- [Show abstract] [Hide abstract] ABSTRACT: This paper presents the characteristics comparison between an HTS air-core transformer (ACT) and an HTS partial-core transformer (PCT) by experiment and simulation. Two types of transformers were fabricated and tested in liquid nitrogen at 77 K. Fundamental characteristics were obtained by no-load, load, and short-circuit tests. The parameters of the equivalent circuits of the two transformers were also obtained. A field-circuit finite-element method was proposed to analyze the electrical characteristics. In addition, the experimental results were compared with the numerically calculated ones. The results show that the measured critical currents of the transformer with and without the iro n core are nearly the same. The measured exciting current and percent voltage regulation of the PCT is smaller than that of the ACT. The PCT has a better short-circuit current limiting function. The numerical results show that the uneven degree of circulating current of ACT is larger than that of the PCT. The loss characteristic was also discussed in this paper.
- [Show abstract] [Hide abstract] ABSTRACT: The coupling factor is a crucial parameter for a higherature superconductor (HTS) air-core transformer. This paper proposes a numerical method for calculating and optimizing the coupling factor, which took the influence of insulation distance into account. To verify this method, a laboratory-scale HTS air-core transformer was fabricated and tested in liquid nitrogen. GdBCO HTS tapes were used for HTS windings. The primary windings were six single pancakes (SPs) in series, and the secondary windings were three SPs in parallel. Each SP has 32 turns. The numerical calculation and the experimental measurement were consistent. Because the HTS air-core transformer has no specific path for the magnetic flux compared with the full-core transformer, more attention should be given to optimizing the coupling factor. The relationship between the windings arrangement, the insulation distance, the radial dimension, and the coupling factor was discussed by the numerical calculation method. The conclusion of this paper will guide the design of the 1-MVA/10-kV/400-V HTS power transformer, which is going to be built later this year.
- [Show abstract] [Hide abstract] ABSTRACT: Overcurrent performance of high-temperature superconducting (HTS) coated conductor (CC) is one of the most crucial parameters for industrial applications, especially in superconducting fault current limiters (SFCLs). Thus, numerical study of overcurrent process becomes desirable in design of these superconducting devices. In this paper, a field-circuit combined model is introduced to study the overcurrent process of HTS CCs. This model is built by both MATLAB and COMSOL. Circuit parameters, electromagnetic, and temperature distributions are individually calculated by different software. Temperature (T), magnetic intensity (B), and generated heat (Q) are used as real-time intermediate exchanging variables. Accuracy of this model is verified by short-fault experiments on straight HTS CC. Further applications, such as reclosing process and superconducting coils are both performed. Results obtained in this paper prove the validity of this model; this model can be helpful in future design of superconducting devices.
- [Show abstract] [Hide abstract] ABSTRACT: Impulse transformers are widely used in high-frequency signal transportation, high voltage, and current impulse generation. Nowadays, with the increase of the design capacity, disadvantages of impulse transformers using normal electrical materials become more obvious. In order to solve this problem, superconducting tapes are investigated into the design and fabrication of impulse transformers. First, this paper presents the design and performance test of a normal air-core impulse transformer. The design process of this transformer is carried out by theoretical calculation and circuit-field coupled method. Then, performance tests of this transformer under both room temperature and liquid nitrogen temperature are carried out, to verify the validity of this design method, as well as explore the influence of the system parameters. Finally, on the basis of this, the concept design of a 5-kA superconducting impulse transformer is presented, and the performance of the superconducting transformer is obtained by the numerical methods.
- [Show abstract] [Hide abstract] ABSTRACT: Given the limited length of ReBCO coated conductors (CCs), a large number of joints is necessary in high-temperature superconducting (HTS) applications. The quality of joints largely determines the stability of the HTS applications, because joints are usually weaker than ReBCO CCs themselves concerning mechanical and electrical characteristics. To address this problem, a novel type of CC joint, which employs a fabrication process similar to CC lamination, is developed (named after "lamination joint" for convenience). In this paper, the behaviors of the lamination joints under axial tension loads and dc/ac overcurrent are investigated. Single fully laminated CCs and joints fabricated by a conventional soldering method are also studied for comparison. The lamination joints exhibit similar critical axial tensile forces (around 700 N) to the single fully laminated CCs, which are much larger than that of the conventional soldering joints. Moreover, overcurrent tests show that the lamination joints have smaller possibilities to burn out than the single fully laminated CCs. Moreover, the lamination joints are much more stable in quality and are mass producible since they are automatically fabricated by a lamination system. Furthermore, a joint resistance of several n Omega can be achieved by simply extending the splicing length to several meters.
