Superconducting DC Cable Research Articles

Study of Magnetic-Field Distribution Around the High-Tc Superconducting DC Cable

Study of Magnetic-Field Distribution Around the High-Tc Superconducting DC Cable

Hydrogen-electricity hybrid energy pipelines for railway transportation: Design and economic evaluation

With the decarbonization and electrification of modern railway transportation, the demand for both the high-capacity electrical energy and hydrogen fuel energy is increasingly high. A novel scheme was proposed from liquid hydrogen production by surplus wind and solar energy, to liquid hydrogen-electricity hybrid energy transmission for railway transportation. The 100 MW hybrid energy transmission pipeline was designed with the 10 kA/1.5 kV superconducting DC cable for electricity and cryogenic layers for liquid hydrogen and liquid nitrogen, showing strong capability in transmitting “electricity + cold energy + chemical energy” simultaneously. Economic evaluation was performed with respect to the energy, equipment capacity, and costs, with sensitivity and profitability analysis. With the discount rate 8%, the dynamic payback period of the hybrid energy pipeline was 7.1 years. Results indicated that the shortest dynamic payback period of the hybrid energy pipeline was 4.8 years, with the maximum transmission distance 93 km. Overall, this article shows the novel concept and design of liquid hydrogen-electricity hybrid energy pipelines, and proves the technical and economic feasibilities for future bulk hybrid energy transmission for railway transportation.

A Review on Insulation and Dielectrics for High- Temperature Superconducting Cables for Power Distribution: Progress, Challenges, and Prospects

The high temperature superconducting (HTS) AC and DC cables are gradually finding their way towards commercialization due to their ability to transfer bulk electric power with negligible dielectric loss. In this context, this paper critically reviews the different aspects of HTS cables used in various applications and related accessories. The operational, lab-scale, and prototype HTS cable projects are discussed in detail. Different dielectrics, electrical insulations, superconductors, cooling systems, cryogens, and upcoming projects of HTS cables are reviewed. The underlying challenges and future directions in AC and DC cables, electric aircraft and ships, associated costs, cryogenic insulation materials, cooling technologies, superconductors, and electrical insulations of HTS cables are discussed in detail. The space charge performance of presently-used polypropylene laminated paper (PPLP) insulation and future insulation, i.e., Kapton material, is experimentally analyzed, to show the future challenges in the insulation design. The technology readiness level, market trends, and relevant test standards are explored. The research and market trends suggest that HTS cables can replace conventional transmission methods. However, their consistent growth and adaptation of HTS cables will depend on design of cryocoolers, raw material cost of superconductors, and large-scale production.

Fault Transient Study of a Meshed DC Grid With High-Temperature Superconducting DC Cables

This paper presents the DC fault transient study of a meshed DC grid with high-temperature superconducting (HTS) HVDC cables to integrate offshore wind power. A four-terminal meshed DC grid model with a ±100 kV DC voltage rating is developed. DC circuit breakers (DCCBs) are implemented in the DC grid to deal with DC cable faults. To conduct the fault transient study, the fault scenarios with different fault locations and protection delays are studied. The sensitivity study versus different DC fault locations reveals that the faults on the faulted cable will not cause the quenching of healthy cables. The sensitivity study considering different DC fault protection delays demonstrates that the HTS cables have a current-limiting effect and the long delay of DC fault protection does not result in large fault current and the quenching of healthy cables. Extensive simulations in PSCAD/EMTDC validate the founding.

Multi-Objective Optimization for Improving Weight and Fault Characteristics of a DC HTS Cable in Cryo-Electric Aircraft

The aim of the presented study is to optimize the different classes of high-temperature superconducting (HTS) DC cables for improving their performances in a cryo-electric aircraft considering their weight, peak temperature during faults, and the ratio of current passing through each tape to the critical current of HTS tapes. These terms were interpreted into three objective functions, and a multi-objective optimization algorithm known as non-dominated sorting genetic algorithm II was used to find the optimal solution clusters. The cable optimization was conducted for different former materials by changing the former thickness and radius. Results showed that the DC HTS cables with aluminum former have the lowest weight while cables with copper formers have the best thermal performance against faults.

