High Temperature Superconducting Power Cable Research Articles

Increased Impedance of Wound HTS Power Cables by Quenching at a Shielding Layer for Fault Current Limiting Function

The use of high temperature superconducting (HTS) power cables is viable solution to address the growing demand for electric power, while ensuring low loss and high electric power transmission. Usually, HTS power cables and circular connection of substations reduce grid impedance, and the fault current is increased over the capacity of the circuit breaker. To protect the electric power grid, the fault current should be reduced below the allowable level. Of late, research on HTS power cables having fault current limiting (FCL) functions has increased. An inductive FCL HTS power cable uses the increased inductive impedance caused by magnetic flux leakage to a neighboring iron yoke when a quench occurs at the shielding layer of the cable core due to electromagnetic coupling with the conducting layer. Therefore, there is less heat generation, and the temperature rise is suppressed. In this study, we describe the increased inductive impedance and reduced fault current using a FCL model coil. To demonstrate the electromagnetic coupling between the conducting and shielding layers of the cable core, two concentrically wound REBCO coils were fabricated. The iron yoke inside the coil increases the inductive impedance when a fault current flows through the conducting layer, and quench occurs at the shielding layer. The results are used to develop an FCL HTS power cable with 154 kV, 600 MVA class.

Vibration Testing and Evaluation of 35-kV km-Level Domestic Second-Generation High-Temperature Superconducting Power Cable in Shanghai, China

The 35-kV km-level domestic high-temperature superconducting (HTS) power cable project in Shanghai, China, is expected to be deployed in 2021. Facing the complex operating environment in the metropolitan area, such as the vibration excitation from the subway, pile driving, and seismic activities, the vacuum degree, sealability, and insulation performance of the HTS cables may be affected by the unavoidable mechanical perturbations. The response to the potential vibration excitation is a key factor for the safety and stability of the HTS cables in operation. Therefore, this paper conducts a set of vibration testings for the HTS cable under four kinds of vibration conditions, which include the subway vibration, pile-driving vibration, typical earthquake vibration, and Shanghai artificial wave vibration. Results show that the leakage and outgassing rate of the HTS cable is 1.3×10−6 Pa m3/s at a static state, and increases to 1.4×10−6 Pa m3/s after the vibration testing. The HTS cable exhibits insignificant changes after the vibration testings in the dielectric spectrum examination. The HTS cable can keep its function after the vibration testing that possibly occurred in the metropolitan area.

Exergetic analysis of reverse Brayton cryocooler with different turbine arrangements for HTS power cables

The cooling systems of High-Temperature Superconducting (HTS) power transmission cables are unable to achieve the desired performance as prescribed in the cryogenic road map. Reverse Brayton cycle based Cryocooler (RBC) has been identified as a refrigerator that has the potential to achieve the desired performance for higher cooling load demand. In this article, an exergy analysis of the basic Reverse Brayton Cycle (RBC) and its modifications has been investigated for identification of appropriate cycle configuration that will meet the demand for practical applications of HTS cables. The modifications in the basic cycle are carried out with different arrangements of turbines such as parallel or series expansion that may increase the exergy efficiency of the cooling system with a heat load of 10 kW @ 65 K. The analysis is carried out using Aspen HYSYS V8.6, and helium is used as a working fluid. The effects of important process parameters such as pressure ratio, compressor inlet pressure, and flow diversion on the performance are investigated and are used to optimize these cycles. The best cycle configuration based on RBC is identified and recommended based on the results of the analysis.

Detection of Series Faults in High-Temperature Superconducting DC Power Cables Using Machine Learning

A novel method to detect series faults in multi-strand high-temperature superconducting (HTS) power cables, based on monitoring the transmission characteristics as an indirect way to detect the magnetic signature, is reported. The efficacy of the non-destructive detection method was studied using finite element analysis (FEA) and measurements on a model cable with a varying number of disconnected strands. An S-parameter model block was implemented, and the changes in the magnetic signature resulting from the failed superconducting strands in the cable were investigated. The transmission characteristics of the cable were experimentally analyzed with various types of series faults. The S-parameters obtained experimentally on the model cable were analyzed, and the output of the implemented linear time-invariant (LTI) system was evaluated with the experimental data. A machine learning tool was developed to predict the type of series fault with a regression algorithm based on the Scikit-learn library. The reported method has the potential of enhancing HTS power system reliability by providing early indications of series faults in HTS cables.

