• 전력전자학회논문지
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• 제19권6호
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• pp.494-502
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• 2014

This paper presents a control method of the modular multilevel converter - high-voltage direct current (MMC-HVDC) system to regulate grid voltage on the basis of the Jeju Island power system. In this case, the MMC-HVDC system is controlled as a static synchronous compensator (Statcom) to exchange the reactive power with the power grid. The operation of the MMC-HVDC system is verified by using the PSCAD/EMTDC simulation program. The Jeju Island power system is first established on the basis of the parameters and measured data from the real Jeju Island power system. This power system consists of two line-commutated converter - high-voltage direct current (LCC-HVDC) systems, two Statcom systems, wind farms, thermal power plants, transformers, and transmission and distribution lines. The proposed control method is then applied by replacing one LCC-HVDC system with a MMC-HVDC system. Simulation results with and without using the MMC-HVDC system are compared to evaluate the effectiveness of the control method.

• Journal of Power Electronics
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• 제18권6호
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• pp.1659-1669
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• 2018

An improved circulating current suppression control method is proposed in this paper. In the proposed controller, an outer loop of the average capacitor voltage control model is used to balance the sub-module capacitor voltage. Meanwhile, an individual voltage balance controller and an arm voltage balance controller are also used. The DC and harmonic components of the circulating current are separated using a low pass filter. Therefore, a multiple quasi-proportional-resonant (multi-quasi-PR) controller is introduced in the inner loop to eliminate the circulating harmonic current, which mainly contains second-order harmonic but also contains other high-order harmonics. In addition, the parameters of the multi-quasi-PR controller are designed in the discrete domain and an analysis of the stability characteristic is given in this paper. In addition, a simulation model of a three-phase MMC system is built in order to confirm the correctness and superiority of the proposed controller. Finally, experiment results are presented and compared. These results illustrate that the improved control method has good performance in suppressing circulating harmonic current and in balancing the capacitor voltage.

• Journal of Power Electronics
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• 제17권4호
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• pp.933-941
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• 2017

In this paper an improved low frequency selective harmonic elimination-PWM (SHE-PWM) technique for Cascaded H-bridge (CHB) converters is proposed. The proposed method is able to eliminate low order harmonics from the output voltage of the converter for a wide range of modulation indices. To solve SHE-PWM equations, especially for low modulation indices, a modified method is used which employs either the positive or negative voltage polarities of H-bridge cells to increase the freedom degrees of each cell. Freedom degrees of the switching angles are also used to increase the range of available solutions for non-linear SHE equations. The proposed SHE methods can successfully eliminate up to $25^{th}$ harmonic from a 7-level output voltage by using just nine switching transitions or a 150 Hz switching frequency. To confirm the validity of the proposed method, simulation and experimental results have been presented.

• Journal of Electrical Engineering and Technology
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• 제12권2호
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• pp.701-710
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• 2017

This paper presents a five-level PWM inverter using series and parallel connection of voltage sources. The alternative connection is done by an auxiliary circuit consisted of a switch, three diodes, and two batteries. The auxiliary circuit is located between input dc voltage source and H-bridge cell. Thanks to the auxiliary circuit, the proposed inverter synthesizes five-level output voltage in an effective way. Topologically both batteries are charged and discharged in the same rate, so it does not need to apply battery voltage balancing control method. Theoretical analysis of the proposed inverter is verified by computer-aided simulation and experiment based on a prototype of 1kW.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2011년도 추계학술대회
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• pp.185-186
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• 2011

