• Journal of the Korean Ceramic Society
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• v.44 no.2 s.297
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• pp.84-88
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• 2007

CdSe hollow spheres with the diameter of about 30-50 nm were synthesized after ultrasonic irradiation in the presence of $Cd(NO_3)_2,\;Na_2SeSO_3$, and polyvinylalcohol(PVA). The characteristics of CdSe hollow spheres were analyzed using X-ray diffraction(XRD), transmission electron microscopy(TEM), UV-vis measurement and PL spectrometer. The characteristics of solvent as water and water-1-propanol mixture in the system played important roles on the controlled synthesis of hollow sphere. Based on the observation of morphological difference of CdSe, the possible mechanism of CdSe hollow sphere formation will be discussed.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.10
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• pp.1471-1476
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• 2012

A study on the dielectric characteristics of the Glass Fiber Reinforced Plastic (GFRP) is important for designing a reliable high voltage superconducting machines such as transmission superconducting fault current limiters, superconducting cables, and superconducting transformers. In this paper, dielectric experiments of the GFRP under lightning impulse voltage are conducted in liquid nitrogen($LN_2$) according to various experimental conditions such as the thicknesses of the GFRP, the diameters of electrode systems and the pressures. The dielectric characteristics of the GFRP are analyzed by using a Finite Elements Method(FEM) according to various field utilization factors. It has been reported that the electrical insulation design of the GFRP would be conducted by considering the mean electric field intensity($E_{mean}$) distributed inside the GFRP. In this study, it is found that the dielectric performance of the GFRP could be explained by not only $E_{mean}$ but also the maximum electric field intensity ($E_{max}$). Finally, the empirical formulae of the GFRP to estimate an electrical breakdown voltage at sparkover under the lightning impulse condition are deduced. It is expected that the presented experimental results in this paper are helpful to design electrically reliable high voltage superconducting machines using the GFRP as an insulation material.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2008.10a
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• pp.253-255
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• 2008

This paper gives a basic data of the surface discharge characteristics for epoxy resin in Dry-Air. Used electrodes are needle to plane, sphere to plane and KS M3015 electrodes. With the changing electrodes in same condition, we can obtain different creeping lengthes, breakdown voltages and dielectric strengths, respectively. Dielectric strengths of Needle to plane electrodes are more higher than the others. Breakdown voltage and dielectric strength increase as the thickness of epoxy resin and creeping strength increase.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.10 s.89
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• pp.961-968
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• 2004

In this paper, we present a set of numerical schemes to solve the Muller integral equation for the analysis of electromagnetic scattering from arbitrarily shaped three-dimensional dielectric bodies by applying the method of moments(Mon. The piecewise homogeneous dielectric structure is approximated by planar triangular patches. A set of the RWG(Rao, Wilton, Glisson) functions is used for expansion of the equivalent electric and magnetic current densities and a combination of the RWG function and its orthogonal component is used for testing. Numerical results for a dielectric sphere are presented and compared with solutions obtained using other formulations.

• Progress in Superconductivity and Cryogenics
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• v.13 no.1
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• pp.27-30
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• 2011

A sub-cooled liquid nitrogen cooling system is known as a most promising method to develop large scale superconducting apparatuses such as superconducting fault current limiters and superconducting cables [1]. Gaseous helium (GHe), gaseous nitrogen ($GN_2$) and sulfur hexafluoride ($SF_6$) are commonly used for designing an high voltage applied superconducting device as an injection gaseous medium [2, 3]. In this paper, the analysis on the dielectric characteristics of GHe, $GN_2$ and $SF_6$ are conducted by designing and manufacturing sphere-to-plane electrode systems. The AC withstand voltage experiments on the various gaseous insulation media are carried out and the results are analyzed by using finite element method (FEM) considering field utilization factors (${\xi}$). It is found that the electric field intensity at sparkover ($E_{MAX}$) of insulation media exponentially decreases according to ${\xi}$ increases. Also, the empirical expressions of the functional relations between $E_{MAX}$ and ${\xi}$ of insulation media are deduced by dielectric experiments and computational analyses. It is expected that the electrical insulation design of applied superconducting devices could be performed by using the deduced empirical formulae without dielectric experiments.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.39 no.10
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• pp.27-37
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• 2002