- [Show abstract] [Hide abstract] ABSTRACT: To improve the poor efficiency of conventional ac induction heaters, which is usually below 50%, a higherature superconducting (HTS) dc induction heater has been proposed as a candidate solution. In previous researches, the air gap structure is fixed, which leads to a less controllable magnetic field distribution along the axial direction of billets, hence a less controllable temperature distribution along the same direction. However, an adjustable temperature distribution along the axial direction of billets is required in the aluminum extrusion industry. In order to solve this problem, a novel adjustable air gap structure is proposed and fabricated in our laboratory-scale prototype of the HTS dc induction heater. To validate the method, heating experiments for uniform and gradient temperature distribution were performed with different air gap structures. In addition, a novel adjustment method, i.e., unilateral adjustment, for air gap structure was proposal and tested, which simplified the adjustment method. A finite-element method model of the heater was built to analyze the temperature distribution and the theoretical availability of adjustable air gap structure.
- [Show abstract] [Hide abstract] ABSTRACT: An experimental high temperature superconducting (HTS) transformer has been designed and built using GdBCO HTS tapes. The transformer is unique in that the magnetic circuit is comprised of air and an iron partial core. The HTS windings and iron core were immersed in a liquid nitrogen environment. Fundamental characteristics were obtained by no-load and load tests. The transformer had a smaller exciting current compared with the HTS air core transformer, and the voltage regulation was 1.67% at full load. A lumped parameter equivalent circuit that takes the ac loss was proposed and testified by tests. A 3-D 'field-circuit' coupled model built in COMSOL was used to successfully analyze the electromagnetic characteristics of the transformer. The experimental results were consistent with the calculated and simulation values. The detail results will describe in this paper.
- [Show abstract] [Hide abstract] ABSTRACT: Given the length limitations of commercial coated conductors, joint is very important for their practical applications. Silver diffusion joints are regarded to be promising in the superconducting applications without operation in persistent current mode. This paper presents a 3-D numerical model of silver diffusion joints. The current distributions in the contact layers and their effect on the joint resistance are systematically studied by varying silver layer thickness, length, and width of contact area. It is investigated that the width of the contact area has an inverse proportion to the simulated joint resistances in the range of 2-12 mm like a normal conductor. However, the relationship between the contact length and the simulated joint resistance has an evident deviation from the inverse proportion, because the current distribution in silver layers is highly nonuniform and gives rise to a smaller 'silver usage ratio' when the contact length is longer. Based on the hypothesis that the sheet resistivity of ReBCO-Ag and Ag-Ag interfaces is uniformly distributed and is affected by the 'silver usage ratio' to the same extent as the bulk resistance of the silver layers, the joint resistances derived from our model are in good agreement with the experimental results.
- [Show abstract] [Hide abstract] ABSTRACT: The recovery time is a crucial parameter to high temperature superconducting tapes, especially in power applications. The cooperation between the reclosing device and the superconducting facilities mostly relies on the recovery time of the superconducting tapes. In this paper, a novel method is presented to measure the recovery time of several different superconducting samples. In this method criterion used to judge whether the sample has recovered is the liquid nitrogen temperature, instead of the critical temperature. An interesting phenomenon is observed during the testing of superconducting samples exposed in the liquid nitrogen. Theoretical explanations of this phenomenon are presented from the aspect of heat transfer. Optimization strategy of recovery characteristics based on this phenomenon is also briefly discussed.
