• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.10
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• pp.1655-1663
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• 2016

Recently the researches on Modular Multi-level Converter (MMC) are being highlighted because high quality and efficient power transmission are key issues in the High Voltage Direct Current (HVDC) transmission system. This paper proposes an improved pre-charging method for the sub-module capacitors in MMC that operates in Nearest Level Control (NLC) modulation. The proposed method does not require additional circuits or Pulse Width Modulation (PWM) techniques. The feasibility of proposed method was verified through computer simulations for a scaled 3-phase 10kVA MMC with 12 sub-modules per each arm. Hardware experiments with a scaled prototype were performed in the lab to confirm the simulation results.

• Journal of Power Electronics
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• v.18 no.2
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• pp.456-466
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• 2018

This paper introduces the development of a de-icer based on a full-bridge modular multilevel converter (FMMC). The FMMC can generate a wide range of DC voltages owing to its modularity, scalability, and redundancy, which makes it suitable for ice-melting applications. First, operating principles and voltage ranges are analyzed when FMMC is applied as a mobile de-icer. Second, two new startup strategies, constant modulation index and constant power startup strategies, are proposed. Third, the main control strategies of the de-icer are proposed. Fourth, a novel rated-current zero-power test scheme is proposed to simplify test conditions. Finally, a 10 kV 1.5 kA mobile MMC de-icer is designed and built, and experiments are carried out to validate the proposed startup, control strategies, and rated-current zero-power test scheme.

• Journal of Power Electronics
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• v.17 no.1
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• pp.96-105
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• 2017

In this paper, a hybrid modular multilevel converter (MMC) topology with an improved nearest level modulation method is proposed for medium-voltage high-power applications. The arm of the proposed topology contains N series connected half-bridge submodules (HBSMs), one full-bridge submodule (FBSM) and an inductor. By exploiting the FBSM, half-level voltages are obtained in the arm voltages. Therefore, an output voltage with a 2N+1 level number can be generated. Moreover, the total level number of the inserted submodules (SMs) is a constant. Thus, there is no pulse voltage across the arm inductors, and the SM capacitor voltage is rated. With the proposed voltage balancing method, the capacitor voltage of the HBSM is twice the voltage of the FBSM, and each IGBT of the FBSM has a relatively low switching frequency and an equalized conduction loss. The capacitor voltage balancing methods of the two kinds of SMs are implemented independently. As a result, the switching frequency of the HBSM is not increased compared to the conventional MMC. In addition, according to a theoretical calculation of the total harmonic distortion of the electromotive force (EMF), the voltage quality with the presented method can be significantly enhanced when the SM number is relatively small. Simulation and experimental results obtained with a MMC-based inverter verify the validity of the developed method.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.12
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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.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.4
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• pp.285-296
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• 2017

This paper proposes an improved modulation scheme for the medium voltage modular multi-level converter (MMC), which operates in the nearest level control and applies in the medium voltage direct current (MVDC) system. In the proposed modulation scheme, the offset (neutral-to-zero output) voltage is adjusted, with the phase voltage magnitude, thereby maintaining a constant value with N+1 level in the controllable modulation index (MI) range. In order to confirm the proposed scheme's validity, computer simulations for the 22.9 kV - 25 MVA MMC were performed with PSCAD/EMTDC, as well as hardware experiments for the 380 V - 10 kVA MMC. The proposed modulation scheme offers to build a constant pole voltage regardless of the MI value, and to build a phase voltage with improved total harmonic distortion (THD).

• Journal of Power Electronics
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• v.16 no.6
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• pp.2294-2305
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• 2016

The introduction of a high-voltage direct-current (HVDC) system based on a modular multilevel converter (MMC) for wind farm integration has stimulated studies on methods to control this type of converter. This research article focuses on the control of the AC voltage and circulating current for a wind-farm-side MMC (WFS-MMC). After theoretical analysis, emotional learning (EL) controllers are proposed for the controls. The EL controllers are derived from the learning mechanisms of the amygdala and orbitofrontal cortex which make the WFS-MMC insensitive to variance in system parameters, power change, and fault in the grid. The d-axis and q-axis currents are respectively considered for the d-axis and q-axis voltage controls to improve the performance of AC voltage control. The practicability of the proposed control is verified under various conditions with a point-to-point MMC-HVDC system. Simulation results show that the proposed method is superior to the traditional proportional-integral controller.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.6
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• pp.476-483
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• 2017

In this paper, a voltage-based model predictive control (MPC) scheme for a modular multilevel converter is used to reduce switching loss. The proposed method calculates an offset voltage that clamps the switching operation of submodules in which the current greatly flows at every sampling period by using the reference phase voltage and the reference phase current. To use the offset voltage, the proposed method converts the current-based MPC to the voltage-based MPC. The proposed voltage-based MPC then generates a new reference pole voltage that clamps the switching of submodules by applying the calculated offset voltage to the phase voltage. Therefore, the proposed method can reduce the switching loss by stopping the switching operation of submodules in which the current greatly flows. The switching loss reduction effect of the proposed method is verified by comparing its loss data with those of the conventional MPC method.

• Proceedings of the KIPE Conference
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• 2017.07a
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• pp.282-283
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• 2017

본 논문은 (주)효성 중공업연구소에서 국책과제를 통해 자체 개발한 MMC(Modular Multi-level Converter) type ${\pm}12kV$ 25MVA HVDC 시스템에 대하여 실증단지 구축 및 commissioning test에 대한 소개를 한다. 제주에 구축된 HVDC 실증단지는 국내 유일의 MMC type 전압형 HVDC 시스템이며 11-레벨의 AC 출력 전압을 형성하는 2개의 컨버터가 Back-to-Back 형태로 구성되어 있다. 각 컨버터의 AC 출력단은 각각 계통과 풍력발전단지에 연계되어 풍력발전단지에서 생산된 전력을 계통으로 전송하는 역할을 한다. 주요 AC/DC Yard 기기들로는 AC 고조파 필터, 밸브리액터, DC 리액터, 변압기, CGIS등이 있다. HVDC 의 commissioning test를 위해 IEEE, IEC, Cigre TB 등의 문서들을 종합 분석하여 당사의 시스템에 알맞은 시험절차서와 검증항목을 준비하였다. 본 논문에서는 당사에서 준비한 시험절차서 및 검증항목에 대하여 소개하고자 한다.

• Proceedings of the KIPE Conference
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• 2013.07a
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• pp.151-152
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• 2013

Multilevel Voltage Source Converter HVDC presents a new method for the energy transmission, The multilevel requires the converter bridge includes more than 100 converter modules, for the EMT simulation file, the simulation time increases to several hours; it can depress the research process. In the paper, author describes a method to simulate the multilevel arm based on time-varying equivalent. The result shows the extremely calculation time reduced with almost the same accuracy.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.289-292
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• 2007

The modular line-connected photovoltaic PCS (power conditioning system) is proposed. The proposed system consists of a step-up DC-DC converter and a full-bridge inverter. A step-up DC-DC converter using a dual series-resonant rectifier circuit and a active-clamp circuit is proposed to achieve a high efficiency and a high input-output voltage ratio efficiently. An IncCond (incremental conductance) MPPT (maximum power point tracking) algorithm that improves MPPT characteristic is used. By control a inverter using a linearized output current controller, a unity power factor is achieved. All algorithms and controllers are implemented on a single-chip microcontroller and the superiority of the proposed algorithms and controllers is proved by experiments.