• Proceedings of the KIPE Conference
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• 2014.07a
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• pp.144-145
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• 2014

본 논문에서는 Buck type의 3상 AC/AC 초퍼를 이용한 3상 voltage sag compensator를 제안한다. 제안된 회로는 sag보상 동작 시 사용된 변압기의 누설리액턴스가 3상 PWM Buck AC/AC converter에 미치는 영향이 작아 동작효율이 높고, sag가 발생하지 않은 상태일 때 입력과 출력이 절연되는 장점이 있다. 본 논문에서는 제안된 회로의 동작 및 특성을 설명하고, 타당성을 입증하기 위해 PSIM으로 시뮬레이션 하였다.

• Journal of the Korean Society for Railway
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• v.19 no.3
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• pp.288-296
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• 2016

A 400km/h simple catenary system was constructed as a test line in Korea. In the design stage of this system, the pre-sag was one of the engineering issues most focused on. It is known that the pre-sag improves the current collection performance in a certain band of high speed. However, the effect of pre-sag at 400km/h has not yet been established. To grasp a better pre-sag in the 400km/h catenary, we transacted the dynamic performance prediction simulation between catenary and pantograph under conditions of 0 and 1/3000 pre-sag. The level of 0 pre-sag was adapted for the 400km/h catenary design after reviewing predictions. We constituted the 1/3000 pre-sag sample section (about 1km) while constructing the 400km/h catenary test-bed (28km) of 0 pre-sag. With a HEMU-430X train, the contact forces were measured in the test-bed including the pre-sag sample section. In this paper, the predicted and measured dynamic performance values (contact forces) for 0 and 1/3000 pre-sag are described and compared. It is conclusively confirmed by analytical and experimental examination that the non pre-sag showed better dynamic (current collection) performance than that of the 1/3000 pre-sag for the 400km/h catenary system.

• The Transactions of the Korean Institute of Power Electronics
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• v.18 no.4
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• pp.322-332
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• 2013

Conventional single-phase series quasi Z-source voltage compensator can not compensate for voltage sag less than 50% that frequently occurs in the industrial field. In this study, single-phase series quasi Z-source voltage sag-swell compensator which can compensate the voltage variation of entire range is proposed. The proposed system is composed of two quasi Z-source AC-AC converters connected in series with output terminal stage. Voltage sag less than 50% could be compensated by the intersection switching control of the upper converter duty ratio and of the upper converter duty ratio. Also the compensation voltage and its flowchart for each compensation mode are presented for entire sag-swell region. To confirm the validity of the proposed system, a DSP(DSP28335) controlled experimental system was manufactured. As a result, the proposed system could compensate for the voltage sag/swell of 20% and 60%. Finally, voltage compensation factor and THD(Total Harmonic Distortion) according to voltage variation and load change were measured, and voltage quality shows a good results.

• The Transactions of the Korean Institute of Power Electronics
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• v.10 no.1
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• pp.102-107
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• 2005

This paper describes a new voltage sag-swell generator for the test of custom power devices such as UPS, DVR, DSTATCOM, SSTS, etc. Voltage sag, swell, outage, and unbalance generation mechanism and the operating principle are described for the proposed scheme. The usefulness of the scheme is proved through simulations and experiments. The proposed scheme has good features of simple structure, high reliability, wide range of sag and swell variation, and easy control. Especially, the scheme can provide a cost-effective implementation of a power quality disturbance generator. Therefore, it is expected that the scheme will contribute to the self implementation of the system with low cost in laboratory.

• Journal of Power Electronics
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• v.10 no.2
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• pp.203-209
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• 2010

This paper describes the development of a three-phase line-interactive DVR with a new sag detection algorithm. The developed detection algorithm has a hybrid structure composed of an instantaneous detector and RMS-variation detectors. The source voltage passes through the sliding-window DFT and RMS calculator, and the instantaneous sag detector. If an instantaneous sag is detected, the RMS variation detector-1 is selected to calculate the RMS variation. The RMS variation detector-2 is selected when the instantaneous sag occurs under the operation of the RMS variation detector-1. The feasibility of the proposed algorithm is verified through computer simulations and experimental work with a prototype of a line-interactive DVR with a 3kVA rating. The line-interactive DVR with the proposed algorithm can compensate for an input voltage sag or an interruption within a 2ms delay. The developed DVR can effectively compensate for a voltage sag or interruption in sensitive loads, such as computers, communications equipment, and automation equipment.

