Hyoungku Kang

Stability testing of a two miniplate fixation technique for mandibular angle fractures. An in vitro study

In the case of mandibular angle fractures, a two miniplate fixation technique in which one plate is applied ventral to the oblique line and a second plate at the inferior border of the buccal cortex was used to stabilize the fracture. In this paper, the stability of the two miniplate fixation technique was tested in a three-dimensional in vitro model. The results of this study showed that in cases of angle fractures this fixation technique provides stable fixation under functional loading.

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.

A Study on the Design of the Stabilizer of Coated Conductor for Applying to SFCL

A superconducting fault current limiter (SFCL) using high temperature superconducting (HTS) wire has been developed in Korea. While the first-generation wire has difficulty in adopting various matrices because it is made by PIT process, the second-generation wire has flexibility in the design of its stabilizer. In general, coated conductor (CC) tape is clad with a thick metal layer as a stabilizer on the superconducting layer such as YBCO. In addition, the metal layer has good electrical conductivity. However, resistive SFCL, when a fault occurs, limits fault current using high resistance of the stabilizer. Therefore, the design of stabilizer is one of the most important parts in the design of SFCL. In this paper, over-current characteristics of CC tapes clad with stabilizers having various specifications were experimentally investigated. It was possible to design the stabilizer of CC based on calculation. An SFCL rated on 13.2 kV/630 A was designed using the optimal design of the stabilizer.

Visualization Study on Boiling of Nitrogen During Quench for Fault Current Limiter Applications

For the development of a 13.2 kV/630 A bifilar winding type high-Tc superconducting fault current limiter (SFCL), a visualization study has been conducted to clarify boiling characteristics during the quench of the high-Tc SFCL. A series of experiments was carried out using a stainless steel strip on a G10 plate as a heating element to simulate the quench state of the high-Tc SFCL. A pulse of DC power input was applied to the strip in saturated and subcooled liquid nitrogen. The magnitude of the heat generation was varied from 10 W/cm2 to 170 W/cm2 and the period of the heat impulse was fixed at approximately 100 ms. Bubble behavior was observed by a high-speed camera through view ports of a cryostat. The boiling phenomena, the temperature rising of the strip and the recovery time were compared for different power densities and liquid nitrogen operating conditions. The bubble suppression was clearly observed with respect to the degree of subcooling

Study on the Breakdown Voltage Characterization of Insulation Gases for Developing a High Voltage Superconducting Apparatus

A sub-cooled liquid nitrogen system is known to be a promising method to develop a high voltage superconducting magnet. Therefore, the investigation of dielectric characteristics of gaseous electrical insulation media is important for the design of a high voltage superconducting apparatus. In this paper, dielectric characteristics of various gases according to utilization factors are investigated for the establishment of insulation design criteria of a high voltage superconducting apparatus. Dielectric experiments on gaseous nitrogen and gaseous helium (GHe) are carried out by using several kinds of sphere-to-plane electrode systems made of stainless steel. As a result, it is found that the dielectric characteristics of gas are determined by as well as gap length of electrode systems. Also, empirical expressions for electrical breakdown performance of gaseous electrical insulation media are derived and formulated. It is considered that the results of the study would be applicable to designing a high voltage superconducting apparatus.

The short-circuit characteristics of a DC reactor type superconducting fault current limiter with fault detection and signal control of the power converter

In general case of DC reactor type superconducting fault current limiter (SFCL), a fault current gradually increases during the fault. It takes above 5 cycles to cut off the fault in the existing power system installed the conventional circuit breakers (CBs). Therefore, the fault current increases during the fault even if the SFCL is installed. This paper proposes a technique for decaying the fault current with the function of the fault detection and control of power converter of the SFCL. Using the proposed method, the fault current can decay after 1-2 cycles when the fault occurs. To analyze this technique, three-phase 6.6 kV/200 A SFCL was fabricated. The SFCL has just one DC reactor, an AC to DC power converter which has thyristors as the rectifying device, and a three-phase transformer, which is called magnetic core reactor (MCR). The short-circuit tests of this SFCL were performed successfully. Comparing the result using the proposed technique with the typical result, the fault current is decreased effectively by the proposed technique. This result shows that this SFCL using the fault detection and control of power converter can be applied to the existing power system which has conventional CBs.

