• 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.

• 정보처리학회논문지:소프트웨어 및 데이터공학
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• 제11권7호
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• pp.299-306
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• 2022

최근 로봇이나 설비, 회로 등에 센서 내장이 보편화 되고, 측정된 센서 데이터를 학습하여 기기의 고장을 진단하기 위한 연구가 활발하게 수행되고 있다. 이러한 고장 진단 연구는 고장 상황이나 종류를 예측하기 위한 분류(Classification) 모델 개발과 정량적으로 고장 상황을 예측하기 위한 회귀(Regression) 모델 개발로 구분된다. 분류 모델의 경우, 단순히 고장이나 결함의 유무(Class)를 확인하는 반면, 회귀 모델은 무수히 많은 수치 중에 하나의 값(Value)을 예측해야 하므로 학습 난이도가 더 높다. 즉, 입력과 출력을 대응시켜 고장을 예측을 할 때, 유사한 입력값이 동일한 출력을 낸다고 결정하기 어려운 불규칙한 상황이 다수 존재하기 때문이다. 따라서 본 논문에서는 주기성을 지닌 입출력 데이터에 초점을 맞추어, 입출력 관계를 분석하고, 슬라이딩 윈도우 기반으로 입력 데이터를 패턴화 하여 입출력 데이터 간의 규칙성을 확보하도록 한다. 제안하는 방법을 적용하기 위해, 본 연구에서는 MMC(Modular Multilevel Converter) 회로 시스템으로부터 주기성을 지닌 전류, 온도 데이터를 수집하여 ANN을 이용하여 학습을 진행하였다. 실험 결과, 한 주기의 2% 이상의 윈도우를 적용하였을 때, 적합도 97% 이상의 성능이 확보될 수 있음을 확인하였다.

• 에너지공학
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• 제14권1호
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• pp.37-45
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• 2005

연료 전지 시스템은 아주 유용한 에너지원 중의 하나이다. 시스템은 재사용이 가능하고 환경 친화적인 에너지원이라는 장점을 갖는다. 연료 전지로부터 교류 성분을 얻기 위해서는 인버터가 필요하다. 멀티 레벨 컨버터는 고 전력 연료 전지 시스템에 대한 인버터로서 사용된다. 고조파 분석을 통하여 멀티 레벨컨버터에서 연료 전지의 전압 강하가 증가할 때 기본파 성분은 감소하는 반면 고조파 성분과 왜형률이 증가하는 것을 알 수 있다. 전압 강하 문제를 해결하기 위해서 연료 전지 출력단에 부스트 컨버터 사용, 펄스폭 제어, 울트라커패시터 사용 등 세 가지 서로 다른 해결 방법을 이 논문에서 제안하였다. 제안된 세 가지 해결 방법을 분석하였고 시뮬레이션 결과를 사용하여 비교 분석하였다.

• Journal of Electrical Engineering and Technology
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• 제9권5호
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• pp.1471-1481
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• 2014

This paper presents the new power dissipation model of individual switching device in a high-level modular multilevel converter (MMC), which can be mostly used in voltage sourced converter (VSC) based high-voltage direct current (HVDC) system and flexible AC transmission system (FACTS). Also, the voltage balancing method based on sorting algorithm is newly proposed to advance the MMC functionalities by effectively adjusting switching variations of the sub-module (SM). The proposed power dissipation model does not fully calculate the average power dissipation for numerous switching devices in an arm module. Instead, it estimates the power dissipation of every switching element based on the inherent operational principle of SM in MMC. In other words, the power dissipation is computed in every single switching event by using the polynomial curve fitting model with minimum computational efforts and high accuracy, which are required to manage the large number of SMs. After estimating the value of power dissipation, the thermal condition of every switching element is considered in the case of external disturbance. Then, the arm modeling for high-level MMC and its control scheme is implemented with the electromagnetic transient simulation program. Finally, the case study for applying to the MMC based HVDC system is carried out to select the appropriate insulated-gate bipolar transistor (IGBT) module in a steady-state, as well as to estimate the proper thermal condition of every switching element in a transient state.

