Naoki Osawa

Generation of low-frequency homogeneous dielectric barrier discharge at atmospheric pressure

We found that generation of a homogeneous dielectric barrier discharge (DBD) in atmospheric-pressure air was possible at the frequency range from 32 Hz to 1.1 kHz using alumina (Kyocera A473) as a barrier material. In this homogeneous DBD, the discharge current was continuous with no pulse, and the luminosity became strong near the anode surface. Therefore, this homogeneous DBD is an atmospheric-pressure Townsend discharge (APTD) characterized by electron avalanche ( effect) and secondary electron emission from the barrier material ( effect). We also found that there is a domain in frequency and voltage of the stable generation of the APTD in air. This domain becomes wider with the increase of the ac frequency. If the applied voltage is a bit higher than the upper voltage limit of the stable generation of the APTD, the discharge mode changes from the APTD mode to an oscillation mode. In order to clarify the decisive factor of the generation of the low-frequency APTD, we investigated the influence of gaseous species (oxygen, nitrogen, and helium) and barrier materials (alumina and soda glass). From these experiments, we found that gaseous species are not a decisive factor in the stable generation of the low-frequency APTD, because the low-frequency APTD is generated in all gases tested. Here, the generation of the APTD in 99.5% oxygen may be the first achievement. On the other hand, we found that the barrier material plays an important role in the stable generation of the low-frequency APTD. This paper describes the detail of the experimental results and discussions of the low-frequency APTD.

Calculation of transient puffer pressure rise takes mechanical compression, nozzle ablation, and arc energy into consideration

Thermal puffer-type gas circuit breaker (GCB) has a high dielectric and current interruption capability. In order to design a good thermal puffer GCB, it is important to know the blast pressure for arc cooling. Although pressure calculation programs have been developed and used for design work, the basic characteristics, such as contribution of nozzle ablation gas to puffer pressure rise, amount of back flow gas to puffer chamber, and pressure distribution along gas passages during current interruption, are not well known. In this paper, pressure rise, mass flow, and temperature calculations were carried out using a new calculation model, which takes mechanical compression by puffer piston, nozzle ablation in the nozzle throat and arc energy into consideration. By analysis of the calculation results, we found the pressure rise mechanism is as follows. While fixed contact located in the divergent part of nozzle, all of the ablation gas generated from the nozzle wall cannot be exhausted from the nozzle and it leads to high-pressure generation in the nozzle throat. This pressure causes transfer of hot ablation gas back to the puffer chamber via gas passage. The puffer pressure increases thermally due to temperature rise by this mechanism. At a longer arcing time, as high puffer pressure was already established in the puffer chamber, the nozzle ablation gas cannot flow back to the puffer chamber. Besides as mass flow through nozzle is limited by low gas density, the puffer pressure rise is obtained by the mechanical compression of puffer piston.

Analysis of Nozzle Ablation Characteristics of Gas Circuit Breaker

Thermal puffer type gas circuit breakers have become a mainstream of high voltage circuit breaker, because they are compact in size and interrupt high current with low driving energy. In this type, ablation gas of nozzle material flows back to a puffer chamber during nozzle clogging, and it increases gas temperature and pressure in the puffer chamber. Namely, ablation gas plays an important role for puffer pressure build-up. However, the nozzle ablation characteristics are not yet well known. In this paper, we developed calculation program of nozzle ablation and using this program, we analyzed the amount of nozzle ablation and throat diameter after current interruption. The results found that the amount of nozzle ablation increases in proportion to the 2.2 power of interrupting current, and this calculation results agree fairly well with experimental results. It was also found that the dc component included in interrupting current greatly influences the amount of nozzle ablation. However, the phase of major current loop does not influence the amount of nozzle ablation.

