• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.16 no.5
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• pp.53-60
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• 2002

This paper presents exact autotransformer-fed AC electric railroad system modeling using constant current mode, and single-phase STATCOM(Static Synchronous Compensator) which has an effect on electric railroad system. An AC electric railroad is rapidly changing single-phase feeding electric power. To avoid voltage fluctuation under single phase loads, electric power should be received from a large source. The system modeling theory is based on the solution of algebraic. The AC electric railroad load model is nonlinear. Therefore this paper is considered nonlinear load using PSCAD/EMTDC. And the proposed modeling method is considered the line self-impedances and mutual-impedances that techniques for the AC electric railroad system modeling analysis, and that single-phase STATCOM can reliably compensate the voltage drop. In the case study, the allowance range of feeding voltage is 22.5∼27.5 kV, AT-fed AC electric railroad system circuit is analyzed by loop equation both normal and extension modes. The simulation objectives are to calculate the catenary and rail voltages with respect to ground, as the train moves along a section of line between two adjacent ATs. The results show that single-phase STATCOM can reduce the voltage drop in the feeding circuit and improve the power quality at AC electric railroad system by compensating the reactive power.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.56 no.1
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• pp.1-7
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• 2007

Generally, an electric railroad feeding system has many problems due to the different characteristics in contrast with a load of general three-phase AC electric power system. One of them is harmonics problem caused by the switching device existing in the feeding system, and moreover, the time-varying dynamic loads of rail way is inherently another cause to increase this harmonics problem. In Korea power systems, the electric railroad feeding system is directly supplied from the substation of KEPCO. Therefore, if voltages fluctuation or unbalanced voltages are created by the voltage and current distortion or voltage drop during operation, it affects directly the source of supply. The trainloads of electric railway system have non-periodic but iterative harmonic characteristics as operating condition, because the electric characteristic of the electric railroad feeding system is changed by physical conditions of the each trainload. According to the traditional study, the estimation of harmonics has been performed by deterministic way using the steady state data at the specific time. This method is easy to analyze harmonics, but it has limits in some cases which needs an assessment of dynamic load and reliability. Therefore, this paper proposes the probabilistic estimation method, moments matching method(MW) in order to overcome the drawback of deterministic method. In this paper, distributions for each harmonics are convolved to obtain the moments and cumulants of TDD(Total Demand Distortion), and this can be generalized for any number of trains. For the case study, the electric railway system of LAT(Intra Airport Transit) in Incheon International Airport is modeled using PSCAD/EMTDC dynamic simulator. The raw data of harmonics for the moments matching method is acquired from simulation of the LAT model.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.24 no.6
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• pp.138-146
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• 2010

DC railway power supply system generally uses the running rails as negative-polarity return conductor for traction load current, and the induced rail potential and stay current cause serious problems to any electrified matter in the underground and also safety problems to human body. This paper presents a new algorithm for the analysis of the rail potential and the stray current in DC railway power system operated under independent/parallel power feeding mode. The effect of load current fluctuation during train operation is also calculated by using TPS(Train Performance Simulation) program to analysis the variation of the railway potential and stray current along railway track. Simulation program is developed based on the proposed algorithm and case studies are provided.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.8
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• pp.653-660
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• 2015

In this study, we conduct power performance and load analyses of two different types of vertical-axis tidal-current turbines using the computational fluid dynamics (CFD) method. To analyze the power output and loads, we perform transient CFD simulations considering the cavitation model using ANSYS CFX. The averaged power output of an H-type rotor was 7.47 kW and 67.6 kW in normal and extreme operating conditions, respectively, which did not satisfy the initial design conditions. However, in the case of the helical-type rotor, the power output under normal and extreme conditions were close to the expected values. The cavitation, which may cause instantaneous power fluctuation, occurred repeatedly at the suction side of the rotors. In order to guarantee a more stable power supply and to prevent fatigue failure, we require a design that minimizes cavitation.

