectively. The AC losses of both HTS cable are measured by a calorimetric method. Our calculations of AC losses of both cables are almost equal to each measurement. The calculations also indicate that the layer’s currents in the conductors of both cables are uniform in a wide transport-current range. The uniform layer’s current causes the lowest self-field loss. Therefore the helical pitch length of the layers in both cables is designed to obtain the uniform layer’s current distribution. These results strongly support a certainty of our calculation method.

交流超電導電力機器基盤技術研究開発 超電導ケーブル用導体の交流損失温度依存性

The use of high Tc superconducting (HTS) cable is expected to realize compact, highly efficient transmission lines. In order to achieve high efficiency, it is important to reduce AC loss. However, accurately estimating the AC loss of a real cable system is very difficult because the cable length is very long and the operation temperature varies according to condition. The measurement of AC loss using a short sample is simple and possible using liquid nitrogen at the temperature of 77 K, but evaluating temperature dependence is very difficult because a refrigeration system is necessary. Moreover, a simple AC loss estimation method has been established for shield-less cable only, and is therefore not applicable to HTS cable with a shield layer. We have developed AC loss measurement equipment that uses calorimetrics and measured the temperature dependence for both critical current and AC loss of HTS cable with a shield layer. From the results, we propose a new simple estimation method for temperature dependence on AC loss by measuring the temperature dependence of critical current and AC loss at one point.

Numerical analysis of the AC loss in a high- TC superconducting cable measured by calorimetric method

We calculated the AC property in a high- T C superconducting (HTS) cable fabricated by Furukawa Electric Industries Ltd (FEI) by using an electric-circuit (EC) model. The AC loss of the HTS cable is measured by a calorimetric method by FEI. The measured AC loss and the calculated one are almost equal. According to the calculated results, the layer’s currents are uniform on the low transport current. On the high transport current, the layer’s current distribution indicates a drift-current phenomenon.

Research on Insulation Design of 22.9-kV High-<tex>$rm T_rm c$</tex>Superconducting Cable in Korea

The electrical insulation design is important for high-Tc superconducting (HTS) cable because the cable is operated in a medium voltage environment. For the insulation design of HTS cable, it is necessary to investigate the ac, impulse breakdown, and partial discharge inception stress of a liquid nitrogen/polypropylene laminated paper composite insulation system. These results were used to design the electrical insulation of 22.9-kV-class HTS cable in Korea and cable was manufactured. A manufactured mini-model HTS cable was evaluated for ac withstand voltage, impulse withstand voltage, breakdown and dielectric tangent loss (tan/spl delta/). From these tests, the cable has satisfied the standard technical specification of KEPCO in Korea. The breakdown voltage was 110 kV.

Electrical characteristics of high- Tc superconducting mini-model cable under mechanical stresses in liquid nitrogen

To develop 22.9 kV class high- T c superconducting (HTS) cable in Korea, we have been studying electrical insulation properties of dielectric paper, such as breakdown voltage, partial discharge, which is one of the HTS cable structure elements. However, the research on the mechanical stress of dielectric paper compared to breakdown properties of dielectric paper is insufficient. A cracking and variation of the electrical insulation due to mechanical stresses during cooling and bending of HTS cables in cryogenic temperature is a serious problem. Thus, we investigated tensile stress and breakdown stress of dielectric paper under mechanical stress. Moreover, we manufactured mini-model cables investigated breakdown stress under bending stress to design a cable drum for conveyance. In the AC, impulse and partial discharge properties, all test results showed a similar tendency, and the suitable bending radius ratio R/ r was decided to be more than 25.

Reduction of alternating current loss in a high- TC superconducting transmission cable by means of helical pitch adjustment

We have reported that the alternating current (ac) losses in a 66 kV rms 3-core high- T C superconducting (HTS) transmission cable fabricated by Tokyo Electric Power Company and Sumitomo Electric Industries Ltd. are calculated correctly by using an electric-circuit model. According to the calculated results, the circumferential field losses are dominant in the total ac losses in compared with the self-field losses and the axial-field losses. The helical pitches of each layer in the HTS cable are designed to obtain almost same layer currents, which gives the minimum self-field losses. We think that the optimum helical pitches giving the minimum total losses are different from the helical pitches designed by the companies and calculate the optimum values in the condition of the same helical direction of each layer in the cable. As a result, for example, it is found that the ac loss of 2.1 W m −1 cc −1 at transporting 1 kA rms can be reduced to 1.8 W m −1 cc −1 (about 14% reductions) after redesigning the cable with the optimum helical pitches. The optimum helical pitches are obtained for each given transport current. After redesigning, the distribution of layer currents is not uniform and the circumferential fields are reduced.

AC loss analysis of 114 MVA high- Tc superconducting model cable

We have calculated the alternating current (ac) losses of a 114 MVA high- T c superconducting (HTS) transmission cable using an electric-circuit (EC) model. The EC model is comprised of a resistive part and an inductive part. The resistive part is obtained by the approximated Norris equation for a HTS tape. The Norris equation indicates hysteresis losses due to self-fields. The inductive part has two components, i.e. inductances related to axial fields and those related to circumferential fields. The layer currents and applied fields of each layer were calculated by the EC model. By using both values, the ac losses of the one-phase HTS cable were obtained by calculation considering the self-fields, the axial fields and the circumferential fields of the HTS tape. The measured ac loss is almost equal to the calculation.

Alternating current loss calculation in a high-TC superconducting transmission cable considering the magnetic field distribution

We have calculated the alternating current (ac) losses of a 114 MVA high-TC superconducting (HTS) transmission cable using an electric-circuit (EC) model. The HTS cable is fabricated by Tokyo Electric Power Company and Sumitomo Electric Industries, Ltd. The EC model is comprised of a resistive part and an inductive part. The resistive part is obtained by the approximated Norris equation for a HTS tape. The Norris equation indicates hysteresis losses due to self-fields. The inductive part has two components, i.e. inductances related to axial fields and those related to circumferential fields. The layer currents and applied fields of each layer were calculated by the EC model. By using both values, the ac losses of the one-phase HTS cable were obtained by calculation considering the self-field, the axial field and the circumferential field of the HTS tape. The measured ac loss transporting 1 kArms is 0.7 W m−1 ph−1, which is equal to the calculation. The distribution of each layer loss resembles in shape the distribution of the circumferential field in each layer, which indicates that the circumferential fields strongly influence the ac losses of the HTS cable.

Development of important elementary technologies for a 66 kV-class three-phase HTS power cable

High T c superconducting (HTS) cables are expected to serve as underground power lines supplying electrical power to densely populated cities in the future. TEPCO and Furukawa have developed compact HTS cables that can replace the old cables in their existing ducts under metropolitan Tokyo. To connect HTS cables with actual electrical networks requires confirmation of long-term reliability of the electrical insulation and the ability to withstand accidents caused by short circuit. This report gives the results of our examination of these problems.

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.