Superconducting Power Cables Research Articles

Thermal and Cooling Characteristic Analysis of a Superconducting DC Power Cable for Railway Application

A superconducting power cable is required to be cooled down and kept at a very low temperature, so it is important to reveal its thermal and cooling characteristics. We have analyzed the heat flow and temperature distribution of a single-core superconducting power cable for railway application. Temperature of the cable increases due to heat flow through the vacuum thermal insulation layer and the current leads. A superconducting cable core is displaced from the cable center axis by gravity and by thermal contraction at a curved section. These forces press the vacuum thermal insulation layer, which would influence its thermal insulation performance. Heat flow through the thermal insulation layer, etc. is evaluated, and FEM analysis is carried out to obtain the temperature distribution. The total heat invasion and the pressure drop are also estimated in the superconducting power cable.

Transposition-Based Connection Scheme and Joint Box Concept for a Tri-Axial HTS Power Cable

Tri-axial high temperature superconducting (HTS) power cables are being developed to maximize their advantages, which include reducing amount of HTS wires, having a low-leakage magnetic field, and being compact compared to different types of HTS power cables. However, a drawback of HTS power cables is their inherent imbalance impedance, which is due to an asymmetrical structure on each phase. To solve this problem, the phases must be transposed. The authors designed a distribution class tri-axial HTS power cable, as well as a transposition-based connection scheme for the cable. The phases of the cable were connected to each other via transposition through their cross-connections. This was achieved with an insulated support structure with three walled-off areas for insulation between three the phases, as well as transposition conductors, including copper base and HTS wires. In this paper, a distribution class tri-axial HTS power cable and a connection scheme for it are designed. The results, including the transposition-based joint box design, are described in detail. The connection scheme will be applied to the joint box of the cable for high-capacity and long-distance transmission.

Concept and Design of 500 Meter and 1000 Meter DC Superconducting Power Cables in Ishikari, Japan

The design and construction of two DC superconducting power cables were started in the spring of 2013 as a national project in Japan. The cable lengths are 500 and 1000 m, respectively. The project is called the Ishikari Project. Four parties, Chiyoda Corp., Sumitomo Electric, Sakura Internet, and Chubu University joined to establish a partnership. The 500-m cable will connect an Internet data center (iDC) to a large scale array of photovoltaic cells to supply dc power. During the design and the construction in the field, test benches have also been constructed to investigate the performance of the individual components of the cable system. The design value of the heat leak of the cryogenic pipe is 1.3 W/m and the experimental result is 0.7 W/m to 1.3 W/m on the test bench. The design value of the heat leak from the current lead is 35 W/kA and the experimental value is 30.5 W/kA at the rated current. Control of the thermal contraction and expansion are important to maintain cable safety and two methods are considered. Construction was started in the field in mid-May, and the 500-m cable will be completed at the end of the fiscal year 2014.

Cryogenic Tests of 30 m Flexible Hybrid Energy Transfer Line with Liquid Hydrogen and Superconducting MgB2 Cable

Recently we reported about first in the world test of 10 m hybrid energy transfer line with liquid hydrogen and MgB2 superconducting cable. In this paper we present the new development of our second hybrid energy transfer line with 30 m length. The flexible 30 m hydrogen cryostat has three sections with different types of thermal insulation in each section: simple vacuum superinsulation, vacuum superinsulation with liquid nitrogen shield and active evaporating cryostatting (AEC) system. We performed thermo-hydraulic tests of the cryostat to compare three thermo-insulating methods. The tests were performed at temperatures from 20 to 26 K, hydrogen flow from 100 to 450 g/s and pressure from 0.25 to 0.5 MPa. It was found that AEC thermal insulation practically eliminated completely heat transfer from room temperature to liquid hydrogen in the 10 m section. AEC thermal insulation method can be used for long superconducting power cables. High voltage current leads were developed as well. The current leads and superconducting MgB2 cable have been passed high voltage DC test up to 50 kV DC. Critical current of the cable at ∼21 K was ∼3500 A. The 30 m hybrid energy system developed is able to deliver up to 135 MW of chemical and electrical power in total.

