High Temperature Superconducting DC Cable Research Articles

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.

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.

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.

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.

Thermal and Electromagnetic Design of DC HTS Cables for the Future French Railway Network

The use of High Temperature Superconducting (HTS) cables as a technology is considered as key to increasing the power of the electrical grid while reducing the volume of installation. In addition, when transmission currents exceed a few kA, DC HTS cables significantly reduce power losses, rights-of-way and total system mass. This article describes the various studies that need to be carried out in order to correctly size HTS DC cables for the new railway network envisaged by the French company SNCF, which must consider ultra-urban needs. The process used to design DC cables for operating powers of 10, 20 and 30 MW at a nominal voltage of 1.5 kV using commercial (RE)BCO tapes is presented. In this sizing process, the critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (B, θ, T) dependence of the superconducting tapes, the thermal properties of the materials and the different cooling modes are taken into account. Several solutions are suggested to ensure that the cable can withstand fault currents and reduce recovery time in the initial state. Finally, with the help of analytical models and industrial data, once the cables have been designed, the various system losses are reported as well as the power or size of the various auxiliary components.

Electrical Insulation Characteristics of LN2/CF4 Mixture at Cryogenic Temperatures

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.

Insulation Characteristics of PPLP in GHe and Design of 10 kV Bipolar Coaxial HTS DC Cable

Electrical insulation is an essential component of high temperature superconducting (HTS) dc cable systems and polypropylene laminated paper (PPLP) and has been widely used as insulation material for cold dielectric (CD) HTS cables due to its excellent dielectric mechanical properties. In this paper, mini-model cables were fabricated to simulate CD-type HTS cables and the insulation tests were carried out. DC and the lightning impulse breakdown strength of the 1% Weibull distribution of PPLP at 0.3 MPa helium gas (GHe) were investigated. Based on these results, the insulation design of a 10-kV bipolar coaxial HTS dc cable was performed. Furthermore, a model cable was manufactured according to the insulation design and tested for dc and impulse withstand voltage, which verified that the insulation design was qualified.

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.

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.

Development of Superconducting Devices for Power Grids in Italy: Update About the SFCL Project and Launching of the Research Activity on HTS Cables

In the framework of the development of High Temperature Superconducting (HTS)-based solutions for modern power grids, Ricerca sul Sistema Energetico - RSE S.p.A. has gained a significant experience in design, testing, and installation of superconducting fault current limiters (SFCLs). After having successfully demonstrated the techno-economic feasibility of the first Italian three-phase BSCCO-based SFCL (9 kV/3.4 MVA) inside the distribution grid of A2A Reti Elettriche S.p.A. in the Milano area, RSE has initiated the development of a new upgraded three-phase SFCL, whose rated power has been enhanced in order to meet the utility's needs. Unlike the first prototype, the new SFCL device (9 kV/15.6 MVA) is YBCO-based: this choice allows reducing the needed tape length and cooling power, saving at the same time a significant amount of operational costs. The first phase was already assembled and tested in 2015, whereas the second phase was tested in February 2016. This paper will show the last outcomes from characterizations and short-circuit tests. Along with the SFCL project, RSE has also been focusing on superconducting cables and, in the framework of the European Project Best Paths, it has been involved in DEMO5, that is dedicated to the development of a HTS DC cable in MgB2 (10 kA/3.2GW). Taking the cue from the DEMO5 requirements, RSE has initiated a parallel research activity that is focused on studying the thermo-fluid dynamic behavior of cryogenic coolants flowing in forced convection regime inside the flexible cryostat. This paper will present the first simulation results and will give some anticipation on next steps.

Transient Thermal Analysis of HTS DC Cables Cooled With Gaseous Helium Using a Volume Element Method

Gaseous helium is being considered as a cryogen for a variety of high-power density applications. It is capable of reaching operational temperature levels as low as 20 K. This paper presents a time efficient method of designing and modeling a high-temperature superconducting dc cable. The analysis is an extension of a previously presented mathematical model in which the volume element method was used to characterize the fluid flow and heat transfer by conduction, convection, and radiation through a flexible cable cryostat. A system of ordinary differential equations was formulated using the fundamental principles of thermodynamics. The computational domain consists of volume elements distributed in three dimensions to account for the spatial dependence of the model. The pressure drop of the gaseous helium was calculated based on the Darcy-Weisbach correlation. Additionally, the temperature distribution was solved numerically using adaptive time-stepping. The obtained solutions converge within a timescale on the order of seconds to minutes, and the steady state results along with a transient analysis is discussed.

Advanced HTS DC Transmission With Self-Protection Function

High-temperature superconducting (HTS) technology provides an effective way to reduce energy loss in power transmission. HTS power cables have great potential in future power transmission because they have high capacity and are lossless as HTS dc cables, which allow dc transmission without ac loss, which cannot be avoided in the HTS ac transmission systems. To more effectively utilize an HTS dc cable and its transmission system, the HTS dc cable self-acting fault current limitation and self-protection feature have been investigated using the system model developed in this paper. The results obtained show that in addition to its highly efficient power transmission, the HTS dc cable with unique feature can effectively limit system short-circuit currents when short-circuit faults occur, either on the dc side or the ac side of the dc transmission system.

