• Journal of Electrical Engineering and Technology
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• v.7 no.3
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• pp.389-395
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• 2012

In this paper, we analyzed the frequency spectrum of the electromagnetic waves radiated by an electric discharge as a basic study to develop an on-line diagnostic technique for power equipment installed inside closed-switchboards. In order to simulate local and series arc discharges caused by an electric field concentration and poor connections, three types of electrode systems were fabricated, consisting of needle and plane electrodes and an arc generator meeting the specifications of UL 1699. The experiment was carried out in an electromagnetic anechoic chamber, and the measurement system consisted of a PD free transformer, a loop antenna with a frequency bandwidth of 150 kHz-30 MHz, an ultra log periodic antenna with a frequency bandwidth of 30 MHz-2 GHz, and an EMI test receiver with a frequency bandwidth of 3 Hz-3 GHz. According to the experimental results, the frequency spectra of the electrical discharges were widely distributed across a range of 150 kHz-400 MHz, depending on the defects, while commonly found between 150 kHz and 10 MHz. Therefore, considering the ambient noise and antenna characteristics, the best frequency bandwidth for a measurement system to monitor abnormal conditions by detecting electromagnetic waves in closedswitchboards is 150 kHz-10 MHz.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.892-901
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• 2018

Because of excellent electrical properties, epoxy resin is widely used in packaging and casting power equipment. Moisture and temperature in the environment are inclined to seriously affect the insulation tolerance of epoxy resin. This work focuses on the aging characteristics of epoxy resin in hygrothermal environment. Scanning electron microscopy images show that there are micro-crack, micro-slit and holes inside aged samples. The moisture absorption process undergoes three equilibrium stages and it does not follow the Fick's second law. Observing the change of hydrogen bonds in the infrared spectra of the dried samples, it is found that chemically moisture absorption immerges when the physical moisture absorption entered the third equilibrium stage. By Debye equation to fit the imaginary part of the dielectric constant, it is concluded that the uniformity of water molecule has a great influence on the electrical conductivity loss. Furthermore, the polarization loss can be more easily affected by water molecules than small free molecules. After the aged samples being dried, their real and imaginary part of the dielectric constant descend, but their original electrical properties cannot completely restored. After chemical moisture absorption appears inside the material, the residual space charges increase significantly and the charge dissipation rate slow down obviously.

• Wind and Structures
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• v.22 no.5
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• pp.573-594
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• 2016

Simultaneous pressure measurements on 2D square prisms with various corner modifications were performed in uniform flow with low turbulence level, and the testing Reynolds numbers varied from $1.0{\times}10^5$ to $4.8{\times}10^5$. Experimental models were a square prism, three chamfered-corner square prisms (B/D=5%, 10%, and 15%, where B is the chamfered corner dimension and D is the cross-sectional dimension), and six rounded-corner square prisms (R/D =5%, 10%, 15%, 20%, 30%, and 40%, where R is the corner radius). Experimental results of drag coefficients, wind pressure distributions, power spectra of aerodynamic force coefficients, and Strouhal numbers are presented. Ten models are divided into various categories according to the variations of mean drag coefficients with Reynolds number. The mean drag coefficients of models with $B/D{\leq}15%$ and $R/D{\leq}15%$ are unaffected by the Reynolds number. On the contrary, the mean drag coefficients of models with R/D=20%, 30%, and 40% are obviously dependent on Reynolds number. Wind pressure distributions around each model are analyzed according to the categorized results.The influence mechanisms of corner modifications on the aerodynamic characteristics of the square prism are revealed from the perspective of flow around the model, which can be obtained by analyzing the local pressures acting on the model surface.

• Wind and Structures
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• v.13 no.6
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• pp.499-517
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• 2010

Advances in structural materials and construction methods have resulted in flexible and light tall buildings, making an assessment of structural safety during strong wind and serviceability/habitability during comparable medium/weak wind important design criteria. So far, lots of studies on suppressing the wind-induced responses have been carried out for tall buildings with aerodynamic modification. Most of the studies on aerodynamic modification have forced on the corner modification, while the studies on taper and set-back are limited. Changes of sectional shape through taper and set-back can modify the flow pattern around the models, encouraging more 3-dimensionalities, which results in reducing the wind-induced excitations. This paper discusses the characteristics of overturning moments and wind-induced responses of the tall buildings with height variations. The reduction of mean along-wind and fluctuating across-wind overturning moments are apparent in the suburban area than in urban area. A series of the response analyses, the rms displacement responses of the tall buildings with height variations are reduced greatly, while the rms acceleration responses are not necessarily reduced, showing dependences on wind direction.

• YAKHAK HOEJI
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• v.52 no.6
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• pp.426-433
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• 2008

The interaction of cadmium (II) ion with protocatechuic acid ($H_3PA$) and ethylenediaminetetraacetic acid (EDTA) were investigated in aqueous solution at different pH. The protocatechuic acid and EDTA/cadmium stochiometries for cadmium (II) binding have been determined by UV-vis spectrophotometric method. The complexation of Cd (II) ion with protocatechuic acid was formed in solution. Among the two potential sites of chelation present in the protocatechuic acid structure, the carboxylic function presents higher complexation power toward Cd (II). 1 : 1 Cd (II)-complex had a maximum absorbance and showed the bathochromic shift of the long-wavelength band of the UV-vis spectra in the alkaline pH when interacted with protocatechuic acid in 0.2 M $NH_3$ - 0.2 M ${NH_4}Cl$ (pH 8.0) buffer. These results suggest that Cd $({H_2}PA)^+$ complex has the optimal condition of chelation in buffer solution at 64.22 ${\mu}M$ protocatechuic acid (A=1.01455).

