• 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.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1994년도 하계학술대회 논문집 A
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• pp.509-511
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• 1994

Cycloconverters can be classified into two types by the main circuit configuration. One is the non- circulating current type required the dead time the other is circulating current type which has the circulating current reactors. The switching devices were highly stressed by the load voltages during dead time on non-circulating current type operation and the circulating currents on circulating current type operation. The overcome those problems, this paper proposes the switching patterns for the partial and the non circulation current type. The PWM methods, which are saw wave comparing method and load current integrating value comparing method, are presented.

• 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.

• 대한전기학회논문지:전력기술부문A
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• 제51권10호
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• pp.525-533
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• 2002

It is common to install multiple lines in the same route. Recently, excessive sheath circulating current was partially measured in underground cable systems of KEPCO. Especially, the installation type, unbalance section length between joint boxes and zero sequence current by distribution cable have an effect on the rising of sheath circulating current in the underground transmission system. If excessive current flows in sheath, sheath loss which is reduced the transmission capacity is produced. This paper describes the relation analysis of sheath circulating current and burying types. And also, a detailed analysis on rising cause and characteristic of sheath circulating current by considering various unbalanced conditions presents using analysis and measurement regarding cable systems which have the problem of excessive sheath circulation current.

• 약학회지
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• 제43권1호
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• pp.98-103
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• 1999

Effects of Astragali Radix extract (AG) on the cell mediated-and nonlpecific immunotoxic responses of zinc chloride (Zn) were studied usign ICR mice. Mice were divided into 4 groups (10 mice/group), and Zn was given to the mice 1 hr after i.p. injection with 0.5g/kg of AG by i.p. injection daily for 10 days at a dose of 25 mg/kg. Immune responses on the responses on the relative weight of thymus, delayed-type hypersensitivity to SRBC (DTH), phagocytic activity and circulating leukocytes were evaluated. Zn treatment decreased body weight gain, the relative weight of thymus, DTH and circulating leukocytes compared with those in controls. AG treatment increased DTH, phagocytic activity and circulating leukocytes compared with those in controls. Combination of AG and Zn increased DTH and circulating leukocytes compared with those in controls, but decreased body weight gain and the relative weight of thymus. These findings indicated that AG decreased immunotoxicity of Zn on the DTH and circulating leukocytes.

• Journal of Electrical Engineering and Technology
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• 제9권4호
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• pp.1137-1144
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• 2014

A parallel-feeding AC traction power system increases the power supply capacity and decreases voltage fluctuations, but the circulating power flow caused by the phase difference between the traction substations prevents the system from being widely used. A circuit analysis shows that the circulating power flow increases almost linearly as the phase difference increases, which adds extra load to the system and results in increased power dissipation and load unbalance. In this paper, we suggest a phase shifter for the parallel-feeding AC traction power system. The phase shifter regulates the phase difference and the circulating power flow by injecting quadrature voltage which can be obtained directly from the Scott-connection transformer in the traction substation. A case study involving the phase shifter applied to the traction power system of a Korean high-speed rail system shows that a three-level phase shifter can prevent circulating power flow while the phase difference between substations increases up to 12 degrees, mitigate the load unbalance, and reduce power dissipation.

• Journal of Electrical Engineering and Technology
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• 제13권3호
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• pp.1241-1250
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• 2018

Parallel three-phase voltage source inverters in a direct connection configuration are widely used to increase system power ratings. A zero-sequence circulating current can be generated according to the switching method; however, the zero-sequence circulating current not only distorts current, but also reduces the system reliability and efficiency. In this paper, a model predictive control scheme is proposed for parallel inverters to drive an interior permanent magnet synchronous motor with zero-sequence circulating current suppression. The voltage vector of the parallel inverters is derived to predict and control the torque and stator flux components. In addition, the zero-sequence circulating current is suppressed by designing the cost function without an additional current sensor and high-impedance inductor. Simulation and experimental results are presented to verify the proposed control scheme.

• 조명전기설비학회논문지
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• 제21권3호
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• pp.56-61
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• 2007

본 논문에서는 연료전지 시스템을 위한 Z-소스 인버터로 구성된 병렬운전 연료전지 시스템의 순환전류 감쇄 방안을 검토하였다. PWM 방식으로는 Carrier phase shifted SPWM을 적용하였으며, 이 방식은 출력 전류의 고조파 성분을 줄일 수 있는 장점을 갖는다. 그러나 이 기법을 Z-소스 인버터로 구성된 병렬운전 시스템에 적용하는 경우 추가적으로 순환전류가 발생한다. Z-소스 인버터로 구성된 병렬운전 시스템이 동작 시 발생하는 순환전류를 감소시키며, 부하전류에 낮은 고조파 성분을 가질 수 있도록 커플링된 순환전류 리액터를 사용하였다. 순환전류 리액터를 상호 커플링시킴으로써 순환전류를 추가적으로 감소시킬 수 있다. 이에 대한 적합성을 시뮬레이션과 실험을 통해 입증하였다.

• 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.

• 전기학회논문지
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• 제57권12호
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• pp.2147-2152
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• 2008

This paper proposes an algorithm to estimate the circulating currents in the transformers in parallel in an ultra high voltage system. For the Y-Y-${\Delta}$ transformers operated in parallel, there exist two kinds of the circulating currents i.e. one is between the tanks and the other between the banks of the delta side. As the former is 90 deg out of phase of the load current, it is estimated by decomposing the line current into the component 90 deg out of phase of the load current in the frequency domain. The latter is estimated in the time domain from applying the Kirchhoff's voltage law on the delta winding which gives a first-order differential equation in terms of the delta winding currents. To estimate the circulating currents between the tanks, the performance of the proposed algorithm is investigated when the impedances of the two transformer tanks are different or the taps of the on-load tap changer of the transformers are mismatched temporarily. To estimate the circulating currents between the banks, the performance of the proposed algorithm is also examined under magnetic inrush and over-excitation. Test results indicate that the algorithm can estimate the two kinds of the circulating currents successfully.