• 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).

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.55 no.6
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• pp.299-305
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• 2006

In this paper, we suggest a new space vector PWM modulation method with very short processing time which does not need identification of nearest three vectors(NTV) and duty ratio for each vector. The suggested PWM method makes mean value of phase voltage to be same as reference during every modulation period by use of a triangle in small hexagon on multi-level vector space. This paper described the suggested modulation method can be successfully applied to the space vector modulation use of multi-level inverter by computer simulations and experiments.

• Proceedings of the KIPE Conference
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• 2016.07a
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• pp.129-130
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• 2016

본 논문에서는 근사 계단 제어 변조(Nearest Level Control Modulation)로 동작하는 모듈형 멀티레벨 컨버터(Modular Multi level Converter)에서 충전 회로나 반송파(Carrier)없이 초기 충전(Pre-charging)하는 새로운 방식을 제안하였다. 이의 성능을 검증하기 위해 PSCAD/EMTDC 소프트웨어를 통해 암(Arm)당 12개의 서브모듈(Sub-Module)로 구성된 3상 10kVA 모듈형 멀티레벨 컨버터를 구현 및 시뮬레이션을 수행하였다.

• 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.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.

• 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.

• Journal of Power Electronics
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• v.19 no.6
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• pp.1496-1504
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• 2019

This paper proposes a new THD reduction algorithm for modular multilevel converters (MMCs) with offset voltage injection operated in nearest level modulation (NLM). High voltage direct current (HVDC) is actively introduced to the grid connection of offshore wind powers, and this paper deals with a voltage generation technique with an MMC for wind power generation. In the proposed method, third harmonic voltage is added for reducing the THD. The third harmonic voltage is adjusted so that each of the pole voltage magnitudes maintains a constant value with a maximum number of (N+1) levels, where N is the number of sub-modules per arm. By using the proposed method, the THD of the output voltage is mitigated without increasing the switching frequency. In addition, the proposed method has advantageous characteristics such as simple implementation. As a part of this study, this paper compares the THD results of the conventional method and the proposed method with offset voltage injection to reduce the THD. In this paper, simulations have been carried out to verify the effectiveness of the proposed scheme, and the proposed method is implemented by a HILS (Hardware in the Loop Simulation) system. The obtained results show agreement with the simulation results. It is confirmed that the new scheme achieved the maximum level output voltage and improved the THD quality.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2006.06a
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• pp.253-254
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• 2006

this paper suggest a new space vector PWM modulation method with short processing time which does not need idenfication of nearest three vectors(NTV) and duty calculation for each vector. The suggested PWM method makes mean value of phase voltage to be same as reference during every modulation period by use of a triangle in small hexagon on multi-level vector space. This paper described the suggested modulation method can be successfully applied to the space vector modulation use of multi-level inverter by computer simulations and experiments.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.63 no.8
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• pp.1046-1054
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• 2014

In this paper, a hardware prototype for the 10kVA 11-level MMC was built and various experimental works were conducted to verify the operation algorithms of MMC. The hardware prototype was designed using computer simulation with PSCAD/EMTDC software. After manufactured in the lab, the hardware prototype was tested to verify the modulation algorithms to form the output voltage, the balancing algorithm to equalize the sub-module capacitor voltage, and the redundancy operation algorithm to improve the system reliability. The developed hardware prototype can be utilized for analyzing the basic operation and performance improvement of MMC according to the modulation and redundancy operation scheme. It also can be utilize to analyze the basic operational characteristics of HVDC system based on MMC.

• Journal of Power Electronics
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• v.18 no.2
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• pp.418-431
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• 2018

This paper presents a new asymmetrical multilevel inverter with a reduced number of power electronic components. The proposed multilevel inverter is analyzed using two different configurations: i) First Configuration (with a switched diode) and ii) Second Configuration (without a switched diode). The presented topologies are compared with recent multilevel inverter topologies in terms of number of switches, gate driver circuits and blocking voltages. The proposed topologies can be cascaded to generate the maximum number of output voltage levels and they are suitable for high voltage applications. Various power quality issues are addressed for both of the configurations. The proposed 11-level inverter configuration is simulated using MATLAB and it is validated with a laboratory based experimental setup.