High Temperature Superconducting Power Cable Research Articles

Temperature Simulation of a 20 m HTS Power Model Cable System in a Fault Current for 275 kV Transmission Lines

A simulation has been developed for estimating the transient temperature and pressure distributions in a high-temperature superconducting (HTS) power cable with a fault current accident cooled by a forced flow of subcooled liquid nitrogen (LN2). This simulation is an essential tool for realizing a practical HTS power cable in order to assess the effects of short-circuit accidents. When a fault occurs in a 275 kV class power transmission system in Japan, an excessive current of 63 kA may flow in the cable for 0.6 s. When faults occur, it is important to estimate the temperature and pressure profiles in a cable cooled by the forced flow of subcooled LN2. The temperature profiles of the LN2 coolant and the cable cores were analyzed by solving nonlinear partial differential equations of the heat transfer phenomenon through heat conduction using finite-difference method. The GASPAK software package (Cryodata) was used to evaluate the fluid properties. The simulation results for 275 kV class 20 m model cable show fairly good agreement with the experimental results obtained by Fukukawa Electric.

Use of partial discharge inception voltage measurements to design a gaseous helium cooled high temperature superconducting power cable

The occurrence of partial discharge is one of the challenges that limit the design options and voltage ratings of gaseous helium cooled high temperature superconducting (HTS) cables. The measurements of partial discharge inception voltage (PDIV) at cryogenic temperatures on several model cables were used in the optimization of the dielectric design of a 30 m long HTS power cable, which was successfully fabricated and tested at DC currents up to 6 kA and voltage of 3.5 kV RMS. Details of the PDIV measurements are described.

Modelling of 3D temperature profiles and pressure drop in concentric three-phase HTS power cables

To meet the increasing power demand in cities under the spatial constraints for cable channels, the application of high temperature superconducting (HTS) cables cooled with liquid nitrogen offers an increasingly attractive alternative to conventional cable solutions. This paper presents a differential equation model for three-phase concentric HTS cables, describing the temperature distribution in the various cable layers and in the liquid nitrogen flow. The design of the 1km long AmpaCity cable is used as a reference, which presently is the longest HTS cable installation in the world. The model considers the AC losses in the superconducting phases in addition to the external thermal load, as well as pressure losses in the coolant flow. The integrity of the algorithm is verified through energy conservation, yielding negligible numerical solver errors. The model is then applied to the operation of the AmpaCity cable. The monitoring of its operating range is explained, based on electrical and coolant properties measured at the cable end. The final application study shows options for extending the cable length up to factor five, using a second cooling unit in combination with a mixed coolant.

Design of non-flammable mixed refrigerant Joule-Thomson refrigerator for precooling stage of high temperature superconducting power cable

A cryogenic Joule-Thomson (JT) refrigerator has many advantages for large industrial applications, including easy cooling power adjustability and high reliability because there are no moving parts at low temperature. In this paper, a single stage and a cascade type of non-flammable (NF) mixed refrigerant (MR) JT refrigerators have been proposed for the precooling process of a neon/nitrogen mixed refrigerant JT refrigerator. The neon/nitrogen MR JT refrigerator is used to maintain a subcooled state of liquid nitrogen coolant for an HTS (high temperature superconductor) cable.Both selected MRs for the 1st stage (low temperature cycle) of the cascade MR JT refrigerator and the single MR JT refrigerator are composed of Nitrogen (N2), Argon (Ar), Tetrafluoromethane (CF4, R14) and Octafluoropropane (C3F8, R218). R410A is selected as the refrigerant for the 2nd stage (high temperature cycle) of the cascade MR JT refrigerator, of which the cooling temperature is approximately 240K. A commercial software with Peng-Robinson equation of state (EOS) is utilized to design the non-flammable MR JT refrigerator. The optimal design is discussed with consideration for various parameters such as the temperature staging, the operating pressure of the compressor and the mass flow rate of the working fluid. The maximum coefficient of performance (COP) and Carnot efficiency of the cascade MR JT refrigerator are obtained as 0.216 and 40%, respectively at 105K. Exergy analysis is also carried out in this paper to reveal the irreversibility of the refrigeration cycle.

