• 한국산학기술학회논문지
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• 제22권5호
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• pp.22-33
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• 2021

태양광전원이 설치된 배전선로의 단선 등으로 결상사고가 발생하는 경우, 연계용변압기의 철심형태 및 결선 방법에 따라 여러 가지 문제점이 발생할 가능성이 있다. 즉, 단선사고 발생 시 태양광전원 인버터의 단독운전 방지기능에 따라 결상을 감지하여 발전을 정지해야 하지만, 결상을 인지하지 못하고 계속 발전하는 사례가 보고되고 있으며, 연계용 변압기의 역조류 현상으로 인해 결상된 상으로 전원을 공급하여 계통에 악영향을 주는 문제점이 발생하고 있다. 하지만, 기존의 결상사고 보호계전기(Open Phase Relay, 47)는 태양광전원 연계용변압기의 결선방식과 철심형태에 따라 결상된 상에 전압이 유지될 수 있어서, 결상사고 검출이 어려워, 새로운 결상사고 검출 알고리즘이 요구되고 있다. 따라서, 본 논문에서는 전류비교방식의 개념을 이용하여, 연계용변압기 1, 2차측의 전류와 위상을 비교하여 결상사고를 검출하는 새로운 개념의 알고리즘을 제안한다. 또한, 전력계통 상용해석 프로그램인 PSCAD/EMTDC를 이용하여 수배전반, 저압배전선로, 부하, 연계용변압기, 태양광전원으로 구성된 배전계통과 결상사고 검출 보호장치의 모델링을 수행하고, 이를 바탕으로 결상사고 검출 보호장치를 구현한다. 상기의 모델링과 결상사고 검출 알고리즘을 바탕으로 태양광전원 연계용 변압기의 결선방식과 철심구조에 따라 결상사고 검출 보호장치의 특성을 분석한 결과, 결상 시 연계용변압기의 동작 기울기와 1차측 전류 불평형률이 보호장치의 정정 값(30[%])을 초과함에 따라 결상사고가 정확하게 검출되어, 제안한 알고리즘이 결상사고 보호방식에 유용함을 알 수 있었다.

• Steel and Composite Structures
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• 제18권6호
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• pp.1493-1515
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• 2015

A new refined hyperbolic shear deformation theory (RHSDT), which involves only four unknown functions as against five in case of other shear deformation theories, is presented for the thermoelastic bending analysis of functionally graded sandwich plates. Unlike any other theory, the number of unknown functions involved is only four, as against five in case of other shear deformation theories. The theory presented is variationally consistent, does not require shear correction factor, and gives rise to transverse shear stress variation such that the transverse shear stresses vary parabolically across the thickness satisfying shear stress free surface conditions. The sandwich plate faces are assumed to have isotropic, two-constituent material distribution through the thickness, and the modulus of elasticity, Poisson's ratio of the faces, and thermal expansion coefficients are assumed to vary according to a power law distribution in terms of the volume fractions of the constituents. The core layer is still homogeneous and made of an isotropic ceramic material. Several kinds of sandwich plates are used taking into account the symmetry of the plate and the thickness of each layer. The influences played by the transverse shear deformation, thermal load, plate aspect ratio and volume fraction distribution are studied. Numerical results for deflections and stresses of functionally graded metal-ceramic plates are investigated. It can be concluded that the proposed theory is accurate and simple in solving the thermoelastic bending behavior of functionally graded plates.

• International Journal of Fluid Machinery and Systems
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• 제6권4호
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• pp.200-205
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• 2013

In order to investigate the characteristics of unsteady flow in a mixed flow pump guide vane under the small flow conditions, several indicator points in a mixed flow pump guide vane was set, the three-dimensional unsteady turbulence numerical value of the mixed flow pump which is in the whole flow field will be calculated by means of the large eddy simulation (LES), sub-grid scale model and sliding mesh technology. The experimental results suggest that the large eddy simulation can estimate the positive slope characteristic of head & capacity curve. And the calculation results show that the pressure fluctuation coefficients of the middle section in guide vane inlet will decrease firstly and then increase. In guide vane outlet, the pressure fluctuation coefficients of section will be approximately axially symmetrical distribution. The pressure fluctuation minimum of section in guide vane inlet is above the middle location of the guide vane suction surface, and the pressure fluctuation minimum of section in which located the middle and outlet of guide vane. When it is under the small flow operating condition, the eddy scale of guide vane is larger, and the pressure fluctuation of the channel in guide vane being cyclical fluctuations obviously which leads to the area of eddy expanding to the whole channel from the suction side. The middle of the guide vane suction surface of the minimum amplitude pressure fluctuation to which the vortex core of eddy scale whose direction of fluid's rotation is the same to impeller in the guide vane adhere.

