• 전력전자학회논문지
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• 제20권3호
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• pp.273-279
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• 2015

A zero vector selection method to reduce switching losses for model predictive control (MPC) of voltage source inverter is proposed. A conventional MPC of voltage source inverter has not been proposed, and a method to select the redundancy of the zero vector is required for this study. In this paper, the redundancy of the zero vectors is selected with generating a zero sequence voltage to reduce switching losses. The zero vector of 2-level inverter is determined by determining sign of the zero sequence voltage. In the proposed method, the quality of the current is retained and switching loss can be reduced compared with the conventional method. This result was verified by P-sim simulation and experiments.

• 전력전자학회논문지
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• 제11권2호
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• pp.172-183
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• 2006

본 논문은 Akagi에 의해 발표된 p-q 이론과 Nabae 및 Peng에 의하여 발표된 크로스벡터 이론의 대비를 통하여 3상 4선 계통에서 순시전력보상방법을 분석한다. 3상 3선 계통에서 영상성분 전압이 없는 경우 두 이론은 똑같다. 그렇지만 3상 4선 계통에서 영상성분 전압과 전류가 포함되는 경우, 두 이론은 순시무효전력과 순시유효전력을 달리 정의한다. 본 논문에서는 이론적 분석과 컴퓨터 시뮬레이션을 바탕으로 영상성분 전압과 영상성분 전류가 존재하는 3상 4선 계통에서 에너지 저장요소의 필요 없이 무효전력만의 보상을 통하여 계통의 중성선 전류를 완벽히 제거할 수 있는 순시전력보상방법을 제시한다.

• 전력전자학회논문지
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• 제7권6호
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• pp.596-603
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• 2002

본 논문에서는 영상고조파 주입법을 사용하여 3상 다이오드 정류기의 입력전류를 개선하는 새로운 방식의 고조파 저감기술을 제안한다. 3고조파 주입을 근간으로 하는 본 방식은 다이오드 정류기의 상측과 하측의 전류를 독립적으로 제어하기 위하여 두 대의 단상 반파 인버터와 두 대의 단상 변압기로 구성되어져 있다. 이렇게 만들어진 영상전류는 zigzag 변압기에 의해서 다이오드 정류기의 상측과 하측으로 주입이 되어진다. 이 결과 3상 다이오드 정류기의 입력전류는 순수한 정현파 전류가 된다. 본 방식의 타당성을 1.5kVA 시작품의 실험을 통하여 입증하였다.

• 전기학회논문지
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• 제63권10호
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• pp.1364-1370
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• 2014

Power system fault analysis has been based on symmetrical component method, which describes power system elements by positive, negative and zero sequence impedance. Obtaining accurate line impedances as possible are very important for estimating fault current magnitude and setting distance relay accurately. Especially, accurate calculation of zero sequence impedance is important because most of transmission line faults are line-to-ground faults, not balanced three-phase fault. Since KEPCO has started measuring of transmission line impedance at 2005, it has been revealed that the measured and calculated line impedances are well agreed within reasonable accuracy. In case of underground transmission lines, however, large discrepancies in zero sequence impedance were observed occasionally. Since zero sequence impedance is an important input data for distance relay to locate faulted point correctly, it is urgently required to analyze, detect and consider countermeasures to the source of these discrepancies. In this paper, development of pre/post processor to ATP (Alternative Transient Program) version of EMTP (Electro-Magnetic Transient Program) for sequence impedance calculation was described. With the developed processor ATP-cable, effects of ground resistance and ECC (Earth Continuity Conductor) on sequence impedance were analyzed.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 추계학술대회 논문집 전력기술부문
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• pp.124-126
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• 2001

A reliable fault type identification (phase selection) plays a very important role in transmission line protection, particularly in Extra High Voltage(EHV) networks. The conventional fault type identification algorithm used the phase difference between positive and negative sequence current excluding load current. But, it is difficult to pick out only fault current since we can not know when a fault occurs and identify the fault type in weak-infeed conditions that dominate zero-sequence current in phase current. The proposed algorithm can identify the accurately fault type using the sum of unit vectors which are calculated by positive-sequence votage and negative-sequence voltage.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2015년도 전력전자학술대회 논문집
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• pp.177-178
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• 2015

