Heung-Jin Ju

The space charge effect on the corona discharge of SF/sub 6/ gas

In this paper, the characteristics of corona discharge in SF/sub 6/ gas used as an insulating medium in lots of high-voltage equipment, are simulated by using the finite-element method (FEM) with the flux-corrected transport (FCT) method. The properties of negative corona discharge in needle to plane electrode are analyzed at different discharge stages. For the accurate results, the secondary electron emission by photon and ion as well as Townsend first ionization and electron attachment are considered. The simulation results show that the electric field intensity between anode and ion group decreases according to the electric field distortion by space charge as time goes by. It is also found that the electron density is reduced sharply by the superior electron attachment effect of SF/sub 6/ gas so that the corona discharge has been extinguished abruptly.

Active Clamping Circuit to Suppress Switching Stress on a MOS-Gate-Structure-Based Power Semiconductor for Pulsed-Power Applications

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 semiconductors 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.

Optimization of a grounded electrode shape in gas insulated switchgear with a reversely elliptical permittivity graded insulator

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.

Optimal design of an elliptically graded permittivity spacer configuration in gas insulated switchgear

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.

Investigation of simple graded permittivity solid spacer shape by electrode modification on gas insulated switchgear

It was investigated and proved that the Functionally Graded Material (FGM) as an insulation material for high voltage applications are able to relax the electric field concentration around the high voltage electrode and along the gas-insulator surface. Based on the previous numerical analysis, it was proved that the FGM spacer, which permittivity has gradually changed, with the modification of a spacer configuration by the Design of Experiment (DOE) showed considerable maximum electric field reduction in comparison with a conventional spacer which has uniform permittivity. However, it is hard to apply a gradual permittivity variation in the FGM spacer for a real product processing due to its complicated shape. Thus, through this paper, it is proceeded to change the electrode shape in the gas insulated switchgear in order to increase the possibility of real FGM insulator manufacturing. To achieve the goal, the optimization processes are used to modify the both shape of electrode and FGM spacer on a commercial gas insulated switchgear configuration. Consequently, the insulation capability of the switchgear with the optimally designed FGM spacer can be efficiently improved.

The characteristic evaluation of brazing joint according to the Ni plating to one-side of the base metal

Vacuum interrupters (VIs) as the most important part of a vacuum circuit breaker play a role to interrupt the fault current by rapidly extinguishing the arc. VIs has been manufactured by the vacuum brazing of a various of materials such as a stainless steel-stainless steel, stainless steel-copper, stainless steel-ceramic, etc. It has been reported by existing studies that wettability and flowability are improved by the Ni plating on a stainless steel surface in brazing of a stainless steel-stainless steel. In this research, in brazing of a stainless steel-stainless steel using AgCu filler metal we performed the Ni plating to only one-side of the base metal, and then observed the microstructure of joints and the melting properties of the base metal according to the brazing time. Additionally, the mechanical properties of the bonded specimens were measured by a tensile strength test and the composition of an image of joints were investigated by SEM and EDS analysis.

Optimization of a cathode configuration in gas insulated switchgear with a permittivity graded insulator

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.

The optimal design and comparison of power supply for dielectric barrier discharge ozone reactor

It has been noticed that Dielectric Barrier Discharging (DBD) has a great potential as ozone radical and light source in medical and environmental industry. But, due to its operating pressure, it is impossible to measure its electrical characteristics in direct. For the first step, it is proceeded electrical modeling of planer types of DBD reactor by using 2-port circuit analysis method with DBD discharging mechanism to estimate the electrical characteristics of DBD reactor. Through the modeling of DBD reactor, 2 types of power supplies of the DBD reactor for achieving high efficient that indicates impedance matching with DBD reactor are analyzed.

Interrupting capability of vacuum interrupter by various parameter

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).

Application of double grid cathode for improvement of neutron yield in IEC device

Recently, in order to improve a neutron yield in an Inertial Electrostatic Confinement (IEC) device operating at a glow discharge mode, various techniques have been performed, such as injection of plasma ion source, introduction of multi-grid configuration, coating electrode with titanium, magnetic assisted IEC device, and so on. However, these techniques lead to the complicated structure of a device which limits the flexibility of the shape design and are not easy to make an IEC device compact and portable. In this research, we have proposed a new configuration for the yield improvement without additionally external ion sources; an application of a double grid cathode. A neutron yield in an IEC device is proportional to the ion current and is closely related to the potential well structure inside the cathode. Therefore, when a double grid cathode is applied to an IEC device, the high electric field strength near the cathode is generated and thus, the high neutron production rate will be obtained. These facts are verified as compared with the ion current calculated at both single and double grid cathode shape. Additionally, ions lives and trajectories are examined in various grid cathode configurations and cathode voltages by using a numerical simulation.