Woo Seung Lee

The effects of co-wound Kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of GdBCO pancake coils

Recently, studies of partially insulated, high-temperature superconducting (HTS) coils have shown application in the design and construction of compact, stable and self-protecting HTS coils. This article presents the electrical characteristics of HTS coils based on the turn-to-turn inserted materials and conditions. Three partially co-wound pancake coils were fabricated and tested. Each coil was partially co-wound with Kapton, stainless steel and copper tape at every fourth turn of the winding. Tested coils were co-wound on every turn with Kapton, stainless steel and copper tape, and coils without turn-to-turn insulation were the control group. Charge–discharge, sudden-discharge and over-current tests were performed to evaluate the performance of the fabricated coils. The experimental results show that the properties of materials inserted into the coil can control the time constant (τ). Therefore, HTS coils can be designed for specific purposes according to the time constant control. The experimental results of the study could be useful in designing HTS coil applications.

Numerical Analysis and Design of Damper Layer for MW-Class HTS Synchronous Wind Turbine Generator

This paper presents a numerical analysis on MW-class high-temperature superconducting (HTS) synchronous generator, especially focusing on damper layer design. Our research program to develop a large-scale HTS wind turbine generator (WTG) core technology, a two-dimensional electromagnetic FEM model of the direct-drive HTS WTG, comprised of 24 air-core type HTS race track coils (24 poles), has been built. The damper is used for stable operation of HTS field coil against varying magnetic field by change of rotation speed. This model is designed to obtain basic operating parameters for HTS WTG, including magnetic field distribution and induced electromotive force, in static and transient condition. First, the parameters in static condition, which means that the rotating speed is constant, are compared with our design parameters to confirm the feasibility of our numerical analysis. Second, in transient condition, we focused on the reaction between rotating magnetic field and its stator components, damper, and armature winding. By changing damper material having different electrical conductivity and magnetic permeability, we concluded our damper design to be applicable to our MW-class HTS synchronous generator model.

Experimental Study on Hysteresis of Screening-Current-Induced Field in an HTS Magnet for NMR Applications

This paper presents a research on magnetic hysteresis of an HTS magnet comprising a stack of pancake coils wound with GdBCO-coated conductors. The hysteresis during a typical charging and discharging process of an HTS magnet occurs mainly due to the screening-current-induced field (SCF). To investigate the hysteresis, an HTS magnet, comprising 16 single-pancake coils with the respective winding i.d., o.d., and height of 100, 124, and 4 mm, was constructed. Magnetic field distributions in a 10-mm cold bore of the HTS magnet were measured by a three-dimensional field mapper, and their spatial harmonics, axial and radial, were obtained using associated Legendre equations. Field mappings were conducted at every 10 A during charging up to 90% of the magnet critical current, and repeated at every 10 A during discharging. The hysteresis loops were obtained in terms of major field gradients, axial of Z1 and Z2 and radial of X and ZX. As a result, strong hysteresis was observed that the magnetic fields were not linearly proportional to the operating current even at the beginning of charging. Even at the identical operating currents of the coil during charging and discharging, a significant difference in the spatial field distribution was observed. The residual field, a primary source to the field discrepancy, was measured 2.11% of the magnet center field at its peak operating current of 50 A, which is significant for some applications including NMR (nuclear magnetic resonance) magnets. More importantly, the residual gradients in some major axial gradients, Z1 and Z2, were much more noticeable, about -9.4% and 10.2%, respectively, of those measured at 50 A. A recommended charging protocol, called “current-sweep-reversal,” in terms of minimizing the hysteresis of SCF was investigated and discussed in detail.

Experimental study of the effects of alternating fields on HTS coils according to the winding insulation conditions

This paper examines the effects of alternating fields on high-temperature superconducting (HTS) coils according to the winding insulation condition. Alternating fields can occur in synchronous machines (armature reaction, faults) and other devices. In superconducting synchronous machines, alternating fields affect the operational characteristics of the machine and the superconducting field coil. Therefore, a method of reducing the effects of alternating fields is necessary in superconducting synchronous design. In this study, the effects of alternating fields on the HTS field coil according to the winding insulation condition were experimentally evaluated. The experimental results show that HTS coils made using the no-insulation technique can be a solution for reducing the effects of the alternating field. These results are expected to suggest useful data for applications of HTS field coils in superconducting synchronous machines.

Experimental Study of the New Type of HTS Elements for Current Leads to be Applied to the Nuclear Fusion Devices

This paper describes new kind of HTS (High Temperature Superconducting) element for current leads, which can transfer large electric current in the nuclear fusion system. Since the HTS tapes have no resistance and the generated Joule heat is also very small compared to the conventional current leads, cooling costs of the current leads can be reduced. For this reason, the various feasibility researches for HTS current leads have been promisingly progressed in the superconducting power applications. However, there is a limitation on the capability of current carrying for single HTS tape. In addition, the cross-sectional area of the current leads should be small enough to minimize the heat conducted through the HTS current leads. In order to improve the current carrying capacity of the current leads, multiple HTS tapes are required. In this paper, we proposed several structures of compact HTS current leads and compare them other 2 kA class current leads. We report on the calculated characteristics of stabilized HTS current leads and on the performance measured during critical current measurement tests.

