Kwang-Cheol Ko

Hanyang University, Sŏul, Seoul, South Korea

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Publications (61)21.19 Total impact

  • Kun-A Lee · Kwang-Cheol Ko
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    ABSTRACT: The virtual cathode oscillator (vircator), which is one of the microwave sources, has been widely studied for about 40 years, since it has advantages, such as its simple structure and high-power radiation. To implement the behavior of the vircator, a simulation model of the axial vircator using circuit elements is studied in this paper. There are two steps in simulation of this paper: 1) obtaining the optimized case in the fixed input power that has a maximum efficiency and 2) at the case, conducting the circuit modeling. Passive elements and switches are used to model the power losses, emission threshold, etc. This circuit analysis is simulated by the Electro Magnetic Transient Program. Through the simulation model of the axial vircator using circuit elements, the high-power electromagnetic system can be handled as an integrated system, which contains a prime power part, a pulsed power part, a microwave source part, etc. This means that the circuit analysis can be carried out from the prime power to the vircator.
    No preview · Article · Oct 2014 · IEEE Transactions on Plasma Science
  • Kun-A Lee · Kwang-Cheol Ko
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    ABSTRACT: The risk of high power electromagnetic (HPEM) pulse is dramatically increasing as the output of HPEM pulse increases and the affected electronic devices becomes smaller [1], [2]. There are largely two ways of coupling HPEM pulse: 1) front door coupling, which is through antennas and sensors to receive signals and 2) back-door coupling, which is through an unintended way, such as punctures and slots. In this paper, we modeled the loss phenomenon of HPEM pulse which happens when HPEM pulse radiated from HPEM system propagates to the receiving antenna. The loss circuit consists of serial-parallel resistors, each value of resistor is formularized as distance. Our modeling consists of two parts: 1) one-stage model, which means initial value for various applications and 2) nth-stage model, which means additional distances. Therefore, we could figure out the effect of HPEM on antenna as the propagation distance changes. After the due simulations, we confirm our hypothesis as the HPEM pulse which reaches to receiving antenna is consistent with the existing theory. This circuit modeling is very remarkable in that it understands two different HPEM system and propagation/receiving antenna as one whole system.
    No preview · Article · Oct 2014 · IEEE Transactions on Plasma Science
  • Sung-Gyen Lee · Young-Maan Cho · Kwang-Cheol Ko

    No preview · Article · Sep 2014
  • Sung-Gyen Lee · Young-Maan Cho · Kwang-Cheol Ko
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    ABSTRACT: In high-speed train, the EMI is generated by pantograph detachment and switching device of motor-block. Especially, the conductive noise flows through rail as return feeder influences unintended results to sub-station, transformer, etc. These phenomena were investigated by PSIM circuit analysis tool and each part of railway system is modeled by circuit elements and switching devices. Consequently, the distorted wave in return feeder current occurs by the high speed switching frequency, and the overvoltage is applied by the pantograph detachment. Also the distortion of return current is high in starting point and it decreases from the proximity of a starting point ro the end of terminal.
    No preview · Article · Dec 2013
  • Heung-Jin Ju · Kwang-Cheol Ko · Dong-Kyue Kim
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    ABSTRACT: An application of a functionally graded material (FGM) to the solid spacer in gas insulated switchgears (GISs) can reduce the electric field concentration at the specific region, for example, the anode or the interface between the spacer and the gas. However, the electric field stress near the triple junction of the grounded electrode with a rounded shape, which remarkably affects the insulation capability of the GIS, inversely related. Therefore, in order to prevent this, it is necessary to modify the grounded electrode geometry in the common Cubicle-GIS and apply the elliptical FGM spacer with a reverse-direction variation unlike the permittivity variations of the existing unidirectional or bidirectional distributions. In this paper, we found the proper distribution of dielectric permittivity, dug a groove in the grounded electrode near the triple junction, and performed the optimization of this grounded electrode configuration by using the design of experiments (DOE). Consequently, the electric field stress in the triple junction of the grounded electrode can be effectively reduced by using the optimally designed grounded electrode shape applying the FGM spacer.
