• Proceedings of the KIEE Conference
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• 2011.07a
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• pp.91-92
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• 2011

Beat is a phenomenon, where the magnitudes and frequencies of the voltage and the current fluctuate. This paper describes a quantitative analysis of the beat phenomenon of a wind farm using the envelope of a current during intertie/grid fault conditions. In this paper, the ratio of the crest to the trough of the envelope curve and the time interval between adjacent troughs are defined and used to evaluate the beat phenomenon quantitatively. Beat phenomena under various fault and wind conditions are analyzed. The proposed quantitative analysis seems simple but effective in the more understanding of beat phenomenon of a wind farm, and thus can be used as a basis for operation and/or protection of an intertie.

• Journal of the Korean Solar Energy Society
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• v.33 no.5
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• pp.34-40
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• 2013

Islanding phenomenon of photovoltaic system is undesirable because it leads to a safety hazard to utility service personnel and may cause damage to power generation and power supply facilities as a result of unsynchronized re-closure. Anti-islanding protection is an important technical requirement for grid-connected PV system. Until now, various anti-islanding methods for detecting and preventing islanding of photovoltaic and other distributed generations have been proposed. Most of them are focusing on the anti-islanding performance of single PV system according to the related international and domestic standard test procedures. There are few studies on the islanding phenomenon for multiple photovoltaic operation in parallel. This paper presents performance analysis of anti-islanding function for grid-connected PV inverter systems when several PV inverters are connected in parallel.

• Journal of Electrical Engineering and Technology
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• v.11 no.3
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• pp.675-682
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• 2016

Sudden voltage drops and outages frequently disturb the operation of sensitive loads for domestic, commercial, and industrial use. In some cases, these events may even impair the functioning of relevant equipment. To maintain power under such conditions, a UPS system is usually installed. Once a disturbance happens at the grid side, the line-interactive UPS system takes over the load to prevent an interruption. But, due to magnetic saturation of the transformer, a significant inrush current may occur for the transformer-coupled loads during this transition. The generation of such transient currents may in turn decrease the line voltage and activates over-current protecting devices of the system. In this work, a cost-effective, line-interactive UPS system is proposed that eliminates the inrush current phenomenon associated with transformer-coupled loads. The strategy was implemented by connecting a standard current-regulated voltage source inverter (CRVSI) to the secondary winding of the load transformer. During any transient condition at the grid side, the load current is monitored and regulated to achieve either seamless compensation of the load current or complete transferal of load from grid to the inverter. Experimental results were obtained for a prototype under all possible operating conditions so as to validate the performance of the proposed topology.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.5
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• pp.307-314
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• 2021

Even though the LCL filters have superior harmonic attenuation ability to L filters, stability has always been an issue. The system could be unstable because of the resonance phenomenon, especially when digital controller is used. Adding a notch filter to the compensator is one approach to solve the problem. Resonance phenomenon can be inhibited by aligning notch frequency to system resonance frequency. However, resonance frequency variation can be obtained because the actual system has a nonstationary characteristic. Therefore, the system could be unstable, where the system parameters are changed when the conventional notch filter is used. An adaptive digital notch filter that stabilizes the system even system parameters are changed. Simulation and experiment results are provided to verify the validity of the proposed adaptive filter.

• Journal of Power Electronics
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• v.18 no.3
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• pp.853-862
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• 2018

Frequency coupling in the phase domain is a recently reported phenomenon for phase locked loop (PLL) based three-phase grid-inverter systems. This paper investigates the mechanism and stabilization method for the frequency coupling to the stability of grid-inverter systems. Self and accompanying admittance models are employed to represent the frequency coupling characteristics of the inverter, and a small signal equivalent circuit of a grid-inverter system is set up to reveal the mechanism of the frequency coupling to the system stability. The analysis reveals that the equivalent inverter admittance is changed due to the frequency coupling of the inverter, and the system stability is affected. In the end, retuning the bandwidth of the phase locked loop is presented to stabilize the three-phase grid-inverter system. Experimental results are given to verify the analysis and the stabilization scheme.

