Tomohiro Furusato

Initiation mechanism of a positive streamer in pressurized carbon dioxide up to liquid and supercritical phases with nanosecond pulsed voltages

In this work, the initiation process of an electrical discharge in pressurized carbon dioxide up to the liquid and supercritical phases was investigated using Schlieren and photomultiplier techniques. A pulsed positive voltage with a rise time of about 40?ns and half-width of 150?ns was applied to a point-to-plane gap. The experimental results showed that the discharge started with a primary streamer consisting of a burst primary streamer and a successive primary streamer, and a back discharge followed them. It was predicted from an analysis of the experimental results that the initiation criterion of a burst primary streamer was electron multiplication on the order of 108 independent of the medium conditions. That is, a direct ionization process without bubble formation is supported as an initiation mechanism of nanosecond positive discharges in pressurized carbon dioxide. The medium state in the burst primary streamer channel was a gas or pseudo-gas of 50?200?kg?m?3 density.

Initiation Mechanism of a Negative Nanosecond Pulsed Discharge in Supercritical Carbon Dioxide

This paper deals with the initiation mechanism of a negative nanosecond pulsed discharge in supercritical (SC) carbon dioxide that was examined in detail using Schlieren and photomultiplier techniques. A negative pulsed voltage with a rise time of about 90 ns and half-width of 410 ns was applied to the point electrode. The experimental results show that: 1) The negative primary streamer was of a form quite different than that of a positive one: a bushlike negative streamer and a filamentlike positive streamer; 2) a drastic change in the density dependence of streamer initiation voltage appeared around the subcritical phase in the characteristics for streamer initiation voltage versus medium density; and 3) a shock wave of speed 1-1.5 Mach began growth following a delay of around 50 ns from the streamer initiation. It was predicted from the analysis of the experimental results that the drastic change in the streamer initiation voltage versus medium density characteristics may be due to a transition between the two types of generation mechanisms of the initial electron: electron detachment from negative ions in the gas phase and field electron emission from the point electrode in the SC and liquid phases. The shock wave was presumed to be caused by thermal relaxation of the vibrational energy in carbon dioxide molecules in a decay process of the primary streamer.

Visualization of positive pulsed streamer in supercritical carbon dioxide by Schlieren Method

This paper deals with visualization of positive pulsed streamers and arc discharge under a needle-to-plane gap in supercritical carbon dioxide (SCCO2) by the Schlieren Method. In addition, differences in streamer branches in gas, liquid and supercritical phases of CO2 are evaluated by means of fractal dimension. The experimental results are summarized as follows: 1) a tree-like streamer growing from the tip of needle was successfully visualized. The field strength for streamer initiation and the mean velocity of the streamer are estimated as approximately 9 MV/cm and 60 km/s, respectively. Pre-breakdown phenomena in SCCO2 are divided into two processes: streamer growth and shockwave expansion. 2) Streamer branches in SC phase were found to be more complicated than in other phase of CO2. 3) Bubble formation in the streamer initiation process was not recognized in SCCO2. For dielectric breakdown, the cylindrical shockwave grows along the streamer channel. A trace of light emission is recognized near the center of the arc column with a residual image of the streamer.

Weibull Statistical Analysis of Pulsed Breakdown Voltages in High-Pressure Carbon Dioxide Including Supercritical Phase

Pulsed discharge plasma in supercritical fluids (SCFs) has attracted a great deal of attention in the field of plasma application for chemical processing. Characteristics of electrical breakdown are one of the important concerns for dielectric reliability of a plasma reactor with SCFs. In this paper, the pulsed breakdown voltages of quasi-uniform gaps were measured by changing the CO2 medium from gas to supercritical phase at a given temperature, and Weibull statistical analysis was applied to the measured breakdown voltages. The area effect on breakdown voltage, which is known to give an equivalent effective constant to Weibull shape parameter m, was examined with three sphere-to-sphere gaps. The experimental results indicate that the dispersion in the pulsed breakdown voltage in pressurized carbon dioxide obeys Weibull distribution, and the Weibull shape parameter m depends on the state of the CO2 medium. The dependence of m on the medium density that was obtained from direct analysis of breakdown voltages for a given electrode system indicated a similar tendency to that of the effective constant by area effect analysis.

Positive Pulsed Streamer in Supercritical Carbon Dioxide

A positive pulsed streamer in a needle-to-plane gap in supercritical carbon dioxide was observed by using a laser schlieren method. The treelike streamer is initiated at a needle electrode when the electric field reaches approximately 9 MV/cm and propagates spatially toward a plane electrode. The streamer is accompanied by the cylindrical and spherical shock waves. A low-density region along the streamer channel decays with time, and no bubble formation occurs finally.

