• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.2
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• pp.95-101
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• 2017

A simple and practical voltage balance method for a solid-state transformer (SST) is proposed to reduce the voltage difference of cascaded H-bridge converters. The tolerance device components in SST cause the imbalance problem of DC-link voltage in the H-bridge converter. The Max/Min algorithms of voltage balance controller are merged in the controller of an AC/DC rectifier to reduce the voltage difference. The DC-link voltage through each H-bridge converter can be balanced with the proposed control methods. The design and performance of the proposed SST are verified by experimental results using a 30 kW prototype.

• Journal of the Korean Magnetics Society
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• v.12 no.1
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• pp.20-23
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• 2002

The local coercivity distribution of CoCrPt alloy films prepared by dc magnetron sputtering has been investigated by means of a magneto-optical microscope magnetometer (MOMM) capable of simultaneously measuring the local properties on 400 nm spatial resolutions. Serial samples of CoCrPt alloy films were prepared with the Pt composition of a range from 6 to 28 at. %. We find that the local coercivity distribution crosses over from Gaussian to non-Gaussian distribution in CoCrPt alloy films with increasing Pt composition, with increasing trends in the width of the distribution as well as the average local coercivity. Transmission electron microscopy (TEM) studies reveal that our findings are closely correlated with the dependences of the grain size distribution and its average size on Pt concentration.

• Journal of Electrical Engineering and Technology
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• v.10 no.1
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• pp.165-175
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• 2015

Energy storage system has been widely applied in power distribution sectors as well as in renewable energy sources to ensure uninterruptible power supply. This paper presents a model predictive algorithm to control a bidirectional AC-DC converter, which is used in an energy storage system for power transferring between the three-phase AC voltage supply and energy storage devices. This model predictive control (MPC) algorithm utilizes the discrete behavior of the converter and predicts the future variables of the system by defining cost functions for all possible switching states. Subsequently, the switching state that corresponds to the minimum cost function is selected for the next sampling period for firing the switches of the AC-DC converter. The proposed model predictive control scheme of the AC-DC converter allows bidirectional power flow with instantaneous mode change capability and fast dynamic response. The performance of the MPC controlled bidirectional AC-DC converter is simulated with MATLAB/Simulink(R) and further verified with 3.0kW experimental prototypes. Both the simulation and experimental results show that, the AC-DC converter is operated with unity power factor, acceptable THD (3.3% during rectifier mode and 3.5% during inverter mode) level of AC current and very low DC voltage ripple. Moreover, an efficiency comparison is performed between the proposed MPC and conventional VOC-based PWM controller of the bidirectional AC-DC converter which ensures the effectiveness of MPC controller.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.5B
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• pp.318-325
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• 2008

HomePlug AV(HPAV) is the standard for distribution of Audio/video content as well as data within the home by using the power line. It uses a hybrid access mechanism that combines TDMA with CSMA/CA for MAC technology. The CSMA/CA protocol in HPAV has two main control blobs that can be used for access control: contention window(CW) size and deferral counter(DC). In this paper, we extensively investigate the impacts of CW and DC on performance through simulations, and propose an adaptive mechanism that adjusts the CW size to enhance the throughput in HPAV MAC. We find that the CW size is more influential on performance than the DC. Therefore, to make controlling the network easier, our proposal uses a default value of DC and adjusts the CW size. Our scheme simply increases or decreases the CW size if the network is too busy or too idle, respectively, We compare the performance of our proposal with those of the standard and other competitive schemes in terms of throughput and fairness. Our simulation and analysis results show that our adaptive CW mechanism performs very well under various scenarios.

• Progress in Superconductivity and Cryogenics
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• v.12 no.4
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• pp.24-30
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• 2010

The renewable energy source is considered as a good measure to cope with the global warming problem and the fossil energy exhaustion. The construction of electric power plant such as an offshore wind farm is rapidly increasing and this trend is expected to be continued during this century. The bulky and long distance power transmission media is essential to support and promote the sustainable expansion of renewable energy source. DC power cable is generally considered as the best solution and the demand for DC electric power has been rapidly increasing. Especially, the high temperature superconducting (HTS) DC cable system begins to make a mark because of its advantages of huge power transmission capacity, low transmission loss and other environmental friendly aspects. Technical contents of DC HTS cable system are very similar to those of AC HTS cable system. However the DC HTS cable can be operated near its critical current if the heat generation is insignificant, while the operating current of AC HTS cable is generally selected at about 50~70% of the critical current because of AC loss. We chose the specifications of the cable core of 'Tres Amigas' project as an example for our study and investigated the heat generation when the DC HTS cable operated near the critical current by some electric and thermal analyses. In this paper, we listed some technical issues on the design of the DC HTS cable core and described the process of the cable core design. And the results of examination on the current capacity, heat generation, harmonic loss and current distribution properties of the DC HTS cable are introduced.