Stories From China

Superconducting DC cable has the advantages of high current carrying capacity and low loss. It needs an ultra-low temperature to maintain operation. The transmission of clean fuel [such as hydrogen, liquefied natural gas (LNG), ethylene, etc.] in liquid form has the advantages of high energy density and large transmission capacity per unit volume. It also requires an ultra-low temperature environment and thermal insulation. The superconducting electricity-gas integrated DC energy transmission pipeline combines the advantages of the two techniques. The DC energy and liquid fuel are transmitted together with the same refrigeration system and thermal insulation pipe. The superconducting core is cooled by low-temperature fuel, and the refrigeration system and thermal insulation pipe are shared. The superconducting electricity-gas integrated DC energy transmission pipeline improves the overall efficiency of energy transmission and reduces the comprehensive cost [1], [2]. It has the potential to be used in offshore wind power transmission, port power, and other conditions.

Effect of Harmonic Voltage On Partial Discharge Properties of LN2/PPLP Insulation for HTS DC Cable

In this paper, the effect of harmonic voltage on partial discharge properties of LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /PPLP (Liquid nitrogen/ Polypropylene-Laminated-Paper) insulation system for high-temperature superconducting (HTS) DC cable is studied. The harmonic components with a frequency from 50 to 350 Hz are superimposed on the DC voltage to analyze the mechanism of the harmonic amplitude and order on partial discharge (PD). The experimental results show that when the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> of the composite voltage remains consistent, the increase in the proportion of the harmonic amplitude will cause the enhancement of partial discharge amplitude, discharge counts and pulse repetition rate, especially as the amplitude of harmonic component exceeds that of DC voltage. The increase of the harmonic order results in variation of the derivative of the applied voltage with time (dv/dt) along the phase, which further causes the pulse repetition rate to increase. It is concluded that the harmonic components superimposed on DC voltage promote the occurrence of PD, which poses a significant threat to the insulation of superconducting DC cable.

Analysis and Methods for HTS DC Cables

High-temperature superconducting (HTS) DC cables with the merits of light weight, large capacity and low loss provide a novel power transmission method for future utility grid. In this paper, the electromagnetic characteristics of unipolar and bipolar coaxial HTS DC cables are compared by finite element simulation. Besides, the 3D magnetic flux density distribution characteristics of HTS DC cable are obtained. The method to investigate the influence of winding angle on HTS DC cable magnetic flux density is also applied to 3D HTS DC analysis.

Electromagnetic Analysis of HTS DC Cables Based on Critical State Model

High-temperature superconducting (HTS) DC cables have the advantages of low transmission loss, high current density and large transmission capacity, which have gained more and more attentions in recent years. The typical structures of HTS DC cables include unipolar and bipolar coaxial that doubles the cable's transmission capacity. Based on the critical state model, this article analyses and contrasts the critical current density distributions and degradations due to magnetic field in the HTS tapes of the unipolar and coaxial bipolar DC cables. The influences of the bipolar cables with different voltage levels are also compared. The results show that the critical current density degradations due to magnetic field are almost the same in both unipolar and coaxial bipolar structures, which are mainly caused by the self-field. For the bipolar cables, the current density distribution in the outer layer HTS tapes changes with the voltage level. To evaluate the influence of the axial magnetic field, a 3-D model of the unipolar cable is established with a helical wound pitch of the HTS tapes. The results show that the axial magnetic field is relatively low and the critical current density would not degrade if the helical angle of the HTS tapes is reasonably small.

Solution to Fault of Multi-Terminal DC Transmission Systems Based on High Temperature Superconducting DC Cables

In this paper, we developed the small-signal state-space (SS) model of hybrid multi-terminal high-voltage direct-current (HVDC) systems and fault localization method in a failure situation. The multi-terminal HVDC (MTDC) system is composed of two wind farm side voltage-source converters (VSCs) and two grid side line-commutated converters (LCCs). To utilize relative advantages of the conventional line-commutated converter (LCC) and the voltage source converter (VSC) technologies, hybrid multi-terminal high-voltage direct-current (MTDC) technologies have been highlighted in recent years. For the models, grid side LCCs adopt distinct two control methods: master–slave control mode and voltage droop control mode. By utilizing root-locus analysis of the SS models for the hybrid MTDC system, we compare stability and responses of the target system according to control method. Furthermore, the proposed SS models are utilized in time-domain simulation to illustrate difference between master–slave control method and voltage droop control method. However, basic modeling method for hybrid MTDC system considering superconducting DC cables has not been proposed. In addition, when a failure occurs in MTDC system, conventional fault localization method cannot detect the fault location because the MTDC system is a complex form including a branch point. For coping with a failure situation, we propose a fault localization method for MTDC system including branch points. We model the MTDC system based on the actual experimental results and simulate a variety of failure scenarios. We propose the fault localization topology on a branch cable system using reflectometry method. Through the simulation results, we verify the performance of fault localization. In conclusion, guidelines to select control method in implementing hybrid MTDC systems for integrating offshore wind farms and to cope with failure method are provided in this paper.