• 한국정보통신학회논문지
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• 제6권1호
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• pp.89-98
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• 2002

본 연구에서는 광계측 기술을 이용하여 배전자동화 개폐기 내에서 발생하는 전자 계측장치의 절연문제 및 측정오차의 문제점을 개선하기 위하여 배선자동화개폐기 내장형 광전류 및 광전압센서를 연구하였다. 자기광학 효과(magneto optic effect)인 페러데이효과(Faraday effect)를 이용하여 최적의 광전류 센서를 설계 제작하여 높은 전류까지 선형성을 유지시키고 다른 상의 전류에 대한 유도현상을 최소로 하기 위해 순철 코아를 사용하여 광전류 센서를 일체화시켜 설계 제작하였다. 설계내에서 결정의 굴절률 및 방위각의 변화에 의해 생기는 전기광학효과(electrooptic effect)인 포켈스 효과(pockels effect)를 이용하여 광전압센서를 구성하였으며 배전 자동화용 개폐기의 협소한 내부구조를 고려하여 공간전계방식을 채택하였다. 또한 다심의 광섬유를 개폐기와 외부의 신호처리 단말기와 연결하기 위하여 기밀형 멀티 광커넥터를 설계 자작하여 접속 손실 및 가스 누설 시험을 하였다. 전기적 특성시험에서 광 전류센서에서는 인가 전류에 대한 선형 특성은 20A 에서 700A까지 변동 오차 2.5% 이내의 선형성을 유지하였으며, 광전압센서에 있어서 인가 전압에 대한 선형 특성은 6.6kV에서 19.8kV까지 거의 변동 오차가 1% 이내로 우수한 선형 특성을 나타냈다.

• Nuclear Engineering and Technology
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• 제26권3호
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• pp.337-344
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• 1994

가압경수형 원자로의 노심장전모형 선정시 제약이 되는 집합체첨두 $F_{{\Delta}H}$$^{N}$ 을 감소시키기 위하여 다중농축도 개념을 적용하여 핵연료봉의 집합체내 출력분포를 평탄화함으로써 첨두봉출력을 감소시키는 방안에 대하여 연구하였다. 다중농축도 핵연료집합체란 기존 집합체의 단일 농축도핵연료봉을 이중농축도 핵연료봉으로 대체한 집합체를 말한다. 농축도의 차이를 변화시켜가며 적절한 배치에 의하여 핵연료봉의 집합체내 배치모형을 최적화 하였고, 이러한 다중농축도 핵연료 집합체에서 첨두봉출력의 감소를 가장 크게하는 농축도의 차이는 약 0.3~0.4w/o 일때가 가장 적절한 것으로 밝혀졌다. 다중농축도 핵연료 집합체의 노심에서의 효과를 알아보기 위하여 고리 4호기를 대상으로 8주기에서 평형주기까지 계산을 수행하였으며 그 결과 약 1.5%의 $F_{{\Delta}H}$$^{N}$ 감소효과를 얻을 수 있었다.

• 한국초전도ㆍ저온공학회논문지
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• 제5권3호
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• pp.34-37
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• 2003

A 1 MVA single-phase high temperature superconducting (HTS) transformer with BSCCO-2223 wire was designed in this paper. The rated voltages of each sides of the transformer are 22.9 kV and 6.6 kV respectively. Double pancake HTS windings arranged reciprocally will be used for the transformer windings, because of the advantages of insulation and distribution of surge voltage in case of a large power and high voltage transformer. Single HTS wire was used for the primary windings and four parallel wires were used for the secondary windings of the transformer with transposition. A core of the transformer was designed as a shell type core separated with the windings by a cryostat made of GFRP with a room temperature bore. The operating temperature of the HTS windings will be about 65K with sub-cooled liquid nitrogen. A cryogenic cooling system using a GM-cryocooler for this HTS transformer by natural convection of liquid nitrogen was designed. This type of cooling system can be a good option for compactness, efficiency, and reliability of the HTS transformer.

• 대한기계학회논문집
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• 제10권6호
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• pp.823-829
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• 1986

본 연구에서는 와동의 대류속도를 H.H. Brunn의 방법과 달리 측정하였으며, 또 이를 이용하여 와동의 중심들 사이의 간격을 구하고자 한다. 이를 위하여 먼저 포텐셜코어영역(potential core region)과 혼합층영역(mixing layer region)의 경계 및 천이영역(transition region)과 난류영역(turbulent region)경계를 구하여야 한다. 각 영역들의 대체적인 구분은 Fig.1과 같다.