• 전력전자학회논문지
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• 제22권1호
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• pp.82-88
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• 2017

This study proposes a new topology for the performance test of a valve consisting of a modular multilevel converter (MMC)-based static synchronous compensator (STATCOM). The conventional valve performance test equipment requires high-voltage AC source of several kV rating because the number of submodules to be tested in a valve should be at least six or eight. However, the power source of the proposed scheme is DC and not AC source. The DC power source voltage range of the proposed test circuit is from several volts to several tens of volts. Therefore, the size and cost for the performance test equipment can be reduced considerably compared with the conventional method. The proposed scheme satisfies the requirements of the IEC 62927 standard. Simulations are conducted for a valve of 50[MVA] MMC-based STATCOM. Experimental results with a scale-downed setup show the validity of the proposed performance test topology.

• 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.

• Journal of Power Electronics
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• 제12권2호
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• pp.344-356
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• 2012

In recent years, multilevel technology has become an effective and practical solution in the field of moderate and high voltage applications. This paper discusses an APF with a three-level NPC inverter. Obviously, the application of such converter to APFs is hindered by the problem of the voltage unbalance of DC capacitors, which leads to system instability. This paper comprehensively analyzes the theoretical limitations of the neutral-point voltage balancing problem for tracking different harmonic currents utilizing current switching functions from the space vector PWM (SVPWM) point of view. The fluctuation of the neutral point caused by the load currents of certain order harmonic frequency is reported and quantified. Furthermore, this paper presents a close-loop digital control algorithm of the DC voltage for this APF. A PI controller regulates the DC voltage in the outer-loop controller. In the current-loop controller, this paper proposes a simple neutral-point voltage control method. The neutral-point voltage imbalance is restrained by selecting small vectors that will move the neutral-point voltage in the direction opposite the direction of the unbalance. The experiment results illustrate that the performance of the proposed approach is satisfactory.

• 한국산업융합학회 논문집
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• 제23권1호
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• pp.65-71
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• 2020

Recently, interest in the DC transmission is rapidly increasing worldwide. In many countries and leading companies are prior to the aggressive development of HVDC technology and application. Especially, VSC-HVDC system has been widely applied to transfer power at long distance between power plant and power consumption area. Therefore in this paper, we analyzed the benefit-cost of VSC-HVDC system which has more advantages than existing transmission system. The proposed system is MMC(Modular Multilevel Converter) VSC-HVDC system that have stability of Power Grid, interconnect Large-scale New Power Generation Plants by prevents Blackout. And MMC VSC-HVDC system Reduced the loss importing foreign systems. And the benefits were calculated in four stages, and the costs were applied to the actual project. By evaluating the various avoidance costs compared to the benefit-cost, it was confirmed that MMC VSC-HVDC system was advantageous in system stability and economic and social benefits.

• Journal of Power Electronics
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• 제18권1호
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• pp.56-69
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• 2018

In this paper, an improved Space Vector Modulation (SVM) based fault tolerant operation on a nine-level Cascaded H-Bridge (CHB) inverter with an additional backup circuit is proposed. Any type of fault in a power converter may result in a power interruption and productivity loss. Three different faults on H-bridge modules in all three phases based on the SVM approach are investigated with diagrams. Any fault in an inverter phase creates an unbalanced output voltage, which can lead to instability in the system. An additional auxiliary unit is connected in series to the three phase cascaded H-bridge circuit. With the help of this and the redundant switching states in SVM, the CHB inverter produces a balanced output with low harmonic distortion. This ensures high DC bus utilization under numerous fault conditions in three phases, which improves the system reliability. Simulation results are presented on three phase nine-level inverter with the automatic fault detection algorithm in the MATLAB/SIMULINK software tool, and experimental results are presented with DSP on five-level inverter to validate the practicality of the proposed SVM fault tolerance strategy on a CHB inverter with an auxiliary circuit.

• Journal of Power Electronics
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• 제18권1호
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• pp.204-211
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• 2018

At present, one of two LCC-HVDC systems is responsible for controlling the grid frequency of the Jeju Island Power System (JIPS). The grid voltage is regulated by using STATCOMs. However, these two objectives can be achieved in one device that is called by a modular multilevel converter-high voltage direct current (MMC-HVDC) system. Therefore, this paper proposes frequency and voltage control strategies for the JIPS based on a MMC-HVDC system. In this case, the ancillary frequency and voltage controllers are implemented into the MMC-HVDC system. The modelling of the JIPS is done based on the parameters and measured data from the real JIPS. The simulation results obtained from the PSCAD/EMTDC simulation program are confirmed by comparing them to measured data from the real JIPS. Then, the effect of the MMC-HVDC system on the JIPS will be tested in many cases of operation when the JIPS operates with and without STATCOMs. The objective is to demonstrate the effectiveness of the proposed control strategy.