• 한국정보과학회논문지:시스템및이론
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• 제33권8호
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• pp.505-513
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• 2006

클라이언트와 서비스 제공자간의 서비스 수준 계약인 SLA(Service Level Agreement)를 만족시키기 위해서는 클라이언트 요청을 우선순위 계층으로 구분하여, 낮은 수준의 서비스를 요청하는 클라이언트 보다는 고수준의 서비스를 요구하는 클라이언트에게 우선적으로 서비스를 제공할 수 있는 기술이 필요하다. 본 논문에서는 서비스 제공자의 웹 서버 노드를 우선 순위에 따라 정적. 동적으로 분할하는 방법 및 다중계층 (Multiclass) 에서의 승인 제어 (Admission Control) 기법을 연구하였으며, 시뮬레이션을 통해 SLA를 고려한 웹 서버 처리율 및 응답시간 성능을 분석하였다.

• Journal of Power Electronics
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• 제14권5호
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• pp.946-956
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• 2014

Cascaded H-Bridge (CHB) converters can be directly connected to medium-voltage grids without using transformers and they possess the advantages of large capacity and low harmonics. They are significant tools for providing grid connections in large-capacity renewable energy systems. However, the reliability of a grid-connected CHB converter can be seriously influenced by the number of power switching devices that exist in the structure. This paper proposes a fault-tolerant control strategy based on double zero-sequence voltage injection and DC voltage optimization to improve the reliability of star-connected CHB converters after one or more power units have been bypassed. By injecting double zero-sequence voltages into each phase cluster, the DC voltages of the healthy units can be rapidly balanced after the faulty units are bypassed. In addition, optimizing the DC voltage increases the number of faulty units that can be tolerated and improves the reliability of the converter. Simulations and experimental results are shown for a seven-level three-phase CHB converter to validate the efficiency and feasibility of this strategy.

• 한국컴퓨터정보학회지
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• 제1권2호
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• pp.95-107
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• 1995

동시성 제어 기업은 트랜잭션 처리 시스템의 성능에 중요한 영향을 미친다. 전통적인 록킹 기법은 대기 트랜잭션이 로크를 가지고 있으면서 진행중인 다른 트랜잭션을 블록시키는 블록킹 현상을 일으킨다. 제안된 방법은 트랜잭션 실행의 다음 단계에서 그들의 블록킹 형태를 지연함에 의해서 블록킹 확률을 감소시킨다. 트랜잭션 실행은 전통적인 록킹같은 블록킹 상태와 트랜잭션이 로크를 기다리지만 다른 트랜잭션은 블록화하지 않는 비블록킹 상태로 나누어 볼 수 있다. 그러나 비블록킹 상태 중에 처리된 데이터는 트랜잭션을 취소 시킬 수 있다. 블록킹과 취소 효과를 적절히 균형을 이루어서 제안된 방식은 모든 데이터 함유와 자원 함유 수준에서 전통적 록킹과 OCC방식보다 더 뛰어난 성능을 가지고 있다.

• 대한전기학회논문지:전력기술부문A
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• 제48권3호
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• pp.278-285
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• 1999

This paper utilizes the method of Q-parameterization control to design a controller which solves the problem of imbalance in magnetic bearing systems. There are two methods to solve this problem using feedback controal. The first method is to compensate for the imbalance forces by generating opposing forces on the bearing surface (imbalance compensation). The second method is to make the rotor rotate around its axis of inertia (automatic balancing);in this case no imbalance forces will be generated. In this paper we deal with only imbalance compensation. The free parameter of the Q-parameterization controller is chosen such that these goals are achieved. After the introduction of a model of the magnetic bearing system, we explain the Q-parameterization controller design of the magnetic bearing system with emphasis on the rejection of sinusoidal disturbance for imbalance compensation design. The design objectives are formulated as a linear equations in the controller free paramete Q. Finally, simulation and experimental results are presented and showed the robustness and effectiveness of the proposed controllers.

• Journal of Power Electronics
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• 제15권4호
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• pp.910-919
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• 2015

The use of a PV grid-connected inverter with non-isolated topology and without a transformer is good for improving conversion efficiency; however, this inverter has become increasingly complicated for eliminating leakage current. To simplify the complicated architecture of traditional three-level dual buck inverters, a new dual Buck three-level PV grid-connected inverter topology is proposed. In the proposed topology, the voltage on the grounding stray capacitor is clamped by large input capacitors and is equal to half of the bus voltage; thus, leakage current can be eliminated. Unlike in the traditional topology, the current in the proposed topology passes through few elements and does not flow through the body diodes of MOSFET switches, resulting in increased efficiency. Additionally, a multi-loop control method that includes voltage-balancing control is proposed and analyzed. Both simulation and experimental results are demonstrated to verify the proposed structure and control method.