Effect of puffer volume and operating force on dielectric recovery performance in thermal puffer-type gas circuit breaker

In order to investigate the influence of puffer volume V/sub P/ and operating force to dielectric performance of GCB at current interruption, we calculated the prospective dielectric withstand voltage V/sub W/ between arcing contacts. The withstand voltage was calculated by the electric field strength appeared on arcing contacts and the allowable electric field strength E/sub W//sup */ of hot gas due to thermal puffer action. We could clarify that large initial puffer volume V/sub P/ can increase withstand voltage V/sub W/ at current breaking point and also that an optimum point exists in operating force F. For example, it was found that in case of the operating force of 1000 kgf, the puffer volume V/sub P/ of 1.2 liter would be best choice in the interrupting condition of i=50 kA and arcing time of 1.35 cycle.

Analysis of nozzle ablation characteristics of gas circuit breaker

Thermal puffer type gas circuit breakers have become a mainstream of high power circuit breaker, because they are compact in size and interrupt high current with low operating force. In this type, ablation gas of nozzle material flows back to a puffer chamber during nozzle clogging, and it increases puffer pressure. Namely, ablation gas plays an important role for puffer pressure build up. However, the nozzle ablation characteristics are not yet well known. In this paper, using a newly developed calculation program of nozzle ablation, we analyzed amount of nozzle ablation and ablation rate. As the results, it was found that nozzle ablation occurs by radiation energy of high temperature arc and ablation rate are greatly influenced by interrupting current. It was also found that they increase with the increase of DC component of interrupting current. The calculation results agree fairly well with experimental results.

Calculation of transient puffer pressure rise taking mechanical compression, nozzle ablation and arc energy into consideration

Thermal puffer type gas circuit breaker has high dielectric and current interruption capability. In order to design a good thermal puffer GCB, it is important to know the blast pressure for arc cooling. Although pressure calculation programs have been developed and used for design work, the basic characteristics such as contribution of nozzle ablation gas to puffer pressure rise, amount of back flow gas to puffer chamber and pressure distribution along gas passages during current interruption are not well known. In this paper, pressure rise, mass flow and temperature calculations were carried out using a new calculation model, which takes mechanical compression by puffer piston, nozzle ablation in the nozzle throat and arc energy into consideration. By the analysis of calculation results, we found the pressure rise mechanism is as follows. While fixed contact located in the divergent part of nozzle, all of ablation gas generated from nozzle wall cannot be exhausted from the nozzle and it leads to high-pressure generation in the nozzle throat. This pressure causes transfer of hot ablation gas back to the puffer chamber via gas passage. The puffer pressure increases thermally due to temperature rise by this mechanism. At a longer arcing time, as high puffer pressure was already established in the puffer chamber, the nozzle ablation gas cannot flow back to puffer chamber. Besides as mass flow through nozzle is limited by low gas density, the puffer pressure rise obtained by the mechanical compression of puffer piston.

Properties of creeping discharge progressed in narrow gap between two solid dielectrics in PFAE oil

The behaviors of creeping streamers progressed in a narrow gap between two solid dielectric plates immersed in palm fatty acid ester (PFAE) oil have been investigated using ±1.2/50 µs and ±1.2/1000 µs lightning impulse voltages with ±140 kVpeak in maximum. It is shown that the growth of positive and negative streamers and the flashover voltage are affected by two interfaces between solid dielectrics, the presence of the back side electrode and the wave tail of applied impulse voltages. The temporal progressing processes of the creeping discharges in PFAE oil are observed using a high-speed image converter camera (ICC). These results on the creeping discharge are compared with those in commercial mineral oil.

Electrical and mechanical properties of nanocomposite materials containing electrically dispersed MWCNTs

Pristine carbon nanotubes (CNTs) aggregate severely due to the strong van der Waals binding energies and the dispersibility of CNTs in solvents is extremely poor. This is a formidable hurdle for fabrication of advanced CNT composite materials. The purpose of this research is to realize optimal dispersion of CNTs in organic solvents and to obtain a better understanding of electrical and mechanical properties in polymeric and elastomeric CNT composite materials. In this paper, we report the results obtained by the experiments.

