High Temperature Superconducting Power Cable Research Articles

Economic Evaluation of 23 kV Tri-Axial HTS Cable Application to Power System

Research on the development of a superconducting power cable for commercialization is currently underway across the world. Many demonstrations of superconducting power cables connected to real power grids have already been conducted with various designs depending on the voltage levels of the distribution and transmission lines. In Korea, the first commercial project for high temperature superconducting (HTS) power cables, “Shingal Project” which is fully funded by Korea Electric Power Corporation (KEPCO), is underway to connect two substations with a 23 kV HTS cable with a distance of 1 km. In addition, the KEPCO Research Center (KEPRI) initiated the project in 2017 to develop the 23-kV tri-axial HTS power cables, including a cooling system, for a distance of 3 km. In this paper, the recent progress and research activities related to the two 23-kV superconducting power cables projects are discussed. An economic evaluation of the HTS power cables for commercialization in consideration of their construction environments and social issues is also presented.

Comparison Between Simulation and Experimental Results of Liquid Nitrogen Coolant Distribution in a 66-kV 40-m Model HTS Power Cable System Experiencing Short-Circuit Accidents

In this study, a computer program was developed to simulate the temperature and pressure distributions of a liquid nitrogen (LN) coolant in a high-temperature superconducting (HTS) power cable. This program is important for realizing a practical HTS power cable. According to the Japanese criterion for a 66-kV power-transmission system, in a worst case short-circuit accident, a fault current of 31.5 kA may flow in a 66-kV cable system for 2 s. In addition, when a short-circuit accident occurs, the temperature and pressure of an LN coolant increase rapidly The temperature behaviors of the cable cores and coolant were analyzed by solving the nonlinear partial differential heat-conduction equations in the cable cores by considering the energy balance of the heat flow by using the finite-difference method. Moreover, the pressure behaviors of the coolant in a cooling system were analyzed, considering the density and volume behaviors of the coolant. The simulation results for the 66-kV 40-m model cable qualitatively reproduced the experimental results satisfactorily.

Recent Progress and Design of Three-Phase Coaxial HTS Power Cable in Korea

High-temperature superconducting (HTS) power cables were developed recently and have several advantages over conventional cables. For high current capacity, two or more layers are required in one phase of the HTS cable. Differences in the radii between each phase and layer cause inherent imbalance and non-uniform current distribution of three-phase currents. These outcomes must be minimized by properly designing the pitches of multi-layer three-phase coaxial HTS cables. In this paper, the authors design the conduction layers of a multi-layer three-phase coaxial HTS cable and analyze the characteristics of the designed cable based on the proposed PSCAD/EMTDC component and simulation method. A 23 kV/60 MVA multi-layer three-phase coaxial HTS power cable is considered. The conceptual design of a multi-layer three-phase coaxial HTS cable includes the radii of each layer, the number of layers in each phase, and the pitch design of each layer of the cable. Using an impedance matching program, current imbalance can be minimized and current distribution can be made uniform in all layers of the phase by adjusting the pitch length and winding direction. The characteristics of HTS cables using the proposed PSCAD/EMTDC component are analyzed. In this component, the dynamic resistance of current- and temperature-dependent HTS cables is added to the HTS model. AC losses are calculated using the some equation. Induced current loss is calculated based on the material properties of the shield layer. A transient simulation is performed for a three-phase fault of the three-phase coaxial HTS cable using a PSCAD/EMTDC-based simulation method.