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.

Electromagnetic Field Analysis of a High Current Capacity DC HTS Cable With Self-shielding Characteristic by 2-D Simulation

Comparing with direct current (dc) conventional transmission lines, dc high-temperature superconducting (HTS) power cables have advantages of high current, low transmission loss, and environmental friendly characteristics. With regard to HTS cables with coaxial configuration and self-shielding functions, this type of cable has a “sandwich” structure that is obtained by interleaving the windings for the minimization of the magnetic field and the degradation of the critical current. This paper proposes a two-dimensional (2-D) model for simulating magnetic field distributions, such that the critical current distribution can be found precisely. The ac loss is also analyzed since the current in the dc transmission is comprised of dc and ac ripple. This 2-D analysis model provides a useful reference for the design and operation of dc HTS cables with large current capacity without magnetic field disturbance.

Three-Dimensional Finite-Element Analysis of Terminations for Gaseous-Helium-Cooled High-Temperature Superconducting Power Cables

A three-dimensional finite-element method computational model has been developed for the terminations of a gaseous-helium-cooled superconducting power cable. The model combines turbulent flow physics coupled with heat transfer in fluids and solids and Joule heating. The model can be used to map the termination to identify the locations with the largest temperature gradients, and the design can be optimized to minimize those gradients in an iterative way through multiple simulations. The model also allows for a drastic reduction in time for new design iterations of the physical termination with optimal thermal and fluid characteristics. This paper presents the details of the modeling methodology, and the results are compared with experimental data to show how modern computing power can solve complicated and highly detailed multiphysics simulations to reduce engineering design time.

Economic Evaluation Method for Fault Current Limiting Superconducting Cables Considering Network Congestion in a Power System

High-temperature superconducting (HTS) power cables offer significant advantages for long-distance high-capacity transmission. In order to meet the increasing demand in a power system, HTS power cables can be utilized for effective transmission expansion planning. However, when a fault occurs in a power system, the low impedance of the HTS cables can have an adverse impact on the system protection. In order to resolve the fault current issue, fault current limiting (FCL) HTS cables can be introduced for transmission expansion planning. The introduction of FCL-HTS cables can lead to an economical operation of a power system. This paper presents an economic evaluation method for the FCL-HTS cables in transmission expansion. A numerical example is presented to show the effectiveness of the proposed method.

Progress in American Superconductor’s HTS wire and optimization for fault current limiting systems

American Superconductor has developed composite coated conductor tape-shaped wires using high temperature superconductor (HTS) on a flexible substrate with laminated metal stabilizer. Such wires enable many applications, each requiring specific optimization. For example, coils for HTS rotating machinery require increased current density J at 25–50K. A collaboration with Argonne, Brookhaven and Los Alamos National Laboratories and several universities has increased J using an optimized combination of precipitates and ion irradiation defects in the HTS. Major commercial opportunities also exist to enhance electric power grid resiliency by linking substations with distribution-voltage HTS power cables [10]. Such links provide alternative power sources if one substation's transmission-voltage power is compromised. But they must also limit fault currents which would otherwise be increased by such distribution-level links. This can be done in an HTS cable, exploiting the superconductor-to-resistive transition when current exceeds the wires’ critical J. A key insight is that such transitions are usually nonuniform; so the wire must be designed to prevent localized hot spots from damaging the wire or even generating gas bubbles in the cable causing dielectric breakdown. Analysis shows that local heating can be minimized by increasing the composite tape's total thickness, decreasing its total resistance in the normal state and decreasing its critical J. This conflicts with other desirable wire characteristics. Optimization of these conflicting requirements is discussed.