- [Show abstract] [Hide abstract] ABSTRACT: The resistive type superconducting fault current limiter (SFCL) is one of the most important superconducting power applications nowadays. As known, this type of SFCL is settled directly in the power transmission line. When a short fault happens, the temperature of the superconductors in the SFCL will increase sharply due to the huge generated heat. This means the superconductors need time to recover the superconducting properties and be ready for the next short fault. So the recovery characteristics become one of the most crucial features of the resistive type SFCL. In this paper, several different kinds of measuring methods are presented to calculate the recovery time of the HTS tapes, and comparison of these methods is also carried out by a standard test. On basis of this, samples with different kinds of stabilizers are used to explore the influence of stabilizer on their recovery characteristics. In addition, the influence of the encapsulation technology is also discussed in this paper.
- [Show abstract] [Hide abstract] ABSTRACT: Joints play an important role in the application of YBaCuO-coated conductors. Given the limited length and various requirements of tapes, large numbers of joints are needed in some applications. Therefore, developing stable joints between tapes and achieving low joint resistance has become an important issue for the application of YBCO tapes. In this paper, we propose a novel superconducting joint production process and fabricate these joints with a lamination machine. A few meters or more than 10 m of joint overlapping length can be achieved, and the joint resistance can be as low as 2.25 nΩ when the overlapping length reaches 8 m. A series of experiments has been carried out to explore the joint resistance changing law, with overlap length increases, and the joint characteristics. The results show that joint resistance is inversely proportional to the overlap length. Although long bridge joints cost more, they have an incomparable advantage in that the joint resistance is distributed over the bridge joint, which allows for more stable operation and less liquid nitrogen boiloff. After joint lamination processing, the burnout current rises from 276 to 523 A. Critical current and joint resistance are not affected by the overcurrent before the joint burns out.
- [Show abstract] [Hide abstract] ABSTRACT: Quenching characteristic is one of the most important characteristics of superconducting fault current limiters (SFCLs). Recently, the designs of SFCLs have been developing rapidly. Because of the influences of electrical inductance and heat dissipation, different types of SFCL could behave quite differently during quenching process. In this paper, the quenching characteristics of several different individual SFCL modules have been investigated. Special attention has been paid to the limited current and structural features between them. The elementary types of YBCO thin-film wires are solenoid and pancake. Different types of superconducting fault current limiting modules were connected in series to evaluate their advantages and disadvantages. Results in this paper could be helpful to optimize the designs of SFCLs in space utilization and energy efficiency.
- [Show abstract] [Hide abstract] ABSTRACT: This paper focuses on no-insulation (NI) HTS pancake coils with iron core for superconducting dc induction heaters. A superconducting coil without turn-to-turn insulation and its conventional counterpart with insulation are wound with YBCO-coated tapes. An iron core is built for a laboratory-scale dc induction heater. The electromagnetic characteristics of the two coils with the iron core are studied focusing on the charging and sudden-discharging processes at 77 K. Three variables are measured, i.e., current, terminal voltage, and magnetic field induced. The results show that NI coil with the iron core incurs a significant delay in the charging and discharging process due to the influence of the iron core. A terminal voltage pulse is observed at the beginning of the sudden-discharging process, which is more than five times of its normal value. A circuit-field coupled method is proposed to analyze the electromagnetic characteristics of the NI coil with an iron core. The results from experiment and simulation exhibits good agreement. The research shows that it is possible to use the NI technique to build a HTS iron core magnet for a dc induction heater, which can simplify the quench protection system.
- [Show abstract] [Hide abstract] ABSTRACT: A voltage-source finite-element model of high-temperature superconducting tapes is presented in this paper. This model is a combined model built on both MATLAB and COMSOL, and it is used to study the overcurrent characteristics of non-uniform HTS tapes. The electrical parameters, magnetic distribution, and temperature distribution are separately calculated by different software. The temperature, magnetic flux intensity, and generated heat are set as the intermediate exchange variables. The model accuracy is calibrated and verified by short-circuit experiments of a uniform five-turn HTS solenoid. Then, the basic model is expanded and used to simulate the overcurrent characteristics of nonuniform HTS tapes. Conclusions are provided by considering both simulation and experimental results from these non-uniform tapes, and the results of this study can be used for the design of superconducting devices.