본 논문에서는 최근 직류송전용 컨버터로 많은 관심이 집중되고 있는 모듈형 멀티레벨 컨버터(Modular Multi-level Converter)에서 출력파형의 고조파를 저감하는 모듈레이션 방법과 각 모듈의 직류전압 불평형을 해소하는 알고리즘에 대해 기술하고 있다. 먼저 본 논문에서 임의의 개수로 반브리지 모듈이 주어졌을 때 고조파 레벨이 최소화되도록 다펄스 형태로 출력파형을 형성하는 방식을 제안하고 그 타당성을 PSCAD 소프트웨어를 이용한 시뮬레이션으로 검증하였다. 이 방식은 다펄스 출력파형의 각 계단을 형성하는 모듈의 턴온과 턴오프 시점을 보편화된 수식으로 정하는 방식으로 알고리즘 구현이 매우 용이하다. 또한 각 모듈의 직류전압 불평형을 바로잡는 알고리즘을 제안하고 그 타당성도 시뮬레이션으로 검증하였는데, 이 방식은 각 모듈이 교류 매 반주기마다 생성하는 펄스의 크기를 순차적으로 형성하는 것으로 알고리즘의 구현이 용이하다. 본 논문에서 제안하는 알고리즘은 향후 국내에서 모듈형 멀티레벨 컨버터를 개발할 때 유용하게 활용될 것으로 보인다.

• Tribology and Lubricants
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• 제25권5호
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• pp.335-341
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• 2009

Elastohydrodynamic lubrication (EHL) analysis shows that film thickness is very flat in the contact area and pressure distribution is somehow similar to that of Hertzian contact pressure except the outlet region with pressure spike. These typical patterns of EHL film thickness and pressure are the cases under the steady contact conditions of applied loads and speeds. However, many engineering contacts are rather under the conditions of varying loads and contact speeds, and therefore the predictions for endurance life and performance of machine elements with steady EHL analysis are not suitable in many occasions. This study shows the differences in film thickness formation and pressure distribution between steady and transient contact conditions in several contact cases.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2001년도 Proceedings ICPE 01 2001 International Conference on Power Electronics
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• pp.369-373
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• 2001

Multilevel inverter has attracted great interest in high-voltage high-power field because of its less distorted output. In this paper, a direct torque control (DTC) technique based on a three-level neutral-point-clamped (NPC) inverter is presented. In order to solve the intrinsic neutral-point voltage-balancing problem and to obtain a high performance DTC, a special vector selection method is introduced and the concept of virtual vector is developed. By using the proposed PWM strategy, a MRAS speed sensor-less DTC drive can be achieved without sensing the neutral-point voltage, The strategy can be verified by simulation and experimental results.

• 한국지능시스템학회:학술대회논문집
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• 한국퍼지및지능시스템학회 2007년도 춘계학술대회 학술발표 논문집 제17권 제1호
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• pp.283-286
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• 2007

임계값을 이용한 영상 분할은 대표적인 영상 분할 기법으로 Otsu의 임계값 결정법, Fuzzy 엔트로피를 이용한 H&W의 기법 및 Clustering을 이용한 Kwon의 기법 등 많은 방법이 있다. 대부분의 임계값 결정 기법은 영상에서 얻어진 빈도수 히스토그램의 분석을 통해서 임계값을 결정한다. 특히 Otsu의 임계값 결정 기법은 빈도수 히스토그램의 분산을 최대화하는 방법으로 임계값을 결정하는 빈도수 히스토그램에 기반한 대표적 기법이다. 하지만 영상 기술이 발전함에 따라서 하나의 임계값으로부터 영상을 이진화 하는 기법은 효용성이 떨어지고 있다. 따라서 다중의 임계값을 결정하는 효과적인 방법이 필요하다. 본 논문에서는 그레이 레벨간의 관계성을 파악하고 이러한 관계성으로부터 다중의 임계값을 결정하는 기법을 제안한다. 제안된 기법의 효용성은 모의실험에서 다중 임계값을 사용한 분할영상을 통해서 보인다.

• 전기학회논문지
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• 제67권2호
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• pp.209-215
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• 2018

This paper present a 7-level PWM inverter adopting voltage balancing control to series-connected input capacitors. The prior proposed 7-level PWM inverter consists of dc input source, three series-connected capacitors, two bidirectional switch modules, and an H-bridge. This circuit topology is useful to increase the number of output voltage levels, however it fails to generate 7-level in output voltage without consideration for voltage balancing among series-connected capacitors. Capacitor voltage imbalance is caused on the different period between charging and discharging of capacitor. To solve this problem, we uses the amplitude modulation of carrier wave, which is used to produce the center output voltage level. To verify the validity of the proposed control method, we carried out computer-aided simulation and experiments using a prototype.

• 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.