• Proceedings of the KIEE Conference
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• 1992.07a
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• pp.145-148
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• 1992

The best index should show the margins to the operating limit at the present operating point without much efforts. Such voltage collapse proximity index has been proposed in this study. Differently from the normal procedure in which every bus load is increased at the same propotion, bus load increase toward the direction of T/L loss increase has been applied in this study. Sample studies show the usefulness and the practical applicability in voltage stability analysis.

• 한국초전도학회:학술대회논문집
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• v.10
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• pp.15-20
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• 2000

We demonstrated a single balanced mixer of the combination of high-T. superconductor (HTS) and III-V GaAs beam lead Schottky diodes operating in the mini-cryogenic chamber. The HTS hybrid mixer was designed with a center frequency of 27.5 GHz and a bandwidth of 1 GHz, and consisted of a rat-race coupler circuit with beam-lead diodes attached to its balanced ports. The HTS hybrid mixer with 1 GHz RF bandwidths exhibits a conversion loss of 6 dB. A LO-to-RF isolation was greater than 40 dB in the range of operating frequencies. Since the HTS/III-V hybrid mixer devices have lower noise and conversion loss, this technique provide us with new capabilities that can be effectively utilized in the field of local-point distribution service (LMDS) systems.

• Journal of Power System Engineering
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• v.19 no.3
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• pp.11-15
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• 2015

In this paper, the multi-point hydrogen detection system based on the combination of the hetero-core optical fiber SPR hydrogen tip sensor and interrogator by pseudorandom noise (PN) code correlation reflectometry has been developed. In a light intensity-based experiment with an LED operating at 850 nm, it has been presented that a transmitted loss change of 0.32dB was induced with a response time of 25 s for 4% $H_2$ in $N_2$ in the case of the 25-nm Au, 60-nm $Ta_2O_5$, and 5-nm Pd multi-layers film. The proposed sensor characteristic shows excellent reproducibility in terms of loss level and time response for the in- and out- $H_2$ action. In addition, in the experiment for multi-point hydrogen detection, all sensors show the real-time response for 4% hydrogen adding with reproducible working. As a result, the real-time multi-point hydrogen detection could be realized by means of the combination of interrogating system and hetero-core optical fiber SPR hydrogen tip sensors.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.26 no.11
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• pp.30-39
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• 2012

The characteristics of I-V and P-V of solar cell nonlinearly changes according to irradiation, temperature and load. Therefore, to use efficiently PV system, operating point must be always operating at maximum power point. Also, PV system is semiconductor, so it generates loss by temperature. But because of conventional MPPT methods are not considering temperature, it has problem which decrease efficiency. This paper proposes temperature measurement based optimal voltage(TMOV) MPPT algorithm using temperature measurement based optimal voltage. It analyzes characteristics of solar cell according to irradiation and temperature and conventional MPPT methods. The TMOV control algorithm proposed in this paper is compared and analyzed conventional MPPT methods. The validity of this paper proves using this result.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.7 no.12
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• pp.2987-3002
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• 2013

The problem of optimal channel sensing in heterogeneous cognitive networks is considered to maximize the system throughput performance. The characteristics of an optimal operating sensing point maximizing the overall system rate are investigated under several rate criteria including the sum rate, the minimum of the primary and secondary rates, and the secondary rate with a guaranteed primary rate. Under the sum rate criterion, it is shown that the loss by imperfect sensing is no greater than half of the sum rate achieved by the perfect time sharing approach in a two user case if the sensing point is optimally designed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.3
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• pp.330-337
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• 2004

Analysis of flow phenomena in a centrifugal compressor impeller has been carried out with numerical simulation to understand the physics of flow near stall. Near stall point, tip leakage flow spills ahead of the leading edge of adjacent blade and other leakage flow passes over the clearance of the adjacent blade instead of rolling up into vortex within the passage. The tip leakage flow at the mid chord of impeller blade impinges against the pressure surface of the adjacent blade and then rolls up into vortex within the passage, which blocks the flow passage and generates viscous loss. The spillage of leakage flow ahead of the adjacent blade generates the recirculation of flow entering the impeller, which causes the power transferred into the flow by the impeller to decrease and blocks the flow passage. Near diffuser hub wall, flow recirculation occurs. As operating point goes to stall point, the core of recirculation approaches the impeller exit The length rises to peak point and then drops with mass flow reduction, while the height steadily rises.