• Journal of Power Electronics
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• 제18권6호
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• pp.1659-1669
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• 2018

An improved circulating current suppression control method is proposed in this paper. In the proposed controller, an outer loop of the average capacitor voltage control model is used to balance the sub-module capacitor voltage. Meanwhile, an individual voltage balance controller and an arm voltage balance controller are also used. The DC and harmonic components of the circulating current are separated using a low pass filter. Therefore, a multiple quasi-proportional-resonant (multi-quasi-PR) controller is introduced in the inner loop to eliminate the circulating harmonic current, which mainly contains second-order harmonic but also contains other high-order harmonics. In addition, the parameters of the multi-quasi-PR controller are designed in the discrete domain and an analysis of the stability characteristic is given in this paper. In addition, a simulation model of a three-phase MMC system is built in order to confirm the correctness and superiority of the proposed controller. Finally, experiment results are presented and compared. These results illustrate that the improved control method has good performance in suppressing circulating harmonic current and in balancing the capacitor voltage.

• Journal of Power Electronics
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• 제19권4호
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• pp.858-868
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• 2019

With the widespread use of modern clean energy, lithium-ion batteries have become essential as a more reliable energy storage component in the energy Internet. However, due to the difference in monomers, some of the battery over-charge or over-discharge in battery packs restrict their use. Therefore, a novel multiphase interleaved converter for reducing the inconsistencies of the individual cells in a battery pack is proposed in this paper. Based on the multiphase converter branches connected to each lithium battery, this circuit realizes energy transferred from any cell(s) to any other cell(s) complementarily. This flexible interlaced converter is composed of an improved bi-directional Buck-Boost circuit that is presented with its own available control method. A simulation model based on the PNGV model of fundamental equalization is built with four cells in PSIM. Simulation and experimental results demonstrate that converter and its control achieve simple and fast equalization. Furthermore, a comparison of traditional methods and the HNFABC equalization is provided to show the performance of the converter and the control of lithium-based battery stacks.

• Journal of Power Electronics
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• 제19권1호
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• pp.108-118
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• 2019

This paper proposes a DC-link voltage balance controller using the fourth-phase of a three-level neutral-point clamped (NPC) PWM converter with medium vector selection (MVS) PWM for common-mode voltage reduction. MVS PWM makes the voltage reference by synthesizing the voltage vectors that cannot generate common-mode voltage. This PWM method is effective for reducing the EMI noise emitted from converter systems. However, the DC-link voltage imbalance problem is caused by the use of limited voltage vectors. Therefore, in this paper, the effect of MVS PWM on the DC-link voltage of a three-level NPC converter is analyzed. Then a proportional-derivative (PD) controller for the DC-link voltage balance is designed from the DC-link modeling. In addition, feedforward compensation of the neutral point current is included in the proposed PD controller. The effectiveness of the proposed controller is verified by experimental results.

• Journal of Medicine and Life Science
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• 제17권3호
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• pp.86-93
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• 2020

The study aimed to understand medical students' experiences of emotional changes, including their method of adapting to experiences, and the effect of the experiences in shaping their identities. We interviewed 12 medical students who were finishing their 1-year clinical internship in 2016. Data on their opinions and reasons for emotional changes during their school life were obtained. The descriptive phenomenological approach was applied to analyze the interviews. Their stress came from disappointment in themselves, competitive environment, observing a change in their personalities, meeting their parents' expectations, and interpersonal relations. The interviewees adjusted to the medical study by exercising self-control in their studies and daily lives, by practicing self-acceptance and observing their state of mind, and by breaking free from the competition-driven environment and obsession with grades. In addition, they cultivated endurance and found external support. Finally, they achieved self-efficacy and were comfortable in their identity as medical students. They still had to address the stress from working relationships and the difficulty in balancing studies and life. The medical students' self-evaluation and compulsive tendencies increased during the medical course due to the burden of studies. They evolved by learning self-control and introspection and seeking ways to adapt. Understanding this growth process of medical students will improve student support in medical schools.

• Journal of Power Electronics
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• 제18권5호
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• pp.1595-1607
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• 2018

In this paper, a hierarchical control strategy is introduced to control a new three-port multidirectional DC-DC converter for integrating an energy storage system (ESS) to a bipolar DC microgrid (BPDCMG). The proposed converter provides a voltage-balancing function for the BPDCMG and adjusts the states of charge (SoC) of the ESS. Previous studies tend to balance the voltage of the BPDCMG buses with active sources or by transferring power from one bus to another. Furthermore, the batteries available in BPDCMGs were charged equally by both buses. However, this power sharing method does not guarantee efficient operation of the whole system. In order to achieve a higher efficiency and lower energy losses, a triple-layer hierarchical control strategy, including a primary droop controller, a secondary voltage restoration controller and a tertiary optimization controller are proposed. Thanks to the multi-functional operation of the proposed converter, its conversion stages are reduced. Furthermore, the efficiency and weight of the system are both improved. Therefore, this converter has a significant capability to be used in portable BPDCMGs such as electric DC ships. The converter modes are analyzed and small-signal models of the converter are extracted. Comprehensive simulation studies are carried out and a BPDCMG laboratory setup is implemented in order to validate the effectiveness of the proposed converter and its hierarchical control strategy. Simulation and experimental results show that using the proposed converter mitigates voltage imbalances. As a result, the system efficiency is improved by using the hierarchical optimal power flow control.