• Journal of Power Electronics
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• v.16 no.1
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• pp.142-152
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• 2016

Common mode voltage (CMV) generation is a major problem in switching power converter fed induction motor drive systems. CMV is the zero sequence voltage generated due to the switching action of power converters. Even a small magnitude of CMV with a high rate of change may circulate large bearing currents which may damage a machine's bearings and shorten its life. There are several methods of controlling CMV. This paper presents 3-level sinusoidal pulse width modulation based techniques to control the magnitude and rate of change of CMV in multilevel AC-DC-AC drive systems. Simulation and experimental investigations have been presented to validate the performance of proposed technique to control CMV in 3-level neutral point clamped inverter based AC-DC-AC system.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.1
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• pp.82-88
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• 2017

This study proposes a new topology for the performance test of a valve consisting of a modular multilevel converter (MMC)-based static synchronous compensator (STATCOM). The conventional valve performance test equipment requires high-voltage AC source of several kV rating because the number of submodules to be tested in a valve should be at least six or eight. However, the power source of the proposed scheme is DC and not AC source. The DC power source voltage range of the proposed test circuit is from several volts to several tens of volts. Therefore, the size and cost for the performance test equipment can be reduced considerably compared with the conventional method. The proposed scheme satisfies the requirements of the IEC 62927 standard. Simulations are conducted for a valve of 50[MVA] MMC-based STATCOM. Experimental results with a scale-downed setup show the validity of the proposed performance test topology.

• Journal of Power Electronics
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• v.18 no.3
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• pp.863-870
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• 2018

The traditional power control theories for the harmonic reduction methods in multilevel inverters are found to be unreliable under unbalanced load conditions. The unreliability in harmonic mitigation is caused by voltage fluctuations, non-linear loads, the use of power switches, etc. In general, the harmonics are reduced by filters. However, such devices are an expensive way to provide a smooth and fast response to secure power systems during dynamic conditions. Hence, the Decoupled Double Synchronous Reference Frame (DDSRF) theory combined with a State Delay Controller (SDC) is proposed to achieve a harmonic reduction in power systems. The DDSRF produces a sinusoidal harmonic that is the opposite of the load harmonic. Then, it injects this harmonic into power systems, which reduces the effect of harmonics. The SDC is used to reduce the delay between the compensation time for power injection and the generation of a reference signal. The proposed technique has been simulated using MATLAB and its reliability has been verified experimentally under unbalanced conditions.

• The Transactions of the Korean Institute of Power Electronics
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• v.13 no.1
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• pp.9-14
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• 2008

Since they use the low switching frequency in multilevel inverter systems, they generate the high low frequency harmonic components. Generally, LC filter is used at the output terminal of inverter systems to solve this problem. But it causes a voltage drop at the output terminal by use of damping resistors, and causes the problem in which system efficiency decreases due to power loss of the damping resistor. In this paper, we proposed an output filter design method for NPC three-level inverter systems with low switching frequency. And we analyzed the efficiency of the proposed filter system, and the effectiveness of the proposed system is verified by simulation and experimental results.

• Human Ecology Research
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• v.61 no.2
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• pp.263-280
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• 2023

This study examined four ecological systems, namely individual, family, school, and media environments. A series of moderator analyses were conducted to examine variations in effect size estimates across the study characteristics. The current study estimated that the effect size results were gleaned from 360 primary studies, including 90 journal articles and 270 thesis/dissertations, published between 2011 and 2022. The current meta-analysis results supported the ecological framework. That is, the impact of each ecological system on the development of positive and negative peer relationships varies depending on age groups and protective-risk factors. Specifically, for positive peer relationships, the largest effect size of the protective factor was found at the individual level for young and school- aged children, but at the school level for adolescents. Regarding the risk factors for positive peer relationships, the media was the ecological system with the strongest effect size for both young children and adolescents, while the individual-level demonstrating the strongest effect for school-aged children. Results from this meta-analysis allow us to identify some vital intervention areas in terms of healthy peer-relationship development, which should be of considerable interest to the educators and policymakers who are responsible for assessing and intervening with at-risk young children, school-aged children, and adolescents. From a practical standpoint, the current meta-analysis results are expected to contribute to developing effective prevention initiatives by targeting specific protective and risk factors for peer relationship development on the ecological system level.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.4B
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• pp.324-330
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• 2002

In this paper, we proposed a simple modification on Fiebig's REC algorithm for multiuser detection of FH/MFSK systems replacing hard limiter with soft limiter. The performance improvement of our modified REC algorithm has been confirmed with computer simulations. for the parameter of interest, the capacity could be increased about 5% or more without degrading the reliability.

• Proceedings of the Korea Database Society Conference
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• 1999.06a
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• pp.161-171
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• 1999

Query language has been widely used as a convenient tool to obtain information from a database. However, users demand more intelligent query processing systems that can understand the intent of an imprecise query and provide additional useful information as well as exact answers. This paper introduces a meta-database and presents a query processing mechanism that supports a variety of intelligent queries in a consistent and integrated way. The meta-database extracts data abstraction knowledge from an underlying database on the basis of a multilevel knowledge representation framework KAH. In cooperation with the underlying database, the meta-database supports four types of intelligent queries that provide approximately or conceptually equal answers as well as exact ones.

• Proceedings of the Korea Inteligent Information System Society Conference
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• 1999.03a
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• pp.161-171
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• 1999

Query language has been widely used as a convenient tool to obtain information from a database. However, users demand more intelligent query processing systems that can understand the intent of an imprecise query and provide additional useful information as well as exact answers. This paper introduces a meta-database and presents a query processing mechanism that supports a variety of intelligent queries in a consistent and integrated way. The meta-database extracts data abstraction knowledge form an underlying database on the basis of a multilevel knowledge representation framework KAH. In cooperation with the underlying database, the meta-database supports four types of intelligent queries that provide approximately or conceptually equal answers as well as exact ones.

• Journal of Sensor Science and Technology
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• v.15 no.5
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• pp.357-361
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• 2006

This paper reports a new technology to implement complex PDMS microchannels, which are simply constructed using three-dimensional photoresist structures as mold for PDMS replica process. The process utilizes LOR resist as a sacrificial layer to levitate the structural photoresist and multi-step exposure to control the thicknesses of photoresist structures. Various shapes of photoresist structures were successfully fabricated. Using the PDMS replica method, the three-dimensional photoresist structures are demonstrated to be applicable for implementing complex microchannels in PDMS. In addition, more complex multilevel microchannels are constructed by bonding two PDMS layers with just single PDMS alignment.

• IE interfaces
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• v.5 no.2
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• pp.29-37
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• 1992

An approach to Just-In-Time for small job-shop-type manufacturers is presented. This is aimed at those who understand the pull production system, however, cannot afford either swift changeover in workcenters or frequent delivery to customers. First, as a production planning technique, a lot-sizing model entitled Multilevel Least Total Cost Model(MLTCM) is developed; The production order quantities of each components be those requiremensts of the end item production lot. The contribution of MLTCM is shown via simulation. Then, a framework of the Kanban operations is designed; A Production Control Kanban is introduced as a communication tool with Production Kanban in a job-shop.