• The Transactions of the Korean Institute of Power Electronics
• /
• v.24 no.5
• /
• pp.334-341
• /
• 2019

This study proposes the control strategy for the seamless mode transfer of indirect current controlled parallel grid-connected inverters. Under the abnormal grid condition, the grid-connected inverter can convert the operation mode from grid-connected to stand-alone mode to supply power to the local load. For a seamless mode transfer, the time delay problems caused by the accumulated control variable error must be solved, and the indirect current control method has been applied as one of the solutions. In this study, the design of control parameters for the proportional-resonant-based triple-loop indirect current controller and the control strategy for the seamless mode transfer of parallel grid-connected inverters are described and analyzed. The validity of the proposed mode transfer method is verified by the PSiM simulation results.

• The Transactions of the Korean Institute of Power Electronics
• /
• v.23 no.4
• /
• pp.231-239
• /
• 2018

The phase-locked loop (PLL) is widely used in grid-tie inverter applications to achieve a synchronization between the inverter and the grid. However, its performance deteriorates when the grid voltage is not purely sinusoidal due to the harmonics and the frequency deviation. Therefore, a high-performance PLL must be designed for single-phase inverter applications to guarantee the quality of the inverter output. This paper proposes a simple method that can improve the performance of the PLL for the single-phase inverter under a non-sinusoidal grid voltage condition. The proposed PLL can accurately estimate the fundamental frequency and theta component of the grid voltage even in the presence of harmonic components. In addition, its transient response is fast enough to track a grid voltage within two cycles of the fundamental frequency. The effectiveness of the proposed PLL is confirmed through the PSIM simulation and experiments.

• 한국전산유체공학회:학술대회논문집
• /
• 2006.10a
• /
• pp.193-198
• /
• 2006

It takes a lot of time and effort to generate grids for numerical analysis of problems with ground effect because the relative attitude and height of airfoil should be maintained to the ground as well as the inflow. A low Mach number preconditioned turbulent flow solver using the overlap grid technique has been developed and applied to the ground effect simulation. It has been validated that the present method using the multi-block grid gives us highly accurate solutions comparing with the experimental data of the RAE 101 airfoil in an unbounded condition. Present numerical method has been extended to simulate ground effect problems by using the overlapped grid system to avoid tedious work in generating multi-block grid system. An extended method using the overlapped grid has been verified and validated by comparing with results of multi-block method and experimental data as well. Consequently, the overlapped grid method can provide not only sufficiently accurate solutions but also the efficiency to simulate ground effect problems. It is shown that the pressure and aerodynamic centers move backward by the ground effect as the airfoil approaches to the ground.

• Journal of radiological science and technology
• /
• v.33 no.4
• /
• pp.341-348
• /
• 2010

By accessing the current status of FPD system use in the hospitals located in Seoul and Gyeonggi Province as well as the entrance skin dose and the image quality evaluation realized by C-D Phantom, and the image assessment by the medical professionals regarding the radiography for the extremity, the following results were derived. 1. According to the evaluation made in the actual use of FPD system (12 machines), the grid ratio varied from 8:1 to 13:1, and 6 machines used the grid ratio with 12:1, realizing the largest number. Among the machines, there were 8 machines that allowed a removable grid while 3 machines did use a removable grid (25.0%). 2. When it came to the equipments used for the experiment, it showed that the amount of the entrance skin dose increased from 4.13 times up to 4.79 times with the grid use. 3. The difference in the entrance skin dose depending on the changes in the exposure condition(0.5times or 2.0times) was not significantly different regardless of the patients' thickness. 4. In terms of the image quality depending on C-D Phantom, the grid use was distinguished well. However, the images were well distinguishable as the exposure condition got increased. 5. In the clinical assessment, the grid use was less effective for the Hand PA, which was considered to shoot a thin body part. It was evaluated that the grid use was preferred for the Knee AP, which was shooting for a relatively thick body part. Nonetheless, 3 out of 5 people said that they would not use the grid if the entrance skin dose to reduced.

• KEPCO Journal on Electric Power and Energy
• /
• v.6 no.4
• /
• pp.413-418
• /
• 2020

It is necessary to manage the prediction accuracy of the machine learning model to prevent the decrease in the performance of the grid network condition prediction model due to overfitting of the initial training data and to continuously utilize the prediction model in the field by maintaining the prediction accuracy. In this paper, we propose an automation technique for maintaining the performance of the model, which increases the accuracy and reliability of the prediction model by considering the characteristics of the power grid state data that constantly changes due to various factors, and enables quality maintenance at a level applicable to the field. The proposed technique modeled a series of tasks for maintaining the performance of the power grid condition prediction model through the application of the workflow management technology in the form of a workflow, and then automated it to make the work more efficient. In addition, the reliability of the performance result is secured by evaluating the performance of the prediction model taking into account both the degree of change in the statistical characteristics of the data and the level of generalization of the prediction, which has not been attempted in the existing technology. Through this, the accuracy of the prediction model is maintained at a certain level, and further new development of predictive models with excellent performance is possible. As a result, the proposed technique not only solves the problem of performance degradation of the predictive model, but also improves the field utilization of the condition prediction model in a complex power grid system.