Effects of Direct Fluorination on Space Charge and Trap Distribution of PI Film in LN2

In recent years, high-temperature superconducting (HTS) cables are quickly being developed because of high efficiency and high current transportation capability. Compared with the HTS ac cable, the HTS dc cable has lower loss and stable properties. Under dc environment, the space charge formed in the polyimide (PI) film would distort the local electric field and further influence the characteristics of partial discharge and accelerate the insulation aging. Direct fluorination, as a solution to change the chemical component, can alter the electrical properties of the polymer surface. This paper concentrates on the effect of the fluorination on the space charge and trap distribution of the PI film in liquid nitrogen (LN 2 ). The polarization and leakage current of the PI film in LN 2 are measured under dc voltage. And then, the space charge mobility, amount, and the conductivity are calculated. Moreover, the trap distribution was obtained by the modified isothermal discharge current test. The results indicated that the fluorination improved the space charge mobility and conductivity. The appropriate fluorination suppressed the space charge accumulation in the PI film. However, excessive fluorination brought in more deep traps leading to significant amount of space charge accumulation in the bulk of polymer. As a consequence, the appropriate fluorination has a potential application for insulation in the HTS dc cable.

Mechanical Bending Characteristics of HTS DC Cable

High-temperature superconducting (HTS) dc cable is expected to be an application as larger transmission of electrical power because of, ultimately, lower loss, as well as large capacity compared with ac cable. In order to secure long life, reliability, and compactness of HTS DC cable, the roles of breakdown properties and insulation design are important. In actual cable, breakdown properties are influenced by not only up-scale effect but also mechanical stress during cable production and installation. For HTS cable, polypropylene laminated paper has been used as insulation material. Polypropylene laminated paper has been widely adopted as insulating material for HTS cable. In this paper, the model cables with different thicknesses were fabricated, and the breakdown tests were performed. Based on the results, an improved insulation design of 250-kV-class HTS dc cable was proposed with volume effect. The breakdown characteristics of bended cables were also investigated with various testing conditions.

Insulation Characteristics of PPLP and Design of 250 kV Class HTS DC Cable

The development of high temperature superconducting (HTS) dc cable system has been promoted for the land long-term high-capacity power transmissions, submarine cables and renewable energies. Polypropylene laminated paper (PPLP) has been widely used as insulating material for HTS cable. To develop the PPLP-insulated dc cable, it is important to study the dc, dc polarity reversal and lightning impulse characteristics at a cryogenic temperature. In this paper, we discuss the electrical insulation characteristics of PPLP-insulated mini-model cable. We investigate the dc, impulse breakdown strength, dc polarity reversal, and dc voltage (V)-numbers of dc polarity reversal (N). Also, based on these results, the insulation design of 250 kV class HTS dc cable is performed.

Critical current measurement of HTS tape relating with cable structure for a DC power cable

In the 200 m high temperature superconducting (HTS) cable test facility at Chubu University, a coaxial power cable is used and composed of two BSCCO (Bi-2223) superconducting layers. The tapes are wound closely to reduce effects on the critical current of BSCCO at self-field. Accordingly, each superconducting layer has a different number of BSCCO tapes. Previously, we have investigated dependence of the critical current (Ic) on the gap in order to optimize the HTS DC cable design. We have been studying the effect on the performance of HTS tapes for the superconducting DC power cables by critical current measurements. In the present experiments several HTS tapes are used and set as a similar structure in the cable with a two-layer structure. The critical current of HTS tapes are measured against the gap between the tapes in the same layer. The experiments show the improvement of the critical current by optimizing the tape arrangements due to magnetic field interaction between the tapes. We will present the experimental results and discuss the design of the HTS DC cable.

The surface discharge and breakdown characteristics of HTS DC cable and stop joint box

A high temperature superconducting (HTS) DC cable system consists of a HTS cable and cable joint. The HTS DC cable should be electrically connected in joint boxes because of the unit length of HTS cable is limited to several-hundred meters. In particular, the stop joint box (SJB) must be developed for a compact cooling system. Polypropylene laminated paper (PPLP) and epoxy maybe used as insulating materials for HTS DC cable and SJB. To develop a HTS DC cable, it is necessary to develop the cryogenic insulation technology, materials and the joint methods. In this paper, we will mainly discuss on the DC and impulse characteristics of epoxy and PPLP in liquid nitrogen (LN2). The surface discharge characteristics of epoxy included fillers, PPLP and epoxy with PPLP composite (epoxy+PPLP) were measured under 0.4MPa. Also, the PPLP-insulated mini-model cable was fabricated and then DC, impulse and DC polarity reversal breakdown strength of mini-model cable under 0.4MPa were investigated.

HTS Power Technology for Future DC Power Grid

The growing environmental pressure resulting from the use of fossil fuels is leading mankind to make a transition from the use of traditional energy sources to that of renewable energy based clean energy. Because renewable energy has the feature of instability, it thus brings significant challenges on real-time power balance and power dispatching. Therefore, to secure the power supply, the grid needs to be upgraded by the selection of a reasonable grid structure and operation mode. In this paper, a multiple-level direct current (dc) loop grid, which would be the suitable mode for the future power grid, is suggested. Then, the high-temperature superconducting (HTS) dc power technology such as the HTS dc power cable and dc fault current limiter for the future power grid are discussed. We also report on the test and operation of a 360-m/10-kA HTS dc cable that is being built and would be used for an electrolytic aluminum plant of Zhongfu Group in Henan Province, China.

The Basic Properties of PPLP for HTS DC Cable

Abstract In order to develop high temperature superconducting (HTS) DC cable, it is important to understand the cooling system, high voltage insulation and materials at cryogenic temperature. Especially, the basic properties of insulating materials must be solved for the long life, reliability and compact of system. In this paper, we will discuss mainly on the electrical and the mechanical properties of polypropylene laminated paper (PPLP) in liquid nitrogen (LN 2 ). The polarity effect of DC and impulse voltage, the volume resistivity and the space charge distribution and frost were studied. Furthermore, the mechanical properties such as tensile strength of PPLP at LN 2 (77 K) and room temperature (300 K) were studied.