Young-Sik Jo

This study explores the application of Smart Insulation (SI) technology in high-temperature superconducting (HTS) coils for rotating electric machines, aimed at enhancing operational stability and efficiency. SI technology acts as an insulator below 160 K and becomes conductive above this temperature, enabling current redistribution during quench events to improve thermal stability. We integrated SI into a 100 kW superconducting motor and conducted extensive tests using a 250 kW universal inverter and EV dynamometer. Despite initial cooling limitations capping output at 30 kW, the SI-equipped motor withstood multiple quench events without damage. Unlike traditional insulated coils prone to overheating, SI coils efficiently prevented hot spots, ensuring stable operation. These results highlight the potential of SI technology to improve the performance and resilience of HTS motors under variable load conditions.

This paper presents the results of a study conducted to experimentally investigate the effectiveness of implementing smart insulation (SI) coils in high-temperature superconducting rotating machines (HTSRMs). The obtained results mainly focus on the design and parameter extraction for the proposed method. Utilizing the stator of an existing 1-MW-class superconducting motor, the electromagnetic design of the new rotor was conducted for an approximate power rating of 100 kW. In addition, a static analysis was performed to extract the parameters required to conduct load testing on an HTSRM. Additionally, the SI field coil was manufactured to conduct critical current measurements; the obtained values were compared and analyzed with respect to the wire characteristics.

Vanadium trioxide (V ₂ O ₃ ) exhibits a negative temperature dependency in its electrical resistivity, a phenomenon known as the metal-to-insulator transition (MIT). This study investigates the implications of this property, particularly noting that V ₂ O ₃ ’s resistivity significantly decreases above approximately 150K, reaching levels comparable to metallic resistivity. Such behavior is pivotal in enabling the automatic adjustment and recovery of contact resistance ( R _ct ) in rare-earth barium copper oxide (REBCO) coils, in response to internal temperature fluctuations. By introducing V ₂ O ₃ insulation, REBCO coils can effectively combine the benefits of both insulated and non-insulated configurations. To enhance the reliability of V ₂ O ₃ insulation in REBCO applications, it is essential to ensure the compatibility of V ₂ O ₃ ’s inherent properties, such as electrical and thermal conductivities, with the quench states of the REBCO coil. The primary aim of this research is to evaluate the feasibility of incorporating metallic powder (molybdenum, Mo) into V ₂ O ₃ as a potential enhancement for REBCO applications. The study measures R _ct values, reflecting MIT behavior, in REBCO conductors integrated with V ₂ O ₃ paste containing micrometer and nanometer-sized Mo powder. These measurements were conducted across various Mo concentrations within a conduction-cooled environment, spanning a temperature range of 20 to 290 K. Additionally, thermal conductivity assessments were performed to ascertain the efficacy of the V ₂ O ₃ + Mo mixture coatings. Spectrometry analysis was also conducted on the samples to examine their micro-morphological characteristics.

Non-insulated coils made of second-generation high-temperature superconductors (HTSs) have been developed to prevent the burning of superconducting wires. Although these coils perform reasonably well in a constant magnetic field, the induced current in the coil is a serious concern in a time-varying magnetic field. Hence, we aimed to determine the characteristics of non-insulated HTS coils in a time-varying magnetic field. We manufactured two HTS coils: 1) non-insulated coil and 2) metal-insulated coil with a higher turn-to-turn resistance. Experiments were performed under a time-varying field generated using the stator of a 1 MW rotating machine. The HTS coil in the rotating machine was severely affected by an asynchronous or transient magnetic field. In addition, we designed a special structure to simulate severe operating conditions. As a result, the magnetic field of the HTS coil decreased in the time-varying field. The metal-insulated coil with a higher turn-to-turn resistance exhibited a lower field reduction than the non-insulated coil. We also performed circuit analyses on the experimental results. Our findings suggest the feasibility of applying HTS coils without insulation in rotating machines.

This study aimed to compare and analyze the output characteristics of second-generation high-temperature superconducting racetrack coils with insulation, metal-insulation (MI), non-insulation (NI), and smart-insulation (SI). First, the output characteristics of each coil were experimentally validated, and circuit equations were examined to analyze these characteristics. The output characteristics were found to be affected by the turn-to-turn resistance. Circuit analysis was used to identify the nature of dependence of output characteristics on turn-to-turn resistance in addition to internal behavior and other mechanisms. In particular, an equation relating the output characteristics of the SI coil with temperature change was derived, and each coil model was simulated using the circuit equation. Then, the simulation results were analyzed, the advantages and disadvantages of each insulation method were discussed, and the validity of the simulation was confirmed. The circuit simulation proposed in this study is expected to facilitate advanced predictions in smart insulation material candidate groups.

