• 전력전자학회:학술대회논문집
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• 전력전자학회 2014년도 추계학술대회 논문집
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• pp.11-12
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• 2014

This paper introduces a scaled hardware model for the 10kVA, 1kV, 11-level MMC (Modular Multilevel Converter), which was manufactured in the lab based on computer simulations with PSCAD/EMTDC. Various experiments were conducted to verify the major operation algorithms of MMC. The hardware scaled-model developed in the lab can be utilized for analyzing the operation analysis and performance evaluation of MMC according to the modulation pattern and redundancy operation scheme.

• 전력전자학회논문지
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• 제25권1호
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• pp.61-67
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• 2020

This study deals with the modulation index (MI) of a voltage source converter (VSC) HVDC system based on a modular multilevel converter (MMC). In the two-level converter, the purpose of the MI is to maximize the achievable AC voltage of the converter from a fixed DC voltage. Unlike that in a two-level converter, the MI in the MMC topology plays a role in making the converter a voltage source using a capacitor. The circulating current in the MMC distorts the AC voltage reference, and the distortion affects the MI. In addition, the AC network conditions, such as AC voltage variation and reactive power, affect the MI. Therefore, the MI should be optimized with consideration of internal and external factors. This study proposes a method to optimize the MI of an MMC HVDC system.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2013년도 전력전자학술대회 논문집
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• pp.528-529
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• 2013

본 논문에서는 모듈형 멀티레벨 컨버터 (Modular Multilevel Converter ; MMC) HVDC의 각 모듈 평균 스위칭 절환 회수를 고려한 커패시터 전압 평형 알고리즘을 제안한다. 일반적으로 MMC HVDC 시스템에서 각 모듈을 선택적으로 운전하는 것은 모듈간 커패시터 전압 평형 유지가 가능하나, 모듈간 스위칭 절환 수의 불평형이 발생하는 단점이 있다. 본 논문에서는 각 모듈이 스위칭 절환 수의 평형을 유지하면서 직류 전압 평형을 유지하는 알고리즘을 제안하고 시뮬레이션으로 유효성을 검증하였다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2015년도 전력전자학술대회 논문집
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• pp.510-511
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• 2015

MMC(Modular Multilevel Converter)형 STATCOM 개발을 위해서는 시스템을 이루는 핵심 구성 요소인 Valve의 설계 및 신뢰성 검증이 매우 중요하다. Valve란 다수의 Sub-module을 직렬로 연결, 조합한 것이며 시스템의 용량에 따라 모듈의 직렬 개수가 정해진다. 그러므로 Valve의 최적 설계 및 제어가 시스템의 성능을 개선할 수 있는 주요 항목으로 작용한다. 본 논문에서는 이러한 Valve를 제작한 내용과 시험 설비를 통해 수행된 제품의 운전 특성에 대해서 기술하였다.

• 전기전자학회논문지
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• 제23권2호
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• pp.652-658
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• 2019

본 논문은 반도체 변압기용 모듈형 멀티레벨 DC-DC 컨버터의 서브모듈 커패시터 전압 균형 제어 기법을 제안한다. 일반적으로 서브모듈 커패시터 전압은 개별적인 피드백 제어기에 의해 균등하게 제어할 수 있으나 모듈 개수에 따라 연산 시간이 증가한다. 본 논문에서 제안하는 전압 균형 제어 기법은 순시적으로 최대/최소 전압 값의 서브모듈만을 선택 및 제어하여 연산 시간을 저감할 수 있다. 시뮬레이션을 통해 제안하는 방법의 성능 및 타당성에 대한 검증을 진행한다.

• Journal of Power Electronics
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• 제16권5호
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• pp.1698-1705
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• 2016

In this paper, a hardware-in-the-loop simulation platform for MMCs is established, which connects a real time digital simulator (RTDS) and a designed MMC controller with optical fiber. In this platform, the converter valves are simulated with a small time step of 2.5 microsecond in the RTDS, and multicore technology is implemented for the controller so that the parallel valve control is distributed between different cores. Therefore, the designed controller can satisfy the requirements of real-time control. The functions of the designed platform and the rationality for the designed controller are verified through experimental tests. The results show that different modulation modes and various control strategies can be implemented in the simulation platform and that each control objective can been tracked accurately and with a fast dynamic response.