In this paper, we present various combined field integral equation (CFIE) formulations for the analysis of electromagnetic scattering from arbitrarily shaped three dimensional homogeneous dielectric body in the frequency domain. For the CFIE case, we propose eight separate formulations with different combinations of testing functions that result in sixteen different formulations of CFIE by neglecting one of testing terms. One of the objectives of this paper is to illustrate that not all CFIE are valid methodologies in removing defects, which occur at a frequency corresponding to an internal resonance of the structure. Numerical results involving far scattered fields and radar cross section (RCS) are presented for a dielectric sphere to illustrate which formulation works and which do not.

• Journal of Electrical Engineering and Technology
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• v.6 no.3
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• pp.397-401
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• 2011

The response of lightning impulse voltage was explored in dielectric liquids employing hydrodynamic modeling with three charge carriers using the finite element method. To understand the physical behavior of discharge phenomena in dielectric liquids, the response of step voltage has been extensively studied recently using numerical techniques. That of lightning impulse voltage, however, has rarely been investigated in technical literature. Therefore, in this paper, we tested impulse response with a tip-sphere electrode which is explained in IEC standard #60897 in detail. Electric field-dependent molecular ionization is a common term for the breakdown process, so two ionization factors were tested and compared for selecting a suitable coefficient with the lightning impulse voltage. To stabilize our numerical setup, the artificial diffusion technique was adopted, and finer mesh segmentation was generated along with the axial axis. We found that the velocity from the numerical result agrees with that from the experimental result on lightning impulse breakdown testing in the literature.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.11
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• pp.2097-2102
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• 2011

Discharge analysis technique for dielectric liquid was presented by using the Finite Element Analysis (FEA) under a lightning impulse incorporating two-phase flow phenomena which described gas and liquid phases in discharge space. Until now, the response of step voltage has been extensively explored, but that of lightning impulse voltage was rarely viewed in the literature. We, therefore, developed an analyzing technique for dielectric liquid in a tip-sphere electrode stressed by a high electric field. To capture the bubble phase, the Heaviside function was introduced mathematically and the material functions for the ionization, dissociation, recombination, and attachment were defined in liquid and bubble, respectively. By using this numerical setup, the molecular dissociation and ionization mechanisms were tested under low and high electric fields resulted from the lightning impulse voltage of 1.2/50 ${\mu}s$. To verify our numerical results, the velocity of electric field wave was measured and compared to the previous experimental results which can be viewed in many papers. Those results had good agreement with each other.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.28 no.6
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• pp.396-399
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• 2015

We analyzed the correlation between breakdown voltage(BDV) of liquid nitrogen(LN2) and factors. The chosen factors affecting the breakdown are the diameter of electrode, gap length, temperature of LN2, and pressure of LN2. The BDV of LN2 was increased with increasing the diameter, the gap length and the pressure. And The BDV of LN2 was increased with decreasing the temperature. However, correlation coefficient was different from each other depending on the situation. The BDV exhibited a very high correlation coefficient of 0.92227 to dependence on the diameter. And a very high correlation coefficient of 0.94980 to dependence on the pressure under sphere(D 7.5 mm)-plane electrode. When the pressure is applied, sphere-plane electrode is the correlation coefficient was higher than that of the needle-plane electrode. It shows the dependence of a temperature coefficient of -0.758290 ~ -0.39946 under needle-plane electrode.

• Proceedings of the KIEE Conference
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• 2000.07c
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• pp.1539-1541
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• 2000

Insulation layers in XLPE power cables may have some patterns generated in the manufacturing process. They are called 'flow patterns' and show flow history of molten polyethylene between inner and outer semiconducting layers. Flow patterns are even seen with naked human eyes and suspected to be inhomogeniety of insulation, weakening insulation performance. Investigated in this paper is electrical treeing resistance and ac breakdown strength according to flow patterns. Experiments of electrical treeing and ac breakdown strength by means of ramp tests were conducted using newly developed electrode system with point-to-plane structure and sphere-to-sphere structure, respectively. All results were analyzed with the application of statistics, showing little differences.