Cold Dielectric High Temperature Superconducting Research Articles

Parametric study of the hydraulic performance of counter flow LN2 cooled cold dielectric HTS cable

A hydraulic analysis is promising to perceive the complex turbulent flow behavior of sub-cooled liquid nitrogen (LN2) through counter flow cold dielectric high temperature superconducting (HTS) cables. Such counter flow HTS cables comprise of an annular (cryostat) and a central corrugated pipe separated by a HTS conductor layer. Flow of sub-cooled liquid nitrogen through central and annular corrugated pipe affects the hydraulic performance of HTS cable due to the sudden expansion and contraction in the corrugated pipes shape. Therefore, in the present work, the hydraulic performance in terms of friction factor, pressure drop, pumping power and flow pattern of LN2 through central (forward flow) and annular (return flow) corrugated pipe (cryostat) are investigated computationally. Moreover, the effect of various parameters (corrugation pitch and depth) on hydraulic performance is also examined. In order to analyze the effect of corrugation pitches (6, 8, 10 and 12mm) and depths (4, 6 and 8mm), ten different two dimensional (2-D) axisymmetric model of counter cooled HTS cable are developed and solved using computational fluid dynamics (CFD). Additionally, two-equation turbulence model (k-ϵ) is being used to accurately predict the complex flow pattern of LN2 in corrugated pipes. Furthermore, the results of pressure drop are validated with available experimental results. Hence, the hydraulic analysis results are beneficial in opting the corrugated pipes configuration and the allied cryo-units for fluid pumping.

Demonstration Project of 35 kV/1 kA Cold Dielectric High Temperature Superconducting Cable System in Tianjin

A cold dielectric high temperature superconducting (HTS) cable system, which consists of 100 m three-phase cables, terminals, cryogenic system, and monitoring system, was developed by Futong Group in Tianjin. The rated voltage and rated current of the HTS cable system was designed to be 35 kV and 1 kA, respectively, and the YBCO tapes provided by SWCC were used as the conductors and shielding layers. The HTS cable system was installed at Futong Science Park and put into operation on February 15, 2017. Since then, the HTS cable system has been running stably for more than 20 months with an operating current of ~200 A at a coolant temperature from 72 to 77 K.

Thermo-insulation characteristics of PPLP used for superconducting cable

High temperature superconducting (HTS) cable has attracted more attention for its high current density, low loss, and small size. The insulation structure design of HTS cable is very important for the safe operation. Many of the cold dielectric HTS cables use PPLP as the insulation material. However, the HTS conductor needs to endure temperature higher than 150°C during the manufacturing process, and the effect of high temperature on the insulation characteristic of PPLP is unclear. The DSC curve of PPLP was analyzed, and the melting temperature of PPLP was 164.3 °C. The oven was used to heat PPLP for 10 min, 20 min, 30 min, 1 h, 2 h. The heated PPLP was subjected to a power-frequency AC breakdown experiment. It was found that the AC breakdown voltage of PPLP is less than that of unheated PPLP, and its insulation performance degrades.

Influence of flow rate and heat flux on the temperature distribution in long length counter flow cooled cold dielectric HTS cables

The technical advantages of using High Temperature Superconducting (HTS) cables have drawn the attention of most of the researchers around the world. However, the cooling design of these cables nevertheless remains a challenge for the researchers and manufacturers. Therefore, in the present work, an attempt has been made to ease the challenge of cooling of long length High Temperature Superconducting (HTS) cables. Counter flow cooling arrangements of liquid nitrogen (LN2) have been proposed and analysed the longitudinal temperature distribution along the superconducting cable using heat balance equations. Also, in the present work the solution for the temperature distribution profiles of LN2 in counter cooled HTS cable are also obtained assuming one dimensional (1-D) model. In particular, nonlinear axial temperature profiles of LN2 in the counter cooled HTS cable are found. The single phase cold dielectric HTS cable with stainless steel corrugated pipes as flow paths are used for the analysis. Liquid nitrogen with varying volumetric flow rates of 20–35 L/min and total heat loss (A.C. loss and exterior heat load) of 2.3–2.6 W/m reveals interesting facts about cooling of cables with longer lengths. The defined cable length range at constant diameter (inner and annulus pipe) for the analysis was considered as 1.5–3 km. As a result, the maximum possible HTS cable length of 2.5 km can be achieved at the flow rate of 30 L/min and heat fluxes of 2.3–2.6 W/m within the defined operating range.