Electrical Breakdown Characteristics of Superconducting Magnet System in Sub-Cooled Liquid Nitrogen

Dielectric characteristics of gaseous helium (GHe) injected into the cooling system to make sub-cooled nitrogen condition with constant pressure is found to be extraordinarily weak in dielectric strength by H. Mitsuii (1998). In high voltage superconducting machines using the sub-cooled nitrogen cooling system, the current lead part in GHe environment could be electrical weak points rather than the superconducting magnet part in liquid nitrogen (LN2). In order to enhance dielectric characteristics of current lead part, three methods are suggested in this paper. The first one is enclosing the current leads with solid dielectric material and the second is exposing the current leads to high vacuum, and the last is topping up the magnet system with dielectric materials such as Glass Bubbles (GB) which is usually used as heat insulator. In addition, the extinction time of bubbles caused by the electrical breakdown is measured to determine the suitable pressure of sub-cooled nitrogen system. It is found that the characteristics of bubble extinction in LN2 are drastically enhanced over 1 bar.

Design, fabrication, and test of high-Tc superconducting DC reactor for inductive superconducting fault current limiter

A high-T/sub c/ superconducting (HTS) DC reactor has been developed as a part of DC reactor type superconducting fault current limiter (SFCL) rated on 6.6 kV/200 A. The DC reactor was a solenoid coil and was wound with a 4-parallel stacked tape. This coil has 5 layers which are connected in series each other. The inductance of the coil is about 84 mH. This paper deals with the design, fabrication and testing of the DC reactor. For this design of the coil, a prototype solenoid coil had been used. Using the experimental result of a prototype solenoid, the number of stacks and the inductance were designed. The winding machine for the HTS solenoid was manufactured. Using this machine, the large-scale HTS solenoid using a Bi-2223 tape was fabricated successfully. Characteristics of the fabricated coil were observed through a measurement of voltage as current transportation.

Design, fabrication and testing of superconducting DC reactor for 1.2 kV/80 A inductive fault current limiter

A superconducting DC reactor protects a power system by limiting the amplitude of fault current with its inductance. Therefore, it is very important to design and simulate the DC reactor precisely for making the power system stable and effective. In this paper, we designed the superconducting DC reactor of an inductive superconducting fault current limiter conceptually and acquired the optimal design parameters by using Finite Element Method (FEM). We manufactured the superconducting DC reactor and tested its characteristics at cryocooler-cooled 20 K temperature. Moreover, we compared experimental characteristics with simulation results and analyzed them. We introduced the design method of the superconducting DC reactor and the fabrication method of a 1.2 kV/80 A class DC reactor for an inductive superconducting fault current limiter. Finally, we performed the short circuit test and discussed the results.

Analysis on the Dielectric Characteristics of Insulation Gases for Developing a High Voltage Superconducting Fault Current Limiter

A sub-cooled liquid nitrogen (LN2) cooling system is known as an appropriate method for developing a high voltage superconducting fault current limiter (SFCL). The pressure of a sub-cooled LN2 cooling system should be controlled by injecting non-condensable gas such as gaseous helium (GHe) and gaseous neon (GNe) into the cryostat. Therefore, the electrical breakdown voltage of current leads is directly influenced by an injected gaseous medium. In this study, electrical insulation experiments on GHe and GNe are conducted and the results are compared with each other. Also, the dielectric characteristics of GHe and GNe are analyzed by using field utilization factors. It is found that the dielectric characteristics of GHe are superior to those of GNe under the same conditions. The experimental results are applicable for designing the current leads of a high voltage superconducting fault current limiter.