An open-source 2D finite difference based transient electro-thermal simulation model for three-phase concentric superconducting power cables

Advances in superconductor technology make the prospect of economical operation of high temperature superconducting (HTS) power cables a practical concept for grid applications in urban centers. With more advanced cable designs being developed and commercialized, their geometrical features and dynamic behavior are becoming increasingly complicated to be modeled. This brings new challenges as the complex structure of HTS power cables significantly increases the computation power needed to perform simulations. In this paper we develop a two-dimensional open source simulation code based on the finite difference method which is solved by means of the alternating direction implicit routine. The algorithm has been written in MatLab programming language. The method improves computational performance and simulation time. In addition, this enables the creation of open-source simulation codes. A three-phase concentric HTS cable design has been chosen for the development of the code, nevertheless the model can be employed for any cable design. The results indicate an efficient, stable and powerful simulation code. During the development no numerical instabilities have been found. Besides that, the model is able to deliver quantities that are experimentally difficult to access. Simulation files are available for the scientific community on the HTS Modeling Workgroup webpage. 1 1Simulation files can be downloaded.here.

Design and Performance Test of 2 kW Class Reverse Brayton Cryogenic System

With the increased commercialization of high-temperature superconducting (HTS) power cables cooled using liquid nitrogen and the use of liquefied natural gas as fuel, the need for large-capacity reverse Brayton cryogenic systems is gradually increasing. In this paper, the thermodynamic design of a reverse Brayton cryogenic system with a cooling capacity of the 2 kW class at 77 K using neon as a refrigerant is described. Unlike conventional reverse Brayton systems, the proposed system uses a cryogenic turbo-expander, scroll compressor, and plate-type heat exchanger. The performance test conducted on the fabricated system is also described. The isentropic efficiency of the cryogenic turbo-expander was measured to be 86%, which is higher than the design specification. The effectiveness of the heat exchanger and the flow rate and operating pressure of the refrigerant were found to be lower than the design specifications. Consequently, the refrigeration capacity of the fabricated reverse Brayton cryogenic system was measured to be 1.23 kW at 77 K. In the future, we expect to achieve the targeted refrigeration capacity through further improvements. In addition, the faster commercialization of HTS power cables and more efficient storage of liquefied natural gas will be realized.

Structural Analysis of 2G HTS Tapes under Different Loading Conditions for HTS Power Cable using Finite Element Modeling

In High Temperature Superconducting (HTS) power cable, HTS tapes are wound helically around a circular former and are subjected to different types of mechanical loads during manufacturing, transportation and installation of cable. The strain developed within the HTS tape during these processes may affect the superconducting properties of the tape. In the present work, 3D structural finite element (FE) models using ANSYS® are developed to obtain the strain behaviour of the tape under different loading conditions, considering the layered sandwich composite structure of 2G HTS tapes. The analysis is carried out with axial tensile and twisting loads on the tape, during helical winding of tape around the former and a combined effect of helical winding with external tensile and bending loads. The result obtained from FE simulation due to applied tensile load is compared with the established experimental result, which shows a good agreement. Further, the strain developed within the tape affects the critical current. Therefore, the strain is coupled with an empirical relation to obtain the critical current degradation of the tape. The FE analysis is carried out for five different winding pitches and four different bending radii with YBCO layer in the downward position and the corresponding critical currents are estimated. Further, a minimum winding pitch is obtained, below which the rapid degradation of critical current is initiated. The results show that the HTS tape with YBCO in downward position can withstand more amount of external load without affecting the critical axial strain of the tape.

Novel gases as electrical insulation and a new design for gas-cooled superconducting power cables

