• Journal of Power Electronics
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• v.19 no.2
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• pp.529-539
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• 2019

The modular multilevel converter (MMC) has become a promising topology for high-voltage direct current (HVDC) transmission systems. To control a MMC system properly, the ac-side current, circulating current and submodule (SM) capacitor voltage are taken into consideration. This paper proposes a low-computation indirect model predictive control (IMPC) strategy that takes advantages of the conventional MPC and has no weighting factors. The cost function and duty cycle are introduced to minimize the tracking error of the ac-side current and to eliminate the circulating current. An optimized merge sort (OMS) algorithm is applied to keep the SM capacitor voltages balanced. The proposed IMPC strategy effectively reduces the controller complexity and computational burden. In this paper, a discrete-time mathematical model of a MMC system is developed and the duty ratio of switching state is designed. In addition, a simulation of an eleven-level MMC system based on MATLAB/Simulink and a five-level experimental setup are built to evaluate the feasibility and performance of the proposed low-computation IMPC strategy.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.1214-1215
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• 2011

현재 HVDC 및 FACTS와 같은 전력전자소자 기반 전력기기의 최신 개발은 능동형 스위치소자의 발전 및 설치 환경의 변화로 인하여 VSC Topology로 이동되고 있다. 하지만 VSC Toplolgy의 스위칭 손실등의 문제로 대용량 전력기기의 경우 MMC(Modular Multilevel Converter) 적용이 가속화되고 있다. 이에 본 논문에서는 MMC Topology의 특징, 구동 원리, 시뮬레이션 과정을 소개한다. Modulation 및 Quantization 그리고 Sorting Algorithm을 통해 7-level, 151-level 삼상 MMC Topology을 시뮬레이션을 수행하였다.

• Journal of Power Electronics
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• v.19 no.6
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• pp.1486-1495
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• 2019

Dealing with the DC link fault poses a technical problem for an HVDC based on a modular multilevel converter. The fault suppressing mechanisms of several sub-module topologies with DC fault current blocking capacity are examined in this paper. An improved half-bridge sub-module topology with double direction control switch is also designed to address the additional power consumption problem, and a sub-module topology called hybrid double direction blocking sub module (HDDBSM) is proposed. The DC fault suppression characteristics and sub-module capacitor voltage balance problem is also analyzed, and a self-startup method is designed according to the number of capacitors. The simulation model in PSCAD/EMTDC is built to verify the self-startup process and the DC link fault suppression features.

• 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.

• 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.

• KEPCO Journal on Electric Power and Energy
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• v.3 no.1
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• pp.17-21
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• 2017

Recently, the Korea Enhanced Power system Simulator (KEPS) was upgraded with new processor cards and various peripheral devices. The purpose of the upgrade is for the large-scale AC/DC simulation studies for different purposes such as FACTS, PMU applications, and SPS. The need for such study is now growing in KEPCO network. Frequently, traditional ways of performing the necessary study had been found to be less adequate for guiding the key decisions in the company. Therefore, the growing needs, as well as the revealed inadequacy of the traditional system studies, had been attributing to the momentum for the upgrade project. This paper explains the details of the upgrade project. As an example of presenting the effectiveness of the upgraded RTDS system in KEPRI-KEPCO, a large-scale AC/DC real time simulation case which includes the entire Korean network and a planned MMC-HVDC system is introduced.

• Journal of the Korean Society of Industry Convergence
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• v.22 no.3
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• pp.365-371
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• 2019

Recently, the demonstration and research on the power transmission using high voltage DC such as HVDC(High Voltage DC), Smart Grid, DC transmission and distribution have been actively conducted. In order to control the power converter in high-voltage DC power transmission system, SMPS(Switching Modulation Power Supply) for power converter control using high-voltage DC input is essential. However, the demand for high-pressure SMPS is still low, so the development is not enough. In the low-output SMPS using the high-voltage input, it is difficult to achieve high efficiency due to the switching transient loss especially at light load. In this paper, we propose a new switching scheme for high power SMPS control for low output power. The proposed method can provide better efficiency increase effect in the light load region compared to the existing PWM method. To verify the feasibility of the proposed method, a 40 W SMPS for HVDC MMC(Modulation Multi-level Converter) was designed and verified by simulation.