• Advances in Energy Research
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• v.6 no.1
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• pp.55-73
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• 2019

The cost incurred by voltage sag effect in power networks has always been of important concern for discussions. Due to the environmental constraints, fossil fuel shortage crisis and low efficiency of conventional power plants, decentralized generation and renewable based DG have become trends in recent decades; because DGs can reduce the voltage sag effect in distribution networks noticeably; therefore, optimum allocation of DGs in order to maximize their effectiveness is highly important in order to maximize their effectiveness. In this paper, a new method is proposed for calculating the cost incurred by voltage sag effect in power networks. Thus, a new objective function is provided that comprehends technical standards as minimization of the cost incurred by voltage sag effect, active power losses and economic criterion as the installation and maintenance costs of DGs. Considering operational constraints of the system, the optimum allocation of DGs is a constrained optimization problem in which Lightning Attachment procedure optimization (LAPO) is used to resolve it and is the optimum number, size and location of DGs are determined in IEEE 33 bus test system and IEEE 34 bus test system. The results show that optimum allocation of DGs not only reduces the cost incurred by voltage sag effect, but also improves the other characteristics of the system.

• Journal of the Korean Society of Groundwater Environment
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• v.6 no.1
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• pp.23-32
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• 1999

A new numerical method using solution-adaptive grids (SAG) is developed to solve the Richards' equation (RE) for unsaturated flow in porous media. Using a grid generation technique, the SAG method automatically redistributes a fixed number of grid points during the flow process, so that more grid points are clustered in regions of large solution gradients. The method uses the coordinate transformation technique to employ a new transformed RE, which is solved with the standard finite difference method. The movement of grid points is incorporated into the transformed RE, and therefore all computation is performed on fixed grid points of the transformed domain without using any interpolation techniques. Thus, numerical difficulties arising from the movement of the wetting front during the infiltration process have been substantially overcome by the new method. Numerical experiments for an one-dimensional infiltration problem are presented to compare the SAG method to the modified Picard method using a fixed grid. Results show that accuracy of a SAG solution using 41 nodes is comparable with the solution of the fixed grid method using 201 nodes, while it requires only 50% of the CPU time. The global mass balance and the convergence of SAG solutions are strongly affected by the time step size (Δt) and the weighting parameter (${\gamma}$) used for generating solution-adaptive grids. Thus, the method requires automated readjustment of Δt and ${\gamma}$ to yield mass-conservative and convergent solutions, although it may increase computational costs. The method can be effective especially for simulating unsaturated flow and other transport problems involving the propagation of a sharp-front.

• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.324-325
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• 2011

This paper addresses the assessment of voltage sag costs based on the stochastic prediction of voltage sags. When voltage sags below a certain voltage threshold occur at sensitive industrial process, the industrial customer will experience financial damage. In order to mitigate voltage sag costs and devise efficient solutions to mitigate damage, a study on the financial loss assessment of voltage sags is basically needed. In order to assess the voltage sag costs, the expected sag frequency at a sensitive load point should be calculated by using the concept of the area of vulnerability and historical fault statistics. Then, financial loss due to voltage sags can be obtained by multiplying the expected sag frequency by the cost per sag event.

• Proceedings of the KIEE Conference
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• 2005.07a
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• pp.307-309
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• 2005

This paper studies the effects caused by symmetrical and unsymmetrical voltage sags on three-phase transformer and induction machine. The voltage sag on transformer and induction machine gives rise to inrush current. This inrush current makes sag more severe. These effects depend or many elements such as sag magnitude and duration, type of sag, and fault and recovery voltage instants.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.1
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• pp.29-34
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• 2015

This paper presents a method to select optimal device for mitigating voltage sags. The method is based on economic evaluation and voltage sag assessment involving sag duration as well as magnitude. The economic evaluation is performed by using the operation cost and economic benefit of the mitigation devices. The optimal device can be determined from the values of NPV (net present value) which is widely accepted in cost-benefit analysis. The proposed method can help sensitive customers to select optimal mitigation device. In this paper, the case study considering two sensitive customers was performed by using the proposed method.