Pressure SF6 Research Articles

Signatures of non-universal large scales in conditional structure functions from various turbulent flows

We present a systematic comparison of conditional structure functions in nine turbulent flows. The flows studied include forced isotropic turbulence simulated on a periodic domain, passive grid wind tunnel turbulence in air and in pressurized SF6, active grid wind tunnel turbulence (in both synchronous and random driving modes), the flow between counter-rotating discs, oscillating grid turbulence and the flow in the Lagrangian exploration module (in both constant and random driving modes). We compare longitudinal Eulerian second-order structure functions conditioned on the instantaneous large-scale velocity in each flow to assess the ways in which the large scales affect the small scales in a variety of turbulent flows. Structure functions are shown to have larger values when the large-scale velocity significantly deviates from the mean in most flows, suggesting that dependence on the large scales is typical in many turbulent flows. The effects of the large-scale velocity on the structure functions can be quite strong, with the structure function varying by up to a factor of 2 when the large-scale velocity deviates from the mean by ±2 standard deviations. In several flows, the effects of the large-scale velocity are similar at all the length scales we measured, indicating that the large-scale effects are scale independent. In a few flows, the effects of the large-scale velocity are larger on the smallest length scales.

Simulation of SF6 adsorption on the bundles of single walled carbon nanotubes

Using the Grand Canonical Monte Carlo simulation technique we study the storage of SF6 on the bundles of single-walled carbon nanotubes. We show that nanotube bundles may be efficient SF6 storage vessels, and there is no influence of chirality on adsorption. In the case of SF6 adsorption from the mixtures containing low concentration of this gas, the smallest tubes are the most promising adsorbents. Also the bundles composed of tubes having diameters 2.7–2.8nm and accessible intestinal porosity are efficient. These tubes are also optimal for the high pressure SF6 volumetric storage.

Influence of Surface-conductivity Nonuniformity on Charge Accumulation Phenomena of GIS Insulator under DC Electric Field

When a dc voltage is applied to an insulating spacer, the electric field distribution around it is determined by the resistivity of the material. Consequently, charges are accumulated on the surface of the insulator, and the breakdown voltage of it may become low. In this paper, the charge density distribution of the insulator is measured. The measured data indicate that the surface conductance is the main factor of the charge accumulation. Therefore, the conductivity of the insulator is also measured, and the influence of the surface conductivity on the charge accumulation is examined from both sides of experiment and numerical computation. It is confirmed that nonuniformity in the surface conductivity is responsible for the charge accumulation and the conductivity of insulator in atmospheric pressure SF6 is estimated to around σ = 1×10-18Ω-1.

Empirical analysis of high pressure SF6 gas breakdown strength in a spark gap switch

It is difficult to predict or measure the breakdown strength of SF6 gas in the high pressure range. Therefore, it is hard to find reference data of the high pressure SF6 gas breakdown strength. This paper attempts to provide empirically a novel criterion for the estimation of SF6 gas breakdown strength in a spark gap switch. For this purpose, J.C. Martin's breakdown field strength of non-uniform gap in air and Kuffel et al's breakdown field strength by exploiting streamer criterion of uniform field in SF6 gas are adopted. The research is made for the empirical method of predicting the high pressure SF6 breakdown strength and compared results have been ascertained with several experimental data from different investigators in a wide range of pd (pressure times distance) and field enhancement factor (FEF). For further confirmation of the empirical approach, we have designed and developed a high pressure spark gap switch to be closely coupled to a high voltage PFN-Marx system. The designed spark gap switch operates above 1 MV. Our tests have verified the prediction of the breakdown strength of the SF6 filled spark gap switch in a pd range of 0.27-1.62 MPa . cm (2.7 to 16.2 bar.cm).

Measuring Picosecond Flashover in Closing Coaxial Spark Gap Switch

Flashover voltage and picosecond risetime of breakdown in pressurized SF6 gas is studied on a coaxial line. Gas pressure is varied from 1 to 19 bar. The homogeneous field gap has an opening from ca. 0.2 to 0.9 mm, and flashover voltages reach 120 kV. Measurements are performed using a D-dot probe with 16 to 20 GHz real-time oscilloscopes and a 50 GHz sampling oscilloscope. Measured risetimes are down to ca. 50 ps, and the dependence of voltage collapse on gas density and electric field between the electrodes is reported. Integrity of data is analyzed and experimental results are compared with present theory found in literature and previous results measured with alternative real-time systems.