• Journal of the Korean Society of Industry Convergence
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• v.26 no.6_2
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• pp.1097-1108
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• 2023

Recently, demand for technology for energy economy and stable supply is increasing due to the increase in power demand of loads. The amount of DC power generation using new and renewable energy is noticeably increasing, and the use of DC power supplies is also increasing due to the increase in electric vehicles and digital loads. During parallel operation to increase the capacity of the power converter, the module bus method or the method using Can communication and serial communication has significant difficulties in smooth operation due to communication time delay for information sharing. Synchronization of information sharing of each power converter is essential for smooth parallel operation, and minimization of communication time delay is urgently needed as a way to overcome this problem. In this paper, a new communication method using pulse width information is proposed as a communication method specialized for parallel operation of power converters to compensate for the disadvantage of communication transmission delay in the existing system. The proposed communication method has the advantage of being easily implemented using the PWM and Capture function of the microcomputer. In addition, the DC/DC converter for DC distribution was verified through simulation and experiment, and it has the advantage of easy capacity expansion when applied to parallel operation of various types of power converters as well as DC/DC converters.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.1
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• pp.768-776
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• 2021

Current protection-coordination methods use the reverse time characteristics of the T-C curve, which is not effective for a LVDC distribution system because the protective operation time of converters and DC circuit breakers is much faster than AC protection devices. Therefore, an algorithm is proposed for fault-section isolation using the fault current slope to minimize the blackout region and coordinate between converters and protection devices in a rapid and accurate manner. The method deals with the slope characteristics of a fault current, which may depend on the fault location in an LVDC distribution system. Thus, an LVDC distribution system can be operated in a stable manner by isolating the fault section selectively before the shutdown of the main converter using slope characteristics, which change in proportion to the line impedance and fault location. A 1.5-kV LVDC distribution system was modeled to verify the effectiveness of the proposed algorithm using PSCAD/EMTDC. The system is composed of a distribution substation, LVDC converter, and distribution lines. The simulation results confirm that the proposed algorithm is a useful tool for minimizing the fault section in an LVDC distribution system.

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.55 no.1
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• pp.47-53
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• 2006

A Voltage Bus Conditioner (VBC) is used to mitigate the voltage transients on a common power distribution bus. The VBC described here utilises inductive storage and unlike its counter part with capacitive storage, it can employ the entire stored energy towards transients' mitigation. The performances of adaptive duty ratio control and sliding mode control have been compared. The simulation results (with the package SABER) indicate that the sliding mode control results in the shortest and the smallest bus voltage excursions.

• Applied Science and Convergence Technology
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• v.27 no.1
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• pp.19-22
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• 2018

The ion energy and angle distributions (IEADs) in the DC magnetron sputtering systems are investigated for the variation of gas pressure using particle-in-cell simulation. Even for the condition of collisionless ion sheath at low pressure, it is possible to change the IEAD significantly with the change of gas pressure. The bombarding ions to the target with low energy and large incident angle are observed at low pressure when the sheath voltage drop is low. It is because the electron transport is hindered by the magnetic field at low pressure because of few collisions per electron gyromotion while the ions are not magnetized. Therefore, the space charge effect is the most dominant factor for the determination of IEADs in low-pressure magnetron sputtering discharges.

• Applied Science and Convergence Technology
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• v.27 no.2
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• pp.26-29
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• 2018

The ion energy and angle distributions (IEADs) in the DC magnetron sputtering systems are investigated for the variation of gas pressure using particle-in-cell simulation. Even for the condition of collisionless ion sheath at low pressure, it is possible to change the IEAD significantly with the change of gas pressure. The bombarding ions to the target with low energy and large incident angle are observed at low pressure when the sheath voltage drop is low. It is because the electron transport is hindered by the magnetic field at low pressure because of few collisions per electron gyromotion while the ions are not magnetized. Therefore, the space charge effect is the most dominant factor for the determination of IEADs in low-pressure magnetron sputtering discharges.