• Journal of Power Electronics
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• v.5 no.2
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• pp.160-165
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• 2005

In a parallel operation of UPS, there are two types of circulating currents between UPS. One is the low order circulating current with a fundamental frequency caused by the amplitude and phase differences of UPS output voltages, and the other is the harmonic circulating current with PWM switching frequency caused by non-synchronized PWM waveforms among UPS. The elimination of the low order circulating current is essential for optimal load sharing in parallel operations of UPS, which can be accomplished by the phase and magnitude control at each UPS. The harmonic circulating current may cause troubles and deteriorate in performance of the controller for optimal load sharing in parallel operation of UPS. This paper presents a PWM synchronizing method to eliminate the harmonic circulation current in parallel operation of UPS. The effectiveness of the proposed scheme has been investigated and verified through experiments by a 50kVA UPS.

• Publications of The Korean Astronomical Society
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• v.15 no.1
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• pp.35-60
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• 2000

We have analyzed the frequency sharing in the range of 71-275 GHz, which was adopted as a main topic of the WARC-2000 at the previous conference WARC-97. Though the technology of the active services has not been fully developed in this frequncy range, the heavy usage of this technology is expected in foreseeable future. To protect the passive services from spurious and out-of-band emissions of active services, realignment of the spectrum between 71 GHz and 275 GHz is strongly required. In addition, some effort should be made to allocate special bands for the radio astronomy service.

• ETRI Journal
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• v.36 no.3
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• pp.502-505
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• 2014

Polarized spectrum sharing (PSS) exploits the spectrum opportunities in a polarized domain. However, when it comes to wideband environments PSS is impaired by the frequency-dependent polarization mode dispersion (PMD); thus, the effective throughput of PSS drops. To combat the PMD effect, this work proposes a cluster-based PSS approach to enable PSS on a narrower frequency span. Simulation results show that the effective throughput of PSS on cluster basis outperforms that of PSS on bandwidth and subcarrier basis.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2000.11a
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• pp.173-178
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• 2000

The time sharing type high frequency inverter using serial-parallel resonant to give VVVF function in the inverter used as power source of induction heating at high frequency is presented in this paper. This proposed inverter can reduce distribution of the switching current because of using the current of serial resonant circuit to the input current of the parallel one. The analysis of the proposed circuit is generally described by using the normalized parameters. Also, according to the calculated characteristics value, a method of the circuit design and operating characteristics of the inverter is proposed. In addition, this paper proves the validity of theoretical analysis through Simulation. This proposed inverter show that it can be practically used in future as power source system for induction heating application, DC-DC converter etc.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.27 no.5
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• pp.33-38
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• 2013

This paper presents a simpler calculation of the generator load sharing according to the change of system frequency and the droop setting of turbine-generator governor. The author firstly draws a graph based on the generator operation at rated MW output and normal frequency, and secondly, builds a proportional expression using the graph in order to obtain the solution in a simpler way rather than using conventional mathematical calculation, to provide the readers with an easier understanding of the droop setting of turbine-generator governor.

• ETRI Journal
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• v.33 no.2
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• pp.279-282
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• 2011

Coexistence analysis is extremely important in examining the possibility for spectrum sharing between orthogonal frequency-division multiplexing (OFDM)-based international mobile telecommunications (IMT)-Advanced and other wireless services. In this letter, a new closed form method is derived based on power spectral density analysis in order to analyze the coexistence of OFDM-based IMT-Advanced systems and broadcasting frequency modulation (FM) systems. The proposed method evaluates more exact interference power of IMT-Advanced systems in FM broadcasting systems than the advanced minimum coupling loss (A-MCL) method. Numerical results show that the interference power is 1.3 dB and 3 dB less than that obtained using the A-MCL method at cochannel and adjacent channel, respectively. This reduces the minimum separation distance between the two systems, which eventually saves spectrum resources.

• Journal of Power Electronics
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• v.13 no.3
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• pp.479-486
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• 2013

This paper presents a control strategy for distributed systems, which can be used in islanded microgrids. The control strategy is based on the droop method, which uses locally measured feedback to achieve load current sharing. Instead of the traditional droop method, an improved one is implemented. A virtual inductor in the synchronous frame for three-phase inverters is proposed to deal with the coupling of the frequency and the amplitude related to the active and reactive power. Compared with the traditional virtual inductor, the proposed virtual inductor is not affected by high frequency noises because it avoids differential calculations. A model is given for the distributed generation system, which is beneficial for the design of the droop coefficients and the value of the virtual inductor. The effectiveness of the proposed control strategy is verified by simulation and experiment results.

• Journal of The Korean Association For Science Education
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• v.38 no.5
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• pp.667-680
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• 2018

The purpose of this study is to investigate 1) the types of knowledge students ask for in their reading comprehension of science textbooks using an online annotation system, 2) the accuracy of the knowledge provided by the students to their peers, 3) the frequency of knowledge sharing behaviors, 4) the evaluation of the effect of collaborative reading, and 5) the trust among peers as knowledge sharers. Questions made by 241 students in the second grade of middle school using an online annotation system in two chapters of the science textbook were analyzed using Bloom's revised taxonomy and their answers were grouped according to five accuracy categories. Also, questionnaires for the evaluation of the effectiveness of collaborative reading comprehension and of trust among the students were used. The students asked their peers 'understanding questions' which comprised almost 80% of the total questions they made and were similar with individual metacognitive strategies for reading comprehension. Of the total threads, 71% has scientifically correct threads shared by the students. The frequency of the knowledge sharing behaviors was high but this was affected by the rewards (point system). Students evaluated that collaborative reading comprehension conducted through an online annotation system were helpful in their learning. In addition, the ratio of students trusting their peers who did the knowledge sharing is over 80%. This study shows that when students use an online annotation system, they can fill one another's cognitive gaps in the reading process by sharing knowledge. Also, collaborative reading using an online annotation system has proved that cognitive individualization is possible through sharing knowledge interactively and dynamically, unlike reading hard copies of textbooks which are a one way information transfer.

• Journal of the Korea Society of Computer and Information
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• v.25 no.3
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• pp.199-206
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• 2020

The purpose of this study is to examine the relationship between intention to participation in sharing economy, digital cultural capital, and social connectedness and then analyze the variables affecting the intention to participate in sharing economy. This subjects were university students. Collected data were statistically processed by PASW 18.0 program using reliability, frequency analysis, T-test, one-way ANOVA, correlation and multiple regression analysis. The results were as follows. First, redistribution participation and cooperative lifestyle participation were relatively high. Second, intention to participate in sharing economy activities differs according to gender, age, monthly allowance, and SNS usage time. Third, intention to redistribution participation increases as the recognition of objectified digital culture capital, embodied digital culture capital, and social connectedness increases. And the intention to cooperative lifestyles participation and information sharing participation increased as digital culture capital increased. Forth, the most significant variable affecting the intention to participate in sharing economic activities was digital cultural capital.

• Journal of the Korean Society for Library and Information Science
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• v.52 no.4
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• pp.27-49
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• 2018

This research empirically analyzed the effect of information sharing through the use of public libraries on the construction and consolidation of social capital of individuals. Through the survey targeting public library users, it was identified that the frequency of public library use and the participation of public library programs have positive effect on the recognition of information sharing and the reliability of information shared. In addition, information sharing among people became more active when public libraries mediate their information activities. This public library-mediated information sharing positively affects the construction of structural and relational social capital. In contrast, it does not have significant meaning on the construction of cognitive social capital, which means that information sharing and the use of public library relatively concentrate on satisfying personal information needs.