• The Transactions of the Korean Institute of Electrical Engineers B
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• v.48 no.10
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• pp.537-543
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• 1999

We fabricated a resistive superconducting fault current limiter (SFCL) of a meander type based on a YBCO film with the meander cross section of 5 $\times$ $10^{-6}$$cm^2$, and performed current limitation experiments. The film was coated quench current was 9.6 Apeak at 60 Hz, and the fast quench time was 0.63 msec. The resistance of the limiter continuously increased for three cycles dut to the temperature rise in the gold layer. The temperature of the current limiting element reached the room temperature in 11 msec, $150^{\circ}C$ in 54 msec after quench, and was saturated afterwards. For $45^{\circ}$and $90^{\circ}$faults the fast quench times were 0.56 msec and 0.26 msec, respectively. The quench time is believed to be reduced because the fault occurred when the current was either increasing or at the peak value. This limiter effectively limited the fault current to about 1/5 of the potential current with no SFCL right after the fault and to about 1/8.5 in three cycles. We confirmed that the gold layer effectively carried out the role of heat dissipation as the SFCL was quenched.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.51 no.9
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• pp.522-529
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• 2002

In this paper, computer simulations of the physical Phenomena occurring in the arc region before and after current zero were carried out to evaluate the thermal recovery characteristics of a Laval nozzle. A commercial CFD program "PHOENICS" is used for the simulation and the user-coded subroutines to consider the arcing phenomena were added to this program by the authors. The computed results were verified by the comparison with the test results presented by the research group of GE Co.(General Electric Company). In order to investigate the state of the arc region after current zero, the simulation was carried out with three steps. They are steady state arc simulation, transient arc simulation before current zero, and transient hot-gas flow simulation after current zero. The semi-experimental arc radiation model is adapted to consider the radiation energy transport and Prandtl's mixing length model is employed as the turbulence model. The electric field and the magnetic field were calculated with the same grid structure used for the simulation of the flow field. The post-arc current was calculated to evaluate the thermal recovery characteristics after current zero. Compared with the results obtained by GE Co., it has been found that the critical RRRV(ratio of rise of recovery voltage) will be determined previously by this study.his study.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.48 no.8
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• pp.431-439
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• 1999

Among the variable-speed AC motor drive systems, the static slip power recovery system has been widely adopted in large power drives because a high efficiency drive can be obtained by recovering the slip power to the AC line. Although many improvements have been made in this system, several problems also remain such as the need of transformer in inverter AC side, which results in limiting speed control range and increasing the losses, production of reactive power by the control of inverter firing angle, harmonics in line currents, and so on. This paper presents the novel high performance slip power recovery system using the boost converter and small size filter in the rotor circuit, which recovers slip power of a wound rotor induction machine to AC supply efficiently with the aid of the boost converter, in which most of the problems in conventional system can be solved. The speed can be controlled by the duty ratio of the converter switch, not by inverter firing angle. As a results, the proposed system can operate in high power factor and the harmonic currents caused by the inverter and rectifier can be considerably suppressed. The validity of the proposed system verified by demonstrating the good agreement in the simulation and experimental results.

• Journal of Sensor Science and Technology
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• v.5 no.5
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• pp.11-20
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• 1996

In this paper, the characteristics of therapy radiation diode sensors have been studied by using therapy radiation from the MM22 microtron accelerator. The linearity, reproducibility and error ratio were measured for feasibility as a radiation detector. Energy dependence, sensitivity change after a amount of irradiation and output value according to a number of diodes were also measured for same purpose. We have formed pulse shaping of diode signal with nuclear instruments for portal image reconstruction. The percent depth dose ratio according to field size and depth was compared with that of the detector of a ion chamber. Using thirteen silicon diodes, we can directly read diode outputs on a computer monitor after A/D conversion with 16 channels analog to digital conversion board with 12 bit resolution. The possibility for portal image with diodes has been suggested from output comparison between output value with a human phantom and that without a human phantom.