Status of the ITER neutral beam injection system (invited)

The ITER neutral beam injectors are the first injectors to be designed to operate under conditions and constraints similar to those that will be encountered with a fusion reactor. The injectors will use a single large ion source and accelerator that will produce 40A D− 1MeV beams for pulse lengths of up to 3600s. The accelerated ion beams will be neutralized in a gas (D2) neutralizer which is subdivided into four vertical channels to reduce the gas flow into the injectors that is needed to produce optimum target for neutralization. These injectors will have to operate in a hostile radiation environment and they will become highly radioactive due to the neutron flux from ITER. The design has been modified recently to have a rectangular vacuum vessel with a removable lid that allows vertical access to, and maintenance of, the beamline components, the incorporation of an absolute all metal valve at the exit of the injector, the choice of a rf driven ion source as the reference design of ion source, and to have a high voltage deck incorporating the various auxiliary power supplies in air rather that under high pressure SF6. A major development is that it has been agreed that a Neutral Beam Test Facility (NBTF) will be set up at Padua, Italy. The NBTF will consist of two test beds: one of which will be capable of operating a complete injector at full performance. The second will be an ion source test bed, which will be used for the development and testing, to full performance, of the large negative ion source.

Fields and Surface Irregularity Factors of Stranded Conductors in Co-Axial Cylinder Configurations

This paper aims to investigate theoretically the electric fields and the surface irregularity factors for stranded conductors in pressurized air and SF6 in co-axial cylinder configurations. The surface irregularity factor is defined as the ratio of the corona onset voltage of a stranded conductor to that of a smooth cylindrical conductor of the same outer diameter. This calls for the assessment of the electric field in the vicinity of the stressed conductor and the corona onset voltage. The investigated gap is a three dimensional field problem. To solve this problem, charge simulation technique (CST) is used, where the simulation charges for stressed and grounded conductors are helical of infinite length. The method of corona onset voltage calculation is based on the criterion developed for the formation of repetitive negative corona Trichel pulses. The effects of varying conductor radius, gas pressure, stranding ratio on the irregularity factor values are investigated. The calculated values of irregularity factor in air agree well with those calculated and measured before.

Pulsed energy storage antennas for ionospheric modification

Abstract. Interesting, "new", very high peak-power pulsed radio frequency (RF) antennas have been assembled at the HIPAS Observatory (Alaska, USA) and also at the University of California at Los Angeles (UCLA, USA); namely, a pair of quarter wavelength (λ/4) long cylindrical conductors separated by a high voltage spark gap. Such a combination can radiate multi-megawatt RF pulses whenever the spark gap fires. The antenna at HIPAS is 53m long (λ/2) with a central pressurized SF6 spark gap. It is mounted 5 meters (λ/21) above a ground plane. It radiates at 2.85MHz. The two antenna halves are charged to ± high voltages by a Tesla coil. Spark gap voltages of 0.4 MV (at the instant of spark gap closure) give peak RF currents of ~1200A which correspond to ~14 MW peak total radiated power, or ~56 MW of Effective Radiated Power (ERP). The RF pulse train is initially square, decaying exponentially in time with Qs of ~50. Two similar but smaller 80-MHz antennas were assembled at UCLA to demonstrate their synchronization with a pulsed laser which fired the spark gaps in the two antennas simultanoeously. These experiments show that one can anticipate a pulsed array of laser synchronized antennas having a coherent Effective Radiated Power (ERP)>10GW. One can even reconsider a pulse array radiating at 1.43MHz which corresponds to the electron gyrofrequency in the Earth's magnetic field at ~200km altitude. These "new" pulsed high power antennas are hauntingly similar to the ones used originally by Hertz (1857-1894) during his (1886-1889) seminal verifications of Maxwell's (1864) theory of electrodynamics.

Generation of Teflon-like layers on cellophane surfaces under atmospheric pressure non-equilibrium SF 6 -plasma environments

In this contribution a novel atmospheric pressure SF6–cold–plasma method is suggested for the conversion of cellophane surfaces into perfluorinated layers. This approach takes advantage of the efficiency of SF x , (x< 6 ) species generated in the discharge for oxygen extraction, dehydro-fluorination and fluorination reaction mechanisms. It was shown that relative surface atomic concentrations as high as 58% can be created under relatively short treatment times (5 minutes), and that CF2 groups are the dominant components of the plasma-generated surface layers. Surface morphology analysis by atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that SF6–discharge environments selectively interact with the amorphous and crystalline zones of cellulose. Contact angle evaluations of plasma–exposed cellophane surfaces show an intense hydrophobic character, with contact angles as high as 120°. Potential applications of perfluorinated cellophane layers are suggested.