Investigation on endurance of hydrophilic property of Carbon Fibers treated by air dielectric barrier discharge

Summary form only given. Recently, thermo plastics such as Polypropylene (PP) or polyimide are going to be used as matrix resin of Carbon Fiber (CF) composite materials because they have an advantage to be rewelded at above 130°C. However, mechanical strength of CF/PP composite materials is low because adhesion strength between CF surface and PP surface is low. Therefore, many researchers have investigated plasma surface modification to enhance the interfacial adhesion strength of CF surface1. On the other hand, from the industrial point of view, we need to investigate how long the effect of plasma surface modification lasts. In this study, we investigated the improvement of the hydrophilic property of CFs by air Dielectric Barrier Discharge (DBD), and also measured the endurance of its effect. Room air (temperature: 20~20.3 °C, relative humidity: 39.1~42.5%) was used as source gas of the DBD. The barrier material is alumina (ε=9.1, Type: A473, Kyocera corporation). Gas flow rate was fixed to 5 L/min. Gap length was fixed to 1.5 mm. Spread CFs (Thickness: 0.1 mm, T700SC-12000, Toray Industries, Inc.) were used in this experiment and they were inserted in the center of the discharge gap. Discharge power was fixed to 50 W. Light emission from air DBD with CFs was observed by a multi channel spectrometer. Hydrophilic property was estimated by the water absorbing property test (Byreck method). DBD treatment time was changed from 10 s to 60 s. In order to clarify the endurance of the DBD effect, hydrophilic property of CFs was measured after 1 day to 3 days from DBD treatment. The results showed that (1) hydrophilic property of CFs increased with the increase of DBD treatment time and the rate of increase was 158% at 60 s DBD treatment in comparison with untreated CFs, (2) since O and OH radical were detected in air DBD with CFs, oxygen containing polar functional groups such as -OH and -COOH seemed to be incorporated on the CF surface, and (3) the hydrophilic property of CF treated by DBD slightly decreased with time and the rate of decrease was 9% at 3 days after the DBD treatment in comparison with untreated CFs.

Decrease in dinitrogen monoxide (N 2 O) generation of air-fed ozone generator using Atmospheric Pressure Townsend Discharge

Summary form only given. Ozone is strong oxidizing agent and it can be applied to air quality control, waste water treatment, etc. So far, we succeeded in generating an Atmospheric Pressure Townsend Discharge (APTD) in air using a simple DBD device and have investigated the difference of the by-products of ozone by the difference of discharge modes. Ozone generation experiments were carried out using APTD and Filamentary Discharge (FD) modes by feeding dry-air. The experimental results showed that in both types of discharges, HNO3, N2O5 and N2O were detected as by-products, however the amount of the by-products by APTD was less than by FD. Since the reduced electrical field of the APTD is lower than that at the tip of streamer heads of FD, the dissociation and excitation of nitrogen molecules and water vapor were weak in APTD. This is a reason that NOx generation was suppressed by APTD. In this study, we investigated the effect of gap length on ozone and N2O generation characteristics because the reduced electric field strength of APTD can be changed by changing gap length. Dry-air (absolute humidity: 119.3 mg/m3) was used as the source gas of APTD ozone generator. The flow rate was fixed to 2.0 L/min using a mass flow controller. Gap length was changed from 1.1 mm to 3.1 mm. The barrier material is alumina (Type: A473, Kyocera Corporation). The amount of ozone was measured by changing the discharge power. N2O concentration was measured by an FTIR spectrometer with long path gas cell (length: 3 m). On the other hand, ozone concentration was measured by a UV absorption type ozone monitor. The results showed that at the same ozone concentration, the N2O concentration decreased with the increase of gap length, which support our idea that the reduced electric field strength will influence the by-product generation.