This paper presents a neutral point deviation compensating control algorithm applied to a 3-level NPC converter. The neutral point deviation is analyzed with a focus on the current flowing out of or into the neutral point of the dc link. Based on the zero sequence components of the reference voltages, this paper analyzes the neutral point deviation and balancing control for 3-level NPC converter. An analytical method is proposed to calculate the injected zero sequence voltage for NP balancing based on average neutral current. This paper also proposes a control scheme compensating for the neutral point deviation under generalized unbalanced grid operating conditions. The positive and negative sequence components of the pole voltages and ac input currents are employed to accurately explain the behavior of 3-level NPC converter. Simulation and experimental results for a test set up of 30kW are shown to verify the validity of the proposed algorithm.

• Journal of Power Electronics
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• 제18권1호
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• pp.70-80
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• 2018

A novel strategy based on a zero common mode voltage pulse-width modulation (ZCMV-PWM) technique and zero-sequence circulating current (ZSCC) feedback control is proposed in this study to eliminate ZSCCs between three-level neutral point clamped (NPC) voltage source inverters, with common AC and DC buses, that are operating in parallel. First, an equivalent model of ZSCC in a three-phase three-level NPC inverter paralleled system is developed. Second, on the basis of the analysis of the excitation source of ZSCCs, i.e., the difference in common mode voltages (CMVs) between paralleled inverters, the ZCMV-PWM method is presented to reduce CMVs, and a simple electric circuit is adopted to control ZSCCs and neutral point potential. Finally, simulation and experiment are conducted to illustrate effectiveness of the proposed strategy. Results show that ZSCCs between paralleled inverters can be eliminated effectively under steady and dynamic states. Moreover, the proposed strategy exhibits the advantage of not requiring carrier synchronization. It can be utilized in inverters with different types of filter.

• Journal of Electrical Engineering and Technology
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• 제13권6호
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• pp.2319-2328
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• 2018

This paper introduces a control method for a transformerless MMC-HVDC system. The proposed method can effectively control the grid currents of the MMC-HVDC system under unbalanced grid conditions such as a single line-to-ground fault. The proposed method controls the currents of the positive sequence component and the negative sequence component without separating algorithms. Therefore, complicated calculations for extracting the positive sequence and the negative sequence component are not required. In addition, a control method to regulate a zero sequence component current under unbalanced grid conditions in the transformerless MMC-HVDC system is also proposed. The validity of the proposed method is verified through PSCAD/EMTDC simulation.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2012년도 전력전자학술대회 논문집
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• pp.273-274
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• 2012

This paper investigates control algorithms for a doubly fed induction generator(DFIG) with a back-to-back three-level neutral-point clamped voltage source converter in medium voltage wind power system under unbalanced grid conditions. Two different control algorithms to compensate for unbalanced conditions are proposed. Evaluation factors of control algorithm are fault ride-through(FRT) capability, efficiency, harmonic distortions and torque pulsation. Zero regulated negative sequence stator current control algorithm has the most effective performance concerning FRT capability and efficiency. Ripple-free control algorithm nullifies oscillation component of active power and reactive power. Ripple-free control algorithm shows the least harmonic distortions and torque pulsation. Combination of zero regulated negative sequence stator current and ripple-free control algorithm control algorithm depending on the operating requirements and depth of grid unbalance presents the most optimized performance factors under the generalized unbalanced operating conditions leading to high performance DFIG wind turbine system.

• 전기학회논문지
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• 제67권12호
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• pp.1705-1710
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• 2018

This paper deals with an analysis of the causes of over current relay(OCR) misoperation in power system with distributed generations(DG). In general, Y-D and Y-Y-D transformer connections are used for grid interconnection of DG. According to the interconnection guideline, the neutral point on Y side should be grounded. However, these transformer connections can lead to OCR misoperation as well as over current ground relay(OCGR) misoperation. Several researches have addressed the OCGR misoperation due to the interaction between transformer connections and zero-sequence voltage of distribution system. Recently, a misoperation of OCR at the point of DG interconnection to the utility system has been also reported. With increasing the interconnections of DG, such OCR as well as OCGR misoperations are expected to increase. In this paper, PSCAD/EMTDC modeling including DG interconnection transformer was performed and various case studies was carried out for identifying the cause of OCR misoperation.