• Bulletin of the Korean Chemical Society
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• v.31 no.12
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• pp.3697-3702
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• 2010

We demonstrate that vertically aligned carbon nanotubes can be synthesized directly on tantalum substrate via water-assisted chemical vapor deposition and evaluate their properties as electrochemical capacitors. The mean diameter of the carbon nanotubes was $7.1{\pm}1.5\;nm$, and 70% of them had double walls. The intensity ratio of G-band to D-band in Raman spectra was as high as 5, indicating good quality of the carbon nanotubes. Owing to the alignment and low equivalent series resistance, the carbon nanotube based supercapacitors showed good rate performance. Rectangular shape of cyclic voltammogram was maintained even at the scan rate of > 1 V/s in 1 M sulfuric acid aqueous solution. Specific capacitance was well-retained (~94%) even when the discharging current density dramatically increased up to 145 A/g. Consequently, specific power as high as 60 kW/kg was obtained from as-grown carbon nanotubes in aqueous solution. Maximum specific energy of ~20 Wh/kg was obtained when carbon nanotubes were electrochemically oxidized and operated in organic solution. Demonstration of direct synthesis of carbon nanotubes on tantalum current collectors and their applications as supercapacitors could be an invaluable basis for fabrication of high performance carbon nanotube supercapacitors.

• Journal of Korean Association for Spatial Structures
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• v.17 no.1
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• pp.77-84
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• 2017

Vortex-induced vibration and instability vibration of tall buildings are very important fluid-structure interaction phenomenon, and many fundamental questions concerning the influence of body movement on the unsteady aerodynamic force remain unanswered. For tall buildings, there are two experimental methods to investigate the characteristics of unsteady aerodynamic forces, one is forced vibration method and the other is free vibration method. In the present paper, a free vibration method was used to investigate the unsteady aerodynamic force on tall building whose aspect ratio is 9 under boundary layer simulating city area. Wind pressures on surfaces and tip displacements were measured simultaneously, and the characteristics of tip displacements and generalized forces were discussed. It was found that variation of across-wind displacements showed different trend between the case when wind speed increases and wind speed decreases, and the fluctuating generalize forces in across-wind direction of vibrating model are larger than that of static model near the resonant wind speed and approach to the static value. And for higher wind speed range, there were two peaks in across-wind power spectra of generalize forces of vibrating model, which means that two frequency components are predominant in unsteady aerodynamic forces.

• Wind and Structures
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• v.15 no.1
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• pp.1-15
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• 2012

High-resolution wind data were acquired from a 100-m high offshore tower during the passage of Typhoon Hagupit in September, 2008. The meteorological tower was equipped with an ultrasonic anemometer and a number of cup anemometers at heights between 10 and 100 m. Wind characteristics of the strong typhoon, such as mean wind speed and wind direction, turbulence intensity, turbulence integral length scale, gust factor and power spectra of wind velocity, vertical profiles of mean wind speed were investigated in detail based on the wind data recorded during the strong typhoon. The measured results revealed that the wind characteristics in different stages during the typhoon varied remarkably. Through comparison with non-typhoon wind measurements, the phenomena of enhanced levels of turbulence intensity, gust factors, turbulence integral length scale and spectral magnitudes in typhoon boundary layer were observed. The monitored data and analysis results are expected to be useful for the wind-resistant design of offshore structures and buildings on seashores in typhoon-prone regions.

• Journal of the Korean Ceramic Society
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• v.53 no.5
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• pp.483-488
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• 2016

Anode supported solid oxide fuel cells (SOFCs), consisting of Ni+YSZ anode, YSZ electrolyte, and LSM+YSZ cathode, were fabricated and constant current tested with direct internal reforming of methane (steam to carbon ratio ~ 2) as well as hydrogen fuel at $800^{\circ}C$. The cell, operated under direct internal reforming conditions, showed relatively rapid degradation (~ 1.6 % voltage drop) for 95 h; the cells with hydrogen fuel operated stably for 170 h. Power density and impedance spectra were also measured before and after the tests, and post-test analyses were conducted on the anode parts using SEM / EDS. The results indicate that the performance degradation of the cell operated with internal reforming can be attributed to carbon depositions on the anode, which increase the resistance against anode gas transport and deactivate the Ni catalyst. Thus, the present study shows that direct internal reforming SOFCs cannot be stably operated even under the condition of S/C ratio of ~ 2, probably due to non-uniform mixture (methane and steam) gas flow.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.49.2-49.2
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• 2011

To investigate whether radio galaxies have systematically different accretion disk compared to radio-quiet AGN, we obtained high quality optical spectra for a sample of 22 young radio galaxies, using the KAST Double Spectrograph at the Lick 3-m telescope. Young radio galaxies are particularly useful since the age of the radio phenomena is comparable to that of accretion disk. Based on the optical emission-line diagnostics of narrow line region, which is thought to be photoionized by the nuclear radiation, we constrain the states of the accretion disk. In addition to strong emission lines, i.e., [O I], [O II], [O III], and [Ne III], we use the [Ar III] line to break the degeneracy between the ionization parameter and the SED shape. We find that young radio galaxies show systematically different emission line ratios compared to radio-quiet Type II AGN, suggesting that young radio galaxies probably have the power-law SED without a strong big blue bump. We will present the main results of the emission-line diagnostics.