Prebreakdown process of a negative nanosecond pulsed discharge in supercritical carbon dioxide

The prebreakdown process in a 100 µm gap in supercritical carbon dioxide was investigated under negative nanosecond pulsed voltage. A bushlike negative streamer was observed by the shadowgraph method. Although currents flowed in the middle of the rising edge, a back discharge current suddenly appeared at the falling edge when a peak voltage exceeding 28 kV was applied. Electron multiplication was estimated to be approximately 108 by integrated calculation of the back discharge current. These results suggest that a negative streamer grows by steps and the inside of the bush streamer is considered to consist of a high-pressure gas.

Dielectric recovery mechanism of pressurized carbon dioxide at liquid and supercritical phases

Estimates of dielectric recovery rates of supercritical (SC) and liquid carbon dioxide (CO2) were derived with focus on highly-repetitive pulsed power switching mediums. Calculated results suggest that recovery time of SC and liquid CO2 are approximately 50 times shorter than that of water and oils. Prior to 10 µs after breakdown, recovery rates in neither SC nor liquid CO2 reached 100%, though the recovery rate in SC CO2 was higher than that of liquid CO2. To examine causes of recovery rate differences, each dielectric recovery process in SC and liquid CO2 was observed by laser shadowgraph technique. These shadowgraph images suggest two factors explaining dielectric recovery rate differences between these medium conditions: 1) thermodynamic property differences between medium conditions, and 2) differences in the low density region recovery mechanism.

Characteristics of Shock Waves Generated by a Negative Pulsed Discharge in Supercritical Carbon Dioxide

The initiation and propagation process of shock waves in pressurized carbon dioxide including supercritical (SC) phase was observed by means of schlieren method. A pulsed laser light source was used for high resolution sequential flow visualization and an ultrahigh speed camera equipped with a flash lamp was used for time-resolved visualization. To generate shock waves, a negative pulsed voltage with a rise time of 90 ns and half-width of 410 ns was applied to a point electrode. After development of a bush-like streamer from electrode tip, a spherical shock wave was generated around the streamer. The shock wave velocities and Mach numbers were calculated from the schlieren images taken at gas, SC, and liquid phases. The largest Mach number was measured in SC phase, though shock waves velocity order, from weakest to strongest, was in the gas, SC, and liquid phases, respectively. The shock wave propagated almost linearly after 1 μs, while the shock front grew increasingly difficult after 6 μs to confirm. To examine the initial process of shock waves, the time-resolved high speed imaging setup was used instead of the pulsed laser optical setup. The measurements indicated that the shock wave sharply decayed within submicroseconds; and in comparison with streamer initiation, the shock wave generation was delayed. This delay time might depend on the medium conditions.

Study of breakdown inside a supercritical fluid plasma switch

Supercritical fluid (SCF), characterized by high pressure and high density, combines the advantage of gas and liquid: high ability of mass transfer (dense, high diffusivity and low viscosity) and high heat transfer (high heat conductivity). Research on discharge behavior of SCF in high electrical field is more recent and less common, and is also motivated by new environmental and economical reasons. SCF has high potential in high power switching area, and is proposed to be the alternative for high voltage switching media, owing to its distinguished advantage of high breakdown strength, fast dielectric recovery, and low impact to environment. In conventional circuit breakers, SCF would be the perfect substitute medium for SF6, which is a powerful greenhouse gas and after use is highly toxic. In this work, the design of a versatile SC switch up to 200 bar with adjustable heavy duty electrodes, and imbedded current and voltage sensor is briefly introduced. The SCF supply (adjustable flow rate from 5-700 Liter/h) and repetitive pulse voltage source (30 kV peak value, up to 1kHz repetition rate) enable measurement of breakdown voltage and breakdown delay time inside SC nitrogen, corresponding to variations of parameters such as voltage repetition rate, gap width and gas flow rate through the gap. Via a simultaneous triggering system, breakdown inside a SC nitrogen switch is pictured by a sub-nanosecond fast CCD camera, providing space and time parameters for theoretical analysis of breakdown and subsequent dielectric recovery inside a SCF.

Improvement of insulation performance of solid/gas composite insulation with embedded electrode

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage sulfur hexafluoride (SF6) gas insulated switchgears. Its main circuit including a vacuum interrupter is coated with epoxy resin. Therefore, a solid/gas insulation system is composed. For further miniaturization and higher stress design of SIS, improvement of the insulation performance is required. In the solid/gas insulation system, the breakdown strength along the surface is lower than that of solid. Then, the authors investigated the surface insulation performance of the solid/gas insulation system containing an embedded electrode. In the present study, the sizes of the embedded electrode and the solid insulation material were varied and the partial discharge inception voltage and the surface breakdown voltage were measured. As a result, it was found that the surface breakdown voltage varied considerably with the radius of the embedded electrode. From the result, it was deduced that the surface insulation performance could be improved by choosing the size of the embedded electrode appropriately. Moreover, the effects of the thickness of the insulation material and of the surface distance on the surface insulation performance were clarified.