• 대한전기학회논문지:전기물성ㆍ응용부문C
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• 제54권6호
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• pp.255-261
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• 2005

Recently, the RF inductive discharge or inductively coupled plasma continues to attract growing attention as an effective plasma source in many industrial applications, the best known of which are plasma processing and lighting technology. To the point of lighting sources, the ring-shaped electrodeless fluorescent lamps utilizing an inductively coupled plasma have been objects of interest and research during the last decades, mainly because of their potential for extremely long life, high lamp efficacies, rapid power switching response. In this paper, maxwell 3D finite element analysis program (Ansoft) was used to obtain electromagnetic properties associated with the coil and nearby structures. The electromagnetic emitting properties were presented by 3D simulation software operated at 250 kHz and some specific conditions. The electromagnetic field in the ferrite core was shown to be high and symmetric. An LS-100 luminance meter and a Darsa-2000 spectrum analyzer were used in the experiment. According to data on the lamp tested using high magnetic field ferrite, the optical and thermal wave fields were shown to be high around the ring-shaped electrodeless fluorescent lamp. The optical or light field was high at the center of the bulb rather than around the ferrite core. The light conditions of the bulb were assumed to be complex, depending on the condition of the filler gas, the volume of the bulb, and the frequency of the inverter. Our results have shown coupling between the gas plasma and the field of the light emitted to be nonlinear.

• Steel and Composite Structures
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• 제43권5호
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• pp.581-601
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• 2022

The main objective of this research work is to investigate the free vibration behavior of annular sandwich plates resting on the Kerr foundation at thermal conditions. This sandwich configuration is composed of two FGM face sheets as coating layer and a porous GPLRC (GPL reinforced composite) core. It is supposed that the GPL nanofillers and the porosity coefficient vary continuously along the core thickness direction. To model closed-cell FG porous material reinforced with GPLs, Halpin-Tsai micromechanical modeling in conjunction with Gaussian-Random field scheme is used, while the Poisson's ratio and density are computed by the rule of mixtures. Besides, the material properties of two FGM face sheets change continuously through the thickness according to the power-law distribution. To capture fundamental frequencies of the annular sandwich plate resting on the Kerr foundation in a thermal environment, the analysis procedure is with the aid of Reddy's shear-deformation plate theory based high-order shear deformation plate theory (HSDT) to derive and solve the equations of motion and boundary conditions. The governing equations together with related boundary conditions are discretized using the generalized differential quadrature (GDQ) method in the spatial domain. Numerical results are compared with those published in the literature to examine the accuracy and validity of the present approach. A parametric solution for temperature variation across the thickness of the sandwich plate is employed taking into account the thermal conductivity, the inhomogeneity parameter, and the sandwich schemes. The numerical results indicate the influence of volume fraction index, GPLs volume fraction, porosity coefficient, three independent coefficients of Kerr elastic foundation, and temperature difference on the free vibration behavior of annular sandwich plate. This study provides essential information to engineers seeking innovative ways to promote composite structures in a practical way.

• Computers and Concrete
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• 제33권1호
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• pp.27-39
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• 2024

In this study, the authors investigate the free vibration behavior of three-phases functionally graded sandwich plates using a novel nth-order shear deformation theory. These plates are composed of a homogeneous core and two face-sheet layers made of different functionally graded materials. This is the novel type of the sandwich structures that can be applied in many fields of mechanical engineering and industrial. The proposed theory only requires four unknown displacement functions, and the transverse displacement does not need to be separated into bending and shear parts, simplifying the theory. One noteworthy feature of the proposed theory is its ability to capture the parabolic distribution of transverse shear strains and stresses throughout the plate's thickness while ensuring zero values on the two free surfaces. By eliminating the need for shear correction factors, the theory further enhances computational efficiency. Equations of motion are established using Hamilton's principle and solved via Navier's solution. The accuracy and efficiency of the proposed theory are verified by comparing results with available solutions. The authors then use the proposed theory to investigate the free vibration characteristics of three-phases functionally graded sandwich plates, considering the effects of parameters such as aspect ratio, side-to-thickness ratio, skin-core-skin thicknesses, and power-law indexes. Through careful analysis of the free vibration behavior of three-phases functionally graded sandwich plates, the work highlighted the significant roles played by individual material ingredients in influencing their frequencies.