• Proceedings of the Korea Water Resources Association Conference
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• 2012.05a
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• pp.154-154
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• 2012

지표에서의 토양수분은 작은 구성비를 가짐에도 불구하고 여러 수문 현상을 연계하는 매우 중요한 인자로써 최근 관련 연구가 활발하게 진행되고 있다. 토양수분은 침투나 침루를 통하여 강우와 지하수를 연결하는 기능을 함과 동시에 강우사상에 따른 유출특성에 직접적인 영향을 미치며 증발산을 통하여 에너지 순환을 연결하는 중요한 기능을 한다. 토양수분을 측정하는 방법에는 세타 탐침(Theta Probe), 장력계, TDR(Time Domain Reflectrometry) 등이 이용되고 있으며, 광역 토양수분자료의 보다 정확한 공간 변동성의 관측을 위하여 항공원격탐사와 인공위성 원격탐사기술이 개발되어 적용되고 있다. 인공위성 영상은 자료의 분석이 간편하며, 공간자료이므로 공간 변화를 분석하는 데 있어 매우 편리하다. 그 중 MODIS(Moderate Resolution Imaging Spectroradiometer) 위성영상은 저해상도 영상으로 극궤도 위성인 Terra와 Aqua 위성에 장착되어 있으며, NASA에서 필요한 정보를 받아 사용할 수 있다. 본 연구에서는 유역의 물리적 지형자료와 같은 방대한 양의 자료 수집 없이도, 모형이 구축되면 인공위성자료와 강우자료만으로도 신뢰성 높은 결과를 단시간 내에 효율적으로 산정할 수 있는 자료 지향형 모형인 ANFIS(Adaptive Neuro-Fuzzy Inference System)를 사용하였다. 사용된 퍼지변수로는 시험유역의 토양수분 관측자료와 강수량 및 인공위성 자료인 MODIS NDVI(Normalize Difference Vegetation Index), MODIS LST(Land-Surface Temperature) 영상을 이용하였다. MODIS NDVI는 시간 해상도 8일, 공간해상도 250 인 Level 3 영상이며, MODIS LST는 시간 해상도 1일, 공간해상도 1 km인 Level 3 영상을 사용하였다. 위성자료를 사용하기 위해 Korea TM 좌표체계로 변환한 뒤, 토양수분 관측지점이 속한 각 셀의 속성값을 추출하였다. 위성자료와 수집된 자료 및 토양수분자료와의 관계를 분석하기 위하여 입력자료를 다양한 방법으로 구성하여 입력 변수를 생성하였다. 생성된 입력 변수와 ANFIS 모형을 연계하여 각각의 토양수분 산정모형을 구축하고 대상지점에 대한 토양수분을 산정 및 비교 분석하였다.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.227-227
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• 2013

본 연구에서는 Si(111) 기판에 성장온도 및 InN 증착양 변화에 따른 InN 양자점(Quantum Dot) 핵성생(Nucleation) 특성에 대해 논의한다. InN 양자점은 Nitrogen-Plasma 소스를 장착한 분자선증착기(MBE)를 이용하여 $0.103{\AA}/s$의 성장속도로 성장하였다. 성장온도를 $700^{\circ}C$에서 $300^{\circ}C$로 변환하면서 형성한 시료에서 lnN 양자점의 공간밀도는 $9.4{\times}10^7/cm^2$부터 $1.1{\times}10^{11}/cm^2$를 나타냈다. 가장 높은 공간밀도인 $1.1{\times}10^{11}/cm^2$는 기존에 보고된 값 ($7.7{\times}10^{10}/cm^2$)보다 상대적으로 높은 값을 갖는다 [1,2]. InN 증착양을 93, 186, 및 $372{\AA}/s$으로 각각 변화시켜 형성하여 양자점의 초기 성장거동을 분석하였다. InN 증착양이 증가함에 따라 양자점의 공간밀도는 $4.4{\times}10^{10}/cm^2$에 $6.4{\times}10^{10}/cm^2$까지 증가하였다. 일반적으로 InP 및 GaAs 기판을 기반으로 한 In(Ga)As 양자점은 증착양이 증가함에 따라 밀도는 감소하고 크기는 증가하는 경향을 보이며, 이는 같은 상 (Phase)을 갖는 물질들끼리 결합하려는 경향이 있기 때문이다. 본 실험에서는 기존 결과와 다른 경향을 보이고 있는데, 이는 Si(111) 기판과 InN 사이의 격자부정합이 상대적으로 크기 때문에 InN 양자구조가 커지는 대신 추가로 새로운 핵생성 메커니즘에 의한 것으로 설명할 수 있다. 이러한 InN 증착양에 따른 InN 양자점 성장거동을 표면에너지를 포함한 이론적인 모델을 통해 논의하고자 한다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.345-353
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• 1987

