• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.794-795
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• 2011

The DC side voltage and current of a HVDC transmission system are directly affected by non-linear switching devices such as the thyristor valve which causes real power losses, even under the superconducting conditions of a high temperature superconducting (HTS) power cable. This paper deals with the development of miniaturized superconducting DC cable system. The authors designed and fabricated two thyristor converters for DC transmission system. One is operated as a rectifier and the other is an inverter. The HTS model cable was connected between the DC side of the rectifier and inverter. Real DC transport current and voltage were applied to the miniaturized HTS DC cable. Experimental results are discussed in detail.

• Proceedings of the KIEE Conference
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• 1995.07b
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• pp.571-573
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• 1995

This paper presents a method for optimal dispatch which minimizes transmission losses and improves voltage profile of power systems using genetic algorithm based on the mechanism of natural genetics and natural selection. The constraints are VAR sources(transformer tap, generator voltage magnitude and shunt capacitor/reactor), load bus voltages and generator reactive power. Real variable-based genetic algorithms which can save coding times and maintain the accuracy are applied for optimal dispatch of reactive power. The genes of genetic algorithm consisted of integers for considering discrete VAR sources. A efficient operator for crossover is proposed to consider the effect of close genes. The algorithm proposed can apply to problems for large scale power systems with multi-variables and complex nonlinear functions efficiently. The proposed method is applied to IEEE 30 buses model system to show its effectiveness.

• Communications for Statistical Applications and Methods
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• v.13 no.3
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• pp.685-699
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• 2006

Commercial banks and other related areas have developed internal models to better quantify their financial risks. Since an appropriate credit risk model plays a very important role in the risk management at financial institutions, it needs more accurate model which forecasts the credit losses, and statistical inference on that model is required. In this paper, we propose a new method for estimating a default rate. It is a Bayesian approach using the power prior which allows for incorporating of historical data to estimate the default rate. Inference on current data could be more reliable if there exist similar data based on previous studies. Ibrahim and Chen (2000) utilize these data to characterize the power prior. It allows for incorporating of historical data to estimate the parameters in the models. We demonstrate our methodologies with a real data set regarding SOHO data and also perform a simulation study.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.6
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• pp.797-803
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• 2012

In this paper, for minimizing the real-time operating load losses of the power transformer, a SCADA optimum operating system was developed, and the economic analyses on the test operation were performed. Transformer loss DB which reflects the economic integration operation criteria was constructed by referring the transformer manufacturer's loss data(iron loss, copper loss). Based on the loss DB, each substation transformer real-time loss was calculated according to the size of the transformer loads, and if integration or separation transformer operating conditions minimizing the loss are met, then a window pops-up and the dispatcher performs the substation equipments operation according to the procedure provided by this system. With the existing SCADA main program, the relation database of the substation facilities and integration/separation operation algorithm were developed and applied to Auto MTR Processor and pconn Processor Task module. Seven stations test data for seven months were analyzed for the economic analyses, and the results showed that Cost-Benefit ratio was 2.64, and IRR(Internal Rate of Return), 36%, which asserted the economic justification of the proposed system.

• Journal of Power Electronics
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• v.17 no.4
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• pp.914-922
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• 2017

Multilevel converters have attracted a lot of attention in recent years. The efficiency parameters of a multilevel converter such as the switching losses and total harmonic distortion (THD) mainly depend on the modulation strategy used to control the converter. Among all of the modulation techniques, the selective harmonic elimination (SHE) method is particularly suitable for high-power applications due to its low switching frequency and high quality output voltage. This paper proposes a new expression for the SHE problem in five-level converters. Based on this new expression, a simple analytical method is introduced to determine the feasible modulation index intervals and to calculate the exact value of the switching angles. For each selected harmonic, this method presents three-level or five-level waveforms according to the value of the modulation index. Furthermore, a flowchart is proposed for the real-time implementation of this analytical method, which can be performed by a simple processor and without the need of any lookup table. The performance of the proposed algorithm is evaluated with several simulation and experimental results for a single phase five-level diode-clamped inverter.

• Journal of Electrical Engineering and Technology
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• v.8 no.1
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• pp.53-63
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• 2013

To guide the decision making of the expert engineer specialized in power system operation and control; the practical OPF solution should take in consideration the critical situation due to severe loading conditions and fault in power system. Differential Evolution (DE) is one of the best Evolutionary Algorithms (EA) to solve real valued optimization problems. This paper presents simple Differential Evolution (DE) Optimization algorithm to solving multi objective optimal power flow (OPF) in the power system with shunt FACTS devices considering voltage deviation, power losses, and power flow branch. The proposed approach is examined and tested on the standard IEEE-30Bus power system test with different objective functions at critical situations. In addition, the non smooth cost function due to the effect of valve point has been considered within the second practical network test (13 generating units). The simulation results are compared with those by the other recent techniques. From the different case studies, it is observed that the results demonstrate the potential of the proposed approach and show clearly its effectiveness to solve practical OPF under contingent operation states.

