• Journal of the korean academy of Pediatric Dentistry
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• v.51 no.1
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• pp.40-54
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• 2024

This study aims to identify the regional distribution in the prevalence of dental caries and related multidimensional factors among 12-year-old children in Korea. Data from the 2018 Child Oral Health Survey were used to calculate the average DMFT index of 12-year-old children in metropolitan cities, and a multi-level regression model was applied to explain the regional distribution of dental caries prevalence and related factors. Factors were divided into two levels by administrative structure. This study finds a significant regional difference in the prevalence of dental caries in 12-year-old Korean children across metropolitan cities. Multilevel analysis showed that district-level factors (average number of pit and fissure-sealed permanent teeth, dental treatment demand rate, preventive treatment rate, sex ratio, and number of dentists per 100,000 people) and metropolitan-level factors (intakes of cariogenic beverages and number of pediatric dental hospitals and clinics per 100,000 people) had a significant effect on dental caries prevalence (p < 0.05). Individual characteristics and local socio-environmental factors influence the prevalence of dental caries. Especially considering the strong dependence on preventive treatment and accessibility to dental care services, it is necessary to provide adequate preventive treatment and expand health care resources in high-risk areas of dental caries.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.9 no.1
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• pp.15-28
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• 1999

In Multilevel Secure Database Management System(MLS/DBMS), we assume that system has a security clearance level for each user and a classification level for each data item in system and the objective of these systems is to protect secure information from unauthorized user. Many algorithms which have been researched have focus on removing covert channel by modifying conventional lock-based algorithm or timestamp-based algorithm. but there is high-level starvation problem that high level transaction is aborted by low level transaction repeatedly. In order to solve this problem, we propose an algorithm to reduce high-level starvation using dynamic adjustment of serialization order, which is basically using orange lock. Because our algorithm is based on a single version unlike conventional secure algorithms which are performed on multiversion, it can get high degree of concurrency control. we also show that it guarantees the serializability of concurrent execution, and satisfies secure properties of MLS/DBMS.

• Journal of the Korean Institute of Intelligent Systems
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• v.17 no.6
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• pp.725-730
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• 2007

Gray-level histogram-based threshold selection methods such as Otsu's method, Huang and Wang's method, and etc. have been widely used for the threshold selection in image processing. They are simple and effective, but take too much time to determine the optimal multilevel threshold values as the number of thresholds are increased. In this paper, we measure correlation between gray-levels by using the Gaussian function and define a Gaussian-type finite mixture distribution which is combination of the Gaussian distribution function with the gray-level histogram, and propose a fast and effective threshold selection method using it. We show the effectiveness of the proposed through experimental results applied it to three images and the efficiency though comparison of the computational complexity of the proposed with that of Otsu's method.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.11 no.1
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• pp.43-54
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• 2001

In this paper we study the use of polyinstantiation for spatial data, for the purpose of solving cover in topology channel in multilevel secure spatial database systems. Spatial database system with topological structure has a number of spatial analysis function using spatial data and neighbored one\`s each other. But. it has problems that information flow is occurred by topological relationship in spatial database systems. Geographic Information System(CIS) must be needed mandatory access control because there ,are many information flow through positioning information And topological relationship between spatial objects. Moreover, most GIS applications also graphe user interface(GUI). In addressing these problems, we design the MLS/SRDM(Multi Level Security/Spatial Relational Data Model) and propose polyinstantiation for spatial data for solving information flow that occurred by toplogical relationship of spatial data.

• Information Systems Review
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• v.22 no.4
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• pp.115-134
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• 2020

Previous studies on willingness to pay for digital content have mainly focused on audience factors and individual level. To complement the limitation of previous research, this study conducts a multilevel analysis to find the factors influence digital content purchases considering two axes: audience/media factors and individual/household levels. Using a sample of 10,172 individuals within 4,313 households, the analysis results show individual media factors including theater-going, experience with cloud services, and multi-screen service usage have the greatest effects on digital content purchases. At the household level, the media ownership factors that the number of laptops, wireless routers, and tablets have a greater influence than audience factors such as household size or household income. Our findings help scholars to enhance the understanding of individuals' media use considering household environmental factors and shed light on the importance of multi-screen service usage, and content providers to improve their digital content sales using multi-screen environment.