Experiments and finite-element analyses were conducted to investigate the characteristics of a 300-turn smart insulation (SI) coil coated with vanadium trioxide (V ₂ O ₃ ) placed in an external fluctuating magnetic field. The external fluctuating magnetic field generator was designed to allow sliding insertion through a fabricated non-rotating liquid-nitrogen tank inside the stator of a 1-MW superconducting rotating machine. The 300-turn SI coil was fabricated in a two-layer design with 150-turn windings in each layer. Experiments were conducted in such a manner that the SI coil did not rotate during the generation of the external fluctuating magnetic field owing to the rotating magnetic field of the stator. The SI coil with many turns showed no magnetic field delay because of the ramping time,thereby demonstrating controllability equivalent to that of a normal insulated coil. The stability of the SI coil was confirmed via repeated overcurrent tests, with and without an external fluctuating magnetic field.

This paper deals with the fabrication and performance testing of a prototype machine for the world’s first implementation of a new type high-temperature superconducting rotating machine (HTSRM), which is charged and operated by the application of a contactless superconducting excitation technique with a rotary-type HTS flux pump based on a permanent magnet. Although this type of flux pump has been actively applied in stationary superconducting applications, its practical demonstration on rotary superconducting applications, namely, rotating machines, has not yet been conducted or reported. Therefore, laboratory-scale hardware implementation was conducted to investigate the feasibility of using an HTS contactless field exciter (CFE) as the field exciter of an HTSRM. Firstly, the various core components were manufactured, assembled, and tested to configure the 1-kW-class high-temperature superconducting generator (HTSG) system. Then, the assembled machine was connected with a conventional induction motor and three-phase resistive load. In non-load tests, the characteristics of the induced voltage of the HTSG, which was generated and affected by contactless field excitation and the operating speed, respectively, were tested and measured. In particular, the charge and discharge behaviors of the field current excited by the HTS CFE were experimentally analyzed based on the induced voltage profiles of the HTSG. Then, the HTSG output characteristics were tested and measured by performing experiments on the characteristics of constant load and constant speed, respectively. The generator test results were satisfactory in terms of output, voltage regulation, and the total harmonic distortion in the voltage and current.Our developed HTS CFE can excite an HTS field coil with 101 A in a noncontact manner, resulting in an operating field current margin of approximately 79% and a decrease of approximately 91.5% in the field excitation loss. In addition, the results demonstrated the possibility of designing HTS magnets with self-protecting features.

This paper presents the results of an electromagnetic design and numerical analysis conducted on a 10-MW-class second-generation high-temperature superconducting generator (2G HTSG) used in an offshore wind power turbine. To report the operation reliability of 10-MW-class HTSGs employed in off-shore environment, various electric insulation techniques (EITs) for HTS field coils (FCs), such as no-insulation, metal insulation, and metal-insulator transition insulation, are considered in this study. Using the time-transient solver of the three-dimensional (3D) electromagnetic finite element analysis (FEA), we investigated the electromagnetic characteristics of the HTSG with the three EITs in terms of the electrical output of the HTSG and the critical current of the FCs. To analyze the charging characteristic of the HTS FCs with the three EITs in steady-state operation as well as the electrical in transient-state operation, electric equivalent circuit models are built with key parameters based on the electromagnetic FEA results. Finally, the performances of the HTS FCs are discussed and evaluated in terms of the electromagnetic response and stability characteristics.

In this study, we developed a novel simulation code to analyze a second-generation high-temperature superconductor (HTS) magnet that was insulated with a polyimide film and vanadium III oxide (V ₂ O ₃ ). In particular, V ₂ O ₃ is one of the metal–insulator transition materials that has resistivity that varies with temperature. For its commercialization for large-scale HTS magnet applications, the HTS magnet that uses V ₂ O ₃ is expected to achieve both of the desired characteristics, specifically, (a) high electrical and thermal stabilities for the non-insulated coils, and (b) prompt controllability of the insulated coils. First, an equivalent electrical-circuit model is proposed to develop a simulation code for the analysis of electrical and thermal characteristics of polyimide and V ₂ O ₃ . Then, all simulated results were compared with experimental results of a small-scale HTS single pancake coil and discussed to verify the reliability of the developed simulation code.