Hysteresis Losses Analysis in 2G HTS Cables

The Russian Scientific R&D Cable Institute is involved in many projects on R&D high-temperature superconducting (HTS) power cables including those made of modern second generation (2G) HTS wires. Evaluation of ac losses and methods for the reduction of the ac losses are important in the design of the cables. A fine experimental technique has been developed and implemented to measure ac losses in representative cable models with a length of ~5 m. However, only calculation can provide ac loss prediction in cases when it could be difficult to make measurements. Thus, we need a comprehensive simulation model to calculate ac losses. In HTS cables made of 2G wires, the main ac loss is a hysteresis loss, which only numerical simulation can predict for full-size 2G cables. We develop a finite-element method model to solve the electromagnetic problem for the calculation of ac losses. The model takes into account the strong nonlinear resistivity of the HTS materials, the hysteresis behavior of the magnetic substrate, the nonuniformity of critical current density across the tapes of the cable, and the possible ferromagnetism of a substrate. The model also takes into account any geometrical aspect of the 2G HTS cables. The details of the model and comparisons of calculations with our measurements of the ac losses are presented.

Development of the First Brazilian Project on Superconducting Power Cable

The SUPERCABO Project is in development in Brazil, and its primary objective is to achieve knowledge on designing, construction, tests, and economic feasibility of high-temperature superconducting power cable (HTS cable) technology for Brazil. Such knowledge is shared with four major Brazilian power utilities and with Brazil's National Electric Energy Agency. The method employed was to exchange knowledge and information with those power utilities to guide the project in giving results that allow for actual use of HTS cable technology in Brazil. The procedures were to evaluate the best geometry of HTS cable according to the operating voltage level, to search for an auxiliary solution for fault current protection, and to improve the terminations of the cable. The main outcomes and results presented here are the complete design of the first Brazilian HTS cable, the proposed termination for use in an HTS cable, and computational models for the superconducting core. As a conclusion, the SUPERCABO Project is in development, with results to improve the feasibility of HTS cable technology in Brazil, using cryocooled aluminum power cables to fault current protection and a proposal of inductive linking for power flow in terminations. Some details are not shown because they are patent pending.

Design of partial emission type liquid nitrogen pump

High Temperature Superconductor power cable systems are being developed actively to solve the problem of increasing power demand. With increases in the unit length of the High Temperature Superconductor power cable, it is necessary to develop highly efficient and reliable cryogenic pumps to transport the coolant over long distances. Generally, to obtain a high degree of efficiency, the cryogenic pump requires a high pressure rise with a low flow rate, and a partial emission type pump is appropriate considering its low specific speed, which is different from the conventional centrifugal type, full emission type. This paper describes the design of a partial emission pump to circulate subcooled liquid nitrogen. It consists of an impeller, a circular case and a diffuser. The conventional pump and the partial emission pump have different features in the impeller and the discharge flow passage. The partial emission pump uses an impeller with straight radial blades. The emission of working fluid does not occur continuously from all of the impeller channels, and the diffuser allows the flow only from a part of the impeller channels. As the area of the diffuser increases gradually, it converts the dynamic pressure into static pressure while minimizing the loss of total pressure. We used the known numerical method for the optimum design process and made a CFD analysis to verify the theoretical performance.

Insulation Design of a Stop Joint Box of 80-kV DC HTS Cables Based on DC Electric Field Analysis

