• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1317-1325
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• 2018

A generator-grid coupling calculation model is established to study the leading-phase operational capability of a 770 MW jumbo hydro-generator in a Chinese ultra-mega hydropower station. The static and dynamic stability of the generator are analyzed and calculated to obtain stability limits under leading-phase operating conditions. Three-dimensional (3D) time-varying nonlinear moving electromagnetic and temperature field models of the generator end-region are also established and used to determine the magnetic field, loss, and temperature of the end-region under the leading-phase operating condition. The simulation results agree with data measured from the actual 770 MW hydro-generator. This paper provides reliable reference data for the leading-phase operation of a jumbo hydro-generator, which will help to improve in the design and manufacture of future hydro-generators.

• Korean Journal of Materials Research
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• v.31 no.8
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• pp.458-464
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• 2021

Tungsten disulfide (WS₂), a typical 2D layerd structure, has received much attention as a pseudocapacitive material because of its high theoretical specific capacity and excellent ion diffusion kinetics. However, WS₂ has critical limits such as poor long-term cycling stability owing to its large volume expansion during cycling and low electrical conductivity. Therefore, to increase the high-rate performance and cycling stability for pseudocapacitors, well-dispersed WS₂ nanoparticles embedded in carbon nanofibers (WS₂-CNFs), including mesopores and S-doping, are prepared by hydrothermal synthesis and sulfurizaiton. These unique nanocomposite electrodes exhibit a high specific capacity (159.6 F g^-1 at 10 mV s^-1), excellent high-rate performance (81.3 F g^-1 at 300 mV s^-1), and long-term cycling stability (55.9 % after 1,000 cycles at 100 mV s^-1). The increased specific capacity is attributed to well-dispersed WS₂ nanoparticles embedded in CNFs that the enlarge active area; the increased high-rate performance is contributed by reduced ion diffusion pathway due to mesoporous CNFs and improved electrical conductivity due to S-doped CNFs; the long-term cycling stability is attributed to the CNFs matrix including WS₂ nanoparticles, which effectively prevent large volume expansion.

• Journal of Convergence Society for SMB
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• v.3 no.2
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• pp.1-8
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• 2013

This study examine the relation between small and medium firm's stability and tax avoidance. We use some financial data of small and medium IT firms in KOSDAQ during 2001 to 2009. Following previous research(Desai and Dhamapala 2006), we estimate corporate tax avoidance. And we use quick ratio and current ratio as a short-term stability and equity ratio as a long-term stability. Empirical results follows as. First, It was not significant statistically between short-term stability and corporate tax avoidance. Second, it was significant statistically between the long-term stability and corporate. As corporate tax avoidance increases, the long-term stability(equity ratio) was low. Our analysis provides the empirical evidence between long-term stability and tax avoidance. It offers implications of corporate tax avoidance to small and medium IT firms in KOSDAQ. But this results is difficult generalization because of sample limits.

• Clean Technology
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• v.13 no.4
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• pp.300-307
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• 2007

The combustion characteristics of diffusion flame formed in the wake of a cylindrical stabilizer with varying fuel injection angle were studied. This study was performed by measuring the flame stability limits, lengths and temperatures of recirculation zones of flames, turbulence intensity in the wake of stabilizer, and concentration distribution of combustion gas, and by taking photographs of flames. The flame stability limits are dependent on fuel injection angle and main air velocity. The length and temperature of recirculation zone are dependent on fuel injection angle. As the length of the recirculation zone is decreased, the flame shows more stable behavior. The temperature of recirculation zone has a maximum value at the condition of theoretical mixture. The flame stability is enhanced when the temperature in the recirculation zone decreases. The turbulence intensity in the wake of stabilizer is independent of the fuel injection angle, but it is affected by stabilizer itself and main air flow condition. If the stabilization characteristics of flame is good, the concentration of $C_3H_8$ is high, but the concentration of $CO_2$ is low at the boundary of recirculation zone. The combustion characteristics of diffusion flame can be controlled by changing the fuel injection angles. The appropriate fuel injection angle should be selected to get high combustion efficiency, high load power, low environmental pollution, and clean combustion condition of fuel.

• The Journal of Advanced Prosthodontics
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• v.7 no.3
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• pp.207-213
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• 2015

PURPOSE. The purpose of this study was to examine the abutment screw stability of screw- and cement-retained implant-supported dental prosthesis (SCP) after simulated cement washout as well as the stability of SCP cements after complete loosening of abutment screws. MATERIALS AND METHODS. Thirty-six titanium CAD/CAM-made implant prostheses were fabricated on two implants placed in the resin models. Each prosthesis is a two-unit SCP: one screw-retained and the other cemented. After evaluating the passive fit of each prosthesis, all implant prostheses were randomly divided into 3 groups: screwed and cemented SCP (Control), screwed and non-cemented SCP (Group 1), unscrewed and cemented SCP (Group 2). Each prosthesis in Control and Group 1 was screwed and/or cemented, and the preloading reverse torque value (RTV) was evaluated. SCP in Group 2 was screwed and cemented, and then unscrewed (RTV=0) after the cement was set. After cyclic loading was applied, the postloading RTV was measured. RTV loss and decementation ratios were calculated for statistical analysis. RESULTS. There was no significant difference in RTV loss ratio between Control and Group 1 (P=.16). No decemented prosthesis was found among Control and Group 2. CONCLUSION. Within the limits of this in vitro study, the stabilities of SCP abutment screws and cement were not significantly changed after simulated cement washout or screw loosening.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.7
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• pp.151-157
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• 1999