Design of Electrical Terminals for High Temperature Superconducting (HTS) Power Cable

Design of Electrical Terminals for High Temperature Superconducting (HTS) Power Cable

Hydraulic analysis of liquid nitrogen flow through concentric annulus with corrugations for High Temperature Superconducting power cable

A hydraulic analysis for turbulent flow of subcooled liquid nitrogen through an annulus within the HTS cable is numerically investigated in terms of friction factor and flow pattern. The control volume for the investigation is confined to the annular space between outer smooth surface of cable core and inner surface of corrugated cryopipe. The corrugation geometries i.e pitch and depth affect the friction factor and thus the pressure drop and pumping power of liquid nitrogen. Nine different geometries of corrugated cryostats with various combinations of pitches (6, 10 and 14 mm) and depths (5, 7 and 9 mm) are considered and compared with smooth (non-corrugated) pipe. The results are useful in selecting the cryostat configuration and the associated fluid pumping system for cooling HTS power cables.

Optimization of a superconducting gas-insulated transmission line

The superconducting gas insulated transmission line (S-GIL) is a new design being proposed for helium gas (GHe) cooled high temperature superconducting (HTS) power cables. The S-GIL design utilizes GHe as both cryogen and insulation medium for the HTS cable. This paper builds upon previous proof of concept measurements of the S-GIL. Through optimization of the S-GIL design parameters increase of 20–30% in electrical breakdown voltages was observed for the proof of concept measurements. The paper also contains details on a prototype S-GIL design which was adopted from a commercial coaxial air dielectric cable. The prototype S-GIL cable was successfully demonstrated at 77 K using both GHe and 4 mol% H2 balanced with GHe as the cryogen/insulation medium. The values obtained by the prototype S-GIL cable are higher than the voltage rating currently capable of other GHe cooled HTS cables which verifies the potential of the S-GIL design concept.

Optimization of a superconducting gas-insulated transmission line

The superconducting gas insulated transmission line (S-GIL) is a new design being proposed for helium gas (GHe) cooled high temperature superconducting (HTS) power cables. The S-GIL design utilizes GHe as both cryogen and insulation medium for the HTS cable. This paper builds upon previous proof of concept measurements of the S-GIL. Through optimization of the S-GIL design parameters increase of 20–30% in electrical breakdown voltages was observed for the proof of concept measurements. The paper also contains details on a prototype S-GIL design which was adopted from a commercial coaxial air dielectric cable. The prototype S-GIL cable was successfully demonstrated at 77 K using both GHe and 4 mol% H2 balanced with GHe as the cryogen/insulation medium. The values obtained by the prototype S-GIL cable are higher than the voltage rating currently capable of other GHe cooled HTS cables which verifies the potential of the S-GIL design concept.

A Simplified Model of Coaxial, Multilayer High-Temperature Superconducting Power Cables with Cu Formers for Transient Studies

Bypassing transient current through copper (Cu) stabilizer layers reduces heat generation and temperature rise of high-temperature superconducting (HTS) conductors, which could protect HTS cables from burning out during transient conditions. The Cu layer connected in parallel with HTS tape layers impacts current distribution among layers and variations of phase resistance in either steady-state or transient conditions. Modeling the multilayer HTS power cable is important for transient studies. However, existing models of HTS power cables have only proposed HTS cables without the use of a Cu-former layer. To overcome this problem, the authors proposed a multilayer HTS power cable model that used a Cu-former layer in each phase for transient study. It was observed that resistance of the HTS conductor increased significantly in the transient state due to a quenching phenomenon, which made the transient current mainly flow into the Cu-former layers. Since resistance of the Cu-former layer has a significant impact on the transient current, detailed modeling of the Cu-former layer is described in this study. The feasibility of the developed HTS cable model is evaluated in the PSCAD/EMTDC program.

Power flow analysis of a 5-bus 10 kV power system with high temperature superconducting cable

High temperature superconducting (HTS) power cable provides a promising solution for the challenges the power grids confront. As most of the existing HTS cable projects operated the cable in an isolated system, we have little knowledge about the power flow of the system when HTS cable integrated, which is essential for planning, operation, and protection of a power system containing HTS cable. In the paper, we build a 10 kV 5-bus power system with one HTS cable line and the same line with normal cable is as the benchmark. We solve the model and analyse the power flow of the system under normal operation, line disconnection, source power off, and load growing conditions. The results show that HTS cable tends to undertake more transmission energy and makes the connected buses more robust in terms of voltage drop. As HTS cable’s behaviour is different, it is necessary to readjust the settings and configurations of the secondary side apparatus of power system.