An analysis is performed to study flow and heat transfer characteristics of mixed free and forced convection about a sphere. Nonsimilar boundary layer equations which are valid over the entire regime of mixed convection are derived in terms of the mixed convection parameter, Gr/Re$^{2}$, through a dimensional analysis. The transformed conservation equations are solved by a finite difference method for the whole range of mixed convection regime. Numerical results for fluids having the Prandtl number 0.7 and 7 are presented. As the mixed convection parameter increases, the local friction coefficient and local heat transfer coefficient increases as well. For small Prandtl number, the friction coefficient is larger, while for large Prandtl number, the heat transfer coefficient is larger. Natural convection effect on the forced flow is more sensitive for small Prandtl number fluid. Flow separation migrates rearward as an increase in the mixed convection parameter. For small Prandtl number, the buoyancy effect is relatively small so that the flow separation occurs earlier.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.2
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• pp.87-95
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• 2017

The concept of entransy has been proposed recently as a potential heat transfer mechanism and could be useful in analyzing and optimizing the heat-work conversion systems. This work presents an entransy analysis for the irreversible Carnot cycle by systematic balance formulations of the entransy loss, work entransy, and entransy dissipations, which are consistent with exergy balances. Additionally, several forms of system efficiency are introduced based on entransy for the appreciation of the optimal system performance. The effects of the source temperature and irreversible efficiencies on the optimal conditions for system efficiencies are systematically investigated for both dumping and non-dumping cases of used source fluid. The results show different trends in entransy efficiencies when compared to the conventional efficiencies of energy and exergy, and represent another method to assess the effective use of heat source in power generation systems.

• Journal of the Korean Society for Nondestructive Testing
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• v.30 no.2
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• pp.85-90
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• 2010

The infrared energy is emitted in the infrared wavelength range that corresponds to the surface temperature of a object which has temperature that is over the absolute the temperature(OK). The infrared thermography (IRT) is a non-destrnctive testing method that provides thermal video for the user in real-time by converting the infrared quantity that is detected by the infrared detector into temperature. The pipes of nuclear power plant(NPP) could be thinned by the corrosion and fatigue and the defect could lead to a big accident. For this reason, the effective non-destructive testing method is necessary. In this study, the relationship between the measured temperature and the defect depth or size of NPP pipes were recognized and that was applied to detect the wall thinning defects of NPP pipes.

• Korean Journal of Optics and Photonics
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• v.31 no.1
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• pp.37-44
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• 2020

In this study, we propose a design method to optimize the electro-optical efficiency of a planar solar cell structure by adjusting one-dimensionally periodic emitter electrodes. Since the aperture ratio of the active layer decreases as the period of the emitter electrode decreases, the amount of light absorption diminishes, affecting the performance of the device. Here we design the optimal structure of the periodic emitter electrode in a simple planar solar cell, by simulation. In terms of optics, we find the condition that shows optical performance similar to that of a reference without the emitter electrode. In addition, the optimized electrode structure is extracted considering both the optical and electrical efficiency. This work will help to increase the utilization of solar cells by suggesting a structure that can most efficiently transfer charge generated by photoelectric conversion to the electrodes.