The research on the current capability and insulating properties of 35 kV/1 kA CD HTS cable witness samples

Due to the high capacity and low loss, high temperature superconducting (HTS) cable using Liquid N2 as the coolant has attracted the widespread attention in the field of the modern power technology. A 35 kV/1 kA cold dielectric (CD) high temperature superconducting (HTS) cable demonstration line, which is in the length of 100 m, has been established. To verify the design of the superconducting cable structure, the witness samples in the length of ∼1.0 m were fabricated to evaluate the current capability and insulating properties, such as lightning impulse voltage and alternating voltage tests. The results showed that the critical current (IC) of the superconducting layer and superconducting shield layer was measured to be 1650 A and 3000 A respectively, and the voltage waveform of superconducting layer revealed no apparent increase during the testing process. Meanwhile, it could be also indicated from the results of high voltage tests that the insulating properties of HTS cable conform to the requirements of the standard GB/T3048.8-94.

Insulation Characteristics of Dielectric Material for CD HTS Cable

The electrical insulation performance of dielectric materials is an important parameter in the insulation design of a cold dielectric (CD) high-temperature superconducting (HTS) cable. Polypropylene laminated paper (PPLP) and Kraft paper are suitable for the insulation design of the CD HTS cable. The experimental study on the cryogenic insulation properties of PPLP and Kraft paper at 0.1 MPa liquid nitrogen were carried out, and the dielectric strength of a mini-model cable, which were assembled as the same structure with CD HTS cable in dc breakdown, lightning impulse breakdown, and ac breakdown, were obtained according to the three-parameter Weibull distribution based on grey model GM(1,1). The results have an important practical value for the insulation design of the CD HTS cable.

Advances in the First Brazilian Project on an HTS Power Cable

This paper presents, in development in Brazil, a research for an ac three-phase cold dielectric (CD) high-temperature superconducting (HTS) power cable. That cable was rated to operate mainly at 69 kV/300 A. The first phase of research was part of a National Strategic Project for power transfer in Brazil. The research over the last two years included an improvement in design in order to enable operation also at higher voltages. The cable design changed and now each phase is inside its own cryostat. The electric isolation also changed. It was selected a laminated dielectric material not yet used (to our knowledge) in any other HTS power cable project. The main outcomes and results presented here are i) the improved cable design, ii) the characterizations of the new isolating material, iii) the facilities for cable construction and high-current tests, iv) one demonstrative model for one phase of the cable, and v) the validation of the new cable geometry by numerical simulation of electromagnetic and thermal behaviors of one of its phases at the rated voltage and current. As a conclusion, the results allowed to improve and disseminate the feasibility of the CD HTS cable technology in Brazil. Some details are not shown because they are patent pending.

Dielectric Characteristics of Laminated Paper and Polymer Sheets Used for Lapping Dielectric Structure at 77 K

110 kV cold dielectric high temperature superconducting (HTS) cable has been developing in China. Generally, the lapped dielectric structure is adapted for cable body, so dielectric characteristics of dielectric sheets are very important. Polyimide, Polypropylene laminated paper (PPLP) and polyimide and poly tetra fluoro ethylene (PTFE) are widespread dielectric sheet materials for HTS equipments with lapped structure. In this paper, the lightning impulse dielectric strengths of three materials under tensile statuses are tested by a special electrode system. The experimental results are processed by method of Weibull plot. As a result, the impulse dielectric strengths of all three materials have good adaptive capacity for tensile stresses in liquid nitrogen. The impulse dielectric strengths of polyimide are highest of three materials under normal using stresses. The AC dielectric strengths of three materials are also concluded from previous papers. Combining experimental results, data in previous papers and design methods related, the multi-layered dielectric thickness and layers of PPLP used for 110 kV cold dielectric HTS cable body are designed.

