• 전력전자학회논문지
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• 제24권6호
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• pp.452-458
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• 2019

This study proposes a new test circuit and control method for the submodules of modular multilevel converter (MMC)-based HVDC systems. The test current of conventional submodule test circuits cannot provide the DC offset components or may have some distortion in the linearized current with the DC offset. The proposed scheme can provide not only the DC component but also linearized current without distortion. Therefore, the submodule test current waveform is relatively similar to that of a real submodule consisting of an MMC-based HVDC system. The validity of the proposed circuit and control method is verified through a simulation and experiment.

• KEPCO Journal on Electric Power and Energy
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• 제4권2호
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• pp.47-53
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• 2018

VSC technology is now well established in HVDC and is, in many respects, complementary to the older Line Commutated Converter (LCC) technology. Despite the various advantages of VSC technology, VSC HVDC stations have higher power losses than LCC stations. Although the relative advantages and disadvantages are well known within the industry, there have been very few attempts to quantify these factors on an objective basis. This paper describes methods to determine the operating losses of every component in the valve of VSC-HVDC system. The losses of the valve, including both conduction losses and switching losses, are treated in detail.

• Journal of Power Electronics
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• 제17권4호
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• pp.942-952
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• 2017

Modular multilevel converters (MMCs) have been receiving extensive research interest in high/medium-voltage applications due to its modularity, scalability, reliability, high-voltage capability, and excellent harmonic performance. Submodule capacitors are usually rather bulky because they have to withstand fundamental frequency voltage fluctuations. To reduce the capacitance of these capacitors, this study proposes a modified MMC with an active power decoupling circuit within each submodule. The modified submodule contains an auxiliary half bridge, with its capacitor split in two. Also, the midpoints of the half bridge and the split capacitors are connected by an inductor. With this modified submodule, the fundamental frequency voltage fluctuation can be suppressed to a great extent. The second-order voltage fluctuation, which is the second most significant component in submodule voltage fluctuations, is removed by the proper control of the second-order circulating current. Consequently, the submodule capacitance is significantly reduced. The viability and effectiveness of the proposed new MMC are confirmed by the simulation and experimental results. The proposed MMC is best suited for medium-voltage applications where power density is given a high priority.

• Journal of Power Electronics
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• 제9권2호
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• pp.133-145
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• 2009

In this paper a modular high performance MV-to-LV rectifier based on a cascaded H-bridge rectifier is presented. The proposed rectifier can directly connect to the medium voltage levels and provide a low-voltage and highly-stable DC interface with the consumer applications. The input stage eliminates the necessity for heavy and bulky step-down transformers. It corrects the input power factor and maintains the voltage balance among the individual DC buses. The second stage includes the high frequency parallel-output DC/DC converters which prepares the galvanic isolation, regulates the output voltage, and attenuates the low frequency voltage ripple ($2f_{line}$) generated by the first stage. The parallel-output converters can work in interleaving mode and the active load-current sharing technique is utilized to balance the load power among them. The detailed analysis for modeling and control of the proposed structure is presented. The validity and performance of the proposed topology is verified by simulation and experimental results.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2016년도 전력전자학술대회 논문집
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• pp.385-386
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• 2016

직접 변조 방식(direct modulation)을 사용한 Modular Multilevel Converter(MMC)의 6개의 암 캐패시터 전압의 자연균형화(Natural Balancing) 현상은 여러 연구 및 실험에서 관측되고 있다. 또한, 이러한 현상의 원리뿐만 아니라, 시스템 파라미터와 밸런싱 동특성사이의 관계가 수학적인 분석을 통하여 검증되었다. 직접 변조 방식은 간접 변조 방식(indirect modulation)과 달리, 입/출력 AC 및 DC 측 제어기 외에 암 캐패시터 전압 밸런싱을 위한 순환전류 제어기가 존재하지 않지만, 캐패시터 맥동 저감을 위해 일반적으로 캐패시터 2 고조파 맥동 제어기를 필요로 한다. 본 논문에서는 이러한 직접 변조방식을 사용한 MMC의 2고조파 전압 맥동 제어기가 암 캐패시터 전압 자연 밸런싱에 미치는 영향을 고찰하고 분석한다. 또한, 2 고조파 전압 맥동 제어기의 유무에 따른 전압 자연 밸런싱의 동특성에 대한 비교를 400MW MMC full-scale 시뮬레이션을 통해 살펴보도록 한다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2017년도 추계학술대회
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• pp.31-32
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• 2017

모듈형 멀티레벨 컨버터(Modular Multilevel Converter, MMC) 토플로지는 HVDC 송전을 위한 전력변환 시스템 또는 전기추진 선박과 같은 대용량 시스템에 주로 사용된다. 본 논문은 MMC 시스템의 출력 전류 센서에서 전류 검출 과정 시, 발생 가능한 오차 성분의 영향을 분석하였다. 또한 전류 센서의 오차 성분을 일반화 하고, 이를 시뮬레이션을 통해 타당성을 검증하였다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2017년도 추계학술대회
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• pp.113-114
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• 2017

본 논문은 모듈형 멀티레벨 컨버터를 이용한 특고압 직류 배전용 지능형 변압기의 운용을 위한 제어 기법을 제안한다. 기존의 저주파 변압기는 부피와 무게가 매우 크며 효율이 낮다는 단점을 갖는다. 이에 반해 다수의 IGBT를 사용하는 지능형 변압기는 시스템의 부피를 저감할 수 있으며 고효율 운영이 가능하다. 또한, 직류 배전 계통의 단락 사고가 발생하였을 때 직류 차단기의 역할이 가능하다. 본 논문에서는 지능형 변압기의 토폴로지, 변조 방법을 제시하고, 스위칭 및 출력 특성을 분석한다. 시뮬레이션을 통해 제안하는 지능형 변압기 운용 제어 기법의 타당성을 확인한다.

• Journal of Power Electronics
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• 제15권6호
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• pp.1499-1507
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• 2015

High voltage direct current transmission based on modular multilevel converter (MMC-HVDC) is one of the most promising power transmission technologies. In this study, the mathematical characteristics of MMC-HVDC are analyzed in a synchronous rotational reference frame. A hybrid current vector controller based on proportional integer plus resonant is used to uniformly control the DC and double-base frequency AC currents under unbalanced grid voltage conditions. A corresponding voltage dependent current order limiter is then designed to solve the overcurrent problems that may occur. Moreover, the circulating current sequence components are thoroughly examined and controlled using a developed circulating current suppressor. Simulation results verify the correctness and effectiveness of the proposed control schemes.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2015년도 제46회 하계학술대회
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• pp.996-997
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• 2015

This paper proposes parameter design principle of the sub-module capacitance, Arm inductance and a control method to reduced the circulating currents in modular multilevel converter(MMC) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence, zero-sequence and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider these all components. This study proposes the control method to reduced the circulating currents under the unbalanced voltage.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2018년도 전력전자학술대회
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• pp.456-457
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• 2018

본 논문에서는 MMC(Modular Multilevel Converter)용 Valve 시험회로의 설계를 위한 커패시터 값 선정에 대한 분석을 하고자 한다. 기존의 HVDC MMC용 Valve 시험회로는 전류를 선형적으로 증가시키고 감소시키기 때문에 적절한 전압과 전류를 공급할 커패시터와 인덕터의 설계가 중요하다. Valve 시험회로의 동작을 분석하고, 최적의 커패시터와 인덕터를 선정하여 각 서브모듈의 전압과 입력 전류의 오차를 낮추었으며 시뮬레이션을 통해 타당성을 검증하였다.