Jeyull Lee

HTS coil with enhanced thermal stability in over-current operation for fast response magnet power application

This paper examines the effect of improved winding geometry on high-temperature superconducting (HTS) coils for use in superconducting power applications. One of the most important functions in such a superconducting magnetic energy storage system is to charge–discharge the superconducting coils as fast as possible to secure sufficient power demand. The HTS coils are vulnerable to the thermal instability caused by cyclic and/or unexpected charge–discharge variation. Therefore, it is necessary to enhance the safety of the HTS coils under fast response operation at the request of varying loads. In this study, improved thermal stability in over-current operation is demonstrated by implementing the proposed insulation scheme. The performance of the HTS coil with the proposed winding geometry was experimentally evaluated in comparison with a conventional insulation (CI) coil. Cryo-stability characteristics of the proposed coil are verified with a circuit analysis under a charge–discharge operation scenario. The results of this study show the usefulness of the proposed winding coil as a replacement for CI coils, which have drawbacks related to thermal recovery rate in over-current operation.

Analysis of a High-Tc Superconducting Power Converting System

This paper presents the analysis of a high-Tc superconducting (HTS) power converting system, as well as its operational characteristics. The converting system can be used to charge and discharge a magnet made of series-connected pancake coils. The HTS converting system consists of two heaters, a primary copper winding, a secondary HTS winding, a series-connected HTS pancake coil, an iron core and a conventional copper load. In the experiments, the charging and discharging periods were 7.5 and 2 s, respectively. A partial region of the superconducting tape in a secondary HTS winding is switched to a normal region by a buried heating coil. To measure the converting-current with respect to the magnet flux changes, a hall sensor was installed at the center of the pancake coil. In this experiment, the charging-current and discharging-energy reached about 51.7 A and 36.8 J, respectively. The experimental results have been compared with theoretical predictions by using the finite difference method.

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.

Test and Analysis of Electromagnetic and Mechanical Properties of HTS Coil During Quench State

This paper deals with the analysis of the electromagnetic and mechanical properties of a high-temperature superconducting (HTS) coil, as well as its characteristic as determined through tests using strain gauges. When an operating current is applied to an HTS coil, strain is generated by electromagnetic force. However, additional strain effects in the HTS coil are formed by the Joule heating during quench. Strain was measured with a full-bridge strain gauge mounted directly over the turns. We fabricated an HTS coil wound in a single pancake type to investigate strain characteristics. We also investigated structural analysis by a finite-element method to calculate the strain of the HTS coil. In tests, the strain in the HTS coil was measured during excitation. Test results of the HTS coil have been compared with the theoretical results. Finally, we performed quench propagation tests and could verify the strain effect during quench. The reaction times of the strain gauge at 30 and 80 A are 31.8 and 4.3 s, respectively.

Methods for Increasing the Saturation Current and Charging Speed of a Rotary HTS Flux-Pump to Charge the Field Coil of a Synchronous Motor

The rotary flux-pump using HTS tape has been studied for superconducting rotating machinery application. The charging speed and saturation current of the rotary HTS flux-pump is closely related to magnetic flux linkage passing through the HTS tape. To analyze charging parameters that effect pumping rate and saturation current of the flux-pump, methods of changing the rotating speed, shape of permanent magnet, width of HTS tape, and magnetic flux intensity have been investigated in previous studies [1]– [3]. In this paper, we have tried to test three cases to investigate the pumping rate and saturation current: 1) two different background materials, iron and Bakelite, were used to compare the magnetic flux linkage reinforcement; 2) two HTS tapes were overlapped to extend the magnetic flux linkage area, and each HTS tape was connected to an HTS coil; and 3) the parallel joint was conducted between the flux-pump and the HTS coil to compose a closed loop for persistent current mode. In order to measure the charging speed and pumping rate, a Hall sensor was installed at the center of the HTS coil.

A Study on the YBCO-Coated Conductor Current Lead With Asymmetric Structure Tape Considering Temperature Distribution

A current lead is a connecting device between the superconducting magnet and the power supply. A large amount of heat transfer ocurrs through the current lead because the cold-end in a superconducting system is thermally linked with the warm-end part. Therefore, high-Tc superconductor (HTS) is used in the current lead for a tokamak system. However, it is hard to consider the dependance of HTSs critical current with respect to temperature because the conventional HTS current leads consist of the HTS tapes with the same width and thickness, and their critical currents can differ from their temperature. For this reason, cold-end of the HTS section in current lead is designed excessively than necessary specifications. This generates the additional heat transfer between cold-end and warm-end within the HTS current lead. In this paper, a study on the HTS current leads with asymmetric structure is conducted to reduce the heat transfer by reducing the cross-sectional area of the HTS current lead. Asymmetric current leads are fabricated considering the design parameters about heat transfer within the HTS current lead. Current transporting tests on the conventional, symmetrical, and asymmetrical HTS current leads are performed to confirm the feasibility of the suggested design. And then, heat load of the asymmetric current lead is compared with the symmetric current lead. From this study, it is found that heat load of the current lead is reduced by using the asymmetric HTS current lead.