Cryogenic design and test results of 30-m flexible hybrid energy transfer line with liquid hydrogen and superconducting MgB2 cable

In this paper we present the development of a new hybrid energy transfer line with 30m length. The line is essentially a flexible 30m hydrogen cryostat that has three sections with different types of thermal insulation in each section: simple vacuum superinsulation, vacuum superinsulation with liquid nitrogen precooling and active evaporating cryostatting (AEC) system. We performed thermo-hydraulic tests of the cryostat to compare three thermo-insulating methods. The tests were made at temperatures from 20 to 26K, hydrogen flow from 70 to 450g/s and pressure from 0.25 to 0.5MPa. It was found that AEC thermal insulation was the most effective in reducing heat transfer from room temperature to liquid hydrogen in ∼10m section of the cryostat, indicating that it can be used for long superconducting power cables. High voltage current leads were developed as well. The current leads and superconducting MgB2 cable passed high voltage DC test up to 50kV DC. Critical current of the cable at ∼21K was 3500 A. It means that the 30m hybrid energy system developed is able to deliver ∼50–60MW of chemical power and ∼50–75MW of electrical power, i.e. up to ∼135MW in total.

Production of silver for use as a global-scale superconducting power cable material

High-temperature superconductivity materials are made out of a kind of ceramics. However, the core of high-temperature superconducting (HTS) power cables consists primarily of layers of copper and silver. One kilometre of second-generation HTS power cable may contain as much as 40 kg of silver. It is envisaged that a global superconducting power grid will be required to interconnect the globally distributed hyper-large-scale solar power generation systems to compensate for output fluctuation of each local system by other local systems throughout the grid. In this paper, the resources and production constraints of silver will be discussed in terms of their impact on the operability of the global electric power network based on second-generation HTS power cables.

Characteristic Analysis of DC Electric Railway Systems with Superconducting Power Cables Connecting Power Substations

The application of superconducting power cables to DC electric railway systems has been studied. It could leads to an effective use of regenerative brake, improved energy efficiency, effective load sharing among the substations, etc. In this study, an electric circuit model of a DC feeding system is built and numerical simulation is carried out using MATLAB-Simulink software. A modified electric circuit model with an AC power grid connection taken into account is also created to simulate the influence of the grid connection. The analyses have proved that a certain amount of energy can be conserved by introducing superconducting cables, and that electric load distribution and concentration among the substations depend on the substation output voltage distribution.

Superconducting Power Cable

Superconducting Power Cable

Design and PHILS-based Transient Analysis of a Tri-axial HTS Power Cable

Tri-axial high-temperature superconducting (HTS) power cables are very efficient compared with other HTS power cables due to their reduced use of HTS wires and cryogenic surface area, resulting from the mutually layered structure of the three phases. However, the operating characteristics of tri-axial HTS power cables differ from other cables in a transient-state condition. In order to install HTS power cables in a real grid, feasibility verification through simulation or experimentation is required in advance. Thus, the authors design a tri-axial HTS power cable and implement a power hardware-in-the-loop simulation that consists of a real time digital simulator-based simulation model and hardware devices including a power supply and a 1 m-long tri-axial HTS model cable. Simulation results show the stability verification under steady-state and transient-state conditions.

Japan's First In-grid Demonstration of a High-<i>T</i><sub>c</sub> Superconducting Cable System

High-temperature superconducting (HTS) power cable has significant merits of compactness and large power transmission capacity. To evaluate its performance, safety and reliability, HTS cable system was constructed at Tokyo Electric Power Company's Asahi substation in Yokohama, and the HTS cable had been connected to a real grid from October 29, 2012 to December 25, 2013 for the first time in Japan. The in-grid operation had continued for more than one year without any accidental interruption of the operation or troubles of the system. It is concluded that the HTS cable system has good performance and stability for long term in-grid operation. This paper describes the result of the HTS cable demonstration and their characteristics.