    No preview · Article · Oct 2013 · IEEE Transactions on Dielectrics and Electrical Insulation
  • Kun-A Lee · Jae-Ho Rhee · Young-Maan Cho · Ji-Eun Baek · Kwang-Cheol Ko
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    ABSTRACT: RF system plays an important role in many applications such as military, telecommunication and etc. Especially an antenna used in the RF system is essential in modern society because it transmits and receives a variety of information by wireless. However it is susceptible to unwished microwaves such as noise and electromagnetic shock wave. Compared to the conventional microwave, electromagnetic shock wave can reach up to 100 MW in peak power and span within the range of frequencies between 1 and 300 GHz, which could destroy the electronic equipment. Therefore it is important to analyze the damage of the RF system affected by electromagnetic shock wave. The damage of RF system falls into four types: (1) Deception, (2) Jamming, (3) Upset, (4) Burnout. Among these, the burnout, which means physical damage to RF system, is fatal. There are two main coupling paths in microwave couplings. One is the front-door coupling which is through intentional receptors such as antennas and sensors and the other is the back-door coupling which is through apertures intended for other purposes or incidental coupling to the construction of the target system such as seams, cracks and etc. The former could strongly give affection the target. In this paper, we predict damages of the radar system by electromagnetic shock wave. Main damage type considered is the burnout and only the front-door coupling is covered. It is simulated from electromagnetic shock wave propagation to coupling the radar system via circuit modeling method. We modeled the equivalent circuit for antenna and sub-electronic equipment. The energy induced by electromagnetic shock wave through antenna flows to weak parts of the radar system. The affected parts are analyzed by EMTP (Electro Magnetic Transient Program). Using the damage prediction, the protection plans can be established.
    No preview · Conference Paper · Jun 2013
  • Sung-Gyen Lee · Kun-A Lee · Jae-Ho Rhee · Kwang-Cheol Ko
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    ABSTRACT: Vircator (Virtual Cathode Oscillator), which is one of the microwave sources, has been stuided widely for more than 20 years since it has such advantages as simple structure and high power radiation 1. As electromagnetic pulse (EMP) emitted by vircator has been developing, the studies about hazard of the EMP on target have been also increasing 2. In these applications it is difficult to build the whole system for EMP protection, because it is costly, dangerous and complex. For this reason, the vircator simulator to be composed circuit elements (R, L, C and etc.) is strongly needed. Numerically, vircator can be analyzed by PIC (Particle In Cell). However it takes too many times in computation. Therefore, the purpose of this paper is to build a vircator simulator via circuit elements, which could have its equivalent behavior with lower computation. In this paper, passive elements and switch are used to model for space-charge limitation, absorptions, oscillation and etc. The number of oscillation between the anode and the virtual cathode is limited. These circuit analysis is simulated by EMTP (Electro Magnetic Transient Program). Through the circuit-elements-composed-simulator, it can be handled with as the whole system for EMP protection. This means that circuit analysis can be carried out from the prime power to vircator. It is expected to help the improvement of EMP system with vircator efficiently.
    No preview · Conference Paper · Jun 2013
  • Source
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    ABSTRACT: There are two winding methods for the interior permanent-magnet synchronous motor for vehicle traction: concentrated winding and distributed winding. Both have merits and demerits, and both influences the motor's performance. In this paper, designs of concentrated-and distributed-winding motors with the same capacities are presented, and each motor's characteristics were analyzed using the finite-element method. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International Conference on Advances in Energy Engineering (ICAEE).
    Preview · Article · Dec 2012 · Energy Procedia
  • Kun-A Lee · Kwang-Cheol Ko
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    ABSTRACT: Tapered transmission line was used for impedance matching, for high voltage pulse, and atmospheric medium is applied to characteristic equation of tapered transmission line and reflection coefficient so that nonlinear load and circuit modeling of atmospheric medium was simulated by electromagnetic transient program (EMTP). A characteristic of atmospheric medium and Time delay are decided by inductance and capacitance of tapered transmission line. For electromagnetic wave propagation modeling, in this paper, tapered transmission line is implemented. It is difficult to model tapered transmission line directly. Other transmission line that can be expressed by the circuit is used. So object of this paper is efficient modeling of tapered transmission line.
    No preview · Article · Sep 2012 · Japanese Journal of Applied Physics
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    Heugn-Jin Ju · Dong-Kyue Kim · Kwang-Cheol Ko
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    ABSTRACT: A functionally graded material (FGM) spacer, which the distribution of dielectric permittivity inside an insulator changes spatially, can considerably reduce the electric field concentration around a high-voltage electrode and along the gas-insulator interface when compared to a conventional spacer with a uniform permittivity distribution. In this research, we propose the FGM spacer with an elliptical permittivity distribution instead of that with a distribution of dielectric permittivity varying along a radial direction only in order to improve efficiently the insulation capability. The optimal design of the elliptical FGM spacer configuration is performed by using the response surface methodology (RSM) combined with the steepest descent method (SDM).