Among the accessories of the dc high-temperature superconducting (HTS) power cable, a stop joint box (SJB) is an essential component for the long-distance transmission cable connections and the separation of the cooling section. The main insulating material of the SJB is a polypropylene laminated paper (PPLP), which is already generally used for dc HTS power cable. In order to separate the cooling section, an epoxy spacer is located at the center of the SJB. Due to the weakness of surface breakdown characteristics compared with that of punctual breakdown and different nature of dc electric field distribution determined by the ratio of conductivities of dielectric materials, the epoxy spacer configuration of the SJB should be carefully designed based on dc electric field analysis. In this paper, the optimum configuration of the epoxy spacer of the SJB was investigated based on dc electric field analysis. On the basis of the initial SJB model, which is already designed for the joint box of the conventional ac oil-filled power cable, a variety of improved SJB models with different epoxy spacer configurations were considered. In dc electric field simulations, the main objective was to reduce the tangential dc electric field intensity at the interface between PPLP and epoxy spacer. To find the optimum epoxy spacer configuration, a parametric sweep technique was applied, and an appropriate SJB model could be identified. However, its tangential dc electric field intensity remained still higher than that of the design criteria for the dc electric field from the experimental results. Therefore, as a novel method, a thin Kraft layer between the PPLP and the epoxy spacer was added to reduce electric field intensification. The simulation result of the newly designed insulation structure revealed the possibility of lowering the tangential electric field. Furthermore, different electrical conductivity values were applied to the additional layer instead of the original Kraft value in order to meet the critical electric field intensity. Finally, 1E-11 S/m, among those values, showed the best simulation result, which was lower than the design criteria for the dc electric field. To verify the effect of the additional layer for dc electric field mitigation, miniature SJB specimens were fabricated, and dc breakdown tests were performed using a step-by-step test method. Based on the experimental results, SJB specimens with an additional layer showed the highest breakdown strength compared with that of SJB specimens without an additional layer. Thus, the optimum configuration of SJB determined by dc electric field analysis was approved by experimental works. Consequently, these insulation design skills based on dc electric field analysis could be usefully adopted in developing the SJB of 80-kV dc HTS power cables.

Dielectric Properties of Cryogenic Gas Mixtures Containing Helium, Neon, and Hydrogen

Past efforts of cooling high temperature superconducting (HTS) power cables by gaseous cryogens focused exclusively on helium. The limited dielectric strength of helium gas necessitated alternatives that could be used in the temperature range suitable for HTS power applications. This paper presents the benefits of gas mixtures containing helium with small concentrations of hydrogen or neon to mitigate the limited dielectric strength of pure helium gas. The expectation was that such gas mixtures could improve dielectric characteristics while maintaining the thermal, non-flammable and non-corrosive properties of pure helium gas. The AC breakdown voltage of helium gas mixtures containing 4 mol% neon or 4 mol% hydrogen respectively have been measured and compared to those of pure helium and pure neon. All measurements were performed at 77 K at gas pressure levels between 0.5 and 2.0 MPa. While the 4 mol% neon mixture did not result in any improvement over pure helium, the 4 mol% hydrogen mixture resulted in 80% higher breakdown strength. This is expected to enable higher operating voltages for gas cooled HTS power devices.

RTDS-based Model Component Development of a Tri-axial HTS Power Cable and Transient Characteristic Analysis

The transient characteristics of the tri-axial High Temperature Superconducting (HTS) power cable are different from those of a conventional power cable depending on whether the cable is under a steady or transient state due to the quench. Verification using simulation tools is required to confirm both the characteristics of the cable and the effect of the cable when it is applied to a real utility. However, a component for the cable has not been provided in simulation tools; thus the RTDSbased model component of the tri-axial HTS power cable was developed, and a simulation was performed under the transient state. The considered properties of model component include resistance, reactance and temperature. Simulation results indicate the variation of HTS power cable condition. The results are used for the transient characteristic analysis and stability verification of the tri-axial HTS power cable. In the future, the RTDS-based model component of the cable will be used to implement the hardware-in-the-loop simulation with a protection device.

Influence of Substrates in HTS Wires on AC Loss Characteristics of Cylindrical Single Layer Conductors

Substrates of high temperature superconductor (HTS) wires can change the magnetic field distribution of the wires, with the consequence that they can affect AC losses of the HTS wires, which is a critical issue in HTS applications. In this study, in order to investigate the effects of the substrates on the AC losses of cylindrical single layer conductors which are usually used in HTS power cables, we first experimentally and numerically evaluated self-field losses of the HTS wires and investigated the influence of the substrates on their self-field losses. Next various HTS cylindrical single layer conductors were prepared using different HTS wires and their AC loss characteristics were discussed from the perspective of magnetic substrate. The main results show that the magnetic materials in HTS wires do affect their losses and the magnetic field distribution. For our pentagonal conductors, magnetic flux does not exist outside of the conductor when the magnetic substrates are located outside. Moreover, the AC losses of the bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO) conductors are quite similar and the AC losses of the NY and YN conductors are larger than the losses of the other two conductors. Especially, the AC loss of the YN conductor is the greatest in the low current region.

