Yeon Suk Choi

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.

Long Term Performance Test of KEPCO HTS Power Cable

A project for verification of HTS power cable system by KEPCO (Korea Electric Power Corporation) was started in 2002. The HTS power cable system of 100 m, 22.9 kV, 1.25 kA has been installed and tested at the KEPCOs Gochang power testing center in Korea since 2006. A liquid nitrogen decompression cooling system with a cooling capacity of 3 kW at 66 K and a closed-circulation system of subcooled liquid nitrogen were employed for this purpose. Several performance tests for the HTS power cable system, such as cooling capacity, electrical loading, heat load, AC loss and temperature stability, were performed at operating temperature of 66.4 K. Thermal cycle test including cool-down to liquid nitrogen temperature and warming up to room temperature, was also performed to investigate thermal cycle influences for 8 times. The reliability through the several demonstration tests over than 8,000 hours was proved.

Conduction-Cooled Superconducting Magnet for Material Control Application

The conduction-cooled superconducting magnet with operating current of 180 A is designed, fabricated, and tested for material control application. The superconducting magnet has the effective standard warm bore of 52 mm and the maximum central field of 3 Tesla. Since magnetic field gradient should be larger at the end rather than at the center of the magnet for material control, we developed design method to optimize magnet for this purpose. The safety of the superconducting magnet is evaluated, taking into account the electro-magnetic field, heat and structure, and the superconducting coil is successfully wound by the wet-winding method. The superconducting coil is installed in the cryostat maintaining high vacuum and cooled by a two-stage GM cryocooler. The performance of the conduction-cooled superconducting magnet is discussed with respect to the supplied current, Joule heating, and cooling medium.

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.

Progress on the Development of a 5 T HTS Insert Magnet for GHz Class NMR Applications

The Korea Basic Science Institute (KBSI) has initiated the development of a 5 T HTS insert magnet system. The objective of this program is the design and fabrication of HTS insert magnet for GHz class LTS/HTS NMR applications. Compact and efficient HTS magnet system is designed to be inserted into 15 T conduction-cooled superconducting magnet with 100 mm room-temperature bore. In this paper, the status of the HTS insert magnet development in KBSI is presented. The design issues of the 5 T HTS insert magnet including 2G HTS conductor properties with additional reinforcement and insulation, coil configuration with supporting structures, magnetic winding and bending stress-strain, detect and activate protection, and spatial and temporal field performance are discussed. In addition, the design of cryostat for 5 T HTS insert magnet is presented.

Closed-Loop Cryogenic Cooling for a 21 T FT-ICR Magnet System

A closed-loop cooling concept for 21 T Fourier transform ion cyclotron resonance (FT-ICR) superconducting magnets is presented. In the magnet system, low temperature superconducting coils are immersed in a subcooled 1.8 K bath, which is connected to the saturated helium reservoir through the weight load relief valve. Saturated liquid helium is refrigerated by a Joule-Thomson (JT) heat exchanger and flows through the JT valve, isenthalpically dropping its pressure to approximately 1.6 kPa, corresponding to a saturation temperature of 1.8 K. Helium gas exhausted from JT pump is liquefied by a two-stage cryocooler located after the vapor purify system. In the present paper, the amount of heat budget is determined and the structural design of cryostat is carried out by the relevant analyses. The position of a cryocooler in the magnet system is investigated, taking into account the requirement of magnetic field for normal performance. Helium liquefaction system, a key component of the closed-loop cooling system, is fabricated and tested in order to demonstrate the feasibility of our new cryogenic cooling for high field magnets.

Conceptual Design of Current Leads for a 21 T FT-ICR Magnet System

A new concept of current leads for a 21 T Fourier Transform Ion Cyclotron Resonance (FT-ICR) superconducting magnet system is presented. The binary current lead, a series combination of a normal conductor in the high-temperature section and an HTS conductor in the low-temperature section, is employed. The HTS section is partly immersed in liquid helium and the joint between the normal metal and HTS conductor is cooled by the contact with a cryocooler. After excitation, the normal metal conductor section is thermally disengaged from the HTS section without disturbance to the insulating vacuum space and without requiring complete removal of the current leads. The present paper describes the so-called semi-retractable current leads for a 21 T FT-ICR magnet system application. The optimized dimensions of the leads are presented in order to minimize the thermal heat load when carrying operational current with some margin.

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...

Natural Convection of Subcooled Liquid Nitrogen in a Vertical Cavity

An experiment to measure the natural convection of subcooled liquid nitrogen between two vertical plates has been performed. The main objective of this study is to confirm the feasibility of our recently proposed design for an HTS power transformer cooled by natural convection of subcooled liquid nitrogen. A liquid nitrogen bath is cooled to nearly the freezing temperature (63 K) at atmospheric pressure by a vertical copper heat transfer plate thermally anchored to the coldhead of a GM cryocooler. A parallel copper plate generating uniform heat flux is placed at a distance so that liquid between the two plates may develop a circulating flow by natural convection. The vertical temperature distribution on both surfaces is measured in steady state, from which the heat transfer coefficient is calculated. The experimental data are compared with the existing correlations for a rectangular cavity where each vertical surface has a uniform temperature. The discrepancy between two data sets is examined by consideri...

Cryogenic cooling temperature of HTS transformers for compactness and efficiency

A comprehensive thermal design to optimize the cryogenic cooling temperature of HTS transformer is presented, aiming simultaneously at compactness and efficiency. As small size and low power consumption are conflicting in determining the operating temperature, we develop a general and systematic model to quantify the effects of the temperature on compactness and efficiency. The procedure includes modeling of the critical property of HTS and the winding size, a heat transfer analysis for cooling load estimate, and a thermodynamic evaluation for cryogenic refrigeration. We demonstrate that there exists an optimum for the operating temperature that minimizes the overall power consumption, while taking into account the size effect of HTS windings. The optimal temperature turns out to be slightly above 77 K for two specific systems considered here: liquid-cooled pancake and conduction-cooled solenoid. The operation at temperatures well below 77 K can be justified, if the amount of ac loss is substantially reduced or the saving in capital investment earned by the compactness is significant in comparison with the operational cost.