Computational investigation on thermohydraulic characteristics of High-Temperature Superconducting (HTS) power cables

High-Temperature Superconducting (HTS) cables are emerging as an effective alternative for conventional power transmission system in reducing transmission losses, electrical impedance and Right-of-Way (RoW). Rare earth based superconducting tapes such as Bismuth strontium calcium copper oxide (BSCCO-2223) and Yttrium barium copper oxide (YBCO-123), whose critical temperatures are 110 K and 90 K respectively, are preferred in manufacturing of HTS cables, due to their potential to handle high current load, in cryogenics environment. Moreover, for retaining the superconductivity, these HTS tapes are preferably cooled by Liquid Nitrogen (LN2) (boiling point ∼77 K), in order to remove the heat flux from different sources such as AC losses in HTS tapes, heat-in-leak due to axial conduction through the current leads and radial heat-in-leak from the ambient through the insulation. Therefore, it is essential to estimate the cooling capacity of HTS cables and associated pumping power required for circulation of LN2 between end terminations. Hence, in the present work, the thermohydraulic analysis relevant to the design of long length HTS cables is presented. The computational model of LN2 flowing through corrugated pipe of HTS cable is developed, followed by the estimation of pumping power and cooling capacity, using the commercial code FLUENT. The obtained results are validated with the experimental measurements available in literature.

Thermo-hydraulic analysis on long three-phase coaxial HTS power cable of several kilometers

A coaxial HTS (high temperature superconductor) power cable has advantage of using less amount of HTS wires than three-phases in one cryostat structure because it does not need each shielding layer. In the coaxial HTS power cable, thermo-hydraulic analysis such as pressure drop across a long cable and thermal characteristics in the cable is also required considering geographical feature of the installation site and the temperature distribution in the cable core, composed of three phases and insulation layers. The operating pressure and temperature are limited by the mechanical strength of a cryostat and the boiling temperature of liquid nitrogen. This paper describes a thermo-hydraulic analysis and design of the coaxial HTS power cable considering the operating temperature and circulation of subcooled liquid nitrogen over several kilometers. Through the analysis, the required specifications of a refrigeration system are determined considering the pressure drop and thermal analysis of cable core for heat losses such as AC loss and heat penetration. A 1-D node network analysis is conducted to investigate detail temperature distributions and variations of cable core in conjugation with computational fluid dynamics.

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.

Evaporating system for cryogenic support of long length HTS power cables

The paper deals with an analysis of the results of theoretical and experimental research on an evaporating system for cryogenic support as supplied to long length thermostatting channels of high-temperature superconducting (HTS) cables and hybrid power transmission lines as well as thermal control systems for cryogenic components in aircraft fuel tanks during long-term spaceflights. Experimental evidence for nitrogen and hydrogen are presented here. The importance of such research for practical application in developing modern cryostatting systems has been highlighted.The design of an experimental hybrid power transmission line for studying thermostatting of superconducting power cables has been considered in the paper. The transmission line contains three sections with different types of thermal insulation and current leads providing high current supply to superconducting threads with minimum external heat inflow. The unique experimental data on heat inflows from the outer surface of the transmission line in different sections has been obtained by the authors. It is shown that it may be possible to compensate fully for external heat inflow to a cryogenic line as well as to lower the temperature of a cryogenic coolant in the section with an evaporating system for cryogenic support. In order to determine the possible length of the cryostatting work field of a long length superconducting cable, estimates of using liquid nitrogen and liquid hydrogen as a working fluid for various mass flow rates of the coolant feed have been made via the mathematical model describing physical processes in a thermostatting channel using an evaporating system for cryogenic support. Calculation data on changes in the length of the long length temperature cryostat, pressure and cooling capacity of the evaporating cryostat system has been obtained.