High Temperature Superconducting (HTS) power cables are being developed for a variety of applications including the electrical power grid, electrification of transportation, and high energy physics. The appeal of superconducting technology is its high current density — generally greater than 100 A/mm2 — compared with conventional copper and aluminum conductors, which generally do not support greater than 5 A/mm2. Thus, HTS wires and cables support high power ratings even at low and medium voltages, which reduce the size and weight of the electrical power systems. The size and weight reductions are essential to realize large all-electric ships and electric aircraft. HTS devices must be operated within the boundary of their respective critical temperature, critical current, and critical magnetic field [1]. Second generation HTS materials possess critical temperatures of ∼90 K. However, operating temperatures in the range of 50–70 K are typically required for HTS cables to support the desired operating current of many power-dense systems and to have a sufficient temperature margin to tolerate unexpected heat loads and fault currents. When a superconductor exceeds any of its three critical parameters, a quench will occur. A quench causes the HTS cable to transition to its normal state where it starts to display its high electrical resistance. The interdependency of the electrical and thermal properties of HTS materials also depends on the cryogen used to achieve the desired operating temperature. There are only a few cryogens compatible with the desired operating temperatures of 2 ) being the most commonly used for HTS cable applications for the electric power grid due to its abundance, low cost, and good electrical insulation and heat transfer properties. LN 2 -cooled HTS cables have been successfully demonstrated in the electrical power grid operating at 10–200 kV [2], [3]. The LN 2 -cooled 10 kV AC HTS cable installed as part of the Ampacity project in Essen, Germany demonstrated the potential of HTS technology to be economically feasible in urban areas. Installing the 10 kV HTS cable instead of a conventional 110 kV cable to supply the downtown area of Essen allowed for 4 out of 10, 110/10 kV transformer substations located in the center of Essen to be decommissioned and removed [2].

Development of a High-Temperature Superconducting Gas-Insulated Power Cable

Superconducting gas-insulated transmission line (S-GIL) concept for high-temperature superconducting (HTS) power cables was developed with practical designs that use tubular insulator spacers. The implications of spacer materials and designs were studied in terms of electric field enhancement and its role in limiting the maximum withstand voltage. Three different tubular insulator materials and two different sizes were explored using the finite-element analysis of electric field enhancement. Model cables were fabricated and tested for their response to high-voltage tests at various pressures up to 2 MPa, with two different gas media and at two different temperatures. The experimental results were compared with the expected behavior based on the finite-element analysis. Implications of the designs on cryogenic gas flow and pressure drop along the length are also discussed. Maximum withstand voltage above 40 kV was achieved for designs suitable for gas-cooled HTS cables operating at 77 K.

Thermo-Hydraulic Analysis of a Tri-Axial High-Temperature Superconducting Power Cable with Respect to Installation Site Geography

Various high-temperature superconducting (HTS) power cables are being developed or are ready for commercial operation to help energy suppliers meet the growing power demand in urban areas. Recently, triaxial HTS power cables have been developed by Korea Electric Power Corporation (KEPCO) and LS Cable & System. Further, KEPCO has been planning to install a 2 km long 23 kV/60 MVA triaxial HTS power cable to connect the Munsan and Seonyu substations and increase the stability of the power grid. The HTS power cables should be cooled down to a cryogenic temperature near 77 K. A thermo-hydraulic analysis of the cooling system considering the geographical characteristics of the installation site is essential for long-distance sections. This paper describes the thermo-hydraulic analysis of the triaxial HTS power cable to determine the proper mass flow rates of subcooled liquid nitrogen that meet the operating temperature and pressure of the cable for four configurations of cooling systems: (1) a single cooling system with an external return path, (2) a dual cooling system with an external return path, (3) a single cooling system with an internal return path, and (4) a dual cooling system with internal return path. Since the flow characteristics in a corrugated cable cryostat differ significantly from those in a typical annular tube, a computational fluid dynamics (CFD) analysis was conducted to estimate the pressure drop along the cable cryostat. With the CFD analysis and given heat loads in the cable, the temperature and the pressure variations along the cable were calculated and their pros and cons were compared for each configuration of the cooling system. This thermo-hydraulic analysis will be referenced in the actual installation of the HTS power cable between the Munsan and Seonyu substations.

Design of HTS power cable with fault current limiting function

As demand for electricity in urban areas increases, it is necessary to improve electric power stability by interconnecting neighboring substations and high temperature superconductor (HTS) power cables are considered as a promising option due to its large power capacity. However, the interconnection of substations reduces grid impedance and expected fault current is over 45 kA, which exceeds the capacity of a circuit breaker in Korean grid. To reduce the fault current below 45 kA, a HTS power cable having a fault current limiting (FCL) function is considered by as a feasible solution for the interconnection of substations. In this study, a FCL HTS power cable of 600 MVA/154 kV, transmission level class, is considered to reduce the fault current from 63 kA to less than 45 kA by generating an impedance over 1 Ωwhen the fault current is induced. For the thermal design of FCL HTS power cable, a parametric study is conducted to meet a required temperature limit and impedance by modifying the cable core from usual HTS power cables which are designed to bypass the fault current through cable former. The analysis results give a minimum cable length and an area of stainless steel former to suppress the temperature of cable below a design limit.