Vibrations in machine tools make many problems in precision, production efficiency, and machine performance. The relative vibration between a workpiece and a tool is very complicated due to many sources. In this study, the dynamic characteristics of a newly developed CNC lathe were analyzed and its chatter characteristics were predicted by a chatter analysis method using finite element analysis and 3 dimensional cutting dynamics. The simulated results showed very complex characteristics of chatter vibration and the borderline of limiting depth of cut was used as the stability limit. To check the validity of this method, cutting tests were done in the CNC lathe using a boring bar as a tool because boring process is very weak due to long overhang . The experimental results showed that the simplified borderline was to be considered as limiting depth of cut at which the chatter vibration starts and the stability limits depended on various cutting parameters such as cutting speed, feed and nose radius of tool.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.3
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• pp.253-263
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• 2004

The problem considered in this paper is about the collocation of sensor and actuator for the active control of sound and vibration. It is well-known that a point collocated sensor-actuator pair offers an unconditional stability with very high performance when it is used with a direct velocity feedback (DVFB) control, because the pair has strictly positive real (SPR) property. In order to utilize this SPR characteristics, a matched piezoelectric sensor and actuator pair is considered. but this pair suffers from the in-plane motion coupling problem with the out-of-plane motion due to the piezo sensor and actuator interaction. This coupling phnomenon limits the stability and performance of the matched pair with DVFBcontrol. As a new alternative, a point sensor and distributed piezoelectric actuator pair is also considered, which provides SPR property in all frequency range when the pair is implemented on a clamped-clapmed beam. The use of this sensor-actuator pair is highly expected for the applications to more practical active control of sound and vibration systems with the DVFB control strategy.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1269-1279
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• 1995

An experimental study is carried out on turbulent diffusion flames stabilized by a circular cylinder in a divergent duct flow. A commercial grade gaseous propane is injected from two slits on the rod as fuel. Flame stability limits, as well as size and temperrature of recirculation zone, are measured by direct and schlieren photographs to clarify the characteristics and structure of diffusion flames and to assess the effect of various divergent angle of duct. The results of the present study are as follows. Temperature in the recirculation zone decreases with increasing divergent angle. The blow-off velocity in parallel duct is higher than that in divergent duct. Critical blow-off velocity is expected to be about 8-12 degree through blow-off velocity pattern. Regardless of divergent angles, the length of recirculation zone is nearly constant, and this length becomes longer with rod diameter. Pressure gradient has an effect on the eddy structure in shear layer behind the rod. With the increase of divergent angle, large scale eddies by dissipated energy in shear layer are split into small scale eddies, and the flame becomes a typical distributedreacting flame.

• Journal of Korean Society on Water Environment
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• v.21 no.2
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• pp.158-168
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• 2005

In this study, the discharging effluents from have been 9 municipal wastewater treatment plants surveyed for 1 year-period. Statistics including probability distribution, cumulative occurrence concentration and other statistical parameters were presented. In addition, treatment performance and its stability were also discussed. Most of the plants, have an operational problem of high soluble organic content in the secondary effluent which may be associated with the integrated treatment of human and livestock manures. Nitrogen concentration in the effluents were usually higher during the period of summer and winter. It was found that this is mainly due to lack of the proper C/N ratio during the summer, or/and the effects of low temperature and less dilution by dry weather during the winter. Phosphorus concentration is sharply increased in June. Discussion with plant operators told that it is due to the dissolution of phosphate from the sludge accumulated in the primary settling tanks from the early spring to june. During this period, usually, sludge treatment line is highly overloaded with flush-outs of the sediments also stored in the bottom of combined sewer due to the low flow during winter season. Most of the plants can meet new effluent discharge limits of the nitrogen and phosphorus, and total coliform without further treatment.

• Proceedings of the Korean Geotechical Society Conference
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• 2009.03a
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• pp.1113-1124
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• 2009

The limit equilibrium method and finite element method have been commonly used to assess the stability of anchored slope. However, the existing limit equilibrium methods do not consider the load transfer of anchor and, if the potential failure surface crosses the bonded zone of anchors, the anchor loads are not included in computing the factor of safety. In this paper, simplified bishop method, one among several methods based on the limit equilibrium is used to develope the Excel Sheet program and the working load of anchor along the failure surface is calculated by Flamant's formula, proposed by Morlier and Tenier(1982). Additionally, the finite element analysis of anchored slope considering the load transfer is performed in order to compare with the result from the limit equilibrium analysis and analyze the reinforced effect of anchor. From this study, new modelling method that limits the reinforcing range and considers the load transfer is proposed and Excel Sheet program is developed. The factor of safety for several cases between PC STABL and Excel Sheet program is also compared for verification of the program.