95/00476 A compact algorithm for the steady-state security assessment of power systems

Widespread applications of pressurized SF6 gas and its mixtures as insulating media in many electric power apparatus are the result of recent advances in modern technologies. Partial discharge (PD) is a natural phenomenon occurring in such gas-insulated electric power systems, which invariably contains tiny defects and non-uniformities. The quality of an electrical insulation system can be characterized by PD measurements, which serve to identify the type and status of a defect. A review on the insulation and PD monitoring for gas-insulated switchgear/transmission lines (GIS/GITL) is presented with focusing on the measurement techniques. Since PD monitoring systems can generate a large amount of data, therefore computer-aided interpretation and classification of defects are also discussed.

大気圧SF&lt;sub&gt;6&lt;/sub&gt;ガス中不平等電界下のバリア効果

Generally, it is known that insertion of a barrier under a nonuniform field in air increases the flashover voltage. This is called the barrier effect. However, there are many uncertainties that remain to be clarified about the barrier effect in SF6 gas. Therefore, the influence of the barrier shapes and positions on the flashover voltage for the lightning impulse voltages and voltages (50 Hz) is examined by inserting a barrier between a hemispheric-end rod and a plate in SF6 gas under near-atmospheric pressure. As a result, the following findings were obtained:. (1) Insertion of a disk-shaped barrier reduces the flashover voltage; a cup-shaped barrier whose surface curves almost along the equipotential line increased the flashover voltage by 30 to 40 percent;. (2) trapped charge on the barrier surface is of the same polarity as the applied voltage and the magnitude of positive charge was greater than that of negative charge. It was found also that there is a correlation between the amount of trapped charge and the flashover voltage; and. (3) application of a barrier under a nonuniform field in the atmospheric pressure SF6 gas helps reduce the size of, for example, a cubic-type gas-insulated switchgear.

Barrier effect under nonuniform field in atmospheric pressure SF6 gas

Generally, it is known that insertion of a barrier under a nonuniform field in air increases the flashover voltage. This is called the barrier effect. However, there are many uncertainties that remain to be clarified about the barrier effect in SF6 gas. Therefore, the influence of the barrier shapes and positions on the flashover voltage for the lightning impulse voltages and voltages (50 Hz) is examined by inserting a barrier between a hemispheric-end rod and a plate in SF6 gas under near-atmospheric pressure. As a result, the following findings were obtained:. (1) Insertion of a disk-shaped barrier reduces the flashover voltage; a cup-shaped barrier whose surface curves almost along the equipotential line increased the flashover voltage by 30 to 40 percent;. (2) trapped charge on the barrier surface is of the same polarity as the applied voltage and the magnitude of positive charge was greater than that of negative charge. It was found also that there is a correlation between the amount of trapped charge and the flashover voltage; and. (3) application of a barrier under a nonuniform field in the atmospheric pressure SF6 gas helps reduce the size of, for example, a cubic-type gas-insulated switchgear.

Statistical time lags in low-pressure SF6 breakdown

Statistical time lags have been measured in a discharge tube containing SF6 at a pressure of 1 Torr, with the aim of determining the source of the electrons which initiate breakdownY″Te,hf cx pressure SF6. Measurements with and without careful shielding of the discharge tube give similar values of mean statistical time lag showing that the initiating electrons are not produced by external radiation. Measurements, in which the time between the 500 successive breakdowns used for a single measurement of mean statistical time lag is increased over the range 1–103 s, provide values of mean statistical time lag which increase over the range 10−5–10−2s: This reveals that the breakdown process is controlled by the time since the discharge produced by a previous breakdown. Data from further such measurements, made for different applied tube voltages, are shown to fit well with the Fowler–Nordheim theory of field emission. The conclusion drawn is that the initiating electrons are produced at the cathode surface by field emission, and that the state of the surface at any time depends primarily on the intensity of the previous discharge and the length of time since it took place. During the time between successive breakdowns, a surface layer on the cathode surface is continually developing and reducing the rate of field emission from the cathode. Although the applied electric fields are well below that required for field emission, microscopic fields exist at protrusions and craters on the cathode surface which are high enough to provide field emission: This is supported by experiments on electron emission made with the discharge tube evacuated below 10−6 Torr.

Ambipolar diffusion process in low pressure electronegative gases - Transition of the arc discharge in low pressure SF6.

The transition of the constricted arc to diffused glow discharge in low pressure, 5 to 30 Torr, SF6 was investigated. Some illuminating cases of the enhancement of spectral emission and simultaneous expansion of the plasma column are presented. The mechanism of this transition is attributed to the formation of negative ions and subsequent enhancement of electron diffusion, which is illustrated by the decrease of the density of the current carrying electrons from 3.4×1014 cm-3 for the constricted arc to1×1013 cm-3 for the diffused glow discharge, and the increase of the drift velocity from2×107CM/S to7×107cm/s.