Design and construction of a high temperature superconducting power cable cryostat for use in railway system applications

The primary objective of the current effort was to design and test a cryostat using a prototype five-meter long high temperature Bi2Sr2Ca2Cu3Oy (Bi-2223) superconducting dc power cable for railway systems. To satisfy the safety regulations of the Govt of Japan a mill sheet covered by super-insulation was used inside the walls of the cryostat. The thicknesses of various walls in the cryostat were obtained from a numerical analysis. A non-destructive inspection was utilized to find leaks under vacuum or pressure. The cryostat target temperature range was around 50 K, which is well below liquid nitrogen temperature, the operating temperature of the superconducting cable. The qualification testing was carried out from 77 down to 66 K. When using only the inner sheet wire, the maximum current at 77.3 K was 10 kA. The critical current (Ic) value increased with decreasing temperature and reached 11.79 kA at 73.7 K. This is the largest dc current reported in a Bi2Sr2Ca2Cu3Oy or YBa2Cu3Oy (Y-123) superconducting prototype cable so far. These results verify that the developed DC superconducting cable is reliable and fulfils all the requirements necessary for successful use in various power applications including railway systems. The key issues for the design of a reliable cryogenic system for superconducting power cables for railway systems are discussed.

Flux-transfer losses in helically wound superconducting power cables

Minimization of ac losses is essential for economic operation of high-temperature superconductor (HTS) ac power cables. A favorable configuration for the phase conductor of such cables has two counter-wound layers of HTS tape-shaped wires lying next to each other and helically wound around a flexible cylindrical former. However, if magnetic materials such as magnetic substrates of the tapes lie between the two layers, or if the winding pitch angles are not opposite and essentially equal in magnitude to each other, current distributes unequally between the two layers. Then, if at some point in the ac cycle the current of either of the two layers exceeds its critical current, a large ac loss arises from the transfer of flux between the two layers. A detailed review of the formalism, and its application to the case of paramagnetic substrates including the calculation of this flux-transfer loss, is presented.

Superconducting Power Cable Application in DC Electric Railway Systems

A superconducting power cable is a prospective application of high-temperature superconductors. Although power cables using Bi2223 or YBCO wires have been developed in the world so far mainly for utility grid application, we focus attention on railway application of a superconducting power cable. We have studied how to introduce superconducting power cables effectively and economically to a dc electric railway. Numerical analysis models based on the MATLAB-Simulink are constructed for a dc electric feeder system of singe- and double-track railways. Tractive force and train resistance characteristics are modeled. Energy saving by the superconducting power cable is investigated with the influences of the number of substations, the train operation interval, etc., taken into account. The results show that the superconducting power cables can improve regeneration rate and system energy saving, and reduce the substation capacity very effectively. It will be also an advantage that if the transportation capacity of a railway line needs to be increased, the introduction of superconducting cables could achieve it without changing the existing substations along the line.

Transient Characteristic Analysis of a Tri-Axial HTS Power Cable Using PSCAD/EMTDC

The characteristics of the tri-axial high temperature superconducting (HTS) power cable are different from a general copper cable, depending on whether the cable is under steady state or transient state, which is caused by quench. Before applying the tri-axial HTS power cable to a real utility, the system should be analyzed using certain simulation tools in order to confirm the effects of the cable on the real utility. However, a component that has the same impedance characteristics as the real tri-axial HTS power cable is not provided in simulation tools. In this paper, a model component for the tri-axial HTS power cable was developed in PSCAD/EMTDC. The developed model component included characteristics of the HTS cable, such as resistance, reactance, and temperature variation. The model component for the tri-axial HTS power cable was applied to a model power system and a simulation was performed under transient state. The simulation results show the effect of the tri-axial HTS power cable on the power system, the variation of quench resistance, and the temperature of the cable under fault conditions according to superconducting characteristics. The results of this work will be used to analyze the transient characteristics of other types of HTS power cables.

Thermal Analysis of HTS Power Cable Using 3-D FEM Model

Thermal stability is an important issue to put high-temperature superconducting (HTS) power cables into practical use. Cooling system breakdown and fault current are the two major factors that might cause overheating or quench in HTS power cables. In this paper, a dynamical model of the HTS cable is built with the 3-D finite element method (3-D FEM), and the dynamic thermal characteristic under different operation conditions is analyzed. The computational results of this model can be used to evaluate the operation parameters of HTS cables and to set the action time of the relay protection system. The electrical and thermal parameters are based on the HTS cable, which is being manufactured in China.