Magneto-thermal Coupling Modeling for the Stability Analysis of a 110 kV/3 kA High Temperature Superconducting Cable under Fault Current Conditions

In the power grid, the cold dielectric hightemperature superconducting (CD HTS) cable frequently subjects to overcurrents caused by the shortcircuit fault. During the fault, a large amount of Joule heat is generated. The heat causes the temperature of the HTS cable to rise and leads to the transition of the HTS cable from superconducting state to normal state which would influence the current carrying capacity of the HTS cable seriously. An algorithm has been proposed to calculate the transient current distributions of superconducting layers in fault conditions. This method couples an equivalent circuit model and a thermal model. So the temperature distributions in HTS cable is obtained based on the heat generated by the fault current. Finally, the current and temperature distributions of a 110 kV/3 kA CD HTS cable are investigated with a fault current impact of 25 kA rms for 3 s. The results show that the currents of the conducting layers decrease while the temperature increases in the HTS cable and the former withstands most of the fault current impacts. The current in the former of the HTS cable changes from zero to about 90 % of the fault current. A current diversion effect in the HTS cable is obvious. The conclusions provide the basis for the protection strategy of superconducting cable considering fault current impact and will be a benefit to the stable operation of a superconducting transmission grid.

Experimental Evaluation of AC Losses in the Cold Dielectric High-Temperature Superconducting Cable

The alternating-current (AC) losses in the 20-cm Bi-2223/Ag tape cold dielectric high-temperature superconducting (CD HTS) test cable are experimentally evaluated. The method used is the electrical method which is based on a lock-in amplifier. Firstly, the AC losses of the CD HTS cable which has two conductor layers and one shield layer are measured. Then, in order to eliminate the error caused by joint resistance, a test is designed to measure the joint resistance of each layer. The measured losses that eliminated the error due to joint resistance are found to be in good agreement with calculated ones with the monoblock model, Carr, and Magnusson equations at the frequencies of 50 ∼ 300 Hz. Finally, the errors caused by joint resistance and other uncertain factors are discussed.

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.

Optimal Design of Current Sharing in Transmission Conductors of a 110 kV/3 kA Cold Dielectric Superconducting Cable Consisted of YBCO Tapes

A cold dielectric high-temperature superconducting (HTS) cable has multilayer transmission conductors to carry a large current. When the conductor layer number increases, the unevenly distributed currents caused by the skin effect in each conductor layer become more and more nonuniform, and would lead to the increased amount of loss and poorer transmission performance. In order to fully exploit the performance of conductors in cables, it is important to understand the current sharing mechanism between layers. An optimal design model for minimizing current difference in each conductor layer was proposed using an improved particle swarm optimization algorithm. A 1 km, 110 kV/3 kA cold dielectric HTS cable consisted of YBCO tapes and with current evenly sharing was optimally designed based on an equivalent circuit model. After optimization, we find that the maximum unbalanced rate of the current in conductor layers is 4.1% and there is a uniform current distribution among the conductors in shield layers. The current calculation result of the designed HTS cable shows the current is evenly shared in different layers and thus this optimal design method is validated.

Magnetic-Thermal Coupling Analysis of the Cold Dielectric High Temperature Superconducting Cable

A finite element analysis model of the cold dielectric high-temperature superconducting (HTS) cable was built considering the feature of current and heat transfer boundary condition, based on which the magnetic-thermal coupled field was studied and simulated. Then the boundary conditions in the finite element analysis model, such as temperature, magnetic field and the current distribution in HTS cable conductor, were iteratively renewed based on the results of the simulation. Also, the magnetic-thermal coupled field in HTS cable was simulated again using the fixed boundary conditions. The change regularity of the magnetic-thermal coupled field in HTS cable was obtained through the cycle of dynamic simulation of the magnetic-thermal coupled field.

A Study on the Composite Dielectric Properties for an HTS Cable

In order to optimize the insulation design of a cold dielectric high temperature superconducting (HTS) cable, the composite insulation system has been investigated according to the arrangement of laminated polypropylene paper (LPP) and kraft paper in liquid nitrogen. LPP is a prominent insulating material with a high dielectric strength and low dielectric loss, which has been used previously as a HTS cable insulating materials. In addition kraft paper has been used for oil-field (OF) cable as insulating materials for a long time. In this paper, the dielectric properties on composite insulation system according to the arrangement of LPP and kraft paper was compared. The economic and dielectric performances of two insulating materials were considered and applied toward a HTS cable. Furthermore, from an economic satisfaction point the most suitable arrangement, and compared the dielectric properties of the mini-model cables were examined.