Feasibility Assessment of the New Configuration of an Electric Power Grid with 23 kV Tri-axial HTS Power Cables for an Urban Power Supply

The advantage of high temperature superconducting (HTS) power cables is that they can transmit large amounts of power at relatively low voltages, which can be used to reduce the number of lines in transmission cables or to configure a power grid at lower voltages. Because the commercial operation of 23 kV HTS power cables has begun, the optimal configuration between the 154 kV transformer substations and the 23 kV switching stations connected by the 23 kV HTS cables must be reviewed. In this paper, the new power supply model using 23 kV tri-axial HTS cables is presented for an urban power supply, and the economic feasibility is reviewed to present the appropriate cable capacity selection that matches the 23 kV switching stations.

Design of a Novel Inductive Type Fault Current Limiting HTS Power Cable

In a power system, fault currents are on the rise and are becoming a common problem. Moreover, several methods for the restriction of fault currents can lead to the instability of power system. In the Republic of Korea, the Korea Electrotechnology Research Institute is carrying out the development of a 154 kV class high temperature superconductor (HTS) cable with a fault current limiting function since May 2017. In this project, a novel inductive type fault current limiting (IFCL) HTS power cable is proposed. This paper presents the design of an IFCL-HTS cable for a transmission system, whose principle is based on change in the distribution of magnetic fields around the cable using a quench of HTS shield layer. The results of the design and simulation for performance prediction are discussed in detail.

AC Losses Analysis in stack of 2G HTS tapes in a coil

The model coil with the racetrack geometry based on second generation (2G) High Temperature Superconducting (HTS) tapes has been developed for an electrical machine where a winding pack is a stack of 2G HTS tapes. It is important to evaluate transport current AC losses and possible methods to reduce them in a winding. In a device made of 2G HTS tapes, the main the AC losses are the hysteresis ones. Only numerical simulation permits to predict them in full. The FEM model for calculation of the hysteresis losses developed before for 2G HTS power cables was modified for a stack of the 2G HTS tapes in a coil. In this paper the methods to increase a computational speed are presented. Possible ways to reduce AC losses are discussed also. The details of the model and comparisons of calculations with measurements of the AC loss are presented.

Performance Analysis of Real-Scale 23 kV/60 MVA Class Tri-Axial HTS Power Cable for Real-Grid Application in Korea

Currently, various types of superconducting power cables are being developed worldwide, and research and development of a tri-axial high-temperature superconducting (HTS) power cable are underway. The tri-axial HTS power cable reduces the amount of HTS wire due to its multilayer structure, has high current characteristics, and has less loss than other superconducting cables. However, since the radii of each phase are different, magnetic coupling makes it difficult to measure power loss and analyze performance. This paper presents the results of the design and performance analysis of a tri-axial HTS power cable. A prototype tri-axial HTS power cable was designed with a rated power of 60 MVA, a rated voltage of 23 kV and a length of 6 m, and was tested by cooling to 77 K with liquid nitrogen. We analyzed the performance of the tri-axial HTS power cable in normal conditions through a finite element method (FEM) simulation and experiment. The alternating current (AC) loss of the tri-axial HTS power cable was calculated using a FEM program based on the Maxwell equation, and the result was used to confirm the AC loss of the tri-axial HTS power cable prototype measured by the electrical measurement method. In conclusion, in the current test of a tri-axial HTS cable designed as 23 kV/60 MVA, the DC critical current was over 6000 A, the AC loss was approximately 0.24 W/m, and the simulation and analysis design values were satisfied. The results of this study will be effectively applied to commercial tri-axial HTS power cable development to be installed in a real power system. This means that the actual tri-axial HTS cable has sufficient capacity for rated current operation in the system where it will be applied, and the actual measurement of the cable loss can be applied as an important factor in the design of the cooling capacity of the entire superconducting cable, which consists of several kilometers.