Design of high-efficiency Joule-Thomson cycles for high-temperature superconductor power cable cooling

Liquid nitrogen (LN2) is commonly used as the coolant of a high temperature superconductor (HTS) power cable. The LN2 is continuously cooled by a subcooler to maintain an appropriate operating temperature of the cable. This paper proposes two Joule-Thomson (JT) refrigeration cycles for subcooling the LN2 coolant by using nitrogen itself as the working fluid. Additionally, an innovative HTS cooling cycle, of which the cable coolant and the refrigerant are unified and supplied from the same source, is suggested and analyzed in detail. Among these cycles, the highest COP is obtained in the JT cycle with a vacuum pump (Cycle A) which is 0.115 at 78 K, and the Carnot efficiency is 32.8%. The integrated HTS cooling cycle (Cycle C) can reach the maximum COP of 0.087, and the Carnot efficiency of 24.8%. Although Cycle C has a relatively low cycle efficiency when compared to that of the separated refrigeration cycle, it can be a good alternative in engineering applications, because the assembled hardware has few machinery components in a more compact configuration than the other cycles.

High‐Temperature Superconducting Cable Design Based on Individual Insulated Conductors

The idea of insulating individual high‐temperature superconducting (HTS) tapes was explored as a dielectric design to reduce the risk and complexity of HTS cable manufacturing. Applying insulation on individual HTS tapes is amenable to continuous manufacturing processes and opens up material choices for the insulation. In this study, heat shrink insulation was selected as the material choice for exploring the possibility of this design philosophy because of its commercial availability in multiple thicknesses. A systematic set of selection criteria was developed for the selection of appropriate heat shrink for given HTS tape dimensions. The cryogenic dielectric characteristics of insulated HTS tapes were evaluated both in liquid nitrogen and gaseous helium environments at 77 K. Dielectric characteristics of tapes with a single layer of thicker insulation were compared with those insulated using multiple layers of thinner insulation to evaluate the relative merits of each method. Several model power cables were fabricated using the PET insulated tapes, and their dielectric behavior was evaluated at 77 K in gaseous helium environment. The results suggest that the explored method is useful for HTS power cables operating at low voltages (<1,000 V) primarily due to the limitation on achieving thick insulation with high quality using heat shrink tubing. The suitable processes of insulating longer lengths of samples with additional dielectric benefits are discussed.

Investigation of solid nitrogen for cryogenic thermal storage in superconducting cable terminations for enhanced resiliency

Low heat capacity of helium makes the helium gas cooled high temperature superconducting (HTS) power devices susceptible to large temperature rises during unexpected heat loads such as electrical faults or cryogenic system failures. Cryogenic thermal storage in the form of solid nitrogen designed in the terminations is explored as a means to increase the thermal stability and operational time of HTS power cables in the event of unexpected heat loads. An external tank containing activated charcoal is used as an adsorption buffer tank for nitrogen gas. The use of activated charcoal minimizes the volume of the buffer tank and prevents pressure rises during melting and boiling of the solid nitrogen. Calculations of the cryogenic thermal storage needed and a description of the experimental setup used to understand the design constraints are discussed.

Study on AC loss measurements of HTS power cable for standardizing

High-temperature superconducting power cables (HTS cables) have been developed for more than 20 years. In addition of the cable developments, the test methods of the HTS cables have been discussed and proposed in many laboratories and companies. Recently the test methods of the HTS cables is required to standardize and to common in the world. CIGRE made the working group (B1-31) for the discussion of the test methods of the HTS cables as a power cable, and published the recommendation of the test method. Additionally, IEC TC20 submitted the New Work Item Proposal (NP) based on the recommendation of CIGRE this year, IEC TC20 and IEC TC90 started the standardization work on Testing of HTS AC cables. However, the individual test method that used to measure a performance of HTS cables hasn’t been established as world’s common methods. The AC loss is one of the most important properties to disseminate low loss and economical efficient HTS cables in the world. We regard to establish the method of the AC loss measurements in rational and in high accuracy. Japan is at a leading position in the AC loss study, because Japanese researchers have studied on the AC loss technically and scientifically, and also developed the effective technologies for the AC loss reduction. The JP domestic commission of TC90 made a working team to discussion the methods of the AC loss measurements for aiming an international standard finally. This paper reports about the AC loss measurement of two type of the HTS conductors, such as a HTS conductor without a HTS shield and a HTS conductor with a HTS shield. The AC loss measurement method is suggested by the electrical method..