• Journal of the Korean Society for Railway
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• v.9 no.6 s.37
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• pp.761-765
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• 2006

At present, harmonic currents cause serious problems in power conversion system using the semiconductor switching device. Also some of the conversion system provokes harmonic currents against to the main power supply system and causes hindrances for the system. Main power impedance of the traditional LC passive filter method, influences on the filter characteristics and amplifies the harmonics when resonance phenomenon is occurred. And the traditional existing 2 level inverter systems show the limit in capacity of voltage and current in case of occurring sudden load change. So, to solve this problem active filter which uses cascaded H-bridge multi level inverter has been designed and ex-filter system circuits were totally investigated. With multi level active filtering system not only the size of filter but also the size of filter for transformer can be reduced by half and so as to the weight, while the capacity of inverter can be double sized and wave forms can be compensated exactly and precisely. Also by the benefit of the increase in rating capacity, the various currents owing to the load fluctuation can be dealt more steadily. In order to simulate the wave form of harmonics based on the measured data on the AC 25kV high speed Domestic Commercial railway, it was simulated with PSCAD/EMTDC and PSIM. Based on the results of this demonstration, the power supply system and inverter system would be more stable and also promoting its efficiency.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.27 no.3
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• pp.137-145
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• 2015

In this study, the actual energy consumption of the secondary side of District Heating System (DHS) with different hot water supply temperature control methods are compared. Three methods are Set-point Control, Outdoor Temperature Reset Control and Outdoor Temperature Prediction Control. While Outdoor Temperature Reset Control has been widely used for energy savings of the secondary side of the system, the results show that Outdoor Temperature Prediction Control method saves more energy. In general, Outdoor Temperature Prediction Control method lowers the supply temperature of hot water, and it reduces standby losses and increases overall heat transfer value of heated spaces due to more flow into the space. During actual energy consumption monitoring, Outdoor Temperature Prediction Control method saves about 7.1% in comparison to Outdoor Temperature Reset Control method and about 15.7% in comparison to Set-point Control method. Also, it is found that at when partial load condition, such as daytime, the fluctuation of hot water supply temperature with Set-point Control is more severe than Outdoor Temperature Prediction Control. Therefore, it proves that Outdoor Temperature Prediction Control is more stable even at the partial load conditions.

• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.198-198
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• 1999

본 연구는 진공상태에서 단단한 물질로 된 diamond stylus로 단결정 MgO 표면에 마모(abrasion)가 할 때 발생되는 Photon emission(PHE), electron emission(EE), 마찰력을 시간의 함수로 동시에 측정하였다. 마모가 일어나는 동안 PHE 와 EE을 시간의 함수로 측정하면 마찰력 신호(signal)와 일치하지 않고 강한 fluctuation을 보여주고 있다. 마모를 가할 때 PHE와 EE의 signal은 wear 실험이 지속되는 동안은 force signal과 관계가 있다. 그러나 변형과 마찰력의 시간의 함수에는 관계하지 않음을 알 수 있다. 본 실험에서 사용된 실험장치는 PhE, EE, frictional force을 동시에 실할 수 있는 장치이다. 광자방출 실험은 공기 중에서도 할 수 있으나 전자방출은 진공에서 얻을 수 있으므로 1$\times$10-4pa하에서 실험하였다. 전자방출은 Channel electron multiplier(bias-100V)로 검출하였고, 광자 에너지는 Gencom photomultiplier를 사용하여 180~600nm의 photon을 측정하였다. 마모는 탐색기에 관계되는 접촉점의 움직임에 관계없이 실험하였다. 시료의 처리과정과 load속도에 따른 PhE, EE, data의 방출은 시료의 표면 상태에 따라 좌우되었다. cleaved 표면은 polished 표면보다 강한 emission을 나타내었다. 이것은 마찰이 표면 상태에 의존됨을 볼 수 있었다. 속도에 따라 emission이 증가하다가 ~0.5m/s이상에서 포화상태에 도달하였다. emission 측정은 열처리한 시료와 열처리 안한 시료를 비교하였다. 발광도(luminescence)는 주로 변형(deformation)에 의해 생겼으며, 전자 방출은 벽개(fracture)에 의해 발생됨을 알 수 있었다. 측정한 3개의 signal을 시간에 따라 분석하면 stick-slip-like 현상을 볼 수 있었다. 이것으로 보아 stick은 변형에 의해 생기고 ms 후에 벽개 현상이 발생됨을 볼 수 있다. 이러한 방출 현상은 마모시 일어나는 세라믹의 급격한 벽개 과정을 이해하는 데 많은 도움을 주었다. PhE와 EE signal은 다이아몬드 stylus로 단결정 MgO 기판에 마모를 가할 때 ms 단위로 검출 할 수 있었다. 방출과 마찰력은 표면조건, load, stylus velocity에 따라 변하였다. 마찰력, PhE, EE의 시간에 따른 분석에서 PhE는 변형 과정에 민감하며, EE는 stylus velocity에 의존하였다. 본 연구의 MgO 마찰 실험에서 표면 변화에 대한 정보를 얻을 수 있었다.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.6
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• pp.962-968
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• 2016