DC Breakdown and Surface Potential Behavior of Epoxy/Al2O3 Nanocomposites at Cryogenic Temperature

For high temperature superconducting (HTS) dc cables, one key issue is the necessity of using polymer dielectrics with higher dielectric strength at cryogenic temperature in liquid nitrogen. Nanocomposites have played an important role in improving the dielectric properties of epoxy resin (EP). This article investigates on the dc breakdown strength and the surface potential behavior of the epoxy/Al2O3 nanocomposites (0, 1, 3, 5 wt%) at the room and cryogenic temperatures. Moreover, the trap distributions are analyzed from the surface potential decay (SPD) measurement. The possible effect of the trap distributions on the dc breakdown strength is discussed with the valid results. The measurements show that the dc breakdown strength and the initial surface potential continually increased with the nanoparticle addition up to 3 wt% at both room and cryogenic temperatures, and decreased thereafter. The enhancement of the breakdown strength and the initial potential is also found at cryogenic temperature. Besides, the addition of the nanoparticles decreased the rate of SPD compared with the pure epoxy. It is proposed that the higher trap density and trap level enhances the dc breakdown strength. Interestingly, under the cryogenic environment, the nanoparticles addition significantly affects the trap distributions, which further influences the dielectric strength. The enhanced dc breakdown strength is attributed to the reduced space charge formation, the charge carrier mobility and the energy of the charge carriers within the sample bulk.

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.

Simulation and experiment on superconducting DC energy pipeline cooled by LNG

Superconducting DC energy pipeline is an energy channel that simultaneously transmits electrical energy as well as the liquid fuel. It improves the efficiency of energy transmission and reduces the overall cost by cooling high-temperature superconducting DC cables with the low-temperature liquid fuel, such as liquid hydrogen, liquefied shale gas, and liquified natural gas (LNG). In order to properly design this, it is very important to know the temperature distribution along the pipeline, which is yet to be carried out by the researchers. In this paper, a 10 m energy pipeline with LNG cooling and having a concentric nested structure is simulated and tested experimentally. The simulation results of the temperature rise along the pipeline were in good agreement with the experimental results. The effect of various parameters on the pipeline temperature were also obtained. Finally, based on the theoretical equations and simulation results, an empirical formula is proposed to calculate the temperature at the outlet of the energy pipeline.

General design of ±100 kV/1kA energy pipeline for electric power and LNG transportation

High Tc Superconducting (HTS) DC cable is a promising solution for large-scale power transmission over long distance. However, the indispensable refrigeration system for HTS cable might be undesirable. Considering that LNG (liquid natural gas) now is one kind of clean energy that is used more widely and widely, it is possible to integrate the HTS cable and LNG transportation into an energy pipeline, resulting an acceptable new energy transportation system. In this paper, the principle and structure of the energy pipeline are discussed firstly, and then the basic physical properties of HTS tapes and liquid insulation media in LNG temperature range are analyzed. On the base as above, the general design of ±100 kV/1kA energy pipeline is made.

Magnetic Field Effect on Critical Current of Different Structural HTS DC Cables

High-temperature superconducting (HTS) dc cables become viable for future dc electric power transmissions due to their low loss, light weight, high current density, and high transmission capacity. According to the structure, HTS dc cables are commonly divided into unipolar, bipolar coaxial and multicore types. The HTS critical current is magnetic field dependent, while different HTS dc cable structures have different field distribution features. The influence of different HTS dc cable structures on the critical current is analyzed and compared, in order to provide a basis for the design of a practical HTS dc cable.

Temperature state of electrical insulation of a superconducting DC cable

The purpose of the study was to build a nonlinear mathematical model governing the steady-state one-dimensional temperature distribution in the electrical insulation layer made in the form of a long hollow circular cylinder whose surfaces are given a constant electric field potential difference. By means of this model, integral ratios, which connecting the parameters of the temperature state of this layer, the heat transfer conditions on its surfaces and the temperature-dependent heat conduction coefficient and the electrical resistivity of the electrical insulating material with a given electrical potential difference, were built. A quantitative assay of the integral ratios is carried out with regard to the electrical insulation layer of a superconducting cable cooled by liquid nitrogen.

