• 전력전자학회논문지
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• 제22권3호
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• pp.263-269
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• 2017

This paper proposes a novel circulating current control method for an MMC-HVDC system based on Vector PI control. The method can suppress second-order harmonics of the circulating currents under balanced and unbalanced grid conditions. The proposed method is robust to grid frequency variation. The effectiveness of the proposed method is verified through frequency response and time domain simulation.

• 전기학회논문지
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• 제64권8호
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• pp.1193-1201
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• 2015

This paper presents a study of circulating current of modular multi-level converter (MMC) based a high voltage direct current (HVDC) system under unbalanced grid conditions. Due to the connection of a dependent DC source in each phase, the MMC system inherently generates the power ripple of double-line-frequency components in the AC-side and as a result, the additional sinusoidal current named circulating current flows through the each arm. Reliability improvement of HVDC system under an unbalanced grid condition is one of the important criteria. Generally, the modeling of the circulating current is based on the power relation between DC-side and AC-side. However, the method is not perfectly matched in the MMC system due to the difference of the structural characteristic. In this paper, improved modeling method of circulating current is proposed, which is based on the inner arm power. The proposed method is verified by several simulations to have good agreement of the circulating current components.

• Journal of Power Electronics
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• 제5권2호
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• pp.160-165
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• 2005

In a parallel operation of UPS, there are two types of circulating currents between UPS. One is the low order circulating current with a fundamental frequency caused by the amplitude and phase differences of UPS output voltages, and the other is the harmonic circulating current with PWM switching frequency caused by non-synchronized PWM waveforms among UPS. The elimination of the low order circulating current is essential for optimal load sharing in parallel operations of UPS, which can be accomplished by the phase and magnitude control at each UPS. The harmonic circulating current may cause troubles and deteriorate in performance of the controller for optimal load sharing in parallel operation of UPS. This paper presents a PWM synchronizing method to eliminate the harmonic circulation current in parallel operation of UPS. The effectiveness of the proposed scheme has been investigated and verified through experiments by a 50kVA UPS.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2007년도 제38회 하계학술대회
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• pp.614-615
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• 2007

There are roughly a hundred types of cables in power plants. The distribution of circuits in a nuclear plant is comprised of 20% instrument cables, 61% control cables, 13% AC power cables, 1% DC power cables, and 5% communication lines. In the nuclear power plant, medium voltage cables are generally included in the scope of systems reviewed for safety and are included in a plant's maintenance program. Medium voltage cables provide power to many critical components in plants, including feed water pumps, circulating water pumps, and condensate pumps. Among these cables, high temperature sections of cables feeding electrical power to the circulating water pump and the condenser extraction pump were found. The evaluation for these cables is performed to find the maximum allowable current and temperature. The result shows that the load current flowed about 85% of the allowable current ampacity, and the temperature of conductor at full load current did not exceed the limited temperature. Therefore, existing cables for circulating water pump and condenser extraction pump system are going to be used during design life.

• 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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• 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.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2010년도 하계학술대회 논문집
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• pp.470-471
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• 2010

In this paper, the coupled inductor with three branches in two interleaved converter are analyzed to effectively suppress the high frequency circulating current since the circulating current controller cannot mitigate the high frequency circulating current. As a result, the novel averaged model including the coupled inductor with zero sequence components is developed for reducing the low and high frequency circulating current simultaneously.

• 전력전자학회논문지
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• 제24권2호
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• pp.71-77
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• 2019

This study proposes a circulating current reduction method that uses high-frequency voltage compensation when carrier phase difference occurs between two inverters in MSIS. In MSIS, inverters are configured in parallel to increase power capacity and to increase efficiency by using inverters only as needed. However, in the parallel inverter structure, circulating current is inevitably generated. Circulating current increases the stress on the switch, adversely affects the current control performance, and renders load sharing difficult. The proposed method compensates for the output voltage reference of the slave module by using the high-frequency voltage so that the switching pattern of each module is matched even in asynchronous carriers. The validity of the proposed method is verified by simulations and experiments with 600 W IPMSM.

• KEPCO Journal on Electric Power and Energy
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• 제2권2호
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• pp.211-221
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• 2016

전세계의 인구 증가와 경제 발전은 지속적인 에너지 수요의 증가를 가져오고 있다. 특히, 전력부분에서는 아시아 및 아프리카, 그리고 중남미 등의 개발도상국을 중심으로 그 수요가 지속적으로 증가하고 있어 주요 에너지원인 석탄의 이용도 지속적으로 증가할 것으로 예측되고 있다. 그러나, 이산화탄소 및 대기오염원, 그리고 미세먼지 등의 배출 등은 석탄을 이용한 전력생산에 있어 환경친화적인 기술로의 대응방안 마련과 더불어, 고효율의 다양한 저급연료를 활용할 수 있는 발전 기술의 개발이 요구되고 있다. 이에, 기술개발 및 그 상용화 시장이 점차 증대되고 있는 초초임계 순환유동층 보일러에 대한 관심이 증대하고 있다. 초초임계 순환 유동층 보일러는 중소형의 아임계의 드럼형 구조에서 벗어나 대용량화의 기본 구조인 Once Through 형태의 증기 순환 구조를 지니고 100-300 MWe의 상업용 모듈의 복제를 통해 600 MW급이 상용화 운전 중에 있으며, 향후 설계가 완성된 800 MWe의 상업화 진행이 기대되고 있다. 초초임계 순환유동층 보일러는 2017년 이후 아임계 순환유동층 보일러 설치 용량을 추월하여 표준형 모델이 될 것으로 전망되고 있어, 본 논문에서의 이의 기술적 배경과 개발 현황 그리고 시장전망 등을 통해 기술적 이해를 도모하고자 한다.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2016년도 추계학술대회 논문집
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• pp.143-144
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• 2016

The circulating current control within the phase legs is one of the main control objectives in a modular multilevel converter (MMC) under different operating conditions. This paper proposes a control strategy of circulating currents in the MMC under unbalanced voltage by using a proportional-resonant (PR) controller. Under the unbalanced voltage, the circulating currents in the MMC consists of three components such as positive-sequence, negative-sequence, and zero-sequence circulating currents. With the PR controller, all components of the circulating current will be directly controlled in the stationary reference frame without decomposing into positive- and negative-sequence components. Thus, the ripples in the circulating currents and the DC current are suppressed under the unbalanced voltage. The effectiveness of the proposed method is verified by simulation results based on PSCAD/EMTDC simulation program.