• Korea Journal of Hospital Management
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• v.20 no.1
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• pp.53-64
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• 2015

Our objective was to evaluate differences in linear regression versus multilevel(cross-level interaction model) modeling for affecting factors lumbar discectomy. The data were used in 2011 patients with HIRA sample data. Total number of analysis is 3,641 patients and 248 hospitals. The results of research model showed that the type and location of the hospital-level factors were significant. However, all factors of patient-level were similar in the two models. Therefore, it requires the selection of an appropriate model for a more accurate analysis of the influencing factors in the daily medical charge.

• Research in Community and Public Health Nursing
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• v.27 no.4
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• pp.337-345
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• 2016

Purpose: This study aims at identifying individual- and school-level factors associated with unintentional injuries to Korean adolescents at school by applying multilevel modeling. Methods: From the database of the eleventh Korea Youth Risk Behavior Web-based Survey (KYRBS), the researchers selected 68,043 adolescents from the $7^{th}$ to the $12^{th}$ grades. Data were analyzed using ${\chi}^2$ test for prevalence and multilevel modeling for related factors of unintentional injuries at school. Results: About 22.9 percent of the adolescents had treatment experiences for unintentional injuries at school in the past 12 months. At the individual level, the significant factors associated with unintentional injuries at school included gender, grade, academic achievement, current smoking, alcohol consumption, frequency of high caffeine intake, depression, and relief of fatigue after sleep. At the group level, the significant factors included number of physical education per week and safety education. Conclusion: School based injury prevention programs should be sensitive to both individual- and school-level factors associated with unintentional injury at school among Korean adolescents.

• Journal of Power Electronics
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• v.13 no.3
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• pp.419-428
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• 2013

The modular multilevel cascade converter (MMCC) is a new family of multilevel power converters with modular realization and a cascaded pattern for submodules. The MMCC family can be classified by basic configurations and submodule types. One member of this family, the Hexverter, is configured as Double-Delta Full-Bridge (DDFB). It is a novel multilevel AC/AC converter with direct power conversion and comparatively fewer required components. It is appropriate for connecting two three-phase systems with different frequencies and driving an AC motor directly from a utility grid. This paper presents the dq model of a Hexverter with both of its AC systems by state-space representation, which then simplifies the continuous time-varying model into a periodic discrete time-invariant one. Then a generalized multivariable optimal control strategy for regulating the Hexverter's independent currents is developed. The resulting control structure can be adapted to other MMCCs and is flexible enough to include other control criterion while guaranteeing the original controller performance. The modeling method and control design are verified by simulation results.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05a
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• pp.340-345
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• 1996

Because of limited resources of time and information processing capability during abnormal situation, diagnosis is difficult tasks in nuclear power plant (NPP) operators. Moreover since minimizing of adverse consequences according to process abnormalities is vital for the safety of NPP, introducing of diagnosis support systems have particularly emphasized. However, considerable works to develop effective diagnostic support system are not sufficiently fulfilled because of the complexity of NPP is one of the major problems. To cope with this complexity, a lot of model-based diagnosis support systems have considered and implemented worldwide. In this paper, as a prior step to development of model-based diagnosis support systems, primary side of pressurized water reactor is functionally modeled by multilevel flow modeling (MFM) concept. MFM is suitable for complex system modeling and for diagnosis of abnormalities. Furthermore, knowledge-based diagnosis process, of NPP operator could be supported because this diagnosis strategy can represent operator's one.

• Journal of information and communication convergence engineering
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• v.1 no.3
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• pp.163-168
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• 2003

This paper presents a cascade multilevel PWM rectifier without the isolation transformers for energy build-up at each inverter modules. The features and advantages of the proposed PWM rectifier can be summarized as follows; I) It realizes the high power high voltage AC/DC power conversion, 2) It uses no transformer which is bulky and heavy, 3) It has hybrid structure so that switching devices can be effectively utilized, 4) It produces high quality AC current even in high power high voltage applications, 5) The input power factor remains unity by simple modulation index control. The multilevel rectifier is analyzed by using the circuit DQ transformation whereby the characteristics and control equations are obtained. Finally, it will be shown that the system simulation reveals the validity of analyses.

