Ho-Myung Chang

Thermodynamic optimization of conduction-cooled HTS current leads

A theoretical optimization is performed for the conduction-cooling method of high Tc superconductor (HTS) current leads, which can be applied to the superconducting systems cooled directly by cryogenic refrigerators without liquid helium. The current lead is a series combination of a normal metal conductor at the warmer part and a HTS at the colder part, and is cooled by a contact with distributed or staged refrigerators instead of boil-off helium gas. An analytical method is developed to derive a mathematical expression for the required refrigerator power. By incorporating the critical characteristics of the HTS, it is demonstrated that there exist unique optimal values for the current density of HTS and the joint temperature of the two parts to minimize the total refrigerator power per unit current, for a given length of the HTS. As results of the study, the absolute minimum in the refrigerator power per unit current is presented as a thermodynamic limit and the leads cooled by a two-stage refrigerator are theoretically optimized. Some aspects in practical design are also discussed with a new and useful graphical method.

Development of a 13.2 kV/630 A (8.3 MVA) High Temperature Superconducting Fault Current Limiter

This paper deals with fabrication and development of a high temperature superconducting (HTS) fault current limiter (FCL) based on YBCO coated conductor (CC) wire for distribution systems. The capacity of the developed HTS FCL is 8.3 MVA and its rated voltage is 13.2 kV which corresponds to a three-phase power equipment voltage class of 22.9 kV. Tests of the developed prototype HTS FCL were conducted at Korea Electrotechnology Research Institute (KERI) accredited as a testing laboratory by the Korea Laboratory Accreditation Scheme (KOLAS). A short-circuit test and an AC dielectric withstand voltage test for the HTS FCL were conducted under sub-cooled liquid nitrogen (LN2 ) conditions of 3 bar and 65 K. The magnitude of an asymmetric short- circuit current without FCL reached 30 kApeak in a short-circuit test. The superconducting coil quenched instantaneously after the fault, and the magnitude of the fault current was limited to 3.6 kApeak within quarter cycle by the developed resistance of the superconducting coil. An AC dielectric withstand voltage test was performed, and the HTS FCL successfully withstood 143 kV for 1 minute. Also, it was found that there was no electrical or mechanical damage on the superconducting coil after the tests.

Cryogenic cooling system of HTS transformers by natural convection of subcooled liquid nitrogen

Abstract Heat transfer analysis on a newly proposed cryogenic cooling system is performed for HTS transformers to be operated at 63–66 K. In the proposed system, HTS pancake windings are immersed in a liquid nitrogen bath where the liquid is cooled simply by colder copper sheets vertically extended from the coldhead of a cryocooler. Liquid nitrogen in the gap between the windings and the copper sheets develops a circulating flow by buoyancy force in subcooled state. The heat transfer coefficient for natural convection is estimated from the existing engineering correlations, and then the axial temperature distributions are calculated analytically and numerically with taking into account the distributed AC loss in the windings and the thermal radiation on the walls of liquid-vessel. The calculation results show that the warm end of the HTS windings can be maintained at only 2–3 K above the freezing temperature of nitrogen, with acceptable values for the height of HTS windings and the thickness of copper sheets. It is concluded that the cooling by natural convection of subcooled liquid nitrogen can be an excellent option for compactness, efficiency, and reliability of HTS transformers.

Development of 220 V/300 A Class Non-Inductive Winding Type Fault Current Limiter Using 2G HTS Wire

As a part of the 21st Century Frontier R&D Program in Korea being performed from 2004, a non-inductive winding type superconducting fault current limiter (SFCL) is being developed. The target of the second year in phase II of the program is to develop a 220 V/300 A class non-inductive winding type SFCL as a prototype for a 13.2 kV/630 A class, the final goal of phase II. This SFCL has three solenoid type non-inductively wound coils in series using a 2G high temperature superconducting (HTS) wire and it was tested in sub-cooled nitrogen of 65 K, 1 atm. A coil which is composed of four parallel windings in a bobbin and winding directions are opposite to have non-inductive characteristics. Three coils were connected in series and the total length of 108 m of 2G HTS wire was used. Short-circuit tests were performed at applied voltage of 220 V and the SFCL limited the fault current to a few kA extents at the tests. Recovery time of the SFCL was measured after short-circuit tests.