Design and Test of HTS Power Converting System With Multiple Magnets Considering Various Sequential Controls of Heater-Triggered Switches

This paper presents experimental results of the designed high-Tc superconducting (HTS) power converting system with multiple GdBCO magnets. The system is tested by various sequential controls of four heater-triggered switches and an electromagnet, which consists of two energy storage magnets (ESMs). The experiments are mainly composed of two modes. The first one is a simultaneously operating mode (Mode1), and another is an alternative operating mode (Mode2). Each two sequences are performed at Mode1 (CS1 and CS2) and Mode2 (CS3 and CS4) for charging the ESMs, respectively. In those experiments, the average pumping rate of each sequence is calculated at about 17.55, 41.26, 10.25, and 25.83 mA/s, respectively. In the discharging test, two sequences are tested. Load energy is calculated from load voltage and current due to the difference between precharged energy to verify which operating mode is more effective. Load energy of each sequence reached about 47.97 and 38.83 J after 2000 s, respectively.

Operational Characteristics of HTS Coils With Flux Diverters in Semipersistent Mode Under Alternating Magnetic Field

This paper deals with the effect of magnetic flux diverters on the current decay behaviors of high-temperature superconducting (HTS) coils which operate in semipersistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machines, the HTS field coil can be designed to operate in PCM to obtain a steady magnetic field during operation. Due to the alternating magnetic field from the armature winding, the HTS field winding experiences an external magnetic field. As a result, the additional magnetization ac loss in the field coil would be induced, and it affects the current decay characteristics. In order to improve the current decay behavior of the HTS field coil, the use of flux diverter made of magnetic material can be considered. In this paper, we experimentally investigated the current decay characteristics of HTS field coils with magnetic flux diverters under alternating magnetic field. The measurement setup, which is composed of HTS field coil and armature coil, was fabricated. The measurement test of the current decay on the HTS field coil in PCM operation was carried out as functions of the amplitude and frequency of the armature coil.

Experimental Study on Thermal Behavior of HTS Coils With Quasi-Insulation Winding Method at Overcurrent Operation

Recently, superconducting power applications by using the high-temperature superconducting (HTS) tape have been widely researched due to the development of the HTS tape. In particular, the HTS coils cowound with insulation materials have been dominantly used for the power applications. However, the insulated HTS coil has low thermal stability at overcurrent operation, which causes much likely to be damaged. To relieve this issue, other insulation methods such as insulation-free or partial insulation have been investigated. These methods have good thermal stability; however, they have weaknesses such as magnetic field saturation and charge-discharge delay as well. In particular, charge-discharge delay is a significant obstacle for superconducting magnetic energy storage due to a frequently variable current operation. Therefore, a quasi-insulation (QI) coil that compensates the disadvantages has been proposed. In this paper, two QI coils according to the width of the insulation tape were fabricated since the performance of this winding method is affected by the exposure area to the coolant. In addition, insulation-free coil and partial insulation coil were fabricated and tested as the control group. The terminal coil voltage and center magnetic field were measured to compare the characteristics of each coil. The experimental result shows the QI coil has good thermal stability at overcurrent operation without magnetic field saturation and degradation. In addition, cooling effect is proportional to the exposed area to the coolant and adjustable with a different width of insulation tape.

Experimental Analysis of Thermally and Magnetically Triggered Switch for High-Tc Superconducting Power Converting System

In this paper, characteristics of a thermally and magnetically triggered switch for a high-Tc superconducting (HTS) power converting system are experimentally analyzed. For verifying the efficiency of the suggested switch, current charging and discharging tests using heater-triggered switch with or without an external magnetic field are performed. Charging tests are performed with two charging sequences, i.e., CS1 and CS2. Each sequence is experimented with two kinds of heater currents to maintain different temperatures. Saturation current and saturation time are detected to calculate the pumping rate. Two discharging sequences, i.e., DS1 and DS2, are used for discharging tests with heater current to maintain 120 K. From the results, normalized load energies are calculated and compared.