• The Journal of Korea Robotics Society
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• v.2 no.3
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• pp.227-233
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• 2007

Representing an environment as the probabilistic grids is effective to sense outlines of the environment in the mobile robot area. Outlines of an environment can be expressed factually by using the probabilistic grids especially if sonar sensors would be supposed to build an environment map. However, the difficult problem of a sonar such as a specular reflection phenomenon should be overcome to build a grid map through sonar observations. In this paper, the NRF(Neighborhood Recognition Factor) was developed for building a grid map in which the specular reflection effect is minimized. Also the reproduction rate of the gird map built by using NRF was analyzed with respect to a true map. The experiment was conducted in a home environment to verify the proposed technique.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.6
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• pp.440-445
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• 2018

This study proposes a novel control and design method for a grid-connected inverter with an LCL filter without damping. The current resonance phenomenon must be considered when designing the grid-connected inverter system with an LCL filter. Passive or active damping is used in the inverter system to reduce the resonant current. However, passive damping reduces the efficiency of the system, and active damping methods are complex. If the resonant frequency is in a specific region, then the system will be unstable. This study examines the process of stabilizing the entire region without resonant damping. The validity of the proposed method is verified through simulation and experimentation.

• Journal of radiological science and technology
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• v.46 no.2
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• pp.89-97
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• 2023

With the introduction of the D.R system, the grid with high removal rate of scattered radiation is selected and used without considering the grid performance evaluation. Despite the high removal rate of scattered radiation belonging to high grid ratio, it is also possible to see the cut-off phenomenon in which the primary ray involved in the formation of image could be removed as well. Thus, the purpose of this study is to provide the basic data for researches on the usefulness of using the grid by presenting the appropriate grid ratio in the D.R system through the measurement of image in grid such as non-grid, 6:1, 8:1, 10:1, 12:1. The equipments used for this study were radiation generator with grid in 12:1, 10:1, 8:1, 6:1, indirect-type detector, and acryl phantom. As the study for image evaluation, this study measured the SNR, PSNR, MSE, and Entropy. In the results of this study, the PSNR was the highest in 6:1 and the lowest in 8:1. The SNR was high in 6:1 and 8:1, and the lowest in 12:1. In case of Entropy, it was high in 8:1 and 10:1, and the lowest in 12:1. Therefore, when the grid is used, it would be more proper to choose the grid in 8:1 or 10:1 with less loss in information content of primary ray rather than the high grid ratio showing the increased patient exposure dose.

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.65 no.1
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• pp.1-6
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• 2016

Recently, proportion of the induction motor load is gradually increased. When a contingency in the power systems, it has been discovered phenomenon that the voltage is delayed recover caused mechanical characteristics of the induction motor load. It can be a serious impact on the voltage stability of the power system considering induction motor load. The scheme to mitigate this phenomenon tripping off the motors to prevent voltage drop and delayed voltage recovery on the load demand side. Fault induced delayed voltage recovery phenomenon is caused by stalling of small induction motor load in transmission level contingencies. In this paper, fault induced delayed voltage recovery phenomenon mitigation method implementation under voltage load shedding on the korean power system considering induction motor load.

• Journal of the Korean Solar Energy Society
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• v.32 no.spc3
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• pp.289-294
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• 2012

In recent years, understanding the dynamics of DC distribution system has become critically important due mainly to the increasing needs for the interconnection of DC distributed generators and the (DC-based) electric vehicle (EV) charging systems. In this paper, the characteristics of the DC grid system connected to the compact proton exchange membrane fuel cell (PEMFC) has been studied. In particular, the voltage and current transient phenomena were measured by varying the load of the DC grid system. Also, the voltage and current ripple were measured at the different load conditions. Our experimental results clearly manifested that the study contributes to the establishment of fundamental method to characterize the small DC grid system including distributed generation.