Modeling and optimization of gaseous helium (GHe) cooled high temperature superconducting (HTS) DC cables for high power density transmission

Superconducting cables are considered a viable technology to meet the increasing global demand of electricity transmission and distribution. This paper presents a transient mathematical model to predict the thermal response of a superconducting cable contained in a flexible cryostat. The model was conceived to be computationally fast so that system response according to variations of physical properties of the materials, and operating and design parameters could be assessed for optimization purposes. A volume element method (VEM) was utilized, which resulted in a system of ordinary differential equations with time as the independent variable. The model is also space dependent, through the establishment of a mesh with a known three-dimensional distribution of the volume elements in the computational domain. Pressure drop in the gas channels and the temperature gradient with respect to space in the flow direction were taken into account. The numerically calculated DC cable heat leak rate under different environmental conditions was initially adjusted and then experimentally validated by direct comparison to actual experimental data. The final part of the study consisted of using the experimentally validated model to perform the DC cable design and operating parameters optimization in order to obtain minimum heat leak rate and pumping power (or total consumed power). By adopting a fixed cable cross sectional area constraint (or total volume for a given length), an optimized helium channels geometry is also found that shows significant improvement in system performance in comparison to an existing system geometry. For example, for a GHe mass flow rate of 3.8 g s−1, the cryostat with the original geometry is shown to consume 20.5% more power than with the optimized geometry. As a result, it is reasonable to state that the combination of accuracy and low computational time allow for the future utilization of the model as a reliable tool for HTS DC cable & cryostat simulation, control, design and optimization purposes.

Multiterminal DC Transmission Systems Based on Superconducting Cables Feasibility Study, Modeling, and Control

This paper deals with the appropriate modeling and control strategy of a multiterminal dc transmission (MTDC) system that incorporates high-temperature superconducting (HTS) dc cables. The system is based on voltage source converters for the interconnection of stiff ac grids. An overview of the high voltage direct current power transfer technology along with the possibilities it enables, as well as an introduction to the concept of multiterminal topology are presented. The operation principles of the superconducting technology are described and the control strategy of HTS-based MTDC networks is presented. To validate the performance and the dynamic response of the system under steady state as well as under fault and load change conditions, a typical four-terminal MTDC network in ring topology is developed in MATLAB/Simulink.

Surface Flashover Characteristics of PPLP According to Its Orientation in Cryogenic Environment

Polypropylene laminated paper (PPLP), which is used as the main insulation material of the stop joint box (SJB), has excellent dielectric characteristics in cryogenic environment. Despite excellent dielectric characteristics of PPLP, the interface between liquid nitrogen (LN2) and PPLP of the SJB is very vulnerable to the surface flashover. Therefore, it is necessary to investigate the surface flashover characteristics of dielectric materials including LN2 and PPLP for the insulation design of the SJB. PPLP has two orientations according to the direction of fiber in Kraft paper that are cross direction (CD) and machine direction (MD). The research on the mechanical properties of PPLP has been reported since there are differences in physical properties depending on its orientation. However, the study of the surface flashover characteristics of PPLP in the cryogenic environment according to its orientation has not been reported yet. Thus, the surface flashover characteristics of PPLP according to its orientations should be examined to ensure the reliability of the SJB connecting the High-temperature superconducting (HTS) dc cable. In this paper, the surface flashover characteristics of the PPLP in cryogenic environment in accordance with its orientation was fully investigated. The dc surface flashover tests were performed with different orientations of PPLP and various gap distances. In addition, the surface leakage currents were measured by increasing voltage at fixed gap distance. From the test results, the dc surface flashover voltage of CD was found to be 15-20% higher than that of MD. In addition, the breaking point of CD was three times higher than MD, and beyond the breaking point, the surface leakage current of CD was lower than that of MD. Consequently, the surface flashover characteristics of PPLP are significantly affected by fiber orientations of PPLP.

Determination of Threshold Electric Field for PPLP Specimen in Liquid Nitrogen Based on the Measurement of Electrical Conductivity

The externally applied electric field causing sudden increase in electrical conductivity is called the threshold electric field. And the information of this threshold electric field is very important for reliable insulation design of dc power apparatus because dc electric field distribution is governed by electrical conductivity of the insulating material. There are several reports on the threshold electric field of XLPE and polymeric materials used for dc power cables, operated at room temperature. However, there are no previous works on threshold field of cryogenic insulating materials, which is necessary for the reliable insulation design of a superconducting dc cable. Therefore, in this study, we tried to determine the characteristics of threshold electric field of a polypropylene laminated paper (PPLP) specimen in liquid nitrogen by varying the number of PPLP layers. From the experiments, we could observe that there exists the threshold electric field of PPLP in a cryogenic environment. Furthermore, it was found that there was no difference in the measured threshold electric field in spite of different number of PPLP layers. In addition, it was found that if electrical stress exceedes the threshold field on PPLP specimen, the conductivity can be varied according to the number of PPLP layers.