- [Show abstract] [Hide abstract] ABSTRACT: With the extensive application of VSC-MTDC (voltage source converter-based multi-terminal direct current) power systems, large DC fault current becomes an important issue due to the bottleneck of DC breaker's capacity. This paper presents an application of SFCL(superconducting fault current limiter) in VSC-MTDC system. A numerical model of SFCL is built in PSCAD and the accuracy of model is verified by a series of experiments. SFCLs are applied in a typical three-terminal HVDC system to study the current limiting effect with different kinds of fault types and fault positions. The simulation results show that the fault current can be limited effectively without affecting the normal operation of the system.
- [Show abstract] [Hide abstract] ABSTRACT: Superconducting fault current limiter (SFCL) could reduce short circuit currents in electrical power system. One of the most important thing in developing SFCL is to find out the maximum permissible voltage of each limiting element. The maximum permissible voltage is defined as the maximum voltage per unit length at which the YBCO coated conductors (CC) do not suffer from critical current (Ic) degradation or burnout. In this research, the time of quenching process is changed and voltage is raised until the Ic degradation or burnout happens. YBCO coated conductors test in the experiment are from American Superconductor (AMSC) and Shanghai Jiao Tong University (SJTU). Along with the quenching duration increasing, the maximum permissible voltage of CC decreases. When quenching duration is 100 ms, the maximum permissible of SJTU CC, 12 mm AMSC CC and 4 mm AMSC CC are 0.72 V/cm, 0.52 V/cm and 1.2 V/cm respectively. Based on the results of samples, the whole length of CCs used in the design of a SFCL can be determined.
- [Show abstract] [Hide abstract] ABSTRACT: A dc resistive type superconducting fault current limiter (SFCL) is presented in this paper. This SFCL is designed for the HVDC system. Experiments are conducted to prove the current limiting ability of superconducting materials in dc system. Uniform current and voltage sharing among the SFCL modules can be observed through contact resistance tests, dc flow-through tests, and ac flow-through tests. Results of tests show that each limiting module has good uniformity in higher current system. Then, system simulation model based on these experimental data is built in PSCAD, and simulations are carried out to determine the value of shunt resistor. Results of simulation show that SFCL has fast responding time and good current limiting performance in dc network.
- [Show abstract] [Hide abstract] ABSTRACT: In a resistive superconducting fault current limiter (SFCL), the electrodynamic force among the YBCO tapes is enormous when experiencing a high short circuit current, which may cause deformation of tapes and damage of their electrical characteristics. The purpose of this paper is to investigate how much electrodynamic force the YBCO tapes can bear. In this paper, an electrodynamic force simulation model is built to simulate the force, which the two parallel tapes experience and their deformation when short-circuit currents of different value flow through. Then we conducted experiments to observe the deformation of the tapes when they are experiencing a short-circuit current and see if the volt-ampere characteristic of the tapes has a noticeable change after that. Combining the results of the simulation and the experiments, we can obtain the electrodynamic force that tapes can bear. The result can guide us to design the layout of the tapes in the SFCL in accordance with the required fault current level.
- [Show abstract] [Hide abstract] ABSTRACT: Over-current performance of YBa2Cu3O7-δ tapes under power frequency has been well-studied by previous researches. However, in power grid, superconducting apparatus will also experience lightning impulse current under special working conditions. In this paper, two kinds of lightning impulse current are applied to superconducting testing samples. Comparisons between superconducting characteristics before and after the testing are presented in this paper. Explanations of performance degradation are presented from both the perspectives of temperature and stress mathematic calculation. A maximum endurable value of lightning current which may not cause degradation is obtained from the testing results.
- [Show abstract] [Hide abstract] ABSTRACT: This paper presents a study of frequency dependence of the overcurrent characteristics of high-temperature superconducting (HTS) tape. Fault currents with different amplitudes and frequencies are applied to HTS tape, and the relationship between the overcurrent characteristics and current parameters are obtained after processing and analyzing the experimental results. In addition, a phase difference between the current peak and voltage peak is found in these experiments. Using both simulations and experiments, studies are carried out to find the reason for this phenomenon. Conclusions given in this paper provide useful information for optimizing the design of superconducting apparatus.
Shanghai Jiao Tong University
Shanghai, Shanghai Shi, China
- • Department of Electrical Engineering
- • School of Electronic, Information and Electrical Engineering
- • Department of Electronic Engineering
Xidian UniversityCh’ang-an, Shaanxi, China