    Preview · Article · Jun 2012
  • Chi-Wuk Gu · Kun-A Lee · Heung-Jin Ju · Kwang-Cheol Ko · Cheol-Kyou Lee
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    ABSTRACT: Vacuum interrupter (VI) has been commercially available in medium voltage switching equipment. Recently, the development of the VIs is focused to reduce the size. Thus, it is necessary to a smaller size contact. In this paper, we investigated the interruption behavior in the real VI with two different contact size of the transverse magnetic field (TMF) type which is widely used for interrupting the arc current in vacuum. Additionally, we investigated the interrupting capability according to the change of opening speed by using contact with the lower interrupting performance obtained from earlier tests. In our experiment, CuCr25 contact was applied, Weil-Dobke voltage superposition circuit was used for a short circuit current test. Through the correlation between the contact size and opening speed, we could design a smaller VI, and it was applied to 17.5kV 25kA vacuum circuit breaker (VCB).
    No preview · Conference Paper · Jan 2012
  • Chi-Wuk Gu · Jae-Ho Rhee · Heung-Jin Ju · Kwang-Cheol Ko
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    ABSTRACT: An application of a functionally graded material (FGM) to the solid spacer in gas insulated switchgears (GISs) can reduce the electric field intensity. Especially, the location of the high electric field concentration moves from the anode to the interface between the spacer and the gas, when the FGM spacer is used. However, the electric field stress near the triple junction of the cathode with a rounded shape, which remarkably affects the insulation capability of a GIS, increases reversely. Therefore, in order to prevent this, it is necessary to modify the cathode geometry in the common C-GIS. In this research, we dug a groove in the cathode near the triple junction, and performed the optimization of this cathode configuration by using the design of experiments (DOE). Additionally, the permittivity graded spacer with the permittivity variation of a reverse direction distribution unlike that of the existing unidirectional or bidirectional distribution was applied. Consequently, both the maximum electric field intensity generating near the inflection point of the spacer geometry and the electric field stress near the triple junction of the cathode can be efficiently reduced by using the FGM spacer and designing the optimal cathode shape.
    No preview · Conference Paper · Jan 2012
  • Kun-A Lee · Jong-Yoon Park · Kwang-Cheol Ko
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    ABSTRACT: In recent years, as RF systems have been increasing rapidly, there is a growing need for studies about disturbance of electromagnetic shock wave propagation generated by high voltage pulse generator. Through frontdoor or back-door, electromagnetic shock wave can cause a upset or a lockup, that is, unsteady state in RF systems, and even cause a latchup or a burnout, which is permanent breakdown of the RF systems. Numerically, electromagnetic wave propagation can be analyzed by FDTD and FEM. However, it is difficult to calculate the entire high power microwave system using FDTD and FEM. Therefore, the purpose of this paper is to simulate the propagation of electromagnetic shock wave by using the electric circuit modeling methods without complicated calculations. In this paper, a concept of tapered transmission line is adopted as the electric circuit modeling methods. The tapered transmission line is used for impedance matching, and impedances of the tapered transmission line depend on the location. Because of its characteristic, the electromagnetic wave in the tapered transmission line is reflected and refracted while it is propagating. The electromagnetic shock wave propagation in air is expected by a reflection and a reflection coefficient in the tapered transmission line, and then we can simulate the electric circuit models with load by using EMTP (ElectroMagnetic Transient Program). We can describe the various electric circuit models of the electromagnetic shock wave in air. By using these models we can deduce the behavior of high power microwave propagation.
    No preview · Conference Paper · Jan 2012
  • Heung-Jin Ju · Bongseong Kim · Kwang-Cheol Ko
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    ABSTRACT: A functionally graded material (FGM) was investigated for use as an insulator in high voltage applications. The FGM was able to relax the electric field concentration around a high voltage electrode and along the gas-insulator surface. The FGM spacer, the permittivity of which was gradually changed, exhibited a considerable reduction in the maximum electric field when compared to a conventional spacer with a uniform permittivity. It is difficult to apply a gradual permittivity variation in the FGM spacer to real product processing due to its complicated shape. Thus, in this work, the electrode shape in the gas insulated switchgear was changed in order to increase the possibility of real FGM insulator manufacturing. To achieve this goal, optimization processes were used to modify the shape of both the electrode and the FGM spacer on a commercial gas insulated switchgear configuration. Especially, a modification of the spacer configuration was performed with the design of experiments (DOE). Consequently, the insulation capability of the switchgear with the optimally designed FGM spacer can be efficiently improved.