The large scaled lead-acid battery is widely used for efficient operation of the photovoltaic system in many islands. However, lithium-ion battery is now being introduced to mitigate the fluctuation of wind power and to replace lead-acid battery. Therefore, hybrid ESS(Energy Storage system) that combines lithium-ion battery with lead-acid battery is being required because lithium-ion battery is costly in present stage. Under this circumstance, this paper presents the optimal algorithm to create composition rate of hybrid ESS by considering fixed and variable costs in order to maximize advantage of each battery. With minimization of total cost including fixed and variable costs, the optimal composition rate can be calculated based on the various scenarios such as load variation, life cycle and cost trend. From simulation results, it is confirmed that the proposed algorithms are an effective tool to produce a optimal composition rate.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.8
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• pp.1038-1045
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• 2000

An experimental study was carried out to delineate the flow characteristics in a closed countescurrent two-phase thermo syphon with concentric tubes. This is to be installed in the HANARO research reactor as a part of a Cold Neutron Source(CNS). In the present investigation, experiments ata room temperature with Freon-II3 as a moderator were performed. Results show that, based on the magnitude of pressure fluctuation, the flow regimes could be divided into 4 distinct ones in the ($V_f,\;Q_i$) plane, where $V_f$ represents the volume of the charged liquid and $Q_i$ the heat load: a stable flow regime, an oscillatory flow regime, a restablized flow regime and a dryout flow regime. For $V_f$>2.5l, the flow is stable at low $Q_i$. However, as $Q_i$ increases, the flow becomes oscillatory and finally restablizes As $V_f$ increases, the oscillation amplitude decreases, reaching to the restablized flow region at low $Q_i$, and the liquid level in the moderator cell remains high. In the oscillatory flow regimes, for a fixed VI; the oscillating period of time varies with $Q_i$, having a minimum value at a certain value of $Q_i$. The heat load, where the oscillating period of time is minimum, decreases as $V_f$ increases.

• Journal of Korea Multimedia Society
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• v.1 no.1
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• pp.67-79
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• 1998

In this paper, we present the design and analyze the transmission rate of a control algorithm for the client buffer on the VOD systems. The design is based on the server's data transmission rate and clients' data consumption rate. The proposed algorithm stabilizes the client's buffer by reducing the oscillation phenomena of the buffer. And it uses the BCT (Buffer Check Time) to reduce the scheduling load of the client system. The client's data consumption rate and the buffer size are calculated on the basis of BCT. In case that the predicted buffer size operates in the overrun or starvation the buffer. As the load fluctuation of the network, the transmission rate control policy cannot stabilize the client's buffer alone. The media scaling policy, therefore, also performs its complementary part in stabilizing the client's buffer, and performance evaluation of the proposed algorithm is provided through simulation.