• Journal of KIISE:Databases
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• v.30 no.2
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• pp.209-224
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• 2003

Recent development of electronic commerce enables the use of Electronic Stock Trading Systems(ESTS) to be expanded. In ESTS, information with various sensitivity levels is shared by multiple users with mutually different clearance levels. Therefore, it is necessary to use Multilevel Secure Database Management Systems(MLS/DBMSs) in controlling concurrent execution among multiple transactions. In ESTS, not only analytical OLAP transactions, but also mission critical OLTP transactions are executed concurrently, which causes it difficult to adapt traditional secure transaction management schemes to ESTS environments. In this paper, we propose Secure One Snapshot(SOS) protocol that is devised for Secure Transaction Management in ESTS. By maintaining additional one snapshot as well as working database SOS blocks covert-channel efficiently, enables various real-time transaction management schemes to be adapted with ease, and reduces the length of waiting queue being managed to maintain freshness of data by utilizing the characteristics of less strict correctness criteria. In this paper, we introduce the process of SOS protocol with some examples, and then analyze correctness of devised protocol.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.4
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• pp.271-276
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• 2021

Arc fault detector based on multilevel DWT with analysis of high-frequency noise components over 100 kHz is proposed in this study to improve the performance in detecting serial arcs and distinguishing them from inverter noise in PV systems. PV inverters generally operate at a frequency range of 20-50 kHz for switching operation and maximum power tracking control, and the effect of these frequency components on the signal for arc detection leads to negative arc detection. High-speed ADC and multilevel DWT are used in this study to analyze frequency components above 100 kHz. Such high frequency components are less influenced by inverter noise and utilized to detect as well as separate DC series arc from inverter noise. Arc detectors identify the input current of PV inverters using a Rogowski coil. The sensed signal is filtered, amplified, and used in 800kSPS ADC and DWT analysis and arc occurrence determination in DSP. An arc detection simulation facility in UL1699B was constructed and AFD tests the proposed detector were conducted to verify the performance of arc detection and performance of distinction of the negative arc. The satisfactory performance of the arc detector meets the standard of arc detection and extinguishing time of UL1699B with an arc detection time of approximately 0.11 seconds.

• Journal of Power Electronics
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• v.19 no.5
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• pp.1171-1181
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• 2019

Modular Multilevel Converter (MMC)-based HVDC power transmission using a real-time simulator is one of the key technologies in power electronics research. This paper introduces the design methodology of a physical MMC-HVDC control system based on a Field-Programmable Gate Array (FPGA), which has the advantage of high-speed parallel operation, and validates the accuracy of MMC-HVDC control when operated with a Real-Time Digital Simulator (RTDS). Finally, this paper compares and analyzes the characteristics of capacitor voltage balancing methods such as Nearest Level Control (NLC), NLC with a reduced switching frequency, and tolerance band modulation implemented on physical control system.

• Journal of Power Electronics
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• v.18 no.2
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• pp.467-481
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• 2018

Modular multilevel converter (MMC)-based high-voltage direct current (HVDC) presents attractive technical advantages and contributes to enhanced system operation and reduced oscillation damping in dynamic MMC-HVDC systems. We propose an advanced small-signal multi-terminal MMC-HVDC based on dynamic phasors and state space for power system stability analysis to enhance computational accuracy and reduce simulation time. In accordance with active and passive network control strategies for multi-terminal MMC-HVDC, the matchable small-signal stability models containing high harmonics and dynamics of internal variables are conducted, and a related theoretical derivation is carried out. The proposed advanced small-signal model is then compared with electromagnetic-transient and traditional small-signal state-space models by adopting a typical multi-terminal MMC-HVDC network with offshore wind generation. Simulation indicates that the advanced small-signal model can successfully follow the electromechanical transient response with small errors and can predict the damped oscillations. The validity and applicability of the proposed model are effectively confirmed.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.688-698
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• 2018

Quasi Z-Source Multilevel Inverter (QZMLI) topology has attracted grid connected Photovoltaic (PV) systems in recent days. So there is a remarkable research thrust in switching techniques and control strategies of QZMLI. This paper presents the mathematical analysis of Phase shift- Pulse Width Amplitude Modulation (PS-PWAM) for QZMLI and emphasizes on the advantages of the technique. The proposed technique uses the maximum and minimum envelopes of the reference waves for generation of pulses and proportion of it to generate shoot-through pulses. Hence, it results in maximum utilization of input voltage, lesser switching loss, reduced Total Harmonic Distortion (THD) of the output voltage, reduced inductor current ripple and capacitor voltage ripple. Due to these qualities, the QZMLI with PS-PWAM emerges to be the best suitable for PV based grid connected applications compared to Phase shift-Pulse Width Modulation (PS-PWM). The detailed math analysis of the proposed technique has been disclosed. Simulation has been performed for the proposed technique using MATLAB/Simulink. A prototype has been built to validate the results for which the pulses were generated using FPGA /SPARTAN 3E.