    No preview · Article · Aug 2011 · IEEE Transactions on Dielectrics and Electrical Insulation
  • Bongseong Kim · Heung-Jin Ju · Kwang-Cheol Ko · Eiki Hotta
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    ABSTRACT: Metal-oxide-silicon (MOS)-gate-structure-based power semiconductors, such as MOS field-effect transistors, insulated-gate bipolar transistors, and MOS controlled thyristors, are widely used as high-voltage switch and power modulator components in pulsed-power applications. The power semicon- ductors are generally connected in series and in parallel in order to increase their maximum switching voltage and current, respectively. It is important to suppress overvoltage or switching stress on power semiconductors connected in series and parallel during an extremely short switching time and at fast operating frequency. Generally, gate drive control techniques and methods for the suppression of high voltage are required. To suppress overvoltage and switching stress, this paper proposes a simple and effective active clamping method rather than the use of a snubber circuit with free switching condition modulation. Based on comparative switching experiments, the active clamping method is expected to suppress switching stress and overvoltage while load and switching conditions are changed without modification of the high-side auxiliary circuit for pulsed-power applications.
    No preview · Article · Aug 2011 · IEEE Transactions on Plasma Science

  • No preview · Article · Jun 2011
  • Soon-Gook Cho · Kwang-Cheol Ko
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    ABSTRACT: We report on the hydrophobic coating for multi-walled carbon nanotubes (MWCNTs) under low pressure as treated with toluene(C6H5CH3) glow discharge plasma. We investigated optimizing time and the hydrophobic surface of the treated MWCNTs. In order to identify a change of the properties about the toluene plasma modification for treatment of MWCNTs powder, which was observed putting the MWCNTs in the water and calculated via measuring the contact angle between the cushion of MWCNTs powder using the formamide, glycerol and water such as probe liquids. The produced carbonaceous species from the toluene glow plasma were identified by optical emission spectroscopy. Also, we conducted the experiments by varying times from 1 to 10 min and we found optimizing time.The total surface free energies were determined by the Owens–Wendt equation and drastically decreased from 72.90 mJ/m2 to 0.79 mJ/m2, respectively.
    No preview · Article · May 2011 · Current Applied Physics
  • Bongseong Kim · Kwang-Cheol Ko · Eiki Hotta
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    ABSTRACT: It is difficult to design a standard gate driver for optimal switching control of a static induction thyristor (SI-Thy) because of its unique internal structure and the direct commutation switching characteristic between the gate and cathode terminals during the transient turn-on and turn-off switching phases. Therefore, it is important to develop a simple and effective gate driver for achieving optimal fast turn-on and stable switching operations in an SI-Thy under hard-switching conditions. To achieve faster turn-on switching time with shorter gate delay time, impedance matching between the SI-Thy and the turn-on driving circuit component in the gate driver can be realized with additional circuit modification through this paper. To ensure higher stabilities in the turn-off switching phase, forced commutation techniques and additional auxiliary circuits in the gate driver have been devised to suppress the latchup and to eliminate potential ringing possibility. We have demonstrated that our designed and tested gate driver for realizing optimal switching characteristics of an SI-Thy is effective, particularly in the gate driving circuits for various pulsed-power switching applications.
    No preview · Article · Apr 2011 · IEEE Transactions on Plasma Science
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    Bong-Seong Kim · Young-Chul Shin · Kwang-Cheol Ko
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    ABSTRACT: Dielectric Barrier Discharge (DBD) reactor with sinsodual AC type of power supply is very widely adopted for its compact size and effective discharging mechanism to generate high density of ozone radicals. However, at the aspect of design on power supply, its optimal switching conditions and topology is achieved by empirical test. Therefore, throughout this paper, it is proposed a design method of DBD power supply to guarantee a maximum ozone yield rate in accordance with DBD reactor modification and impedance variation when rapid gas discharging in the DBD reactor is proceeded.
    Preview · Article · Jan 2011
  • Soon-Gook Cho · Kwang-Cheol Ko
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    ABSTRACT: In this work, the super-hydrophobic coating for multi-walled carbon nanotubes (MWCNT) was treated with a 3:1 trimethylchlorosilane (TMCS)/toluene glow discharge plasma under low pressure and the super-hydrophobic surface of the treated MWCNTs was also investigated. In order to identify the effects of 3:1 TMCS/toluene plasma modification for treatment of MWCNTs powder, an investigation was conducted on the total surface free energies of the MWCNTs powder, which was calculated by measuring the contact angle between the cushion of MWCNTs powder and the probe liquid. The total surface free energies were determined by the Owens–Wendt equation and drastically decreased from 72.90mJ/m2 to 0.041mJ/m2, respectively.
    No preview · Article · Sep 2010 · Thin Solid Films