Z. Hong

An equivalent circuit grid model for no-insulation HTS pancake coils

An equivalent circuit grid (ECG) model is proposed to analyse the time-varying characteristics of no-insulation (NI) ReBCO pancake coils. In the model, each turn of the coil is subdivided into fine elements in the azimuthal direction, and each element is equivalent to a circuit parameter. Then, the coil is equivalent to a circuit grid. A math model based on Kirchhoffs law is proposed to solve the circuit grid model. The distribution of the electrical current inside the NI coil is analysed for the charging and discharging process. A finite element method (FEM) model is coupled to calculate the magnetic field induced by the coil. To validate the model, a double pancake (DP) coil is fabricated by coated conductor ReBCO tapes. Charging and discharging tests are performed on the coil at 77 K. The results from simulations and experiments exhibit a good agreement. Then, this model is used for more studies on the current distribution inside the NI coil in the charging and discharging process. The charging and discharging delay of NI coil is analysed and explained by the model. The model can also be applied to partial insulated (PI) coils and magnets consisting of NI coils.

The Structure, Performance and Recovery Time of a 10 kV Resistive Type Superconducting Fault Current Limiter

This paper presents the design and performance test of a 10 kV, 200 A resistive type superconducting fault current limiter prototype. This is the continued study of the test towards an individual current limiting module, which was done and published in August 2011. The construction of the whole prototype including the cryostat has finished. A series of tests including short circuit test, recovery test, auto-reclosure test, and LN2 boiling test have been performed and the results are presented in this paper. This single phase superconducting fault current limiter is made from 15 current limiting modules. Several 1-m long YBCO coated conductors prepared by Shanghai Jiaotong University are used to build the current limiting module. Each module has 2 tapes connected in parallel to carry 200 A rated current (Ic of each tape is about 150 A) and 6 tapes connected in series to withstand 700 ~ 800 V voltage drop.

Design and Application of a Superconducting Fault Current Limiter in DC Systems

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.

Design and Experimental Results of a DC Induction Heater Prototype for Aluminum Billets

The advantages of dc induction heating in high efficiency and good heating quality have been proved and studied worldwide in past decades. In this paper, a laboratory-scale dc induction heater with iron yoke for aluminum billets is designed and tested in detail. The billet to be heated is placed in the air gap of the yoke and is driven by a motor. The magnetic field in the gap is built up by copper windings with direct currents. The whole heating processes under different conditions are observed. Various surface temperature distributions have been achieved by changing several heating parameters including heating time, rotating speed, and dc magnetic field. Numerical analysis for calculating the heating process is also carried out in a commercial finite-element software. Results show that all the heating parameters will affect the final heating effects. Also, the numerical results agree well with the experimental results.

Study on a Numerical Method for Calculating the Heating Process of HTS DC Induction Heater

A commercial HTS dc induction heater for aluminium billets is going to be built in Shanghai Jiao Tong University, China. To achieve an optimal design, a numerical model is proposed to accurately calculate the heating process of the billets due to induced current. The model is based on two physics fields i.e. including thermodynamics and electromagnetism. The changes of relevant parameters with increase of billets temperature are taken into account. In the model, the billets rotate in a dc magnetic field generated by a HTS magnet with iron cores. The induction heating power and temperature distribution inside the billets can be calculated accurately. In order to validate the calculation, a small laboratory-scale prototype dc inductive heater for aluminium billets is built. The coils are immersed in liquid nitrogen to flow higher current, and the billet is rotated by a motor. Comparison is carried out on the results of numerical model and experiment and analysis is made for the discrepancy between them. The discrepancy is acceptable for engineering applications. Finally, some examples of using the numerical model to calculate the electromagnetic behaviour for a commercial industry-scale dc induction heater are given.

Fabrication and Characteristic Tests of a Novel Low-Resistance Joint Structure for YBCO Coated-Conductors

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.

Numerical and Experimental Analysis of AC Loss of YBCO Coated Conductor Carrying DC and AC Offset Transport Current

Because of the complexity of the electrical system, dc offset current and ac ripple current have generally existed. However, they are usually undesirable, and their influence to ac loss is also an important issue for superconducting devices built from YBaCuO-coated conductors. In this paper, ac loss of YBCO tapes with nonmagnetic substrate carrying dc or ac offset current have been both measured and calculated. Experiments based on “electrical method” composed of a high-performance amplifier and a wave recorder with filtering function has been carried out. To verify the measurement, a two-dimensional finite element method model based on finite element method and power law is applied to calculate the ac loss with current combined with ac and dc components. The results show that, a small value of dc offset current in a large ac component has minor effect to the ac loss. In addition, when the total current (combined with ac and dc components) approaches critical value, the ac loss would increase significantly.

Design of a HTS Magnet With Iron Core for DC Induction Heater

This paper presents an electromagnetic design of a HTS magnet with iron core for DC induction heater. The heater is designed for aluminium billets with diameter of 60 mm and length less than 500 mm. The magnet is designed to generate 0.5 T DC magnetic fields in its air gap. The coil is wound with YBCO tapes produced by AMSC, and its operating temperature is 50 K. A numerical model is proposed to get an optimal design. The model couples MATLAB code and FEM model and considers the nonlinearity of iron core and the field dependence of tapes Jc. The FEM model is built from commercial software. It can analyze the electromagnetic characteristics of magnet with complex geometry easily. The MATLAB code is used to analyse results from the FEM model, update the design variables to get an optimal solution. Finally, the specifications of design results are shown. The accuracy of the FEM model is the key of the optimization process. A magnet by cooper wires is built to validate the FEM model. Its iron core has same dimension as the designed HTS one. The data from FEM model and experiment show a good agreement.

Numerical Study on Silver Diffusion Joints of Coated Conductors by Finite-Element Method

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.

Characteristic Tests and Electromagnetic Analysis of an HTS Partial Core Transformer

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.