Kwang Lok Kim

Investigation of HTS Racetrack Coil Without Turn-to-Turn Insulation for Superconducting Rotating Machines

This paper reports a study of two HTS racetrack coils wound with GdBCO coated conductors, one without turn-to-turn insulation and the other with Nomex insulation. The coils were characterized by charge-discharge, over-current, and sudden discharge tests. Thermal and electrical characteristics of the GdBCO racetrack coil without insulation outperformed the insulated one. The test results confirmed that although the coil without insulation has a slow charging/discharging time constant, the no-insulation winding technique may enable a compact and protection-free racetrack-typed coil with enhanced mechanical integrity as well as better thermal and electrical stabilities.

Effects of Impregnating Materials on Thermal and Electrical Stabilities of the HTS Racetrack Pancake Coils Without Turn-to-Turn Insulation

A high temperature superconducting (HTS) coil without turn-to-turn insulation is proposed for the field coil of a wind turbine. In the case of the field coil, epoxy impregnation is generally necessary to protect the coil from mechanical disturbances by time-varying magnetic fields and rotational vibrations to provide high mechanical integrity. This study examined the thermal and electrical stabilities of non-insulated GdBCO racetrack pancake coils impregnated with CTD-521, Stycast 2850 FT, and paraffin through cool down, over-current, and repetitive cooling tests. Among the three epoxy impregnated coils, the Stycast 2850 FT-impregnated coil exhibited the best thermal and electrical stabilities during over-current testing. In repetitive cooling conditions, the CTD-521-impregnated coil exhibited less degradation of its superconducting property due to the well-matching of the thermal contraction between the GdBCO racetrack pancake coil and the epoxy.

Effect of Winding Tension on Electrical Behaviors of a No-Insulation ReBCO Pancake Coil

This paper presents a study on the effects of winding tension on the characteristic resistance of a no-insulation (NI) coil. Two ReBCO NI test pancake coils, having the same winding i.d. (60 mm), o.d. (67.6 mm), and number of turns (60), were sequentially prepared in a way that the first test coil was wound with a winding tension of 12-N, tested, and then rewound with a new winding tension of 20-N for the same tests. In each test, the test coil was energized at a target current, the power supply was “suddenly” disconnected, and then the temporal decay of the coil center field was measured, from which the time constant of the test coil and the consequent characteristic resistance were obtained. To check the reproducibility of experimental data, each test was repeated four times and each time the test coil was unwound and rewound with a given winding tension. The experimental results were analyzed with equivalent circuit analyses. Correlation between the winding tension and the characteristic resistance was discussed in detail.

Influence of stabilizer thickness on over-current test of YBCO-coated conductors

The increased use of distributed power generation has led to increasingly high fault current levels. A superconducting fault current limiter (SFCL) is a potential solution to prevent the problem of short-circuit currents. YBCO-coated conductors (CCs) are one of the most promising superconducting materials for SFCLs. Most YBCO CCs have stabilizers, which play a significant role in limiting the fault current in the SFCL. Therefore, the selection of the appropriate material and the thickness of the stabilizer of the CC used for the SFCL may affect its quench/recovery characteristics. In this paper, the quench/recovery characteristics of YBCO CC tapes having stabilizers with various thicknesses were investigated. The quench/recovery test results showed that, as the thickness of the stabilizer decreased, both the final approach temperature and the recovery time decreased.

High-

This paper presents recent progress in the fabrication and test of a 3 T/60 mm room temperature bore high- Tc superconducting high gradient magnetic separator (HGMS) using a solid nitrogen (SN2) cooling system. The aim of the study was to develop an inexpensive, compact, and stable superconducting HGMS for the purification of chemical-mechanical-planarization wastewater in the semiconductor industry. The HGMSs magnet, which was cooled by SN2, was comprised of an assembly of 18 GdBCO double pancake coils, each of which was wound without turn-to-turn insulation. Cooling test results showed that the thermal stability of the high-Tc superconducting-HGMS magnet with SN2 was enhanced due to the large heat capacity of SN2. Moreover, in the filtration tests, the suspended solids value of filtrated wastewater at 2 T in a flow rate of 400 ml/min decreased to 1.6 mg/L, resulting in a 99.6% reduction in SS value compared to raw wastewater.