Development and Experimental Evaluation of a Prototype of the TF Secondary Quench Detection System for KSTAR Device

A prototype of the toroidal field (TF) secondary quench detection (SQD) system was developed and implemented with the Korea Superconducting Tokamak Advanced Research (KSTAR) device to carry out the design verification of the SQD. The SQD can detect a quench based on the change of absolute pressures (APs) and mass flow rates of helium in the cooling lines of the TF coils. If the primary quench detection system of the TF coils cannot detect a quench or the fast discharge of the TF coils cannot be carried out as planned, SQD should work to prevent the TF coils and the peripheral structures from severe damages. In addition, SQD should operate in the reliable and stable condition against the disturbances caused by the poloidal field coil discharge, plasma perturbation, and any faults of subsystems of the KSTAR device. The 2-out-of-3 (2oo3) voting configuration was applied to the SQD to enhance the reliability and stability of quench detection. The prototype SQD consists of AP and differential pressure transducers, signal interfaces, logic solvers, and interlock systems. All the transducers were selected from metallic types with no electronic circuit in order to reduce the failure rates caused by strong electromagnetic field and radiation around the tokamak. The transducers were installed in the manifolds of the helium inlet lines of the KSTAR TF coils. Their signals were amplified and compared with the reference voltage for quench decision in the signal interface unit. The quench signal generated by the signal interface unit was transmitted to the 2oo3 voting modules of the logic solvers. The design requirements of SQD were verified through testing the prototype SQD in the 2014 KSTAR campaign.

Quench Detection Method for HTS Coils Using Electromagnetically Coupled Coils

Generally, in order to detect the quench of the superconducting field coil for the superconducting rotating machine, signal wires for voltage taps are indispensably needed. However, the problem of the conventional method is wire twisting as the superconducting field coil rotates. Therefore, in this paper, authors suggest a new method to detect the quench of the superconducting field coil using electromagnetically coupled coils. The suggested method consists of two superconducting pancake and copper solenoid coils. Two superconducting pancake coils play roles of a field coil and external (alternating) field generation for the rotating machine. In addition, two copper solenoid coils are used to transmit the voltage wirelessly and receive the transmitted voltage using electromagnetic induction. In order to verify the feasibility of this method, experiments are performed according to the transporting current flowing through two superconducting pancake coils and separation distance between two copper solenoid coils. The result shows that it is expected to apply this method to the superconducting rotating machines for the detection of quench without direct connection of voltage taps.

Evaluation on Electrical and Thermal Characteristics of Multi-Stacked HTS Coated Conductor With Various Stabilizers

Transporting large currents through High- Tc (HTS) current leads (CLs) inevitably involves using several HTS tapes in multi-stacked form to obtain a high current density. For safe operation of HTS CLs, the thermal stability of stacked HTS tapes should be considered. In this paper, we investigated electrical and thermal properties of multi-stacked HTS tapes using 2G coated conductor (CC). Three kinds of commercial CC with different specification of stabilizer were used for the experiment. Each stack sample was fabricated by soldering together CC tapes using In-Bi alloy. To verify the effects of a stabilizer on thermal stability for multi-stacked CC tapes, we performed an experiment of minimum quench energy measurements. In addition, transient electrical and thermal behavior were measured as a function of the external heat input energy. Each stack sample has a heater made of nickel-chromium (Ni-Cr) wire to make the external heat disturbance and a type-E thermocouple to measure the temperature. Based on the test results, electrical and thermal characteristics of multi-stacked CC tapes were evaluated.

Evaluation of Electrical and Thermal Properties of Stacked YBCO Coated Conductors for Current Lead Application

This paper deals with the electrical and thermal characteristics analysis of stacked coated conductors (CCs) for current lead application. A test sample is fabricated using YBCO CCs with a stainless steel laminated stabilizer and has a heater made of the Ni-Cr wire to generate external heat disturbance. In order to evaluate a thermal stability behavior, minimum quench energy, voltage, and temperature trace are measured as a function of operating current. To verify thermal behavior in stacked CCs, numerical analysis on temperature trace is carried out using the finite-element method. The numerical simulation results are compared with the experimental data to check the validity of the simulation model.

Operational Characteristics of a Small-Scale Novel Hybrid Resistive-Type SFCL With Controlled Power Electronics

As electric power systems become more complicated, the fault current experiences more frequent changes. To solve this problem, we suggest a novel hybrid-type superconducting fault current limiter (SFCL). Some issues accompany application of resistive-type SFCLs to electric power systems, including initial installation price, operation and maintenance costs, and high-current problems. To overcome these problems, hybrid SFCLs have been developed in some institutions. This paper presents experimental tests of a novel hybrid-type SFCL composed of a resistive superconducting coil, two thyristors, and a control circuit. The superconducting coil limits the first peak of the fault current, and the thyristors control the magnitude of the fault current other than the first peak by adjusting the firing angles. The interactions between the superconducting coil and the thyristors reduce the use of superconducting wire.