Development and Performance Analysis of a 22.9 kV/50 MVA Tri-axial HTS Power Cable Core

Tri-axial high-temperature superconducting (HTS) power cables are being developed to maximize their advantages such as reduced amount of HTS wires, a low-leakage magnetic field, and compactness compared with the different types of HTS power cables. The authors designed a 22.9 kV/50 MVA-class tri-axial HTS power cable core to apply to the distribution system in Korea. The inherent imbalance in the three-phase currents of the tri-axial HTS power cable core was calculated using the impedance matching program. A tri-axial HTS power cable core was fabricated using second-generation YBCO wires and the cable core was tested under 77 K liquid nitrogen to verify the performance of the cable core through obtaining its electrical characteristics data. This paper describes the results of the design, fabrication, and evaluation of the 22.9 kV/50 MVA-class tri-axial HTS power cable core. They will be used to develop a tri-axial HTS power cable for the distribution system.

The Research of Protection Strategy for 110 kV Cold Dielectric High Temperature Superconducting Power Cable

The protective relay is of important significance to ensure the high-temperature superconducting (HTS) cable in safe and reliable operation, which is also the key technology problem that needs to be solved in the process of HTS cable's application. In this paper, the 110 kV cold dielectric (CD) HTS cable is chosen as the research object. First, according to the 110 kV CD HTS cable's technical characteristics and application in power grids, the fault of operation is discussed. Based on the above, the paper presents a protection strategy for the 110 kV CD HTS cable, which combines the advantage of the electric and nonelectric parameters in the detection and could be easily put into use in the projects. Finally, the whole protection strategy is presented. The protection strategy proposed in this paper provides a feasible idea for the protection of HTS cables, which is of positive significance on the promotion of the superconducting cable.

Experiments and Numerical Simulations on Local Degradation Characteristics of Coated Conductor Due to Overcurrent

Coated conductors are subjected to short-circuit currents 10 to 30 times greater than the operating current in the use of superconducting power cables. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and thickness of the stabilizer are considered to be nonuniformly distributed in the real manufacturing process. Thus, the coated conductor may be damaged locally by the hot spot due to the overcurrent. Therefore, it is important to clarify the local degradation characteristics of the coated conductor and determine its tolerance against the fault current in the actual operation. In a previous study, overcurrent experiments and numerical simulations on thermal stress-strain were carried out on a coated conductor with a nonuniformly distributed <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In this study, two samples with nonuniformly distributed copper plating thickness were tested. Numerical simulations based on the finite element method were performed to simulate the temperature and thermal stress-strain distribution caused by the overcurrent. The nonuniform thickness of the copper plating and the longitudinal length of the nonuniform area were considered as the simulation parameters.

Numerical analysis of the stability of HTS power cable under fault current considering the gaps in the cable

During the operation of a high temperature superconducting power cable in a real grid, the power cable can be impacted inevitably by large fault current. The study on current distribution and thermal characteristics in the cable under fault current is the foundation to analyze its stability. To analyze the operation situation of 110kV/3kA class superconducting cable under the fault current of 25kArms for 3s, the equivalent circuit equations and heat balance equations were established. The current distribution curves and the temperature distribution curves were obtained. The liquid nitrogen which exists in the gaps of HTS cable was taken into consideration, the influence of gap liquid nitrogen on the thermal characteristics was investigated. The analysis results can be used to estimate the security and stability of the superconducting cable.

Towards Solving the Problem of Transmission and Distribution of Electric Power in Nigeria via Superconductor Power Cables

Data on transmission and distribution losses in some African countries on conventional cables show that Nigeria experiences the highest transmission and distribution losses (=6.81 x 10 15 kWh) among African countries and, certainly, among the highest in the world with per capita consumption of 0.03 kWh/person. The total installation cost comparison between conventional and superconducting cables (SC) has been done and the comparison of the losses in SC cables and the conventional ones has also been analyzed. Some of the parametric properties of superconducting materials and benefits of high temperature superconducting (HTS) cables are discussed. Transmission lines in Nigeria have been found to experience high voltages and very low voltages under light loading and high loading, respectively. It has also been found that with the use of HTS cables, these losses, which are mainly the I2R losses, are negligibly small and almost zero. It is also found that HTS cables would be panacea for these high transmission and distribution losses in Nigeria. KEYWORDS : Transmission, distribution and superconducting cables.