Manufacture and Test of Small-Scale Superconducting Fault Current Limiter by Using the Bifilar Winding of Coated Conductor

Superconducting fault current limiters (SFCLs) have been developed by many research groups. However, there is no standard for current limiting device. Recently, YBCO coated conductor (C.C.) which is named as 2nd-generation wire has been developed rapidly. YBCO C.C. has many advantages for applying to fault current limiting material. In this paper, a bifilar winding type SFCL was manufactured using YBCO C.C. The bifilar coil was wound as pancake type, and the length of C.C. tape used was 8 m. The short-circuit test of the SFCL was performed successfully rated on 30V/80A. The SFCL had a very low impedance in normal operation and limited the fault current effectively when a fault occurred. From the result, it could be confirmed that the bifilar winding type FCL using YBCO C.C. is feasible. Large-scale SFCL using C.C. should be developed in the future

Recovery Characteristics of Resistive SFCL Wound With YBCO Coated Conductor in a Power System

Since resistive superconducting fault current limiters (SFCLs) are inserted to a power system directly, it is necessary to recover instantly after the clearance of a fault. Most resistive SFCLs using BSCCO bulk or thick film have a recovery time of tens of seconds. An SFCL using YBCO coated conductor (CC) has a large surface area contacted to liquid nitrogen. Joule heat flux of the SFCL is smaller than that of other types when a fault occurs. Therefore, it is important for the SFCL employing CC to investigate a recovery time. In this paper, the recovery characteristics of SFCL with respect to applied voltage were analysed in a power system. All tests were performed in liquid nitrogen and sub-cooled nitrogen. From this result, the parameters for recovery time were obtained.

Effect of gap flow on shuttle heat transfer

A comprehensive analysis is performed to investigate the effect of the oscillating flow of gap fluid on the shuttle heat transfer in reciprocating expanders. For a sinusoidal motion of displacer over cylinder having an axial temperature gradient, a new exact expression for shuttle heat transfer is derived from the analytical solution of the velocity and the temperature distributions for the gap fluid and the two walls. Through a rigorous analysis, the most significant parameter in the shuttle phenomena is proven to be the ratio of the inertial force to the viscous force in the oscillating fluid. For the ratio values smaller than unity, the predicted shuttle heat transfer from the present expression is in good agreement with the previously published results. For large ratios, however, a notable discrepancy exists between the results of the present analysis and the previous investigations. The reason for this discrepancy is that the wall-to-wall heat flux is not in phase with the temperature difference because of the fluid motion that was not included in the previous investigations.

An exact solution for shuttle heat transfer

A new analytical solution has been obtained for the shuttle heat transfer rate in displacer/cylinder systems having an axial temperature gradient. The temperature oscillations of both the displacer wall and the cylinder wall were considered in the analysis. After the cyclic steady state solution was obtained for the temperatures of the walls by introducing complex temperatures, a simple mathematical expression was derived to calculate the shuttle heat transfer. The usefulness of the results was justified by approximate solution of previous work.

Optimization of operating temperature in cryocooled HTS magnets for compactness and efficiency

A new concept of thermal design to optimize the operating temperature of high temperature superconductor (HTS) magnets is presented, aiming simultaneously at small size and low energy consumption. The magnet systems considered here are refrigerated by a closed-cycle cryocooler, and liquid cryogens may or may not be used as a cooling medium. For a specific magnet application, the size of required HTS windings could be smaller at a lower temperature, by taking advantage of a greater critical current density of HTS. As the temperature decreases, however, the power input to the cryocooler increases dramatically because of the heavy cooling load and the poor refrigeration performance. Through a rigorous modeling and analysis incorporating the effect of magnet size into the load calculation, it is demonstrated that there exists an optimum for the operating temperature to minimize the power required. The optimal temperature is strongly dependent upon the magnitude of AC loss in the magnets and the assistance of heat interception.

Optimization of Current Leads Cooled by Natural Convection of Vapor

An optimization theory for current leads passing through a closed vapor‐filled space is introduced. These leads are applicable mainly to HTS power systems where liquid nitrogen is continuously refrigerated by a cryocooler. The design of such leads is basically similar to that of conduction‐cooled leads, because no boil‐off gas flows out of the system. The purpose of this study is to determine whether the cooling by natural convection of vapor requires any modification in optimizing the lead design. In the case that the leads are located in a wide vapor space (called boundary layer flow), the energy balance equations for the lead and the surrounding vapor are solved by the method of perturbation series, as they are weakly coupled by natural convection. The analytical solution shows that the optimal lead parameter does not need to be changed in spite of the convective cooling. In the case of the leads passing through a narrow vapor space (called fully developed flow), on the other hand, the axial conduction...