• 정보통신설비학회논문지
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• 제10권4호
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• pp.121-127
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• 2011

Many peer-to-peer (p2p) systems have been studied in distributed, ubiquitous computing environments. Distributed hash table (DHT)-based p2p systems can improve load-balancing even though locality utilization and user mobility are not guaranteed. We propose a mobile locality-based hierarchical p2p overlay network (MLH-Net) to address locality problems without any other services. MLH-Net utilizes mobility features in a mobile environment. MLH-Net is constructed as two layers, an upper layer formed with super-nodes and a lower layer formed with normal-nodes. Because super-nodes can share advertisements, we can guarantee physical locality utilization between a requestor and a target during any discovery process. To overcome a node failure, we propose a simple recovery mechanism. The simulation results demonstrate that MLH-Net can decrease discovery routing hops by 15% compared with JXTA and 66% compared with Chord.

• Journal of Power Electronics
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• 제17권1호
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• pp.212-221
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• 2017

Permanent-Magnet Synchronous Generators (PMSGs) are used widely in Wind Power Generation Systems (WPGSs), and the Vienna rectifier was recently proposed to be used as the generator-side converter to rectify the AC output voltage in PMSG-based WPGS. Compared to conventional six-switch two-level PWM (2L-PWM) converters, the Vienna rectifier has several advantages, such as higher efficiency, improved total harmonic distortion, etc. The motivation behind this paper is to verify the performance of direct-driven PMSG wind turbine system based-Vienna rectifier by using a simulated direct-driven PMSG WPGS. In addition, for the purpose of reducing the reactive power loss of PMSGs, this paper proposes an induced voltage sensing scheme which can make the stator current maintain accurate synchronization with the induced voltage. Meanwhile, considering the Neutral-Point Voltage (NPV) variation in the DC-side of the Vienna rectifier, a NPV balancing control strategy is added to the control system. In addition, both the effectiveness of the proposed method and the performance of the direct-driven PMSG based-Vienna rectifier are verified by simulation and experimental results.

• 한국소음진동공학회논문집
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• 제26권7호
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• pp.857-864
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• 2016

In the present work, vibration analysis of high speed rotary bell cup model for paint atomizer application is carried out through numerical simulation. At first, eight bell cup models, considering four different cup shapes and two different cup diameters, are proposed and corresponding dynamic characteristics are investigated. To evaluate the operating stability, critical speed analysis is conducted using Campbell diagram and separation margin between operating speed and critical speed is identified. Unbalance vibration responses are also studied according to operating speed and balancing quality grade of G. Finally, the stability and adequacy of the proposed bell cup models are discussed for field application.

• 디지털산업정보학회논문지
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• 제10권3호
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• pp.35-40
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• 2014

Many researches have been conducted to achieve improvement in distributed system that connects multiple computer systems via communication lines. Among others, the load balancing and file migration are considered to have significant impact on the performance of distributed system. The dynamic file migration algorithm common in distributed processing system involved complex calculations of decision function necessary for file migration and required migration of control messages for the performance of decision function. However, the performance of this decision function puts significant computational strain on computer. As one single network is shared by all computers, more computers connected to network means migration of more control messages from file migration, causing the network to trigger bottleneck in distributed processing system. Therefore, it has become imperative to carry out the research that aims to reduce the number of control messages that will be migrated. In this study, the learning automata was used for file migration which would requires only the file reference-related information to determine whether file migration has been made or determine the time and site of file migration, depending on the file conditions, thus reflecting the status of current system well and eliminating the message transfer and additional calculation overhead for file migration. Moreover, mathematical model for file migration was described in order to verify the proposed model. The results from mathematical model and simulation model suggest that the proposed model is well-suited to the distributed system.

• 한국정밀공학회지
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• 제19권7호
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• pp.43-50
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• 2002

This paper presents the new gait implementation of a biped robot with smooth walking using 3-dimensional continuous trunk motion and kick action of ankle joints. Trajectory generation ova trunk is performed not on a unit gait but on a whole walking interval. In applying kick action such as heel-touch or toe-off, varying coordinate system was employed for the simplification of the kinematic analysis. Desired ZMP (zero moment point) is also changed to implement the efficient kick action. As a result, balancing motion of the proposed gait was much more decreased than that of conventional one. Moreover, robot\\`s walking behavior is very smooth, natural and similar to the pace of a human. The walking experiment system is composed of eight AC servo motors and a DSP controller. The walking simulation and the experimental results are shown using the proposed new walking algorithm.