• Journal of Power Electronics
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• v.17 no.3
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• pp.601-609
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• 2017

Gallium nitride (GaN) power switching devices are promising candidates for high switching frequency and high efficiency power conversion due to their fast switching, low on-state resistance, and high-temperature operation capability. In order to facilitate the use of these new devices better, it is required to investigate the device characteristics and performance in detail preferably by comparing with various conventional silicon (Si) devices. This paper presents a comprehensive study of GaN high electron mobility transistor (HEMT) based non-isolated point-of-load (POL) synchronous buck converter operating at 2.7 MHz with a high step-down ratio (24 V to 3.3 V). The characteristics and performance of GaN HEMT and three different Si devices are analytically investigated and the optimal operating point for GaN HEMT is discussed. Zero-voltage switching (ZVS) is implemented to minimize switching loss in high switching frequency operation. The prototype circuit and experimental data support the validity of analytical and simulation results.

• Proceedings of the KIPE Conference
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• 2011.11a
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• pp.73-74
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• 2011

Phase-shedding, one famous technology for multi-phase converters, is implemented in a bi-directional multi-phase converter for ESS. It reduces active operating modules at light load to compensate efficiency. Shedding point, the load level where the converter changes the number of active modules, is important factor that affects the effect of phase-shedding. Loss analysis is done for determining shedding point. Phase-shedding hysteresis is applied so that excessive phase transition is avoided. This paper proposes shedding point correction where the shedding point is adaptively corrected by calculating a new shedding point.

• The KSFM Journal of Fluid Machinery
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• v.6 no.3 s.20
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• pp.36-43
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• 2003

Operating performance of industrial gas turbines in combined cycle power plants depends upon atmospheric conditions. Compressor fouling caused by airborne particles in the atmosphere and their adhesions on compressor blades is one of critical phenomena related to the performance degradation of industrial gas turbines. Compressor fouling provokes increase of pressure loss in inlet duct, decrease of mass flow rate of intake air and decrease of compressor stage efficiency. In this study, impacts of compressor fouling on the performance of an industrial gas turbine operating at design point condition are investigated analytically. As results, it is found that the reduction of produced power with decreased mass flow rate of intake air caused by narrowed flow area by the adhesion of airborne particles on compressor blades is the most dominant impact on the gas turbine performance by the compressor fouling phenomena.

• Journal of Korean Academy of Nursing
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• v.41 no.6
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• pp.758-767
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• 2011

Purpose: This study was done to examine the threshold value of estimated height loss at which the risk of osteoporosis increases and to verify its discriminative ability in the detection of osteoporosis. Methods: It was conducted based on epidemiological descriptive methods on 732 Korean women at a public healthcare center in Seoul between July and November 2010. ANOVA, Pearson correlation, logistic regression analysis and receiver operating characteristics (ROC) curve were used for data analysis. Results: There was an age-related correlation between bone mineral density (lumbar spine: F=37.88, p<.001; femur: F=54.27, p<.001) and estimated height loss (F=27.68, p<.001). Estimated height loss increased significantly with decreasing bone mineral density (lumbar spine: r=-.23, p<.001; femur: r=-.34, p<.001). The odds ratio for the point at which the estimated height loss affects the occurrence of osteoporosis was found to increase at a cut-off value of 2 cm and the area under ROC curve was .71 and .82 in lumbar spine and femur, respectively. Conclusion: The optimal cut-off value of the estimated height loss for detection of osteoporosis was 2 cm. Height loss is therefore a useful indicator for the self-assessment and prognosis of osteoporosis.