• Research in Community and Public Health Nursing
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• v.28 no.4
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• pp.482-493
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• 2017

Purpose: This study aims to investigate the individual and environmental factors related to emotional/behavioral problems to early adolescence in Korea by applying multilevel modeling. Methods: From the database of the 2014 Korean Child and Youth Panel Survey (KCYPS), the researchers selected 1,977 adolescents who are in the second year of middle school. Multilevel model analysis was performed to estimate the impact of relevant factors at the individual and environmental levels. Results: At the individual level, the significant factors associated with emotional/behavioral problems included BMI and study tendency in boys, and drinking, study tendency and economic levels in girls. At the environmental level, the significant factor associated with emotional/behavioral problems included relationship with the teacher. Conclusion: The emotional/behavioral problems of early adolescence are influenced not only by the individual factors but also by the environment factor. Therefore, the environment surrounding the adolescents should also be considered to prevent emotional/behavioral problems.

• The Transactions of the Korean Institute of Power Electronics
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• v.21 no.1
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• pp.49-57
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• 2016

A modular multilevel converter is widely adapted for a high-voltage direct current power transmission system. This study proposes a design methodology for a novel digital control that mitigates the negative effects caused by time delay, including communication transport delay for a modular multilevel converter. The modeling and negative effect of time delay are analyzed theoretically in a frequency domain, and its compensation methodology based on an inverse model is described fully with practical considerations. The proposed methodology is verified through several simulation results using a modular 21-level converter system.

• Journal of Power Electronics
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• v.15 no.2
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• pp.504-517
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• 2015

This paper presents a novel switching voltage model and an offset-based pulse width modulation (PWM) scheme for multilevel inverters with unbalanced DC sources. The switching voltage model under a DC voltage imbalance will be formulated in general form for multilevel neutral-point-clamped topologies. Analysis of the reference switching voltages from active and non-active switching voltage components in abc coordinates can enable voltage implementation for an unbalanced DC-source condition. Offset voltage is introduced as an indispensable variable in the switching voltage model for multilevel voltage-source inverters. The PWM performance is controlled through the design of two offset components in a subsequence. One main offset may refer to the common mode voltage, and the other offset restricts its effect on the quality of PWM control in related DC levels. The PWM quality can be improved as the switching loss is reduced in a discontinuous PWM mode by setting the local offset, which is related to the load currents. The validity of the proposed algorithm is verified by experimental results.

• The Journal of Korean Institute for Practical Engineering Education
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• v.4 no.2
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• pp.129-135
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• 2012

There are many ways in which electric vehicles are mathematically modeled and simulated. The components have different physical background and models, but have to fit into one mathematical model. A multiphysics model structure is required. Depending on the goal of the simulation, there are various levels on which the simulation can be performed. This is called multilevel, consisting of a conceptual system level, a circuit level and a more detailed component level. This paper discusses which multiphysics models and multilevel simulations are required for the various components in an electric vehicle. Also, this simulation approach could improve the effectiveness of learning in engineering education.

• Nuclear Engineering and Technology
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• v.55 no.2
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• pp.756-768
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• 2023

Neutron transport calculations are extremely challenging due to the high computational cost of large and complex problems. A multilevel octree grid algorithm (MLTG) of discrete ordinates method was developed to improve the modeling accuracy and simulation efficiency on 3-D Cartesian grids. The Balakovo-3 VVER-1000 neutron dosimetry benchmark is calculated to verify and validate this numerical technique. A simplified S₂ synthetic acceleration is used in the MLTG calculation method to improve the convergence of the source iterations. For the triangularly arranged fuel pins, we adopt a source projection algorithm to generate pin-by-pin source distributions of hexagonal assemblies. MLTG provides accurate geometric modeling and flexible fixed source description at a lower cost than traditional Cartesian grids. The total number of meshes is reduced to 1.9 million from the initial 9.5 million for the Balakovo-3 model. The numerical comparisons show that the MLTG results are in satisfactory agreement with the conventional S_N method and experimental data, within the root-mean-square errors of about 4% and 10%, respectively. Compared to uniform fine meshing, approximately 70% of the computational cost can be saved using the MLTG algorithm for the Balakovo-3 computational model.