Efficient stimulated Brillouin backscattering of multiple pulses in the highly transient regime

Stimulated Brillouin scattering in the highly transient regime is experimentally investigated using a multipulse reflection scheme. In high pressure N2 and SF6 backward pulses with reflection coefficients approaching 1 are demonstrated employing 1 ns pump pulses at 1.32 μm. The experimentally observed reflection behavior is satisfactorily reproduced by a numerical solution of the equations describing the scattering process. The phase conjugated fractions in the backscattered light is large.

Mass spectrometry of arcs in SF6 circuit breakers

Today, SF6 is used to a great extent as insulating and arc-quenching medium in high-voltage gas-blast circuit breakers. The arcing in SF6 during current interruption forms decomposition products. These can influence the arc-quenching properties of the circuit breaker. Furthermore, they can cause corrosion of the circuit breaker housing. In this comprehensive study we present results obtained for the first time from a direct mass spectrometric investigation of the exhaust gases of a high pressure SF6 arc in a model circuit breaker. Our mass spectrometric system consists of a time-of-flight mass spectrometer (TOFMS) equipped with a molecular beam sampling systems. This device allows us to measure mass spectra of high pressure sources with a time resolution of up to 10,000 spectra per second. We have determined the formation rate of the most abundant decomposition products in a SF6 arc at 1 bar. These products are SF4, CF4, WF6, SOF2, SO2, CS2 S2F2 and HF. The fast detection time inherent to our system permits also the determination of the formation of SF4, which is 0.45–0.50 Vol. %/(kJ/1SF6). In addition, we have studied the influence of water and oxygen impurities which are responsible for the production of highly corrosive HF. Finally, we have considered the influence of the thermal degradation of teflon (P.T.F.E.), which is used as nozzle and insulating material in circuit breakers. On this occasion we have demonstrated that CF4, which exhibits dielectric properties similar to SF6, is the main decomposition product formed from teflon. However, we have found that besides CF4 also excess carbon is formed, which is deposited on insulators of the model circuit breaker.

Ionisation waves in low pressure SF6 gas

Two modes of ionisation waves were observed in the pressure range 0.5-1 Torr and current range 0.1-0.3 mA. With Mode I waves the frequency changed from 20 to 80 kHz with the discharge current. The wave showed a B- type property with both phase and group velocities above 106 cm s-1. The other mode (Mode II) was a kHz wave with a group velocity less than 2*103 cm s-1, which changed from the forward wave to the backward wave with the discharge current. The forward wave showed an exponential increase of both phase velocity and wavelength. Numerical analysis taking into account four species of charged particles agrees qualitatively with the experimental results, and suggests that Mode I was induced by positive ions and Mode II by heavy negative ions.

Studies of Power Arc Faulta in SF6 Insulated Equipment

Power arc behaviour in gas insulated equipment and the influence of arc motion on damage resulting from simulated faults were investigated in the laboratory. Experiments were carried out with power-frequency currents up to 22 kA in a co-axial test chamber insulated with pressurized SF6 and air. Arc motion was measured with time-of-flight and infrared thermal imagery techniques for various fault currents, geometries and pressures. A linear, relationship between the axial arc velocity and fault current was established. For a typical 138 kV SF6 bus duct, this velocity is 100 cm/s·kA. The influence of spacers on the motion of arcs in the gas insulated equipment was studied with respect to temperature rise of and damage to the sheath

Factors Controlling Surface Flashover in SF6 Gas Insulated Systems

Various factors controlling the flashover of solid insulators in pressurized SF6, are reviewed and their influences in gas insulated systems are discussed from a practical point of view. Flashover voltage of clean insulator surface is under the influence of the insulator-metal contact as well as the macroscopic electric field distortion due to the high dielectric permittivity of solid insulator. Conducting particles or even fine metal powder can reduce the flashover voltage. Their effects are strongly dependent on the position they are located, the size of the insulator and gas pressure. Humidity of SF, gas should be strictly governed in SF, gas insulated apparatuses, since the condensation of water can decrease flashover voltage considerably. Decomposition products of SF6 due to the arcing in switchgears are deleterious to epoxy insulators particularly when silica is used as their filler. The decomposition products decrease the leakage resistance on the insulation surface. The field strength near positive electrode is enhanced by the electrolytic effect in the surface conduction layer. In some extreme condition, it initiates tracking on the insulator surface.

High pressure SF6 pulse transmission near 10.4 μm

Measurements were performed to determine the dynamical characteristics of high pressure SF6 absorption at the 10.4 μm band of CO2 as a function of laser pulse intensity. Both long (~ 200 ns FWHM) and short (~ 5 ns FWHM) CO2 laser pulses were employed. The observed pulse transmission characteristics can be accounted for using a vibrational bath model, which allows for the very large number of levels in the absorption spectrum of SF6.