• Journal of Power Electronics
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• 제18권5호
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• pp.1608-1617
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• 2018

The rapid and correct isolation of faulty submodules (SMs) is of great importance for improving the reliability of modular multilevel converters (MMCs). Therefore, a fast diagnosis method containing fault detection and fault location determination was presented in this paper. An improved incremental predictive model of arm current was proposed to detect failures, and the multi-step prediction method was used to eliminate the negative impact of disturbances. Moreover, a control method was proposed to strengthen the fault characteristics to rapidly locate faulty arms and faulty SMs by detecting the variation rate of the SM capacitor voltage. The proposed method can rapidly and easily locate faulty SMs under different load conditions without the need for additional sensors. The experimental results have validated the effectiveness of the proposed method by using a single-phase MMC with four SMs per arm.

• Journal of Power Electronics
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• 제14권6호
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• pp.1100-1108
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• 2014

Modular multilevel converters (MMCs) have emerged as the most promising topology for high and medium voltage applications for the coming years. However, one particular negative characteristic of MMCs is the existence of circulating current, which contains a dc component and a series of low-frequency even-order ac harmonics. If not suppressed, these ac harmonics will distort the arm currents, increase the power loses, and cause higher current stresses on the semiconductor devices. Repetitive control (RC) is well known due to its distinctive capabilities in tracking periodic signals and eliminating periodic errors. In this paper, a novel circulating current control scheme base on RC is proposed to effectively track the dc component and to restrain the low-frequency ac harmonics. The integrating function is inherently embedded in the RC controller. Therefore, the proposed circulating current control only parallels the RC controller with a proportional controller. Thus, conflicts between the RC controller and the traditional proportional integral (PI) controller can be avoided. The design methodologies of the RC controller and a stability analysis are also introduced. The validity of the proposed circulating current control approach has been verified by simulation and experimental results based on a three-phase MMC downscaled prototype.

• Journal of Power Electronics
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• 제14권6호
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• pp.1147-1156
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• 2014

This study proposes a quasi-fixed-frequency hysteresis current tracking control strategy for modular multilevel converters (MMCs) on the basis of voltage partition principle. First, by monitoring the grid voltage and the deviation between the output and reference currents, the output voltage is determined, thus prompting the output current to quickly and efficiently track the given current. Second, the voltages of the upper/lower capacitor of the arm and the voltages between the upper and lower arms are balanced by combining these arms with virtual loop mapping and arm voltage balance control, respectively. In particular, the proposed method is designed for any level and number of sub-modules. The validity of the proposed method is verified by simulations and experimental results of a five-level MMC prototype.

• Journal of Power Electronics
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• 제18권6호
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• pp.1683-1696
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• 2018

Under the conventional control strategy, the asymmetry of arm impedances may result in the poor operating performance of modular multilevel converters (MMCs). For example, fundamental frequency oscillation and double frequency components may occur in the dc and ac sides, respectively; and submodule (SM) capacitor voltages among the arms may not be balanced. This study presents an enhanced control strategy to deal with these problems. A mathematical model of an MMC with asymmetric arm impedance is first established. The causes for the above phenomena are analyzed on the basis of the model. Subsequently, an enhanced current control with five integrated proportional integral resonant regulators is designed to protect the ac and dc terminal behavior of converters from asymmetric arm impedances. Furthermore, an enhanced capacitor voltage control is designed to balance the capacitor voltage among the arms with high efficiency and to decouple the ac side control, dc side control, and capacitor voltage balance control among the arms. The accuracy of the theoretical analysis and the effectiveness of the proposed enhanced control strategy are verified through simulation and experimental results.