• Journal of Power Electronics
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• v.17 no.1
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• pp.127-135
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• 2017

Recently the researches on modular multi-level converter (MMC) are being highlighted because high quality and efficient power transmission have become key issues in high voltage direct current (HVDC) systems. This paper proposes an improved pre-charging method for the sub-module (SM) capacitor of MMC-based HVDC systems, which operates in the nearest level control (NLC) modulation and does not need additional circuits or pulse width modulation (PWM) techniques. The feasibility of the proposed method was verified through computer simulations for a scaled 3-phase 10kVA MMC with 12 SMs per each arm. Hardware experiments with a scaled prototype have also been performed in the lab to confirm the simulation results.

• Proceedings of the KIEE Conference
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• 2015.07a
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• pp.996-997
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• 2015

This paper proposes parameter design principle of the sub-module capacitance, Arm inductance and a control method to reduced the circulating currents in modular multilevel converter(MMC) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence, zero-sequence and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider these all components. This study proposes the control method to reduced the circulating currents under the unbalanced voltage.

• Proceedings of the KIPE Conference
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• 2003.07b
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• pp.588-592
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• 2003

In recent years, the multilevel inverter synthesizing the output voltage with step pulse has been widely used as a solution for high power and high voltage applications. This paper proposes a simple method to obtain the conducting . angle. It is calculated by using voltage-second areas of the divided reference voltage according to the output voltage levels and these areas have influence on output step pulse waves. It is possible to reduce an amount of calculation because it is not required to solve the simultaneous equations by an iterative method. Also, the proposed method can get the conducting angle by means of on line.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.3
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• pp.263-269
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• 2017

This paper proposes a novel circulating current control method for an MMC-HVDC system based on Vector PI control. The method can suppress second-order harmonics of the circulating currents under balanced and unbalanced grid conditions. The proposed method is robust to grid frequency variation. The effectiveness of the proposed method is verified through frequency response and time domain simulation.

• Nuclear Engineering and Technology
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• v.37 no.1
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• pp.118-126
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• 2005

While emergency operating procedures (EOPs) occupy an important role in the management of various abnormal situations in nuclear power plants (NPPs), current technology for the validation of EOPs still largely depends on manual review. A validation method for EOPs of NPPs is thus proposed based on dynamic multi-level flow modeling (MFM). The MFM modeling procedure and the EOP validation procedure are developed and provided in the paper. Application of the proposed method to EOPs of an actual NPP shows that the proposed method provides an efficient means of validating EOPs. It is also found that the information on state transitions in MFM models during the management of abnormal situations is also useful for further analysis on EOPs including their optimization.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2008.04a
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• pp.356-361
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• 2008

Eigenvalue reduction schemes approximate the lower eigenmodes that represent the global behavior of the structures. In the previous study, we proposed a two-level condensation scheme (TLCS) for the construction of a reduced system. And we have improved previous TLCS with combination of the iterated improved reduced system method (IIRS) to increase accuracy of the higher modes intermediate range. In this study, we apply previous improved TLCS to multi-level sub-structuring scheme. In the first step, the global system is recursively partitioned into a hierarchy of sub-domain. In second step, each uncoupled sub-domain is condensed by the improved TLCS. After assembly process of each reduced sub-eigenvalue problem, eigen-solution is calculated by Lanczos method (ARPACK). Finally, Numerical examples demonstrate performance of proposed method.

• ETRI Journal
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• v.35 no.6
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• pp.1160-1163
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• 2013

Herein, we propose a novel method to create multidirectional and multilevel pointing input using only a hardware keyboard. The relative position vectors among pressed keys are calculated into an input vector. The pointing performance of this method is evaluated based on the ISO 9241-9 recommendations. We find that the performance of the method is sufficient to create moderate pointing input, as the measurement of the throughput is close to that of a touchpad, as measured in past studies also using the ISO 9241-9 recommendations.

• Journal of the Korean Data and Information Science Society
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• v.24 no.4
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• pp.931-940
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• 2013

For analysis of clustered survival data, the inferences of parameters in semi-parametric frailty models have been widely studied. It is also important to investigate the potential heterogeneity in event times among clusters (e.g. centers, patients). For purpose of this analysis, the interval estimation of frailty is useful. In this paper we propose a visualizing method to present confidence intervals of individual frailties across clusters using the frailtyHL R-package, which is implemented from h-likelihood methods for frailty models. The proposed method is demonstrated using two practical examples.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.67 no.7
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• pp.846-853
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• 2018

In this paper, an improved model predictive control (MPC) method is proposed, which reduces the amount of calculations caused by the increased number of candidate voltage vectors with the increased voltage level in multi-level inverters. When the conventional MPC method is used for multi-level inverters, all candidate voltage vectors are considered to predict the next-step current value. However, in the case that the sampling time is short, increased voltage level makes it difficult to consider the all candidate voltage vectors. In this paper, the improved MPC method which can get a fast transient response is proposed with a small amount of the computation by adding new candidate voltage vectors that are set to find the optimal vector. As a result, the proposed method shows faster transient response than the method that considers the adjacent vectors and reduces the computational burden compared to the method that considers the whole voltage vector. the performance of the proposed method is verified through simulations and experiments.

• Proceedings of the KIPE Conference
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• 2001.10a
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• pp.286-290
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• 2001

This paper describes an improved short-circuit protection method with a boost type rectifier using a multilevel ac/dc power converter. The output dc power of the proposed converter can be disconnected from the load within several hundred microseconds at the instant of short-circuit fault. Once the fault has been cleared the dc power is reapplied to the load. The rising time of the dc load voltage is as small as several hundred microseconds, and there is no overshoot of the dc voltage because the dc output capacitors hold undischarged state. The converter, which employs the proposed method, has the characteristics of a simplified structure, reduced cost, weight, and volume compared with a conventional power supply, which has frequent output short-circuits. Experimental results are presented to verify the usefulness of the proposed converter.