T_{\rm c}

This paper proposes a numerical approach to calculate the characteristic resistance (Rc) of partial-insulation (PI) and no-insulation (NI) high-temperature superconductor pancake coils with the nonuniform current path in such coils taken into consideration. Recently, an analytic approach has been proposed to estimate (Rc) of an NI coil, where the coil current is assumed to be “uniform” over the entire coil. This model, however, is not effective to explain the increase of (Rc) when a coil is modified from NI to PI. In this paper, we first introduce our numerical approach based on a finite element analysis. Then, the charging characteristics of selected PI and NI coils that we had previously reported are analyzed by the proposed approach. Reasonable agreement between the measured and calculated data validates the proposed approach to estimate (Rc) of a PI as well as an NI coil.

Superconducting High Gradient Magnetic Separator Using Solid Nitrogen Cooling System for Purification of CMP Wastewater

It is important to analyze the transient characteristics of large synchronous generators according to the trend of the extension of wind turbines. The fabricated field coil of a synchronous generator using high-temperature superconducting (HTS) wire is somewhat influenced by the external magnetic field of the stator in a real environment. This paper describes the electromagnetic analysis of an HTS field coil of a 10-MW superconducting synchronous generator during stator winding faults. The real-time digital simulator is used to simulate a fault condition of the 10-MW SCSG. Also, the finite-element method is used in each operating condition, including normal and transient conditions, to analyze the influence of the HTS field coil. Based on the simulation results discussed here, there do not appear to be any significant problems regarding the electromagnetic operation of the HTS field coil during transient conditions of the stator. This paper will be used in the analysis of the transient characteristics of a large superconducting synchronous generator during stator winding faults.

Characteristic Resistance of No-Insulation and Partial-Insulation Coils With Nonuniform Current Distribution

In 2010, we reported, for the first time, the no-insulation (NI) winding technique for high-temperature superconducting (HTS) pancake coils. Based on our test results of small NbTi NI coils, reported in 2011, the key benefits, i.e., enhanced mechanical integrity, compactness, and thermal stability, of the NI winding technique, appear intact for low-temperature superconducting (LTS) windings. However, the intrinsic charging delay observed in NI coils, caused by the lack of insulation, is more pronounced in LTS coils of a bare round wire than in HTS pancakes of thin, wide tape. Thus, to significantly reduce the charging delay in LTS coils of a bare round wire, we proposed a partial-insulation (PI) winding technique, a variation of the NI technique. In the PI winding of a bare round wire, a thin insulation sheet is introduced every few layers-note that in the PI winding, there are no turn-to-turn insulations. This paper reports results, experimental and analytical, of the PI winding technique in which bare-round-wire NI and PI coils were prepared to quantify the effects of PI winding technique. Three LTS coils of the identical dimension and magnet constant were wound with 0.4 mm diameter NbTi mono-filament wire and tested in a bath of liquid helium at 4.2 K, respectively, with three winding techniques: insulated (INS); NI; and PI. We analyzed the experimental results by applying an equivalent circuit model that had earlier been successfully applied to another set of experimental results. A graph model of resistance matrix was applied to estimate characteristic resistance of both NI and PI coils.

Stator Winding Fault Influence on the Field Coil of a 10 MW Superconducting Synchronous Generator

The thermal and electrical stabilities of two epoxy-impregnated GdBCO coils, one wet-wound with epoxy resin every turn and the other impregnated with epoxy resin using vacuum pressure impregnation (VPI) after dry-winding, were evaluated through cool-down, over-current, and repetitive cooling tests. During the cool-down test, the VPI coil showed a more rapid cooling rate compared to the wet-wound coil. The VPI coil also exhibited better thermal and electrical stabilities in the over-current tests because the quench heat could easily be dissipated due to the absence of epoxy between turns. Moreover, in the repetitive cooling tests, the VPI coil exhibited reduced degradation in the superconducting property compared to the wet-wound coil due to lower vulnerability to the discrepancy of thermal contraction between the GdBCO tape and the epoxy resin.

Partial-Insulation Winding Technique for NbTi Coils

This paper reports the quench initiation and propagation characteristics of two GdBCO single-pancake coils, one wound with Kapton tape every two turns and the other completely wound with Kapton tape, to investigate the effect of a partial insulation (PI) winding technique. The thermal/electrical stabilities of the PI coil were improved by turn-to-turn contacts between the partially uninsulated turns, leading to an increased minimum quench energy. Furthermore, transverse normal zone propagation velocities of the PI coil increased because local heat was well transferred through uninsulated turns during quench events.