Temperature and Pressure Distribution Simulations of 3-km-Long High-Temperature Superconducting Power Cable System With Fault Current for 66-kV-Class Transmission Lines

A computer program has been developed by numerically solving the nonlinear heat conduction and heat transfer equations, which are discretized using the finite difference method, to calculate the temperature and pressure distributions in high-temperature superconducting (HTS) power cables cooled using liquid nitrogen (LN2). The LN2 coolant properties are estimated using Cryodata's GASPAK software package. This evaluation of the temperature and pressure distributions is important to understand how we can make practical use of HTS power cables and to assess the effects of short-circuit accidents. To incorporate HTS power cables into real grids, it is necessary to evaluate HTS power cables that are a few kilometers long. In this study, 3-km-long HTS cables were evaluated using our proposed computer program. This program was used to model the cooling system, ac loss, dielectric loss, and intermediate-joint losses. The results revealed that the temperature of the LN2 coolant at the cable outlet could reach the saturation temperature when a fault current of 31.5 kA flowed for 2 s (worst case for the 66-kV-class transmission lines in Japan). From the results, it was also clear that the effects of the ac loss, dielectric loss, and intermediate-joint losses on the increase in the temperature of the LN2 coolant could not be ignored.

Fundamental Study of Ground Fault Accident in HTS Cable

High-temperature superconducting (HTS) power cable has significant merits of compactness and large power transmission capacity. Although the stability of the HTS cable system was verified by in-grid operation, the verification of its safety and reliability against various accidents is required for practical use of this system. A ground fault accident is one of the typical accidents of a conventional cable. If this fault occurs by breakdown of the dielectric layer, the generated arc energy is dissipated into the environment in various forms. As a result, the safety of the public may be jeopardized. Arc energy relates to arc voltage, which is dependent on the inherent physical properties of the faulted equipment. The use of coolant and the structure of the HTS cable differentiate it from a conventional cable, so it should be verified how these differences influence the arc voltage. Accordingly, ground fault tests using the HTS cable were conducted and the arc voltages were compared to that of a conventional cable. The results obtained proved that the arc voltage on the HTS cable is similar to that of a conventional cable, in spite of the differences concerning the use of coolant and the cable structure.

Effects of HTS Tape Arrangements on AC Loss in Self-Shielding DC HTS Cable With AC Ripple Current

Critical current and ac loss of high-temperature superconducting (HTS) tapes are two important factors for the application of ac HTS power cables. Generally, the current in the dc HTS cable is a combination of main dc current and ac ripple current due to the use of rectifier. The ac ripple current distribution without proper adjusting would be nonuniform in each layer especially when the dc HTS cable layer increases; it may lead to high ac loss and poor transmission capacity. In order to understand the ac ripple current sharing among dc HTS cable layers, the inductance matrix of the cable layers was deduced based on the geometry configuration of a self-shielding dc HTS cable. In this paper, in addition to the uniform current design principle, a self-shielding dc HTS cable is also designed with the HTS tape arrangements of each layer optimized by taking the ratios of ac ripple current amplitude to critical current in each layer as equal for minimizing the ac loss. According to the design configuration and deduced inductance matrix, current in each layer and ac loss are calculated. The results show that it is necessary to use the optimized design in the dc HTS cable design and the equal ratio design is better than the uniform current design.