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.

Development of a 66 kV-5 kArms Class HTS Power Cable Using REBCO Tapes With High Critical Current

High temperature superconducting (HTS) cables are able to achieve large power capacity and low-loss power transmission. Fujikura Ltd. worked on developing a 66 kV-5 kArms HTS power cable using high critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) REBCO tapes in the Japanese national project. One of the technical targets in this project was to reduce AC loss to less than 2 W/m at 5 kArms. REBCO tapes with 240 A/4 mm-width of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at 77 K, self field, which were fabricated by ion-beam-assisted-deposition (IBAD) and pulsed laser deposition (PLD) methods, were applied to the HTS power cable. As a result, we succeeded in developing the 20 m-long 66 kV-5 kArms HTS power cable. The measured AC loss was achieved 1.4 W/m at 77 K and 1.0 W/m at 67 K at 5 kArms. On the other hand, it is considered that the eddy current loss of the stainless-steel cryostat pipe of HTS cable become obvious in the case of the single-core cable with the large current capacity. It is suggested that the eddy current loss of the stainless-steel cryostat pipe is due to the internal magnetic flux. Therefore, the eddy current loss of the stainless-steel cryostat pipe as well as uniform current distribution of HTS conductor and shield layer should be taken into account for designing the winding direction and pitch of HTS conductor and shield layer of HTS cable. In this work, the measured eddy current loss of the stainless-steel cryostat pipe is also compared with the calculated one.

Temperature and Pressure Simulation of a High-Temperature Superconducting Cable Cooled by Subcooled &lt;named-content content-type="math" xlink:type="simple"&gt; &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mbox{LN}_2$&lt;/tex-math&gt;&lt;/inline-formula&gt;&lt;/named-content&gt; With Fault Current

A computer simulation has been developed to estimate the transient temperature and pressure distributions in a high-temperature superconducting (HTS) power cable cooled by a forced flow of subcooled LN 2 . This simulation is critical for assessing the effects of short-circuit accidents in practical HTS power cables. When a fault occurs, an excessive current of 31.5 kA may flow in a cable for 2 s, and it is important to understand the temperature and pressure profiles in a cable cooled by the forced flow of LN 2 when these faults occur until the disturbance flows out from a cable. The temperature profile of the LN 2 coolant and the cable cores was analyzed by solving heat conduction and heat transfer equations using the finite-difference method. The Cryodata GASPAK software package was used to estimate the fluid properties. The simulation results show a fairly good agreement with the experimental results. By employing a new model of the induction refrigeration system and the circulation pump, a small discrepancy is resolved in pressures between simulation and experiment. The analysis results show that the pressure in the cable significantly changes depending on the initial gas volumes in the terminals and the volume of the LN 2 gas that evaporates from the copper former in the cable.

Long-term operating characteristics of Japan’s first in-grid HTS power cable

Tokyo Electric Power Company, Sumitomo Electric Industries, Ltd and Mayekawa Mfg. Co., Ltd have jointly conducted the first in-grid demonstration test of a high-temperature superconducting (HTS) cable in Japan, from FY2007 to FY2013. The objective of this project is to evaluate the reliability, stability and other characteristics of the system. The cable structure used in this project is the type of three-in-one cable. As a coolant, sub-cooled liquid nitrogen flows through the gap between the corrugated cryostat and the three cable cores. This structure can realize compactness and reduce heat invasion compared with three single-core HTS cables housed in separate cryostats. The cooling system consists of six refrigerators, two circulation pumps and a reservoir tank. Each refrigerator has a cooling power of 1.0kW at 77K, 0.8kW at 67K. The number of operating refrigerators is controlled so that the coolant temperature at the cable inlet is kept to preset value. The HTS cable was connected to the live electricity grid from October 29, 2012 to December 25, 2013. In-grid operation continued for more than one year without any accidental interruption of operation or other operating issues. During this time, we studied the operating performance of the HTS cable in dependence on the sub-cooled LN2 temperature.