• 한국ITS학회 논문지
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• 제9권6호
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• pp.129-135
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• 2010

본 논문은 애드 혹 네트워크에서 다중 경로 설정을 위한 탐색 기법을 제안한다. 데이터 경로 설정은 요구기반 방식으로 동작하며, 소스와 목적지 노드 간의 다중 경로는 초기 경로 설정 과정에서 함께 구해진다. 이러한 다중 경로는 데이터 경로의 백업이나 부하 분산에 사용될 수 있다. 제안 방식에서는 소스와 목적지 노드 간 다중 경로를 찾기 위해, 사전 질의 방법을 사용한다. 이 방안은 중간 노드에서 응답 메시지를 처리할 때, 주변 노드와의 정보 교환을 통해 최적의 다음노드를 선택하는 방안이다. 시뮬레이션 결과를 통해, 제안된 다중 경로 체계가 경로간의 만나는 횟수를 감소시켜 다중경로의 더 좋은 분리도를 주고, 노드들이 이동하는 환경에서 더 높은 경로 생존 확률을 달성함을 알 수 있었다.

• 전기학회논문지
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• 제63권12호
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• pp.1640-1648
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• 2014

This paper proposes a switching frequency reduction method for MMC (Modular Multilevel Converter) utilizing redundancy operation of sub-module, which can offer reduction of voltage harmonics and switching loss. The feasibility of proposed method was verified through computer simulations with PSCAD/EMTDC software. Based on simulation analysis, a hardware scaled-model of 10kVA, DC-1000V MMC was designed and manufactured in the lab. Various experiments were conducted to verify the feasibility of proposed method in the actual hardware system. The hardware scaled-model can be effectively utilized for analyzing the performance of MMC according to the modulation scheme and redundancy operation.

• Journal of Power Electronics
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• 제9권2호
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• pp.133-145
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• 2009

In this paper a modular high performance MV-to-LV rectifier based on a cascaded H-bridge rectifier is presented. The proposed rectifier can directly connect to the medium voltage levels and provide a low-voltage and highly-stable DC interface with the consumer applications. The input stage eliminates the necessity for heavy and bulky step-down transformers. It corrects the input power factor and maintains the voltage balance among the individual DC buses. The second stage includes the high frequency parallel-output DC/DC converters which prepares the galvanic isolation, regulates the output voltage, and attenuates the low frequency voltage ripple ($2f_{line}$) generated by the first stage. The parallel-output converters can work in interleaving mode and the active load-current sharing technique is utilized to balance the load power among them. The detailed analysis for modeling and control of the proposed structure is presented. The validity and performance of the proposed topology is verified by simulation and experimental results.

• Journal of Power Electronics
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• 제16권2호
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• pp.654-665
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• 2016

The pumping of electric power from photovoltaic (PV) farms is normally carried out using transformers, which require heavy mounting structures and are thus costly, less efficient, and bulky. Therefore, transformerless schemes are developed for the injection of power into the grid. Compared with the H4 inverter topology, the H6 topology is a better choice for pumping PV power into the grid because of the reduced common mode current. This paper presents how the perturb and observe (P&O) algorithm for maximum power point tracking (MPPT) can be implemented in the H6 inverter topology along with the improved sinusoidal current injected to the grid at unity power factor with the average current mode control technique. On the basis of the P&O MPPT algorithm, a power reference for the present insolation level is first calculated. Maintaining this power reference and referring to the AC sine wave of bus bars, a sinusoidal current at unity power factor is injected to the grid. The proportional integral (PI) controller and fuzzy logic controller (FLC) are designed and implemented. The FLC outperforms the PI controller in terms of conversion efficiency and injected power quality. A simulation in the MATLAB/SIMULINK environment is carried out. An experimental prototype is built to validate the proposed idea. The dynamic and steady-state performances of the FLC controller are found to be better than those of the PI controller. The results are presented in this paper.

• Journal of Power Electronics
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• 제16권5호
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• pp.1743-1751
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• 2016

The increasing demand for high voltage DC grids resulting from the continuous installation of offshore wind farms in the North Sea has led to the concept of multi-terminal direct current (MTDC) grids, which face some challenges. Power (current) flow control is a challenge that must be addressed to realize a reliable operation of MTDC grids. This paper presents a reduced switch count topology of a current flow controller (CFC) for power flow and current limiting applications in MTDC grids. A simple control system based on hysteresis band current control is proposed for the CFC. The theory of operation and control of the CFC are demonstrated. The key features of the proposed controller, including cable current balancing, cable current limiting, and current nulling, are illustrated. An MTDC grid is simulated using MATLAB/SIMULINK software to evaluate the steady state and dynamic performance of the proposed CFC topology. Furthermore, a low power prototype is built for a CFC to experimentally validate its performance using rapid control prototyping. Simulation and experimental studies indicate the fast dynamic response and precise results of the proposed topology. Furthermore, the proposed controller offers a real solution for power flow challenges in MTDC grids.