• The Transactions of the Korean Institute of Power Electronics
• /
• v.26 no.2
• /
• pp.75-81
• /
• 2021

This study analyzes the resonance characteristics caused by the mutual interference between LCL filters and the grid impedance under the parallel operation of the grid-connected inverter using the LCL filter. These characteristics are verified through simulation and experiment. Two inverters are used to connect to the grid in parallel, and the system parameters, including the LCL filter, are set to the same conditions. In the case of inverters running in parallel at the point of common coupling, the presence of grid impedance causes mutual interference between the LCL filters of each inverter, and the deviation of the filter resonance frequency is analyzed to understand the parallel inverter. The correlation between the number of devices and the size of grid impedance is simulated by PSIM and verified by MATLAB. By connecting the real-time digital simulator Typhoon HILS to the DSP 28377 control board, the mutual interference characteristics are tested under the condition of two inverters running in parallel. The experimental and analysis results are the same, indicating the validity of the analysis.

• Solar Energy
• /
• v.19 no.4
• /
• pp.63-69
• /
• 1999

The conventional grid connected PV(Photovoltaic) system has a unstable output pattern due to its dependence on the weather condition, although solar-cell array averagely has a regular output characteristics to have a peak output nearly at noon. Therefore assuming the high density grid connection in the future, this unstable output pattern can be one of the main reasons to generate power disturbance such as voltage variation, frequency variation and harmonic voltage generation in low voltage distribution line. However general grid connected solar-cell system do not have functions to cope with these disturbances. Therefore this study proposed a advanced type grid connected PV system with functions to suppress output power fluctuation due to solar-cell array output variation and showed the levelling effect of fluctuation due to instantaneous array output variation.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
• /
• 2002.10b
• /
• pp.195-196
• /
• 2002

Studied is fretting wear behaviour of transversely vibrating tube which is supported by springs and dimples. This simulates the fuel rod fretting due to flow-induced vibration in a nuclear reactor. The contact between spacer grid springs and fuel cladding tubes arc brought into focus in this paper. From the mechanical viewpoint, a concave contact shape of spring is considered to perform a wider distribution of the contact stress. Sliding/impacting experiments are conducted in air at room temperature with the conditions of positive contact force and gap existence to accommodate the mechanical condition between the fuel rod and the grid spring during reactor operation. It is found that wear region is separated and wear volume becomes larger as the supporting condition becomes poorer. Spring and dimple cause similar wear.

• KSTLE International Journal
• /
• v.10 no.1_2
• /
• pp.27-32
• /
• 2009

In this study, fretting wear tests of nuclear fuel rods have been performed by using two kinds of spacer grid springs with a concave and a convex shape in room temperature dry and distilled water conditions. The objectives were to examine the variation of the wear mechanism with increasing fretting cycles and to evaluate the difference of the wear debris detachment behavior at each test environment. From the test results, the wear volume of each spring condition increased with increasing fretting cycles regardless of the test environments. However, the wear rate did not show a regular tendency and apparently changed with increasing fretting cycles. This is because the formation of the wear particle layer and/or the variation of the contact condition between the fuel rod and spring surfaces could affect a critical plastic deformation for detaching the wear debris. Based on the test results, the relationship between the wear behavior of each spring shape and test environment condition, and the variation of the surface characteristics are discussed in detail.

• Journal of the Korea Society for Simulation
• /
• v.28 no.2
• /
• pp.23-33
• /
• 2019

A sub-munition which has low aspect ratio does not have flight stability and control of drag force under free-fall condition. In order to satisfy those problems, fin, which is called grid-fin, is designed instead of conventional flight fins and adapted to the sub-munition. The base model of the sub-munition is firstly set and numerical simulation of the model is conducted under transonic condition that is free-fall range of the sub-munition. Wind test is secondly performed to verify the simulation result. The result shows that grid fin adapted sub-munition has high drag force, but the flight stability is still needed. In order to enhance the flight stability, two additional grid-fins are designed which modify web-thickness and numerical simulations of modified models are conducted. As the results, the thinnest web-thickness grid-fin has the highest flight stability and still maintains high drag coefficient. Based on these results, design of grid-fin adapted sub-munition is completed, the path trajectory of the sub-munition can be predicted with acquired aerodynamic datum and it is expected that grid fin can be used to various shape of the flight vehicle and bomb.