Zhijian Jin

Study on No-Insulation HTS Pancake Coils With Iron Core for Superconducting DC Induction Heaters

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.

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.

Electrical-Thermal Coupled Finite Element Model of High Temperature Superconductor for Resistive Type Fault Current Limiter

A multi-physics finite element model of high temperature superconductors (HTS) is presented in this article. An electrical model based on a set of Maxwells equations and E-J power law is used to solve the critical state of the superconductor. A heat transfer model is added to the electrical model to calculate the temperature distribution and therefore investigate the Jc (T) dependence of the superconductor. The model is used to study the quench behavior of YBCO-coated conductors for the application of resistive type fault current limiters. Some numerical techniques are applied and assumptions are made to simplify the calculation and improve convergence. An equivalent heat transfer coefficient which is much larger than the normal heat transfer coefficient is applied to the region surrounding the superconductors. This equivalent coefficient represents the drastic heat exchange during the boiling of the liquid nitrogen. The cross-section of YBCO tapes is divided into several sub-domains. The temperature is assumed to be uniform in each sub-domain. This simplification significantly improves the convergence of model and still is able to keep a reasonable level of accuracy. The model is then able to simulate the whole process of YBCO tapes quenching and recovering to superconducting state. The numerical results are compared with the fault current experiments and excellent agreement is obtained.

The Development and Performance Test of a 10 kV Resistive Type Superconducting Fault Current Limiter

This paper presents the design of a 10 kV, 200 A resistive type superconducting fault current limiter prototype. Several one-meter 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 reach 400 A rated current ( of each tape is 150 A ~200 A) and 6 tapes connected in series to withstand 700~800 V voltage drop. A series of tests and measurements have been carried out to investigate the performance of the fault current limiting module. Short circuit test of the module has been performed on a specially designed transformer which is able to withstand large current on the secondary side. The power capacity of the lab allows performing the test up to 40 V and a few thousand Amperes. The whole process of a fault, including fault occurring, current limiting, circuit breaker opening and superconductors recovery, has been tested and the current and voltage of YBCO tapes are monitored. The current limiting module is tested with different shunt resistors to demonstrate its ability to vary its limited current.

Test of Maximum Endurable Quenching Voltage of YBCO-Coated Conductors for Resistive Superconducting Fault Current Limiter

Voltage tolerance during quench is one of the most important parameters for YBCO-coated conductors, especially for resistive type superconducting fault current limiter (SFCL) applications. How much voltage can be sustained per unit length of YBCO tape when experiencing fault current indicates the total required amount of the tapes to construct a resistive SFCL. In this paper, a step-down transformer is used to provide the short-current. As the voltage of the secondary side is not continuously variable, we change the voltage per unit length of the YBCO tape by varying the length of the tape. And the effect of the duration of short-current is also considered. A series of test have been performed to determine the maximum voltage drop that each YBCO tape can stand at fault current. The V-t curve has been measured after the tape experiencing successive quenches. Through a large number of experiments we will acquire the maximum endurable quenching voltage per unit length of YBCO tapes when experiencing short-current for different durations. The results presented in this paper would provide useful information for optimizing the design of a resistive SFCL.

AC Loss and Contact Resistance of Resistive Type Fault Current Limiter Using YBCO Coated Conductors

Heat loss is one of the most important issues for resistive type superconducting fault current limiters (SFCL) built from YBaCuO-coated conductors. A newly designed 400 V SFCL module by Smart Grid Center, Shanghai Jiaotong University (SJTU) is tested and modeled in order to get a clear understanding of its characteristics of heat losses. Two main losses, including AC loss of superconducting tapes and the losses due to contact resistance inside the SFCL module are investigated. Experiments based on “electrical method” are set up to measure the AC loss at 77 K with lock-in amplifier conditions. Meanwhile, losses from the contact resistance are calculated by analyzing their V-I curve. To better understand the behavior of the AC loss in SFCL module and verify the accuracy of the experimental results, modeling analysis is performed by means of a 2-dimensional finite element modeling (FEM). Numerical results show that the results are very close to what we get from experimental method. Detailed results will be presented and the heat losses of 10 kV SFCL consisting of about 25 modules in series are going to be discussed.

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.

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.

Bubble Behavior and Its Influence on Insulation Properties of Liquid Nitrogen for Superconducting Power Apparatus

In superconducting power apparatus, electric insulation is one of the most important deciding factors on their performance. Bubbles produced during quench condition of superconducting tapes will degrade the insulation performance of liquid nitrogen and reduce the electrical breakdown voltage of the apparatus. This paper presents the bubble behavior and the influence on insulation properties of liquid nitrogen ( LN2) for superconducting power apparatus. The result will provide useful information for superconducting power devices in transmission voltage level, such as fault current limiter, transformers, and cables. Measurements are made to get the ac breakdown voltages in LN2 using spherical brass electrodes as quasi-uniform electric field. Different structures of HTS tapes are used as electrodes to simulate real working conditions. Bubbles are created by heaters, and their influences on the breakdown voltage are measured. How the breakdown strength would be affected by the bubbles is studied by changing the length of the gap of electrodes and the amount of bubbles. Breakdown voltage of G10 slice is also tested. Experiments show that the breakdown strength of LN2 is sensitive to the behavior of bubbles. Different structures of HTS tape have different breakdown properties under bubbling condition. G10 material is a good choice to provide insulation strength between HTS tapes.

Characteristics Comparison Between HTS Air Core and Partial Core Transformers

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 iron 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.