• Transactions of the Korean hydrogen and new energy society
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• v.33 no.1
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• pp.95-104
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• 2022

Micro hydropower is a readily available renewable energy source that can be harvested utilizing hydrokinetic turbines from shallow water canals, irrigation and industrial channel flows, and run-off river stream flows. These sources generally have low head (<1 m) and low velocity which makes it difficult to harvest energy using conventional turbines. A horizontal-axis screw turbine was designed and numerically tested to extract power from such low-head water sources. The 3-bladed screw-type turbine is placed horizontally perpendicular to the incoming flow, partially submerged in a narrow water channel at no-head condition. The turbine hydraulic performances were studied using Computational Fluid Dynamics models. Turbine design parameters such as the shroud diameter, the hub-to-shroud ratios, and the submerged depths were obtained through a steady-state parametric study. The resulting turbine configuration was then tested by solving the unsteady multiphase free-surface equations mimicking an actual open channel flow scenario. The turbine performance in the shallow channel were studied for various Tip Speed Ratios (TSR). The highest power coefficient was obtained at a TSR of 0.3. The turbine was then scaled-up to test its performance on a real site condition at a head of 0.3 m. The highest power coefficient obtained was 0.18. Several losses were observed in the 3-bladed turbine design and to minimize losses, the number of blades were increased to five. The power coefficient improved by 236% for a 5-bladed screw turbine. The fluid losses were minimized by increasing the blade surface area submerged in water. The turbine performance was increased by 74.4% after dipping the turbine to a bottom wall clearance of 30 cm from 60 cm. The final output of the novel horizontal-axis screw turbine showed a 2.83 kW power output at a power coefficient of 0.63. The turbine is expected to produce 18,744 kWh/year of electricity. The design feasibility test of the turbine showed promising results to harvest energy from small hydropower sources.

• Nuclear Engineering and Technology
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• v.56 no.1
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• pp.56-63
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• 2024

In high radiation fields, gamma cameras suffer from pulse pile-up, resulting in poor energy resolution, count losses, and image distortion. To overcome this problem, various methods have been introduced to reduce the size of the aperture or pixel, reject the pile-up events, and correct the pile-up events, but these technologies have limitations in terms of mechanical design and real-time processing. The purpose of this study is to develop a real-time gamma camera to evaluate the radioactive contamination in high radiation fields. The gamma camera is composed of a pinhole collimator, NaI(Tl) scintillator, position sensitive photomultiplier (PSPMT), signal processing board, and data acquisition (DAQ). The pulse pile-up is corrected in real-time with a field programmable gate array (FPGA) using the start time correction (STC) method. The STC method corrects the amplitude of the pile-up event by correcting the time at the start point of the pile-up event. The performance of the gamma camera was evaluated using a high dose rate ¹³⁷Cs source. For pulse pile-up ratios (PPRs) of 0.45 and 0.30, the energy resolution improved by 61.5 and 20.3%, respectively. In addition, the image artifacts in the ¹³⁷Cs radioisotope image due to pile-up were reduced.

• Journal of Energy Engineering
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• v.11 no.4
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• pp.291-298
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• 2002

This paper presents a practical methodology that can analysis power flow and marginal factors based on optimal power flow (OPF) of power systems under restructuring environment. First of all, to evaluate useful marginal factors, nonlinear optimization problems of minimum fuel cost and minimum transmission loss are formulated and solved by nonlinear primal-dual interior point method. Here, physical constraints considered in the optimization problems are the limits of bus voltage. line overloading, and real & reactive power generation. Also, an evaluation method of marginal price and marginal transmission loss is presented based on sensitivities calculated by the two OPF problems. Especially, to reflect the cost related to transmission losses in the competitive electricity market, an analysis method of MLF (marginal loss factor) is pro-posed. Numerical results on IEEE RTS 24 show that the proposed algorithm is effective and useful for analysis of power market price.

• Journal of Power Electronics
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• v.13 no.5
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• pp.841-853
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• 2013

An application of doubly-fed induction generator (DFIG), which is one of adjustable speed generators, to a gas engine cogeneration system has been investigated. To operate during a blackout as an emergency power supply is one of important roles for the gas engine cogeneration system. However, the DFIG requires initial excitation for startup during a blackout because the DFIG has no excitation source. In this paper, we propose the "blackout start" as a new excitation method to generate a rated voltage at the primary side during a blackout. In addition, a stand-alone operation following a blackout has been investigated by using experimental setup with a real gas engine. Power flows in the generating set with the DFIG at the stand-alone operation have been investigated experimentally. Experimental investigation of the power flow suggests that the generating set with DFIG has optimal speed in minimizing whole system losses.