Impacts of a LVRT Control Strategy of Offshore Wind Farms on the HTS Power Cable

High temperature superconducting (HTS) power cables are a potential solution for the grid integration of offshore wind farms since the HTS cable can conduct bulk wind power at low voltage levels. However, the transient current through the HTS cable in cases of low voltage ride through (LVRT) operation has a negative impact on the HTS cable operation due to the quenching phenomenon. This paper analyzes the impact of LVRT control strategies on the HTS cable operation. In addition, a coordinated control of wind turbines for LVRT improvement of an offshore wind farm is proposed. The feasibility of the HTS cable application for the grid connection of offshore wind farms is also discussed in this study. The proposed controller is designed for the wind turbine generator based on a type-4 permanent magnet synchronous generator. In the proposed controller, the transient current through the HTS cable is reduced by regulating the machine side power during fault conditions. The feasibility of the proposed controller is validated in the PSCAD/EMTDC program (Manitoba Hydro International Ltd., Winnipeg, Manitoba, Canada, version 4.2.1). The effects of transient current on the cable temperatures and resistances are analyzed in this study. Simulation results show that the proposed control strategy could reduce the transient current and temperature rise of the HTS cable.

Electrical faults in high temperature superconducting power cables for MVDC power systems of all-electric ships

High temperature superconducting (HTS) power cables are expected to be used in the future in electric ships that will have integrated power and energy systems (IPES). There have been a few studies on understanding electrical faults in power systems consisting of HTS cables; however, there are no comprehensive studies on the response of HTS cables for various types of electrical faults. We have recently initiated a research project on understanding various electrical faults in shipboard medium voltage direct current (MVDC) power systems and the duration of the fault, maximum voltage, and maximum current that the HTS cables will encounter during the fault. This paper presents investigations on the potential cable architectures being developed for MVDC power systems, and the type of faults that HTS cables will encounter in such systems. The paper will assess the relative merits of these architectures in terms of their suitability for accommodating the limitations of cryogenically cooled HTS cables and offer design suggestions for resilient power systems. Electrical and cryogenic thermal models of HTS cables that are suitable for assessing the response of HTS cables for electrical faults, as well as HTS cable designs that can endure electrical faults without catastrophic damage will also be discussed.

DC surface flashover characteristics of ZnO/Epoxy composites at room temperature and 77 K

Epoxy (EP) resin based nano-composites are widely used in high voltage direct current (HVDC) high temperature superconducting (HTS) power cable. In this paper, the DC surface flashover characteristics of ZnO/EP composites were studied at both room temperature and 77 K. The samples were made by dispersing ZnO nano-particles into EP resin with various weight percentages of 0%, 1%, 3%, 6% and 10%, respectively. The experiments were carried out with a cryogenic system in which DC high voltages ranging from 0 kV to 100 kV were supplied. The results showed that the surface flashover voltages increased with the increase of ZnO content at both room temperature and 77 K, and the surface flashover voltages at 77 K were higher than that at room temperature for composites with the same ZnO content.

Progress of the first commercial project of high-temperature superconducting cables by KEPCO in Korea

Korea Electric Power Corporation (KEPCO) has fully funded and completed the first commercial project of high-temperature superconducting (HTS) power cables, called the Shingal Project, to connect two substations with a 23 kV HTS cable over a distance of 1 km. Now, commercial operations are ready to start. In this paper, the configuration of the project is presented, and photos related to the installation and the results of the test operation are described in detail. As a next step, KEPCO is considering a second commercial application project of 154 kV HTS cables to enhance the power supply reliability of the 154 kV radial substations. KEPCO is also developing a demonstration project of 23 kV tri-axial HTS power cables to prove the feasibility of a 23 kV switching station for urban power supply to replace the new 154 kV substation; we call this switching station concept the 23 kV HTS power platform. The Shingal Project has great global significance as it is the first superconducting project to start using 23 kV HTS cables commercially in the real grid.

Fault Analysis and Design of a Protection System for a Mesh Power System with a Co-Axial HTS Power Cable

The uses of high-temperature superconducting (HTS) cables pose a challenge of power system protection since the impedance of the HTS cable is varied during fault conditions. The protection systems should be designed properly to ensure the reliability and stability of the whole system. This paper presents a fault analysis of the co-axial HTS cable in the mesh system and proposes a coordinated protection system. In the proposed protection system, the main protection is the differential current relay whereas the backup protections are the overcurrent and directional overcurrent relays. The normal and abnormal relay operations are considered to analyze the transient fault current in the HTS cable and evaluate the performance of the proposed coordinated protection system. Characteristics of cable impedances and temperatures under various fault conditions are presented. The proposed protection scheme is validated by the simulation in the PSCAD/EMTDC program. Simulation results show that the coordinated protection scheme could successfully protect the HTS cables in both normal and abnormal relay operations.