SF6-N2 Mixtures Research Articles

Adsorption and gas sensing properties of Co modified InN to SF6-N2 mixture decomposition components

• It is the first time that Co-InN is used as the adsorbent to remove the decomposition components of SF 6 -N 2. • Co-InN shows good adsorption properties to NO 2 , SOF 4 , SOF 2 , SO 2 F 2 by chemisorption. • The theoretical calculation results are important to guide the development of resistive gas sensor based on the Co-InN. This paper presents a detection method of partial discharge decomposition components of SF 6 -N 2 mixture by Co-doped InN. The severity of partial discharge in gas insulated switchgear (GIS) can be judged by the gas components when SF 6 -N 2 decomposes. Exploring sensors with excellent gas-sensing detection capabilities can provide method to partial discharge decomposition component detection and fault diagnosis of GIS. Undoped InN is a wide bandgap semiconductor with good sensing properties. In this paper, the adsorption of SF 6 -N 2 decomposition components on Co-doped InN monolayer, as well as the adsorption mechanism and gas sensitivity characteristics were investigated by density functional theory (DFT), including adsorption energy, charge transfer, the density of state theory and molecular orbital theory. Our calculations found that Co-doped InN has the optimal structure at the T N site and has good adsorption performance for NO 2 , SOF 4 , SOF 2 , and SO 2 F 2 gas molecule. All adsorption reactions belong to chemisorption. These results could direct the development of gas sensor based on the Co-doped InN monolayer, and provides methods to online monitoring of GIS discharge failures.

DFT Insight to Ag2O Modified InN as SF6-N2 Mixture Decomposition Components Detector

In gas-insulated switchgear (GIS), partial discharge (PD) can be monitored by detecting sulfur hexafluoride-nitrogen (SF6-N2) decomposition components. In this paper, silver oxide (Ag2O) modification was introduced to improve the gas-sensing properties of graphene-like indium nitride (InN). The adsorption process of NO2, SO2F2, SOF2 and SOF4 on Ag2O-InN was simulated based on the first principles calculation and density functional theory (DFT). The gas sensing mechanism was revealed by density of states theory and molecular orbital theory. It is found that Ag2O doping greatly improves the adsorption properties of InN to NO2 and SOF2 molecules. The adsorption capacity of Ag2O-InN to the four gas molecules is: NO2 > SOF2 > SOF4 > SO2F2. All adsorptions can proceed spontaneously, and the gas molecules are electron donors and Ag2O-InN is an electron acceptor. Through the analysis of recovery time, it is found that NO2 is difficult to desorb from the substrate due to the significant adsorption energy of −2.201 eV, while SOF4, SOF2 and SO2F2 have a moderate adsorption energy of −0.185 eV, −0.754 eV and 0.173 eV and extremely short desorption time. The conductivity of the whole system changed after these four gases were adsorbed on the Ag2O-InN monolayer. In summary, Ag2O-InN can be used as NO2 adsorbent and gas sensors to detect SOF4, SOF2 and SO2F2. This paper provides a method for on-line monitoring of partial discharge in GIS.

CF4 Capture and Separation of CF4-SF6 and CF4-N2 Fluid Mixtures Using Selected Carbon Nanoporous Materials and Metal-Organic Frameworks: A Computational Study.

The adsorption of pure fluid carbon tetrafluoride and the separation of CF4–SF6 and CF4–N2 fluid mixtures using representative nanoporous materials have been investigated by employing Monte Carlo and molecular dynamics simulation techniques. The selected materials under study were the three-dimensional carbon nanotube networks, pillared graphene using carbon nanotube pillars, and the SIFSIX-2-Cu metal–organic framework. The selection of these materials was based on their previously reported efficiency to separate fluid SF6–N2 mixtures. The pressure dependence of the thermodynamic and kinetic separation selectivity for the CF4–SF6 and CF4–N2 fluid mixtures has therefore been investigated, to provide deeper insights into the molecular scale phenomena taking place in the investigated nanoporous materials. The results obtained have revealed that under near-ambient pressure conditions, the carbon-based nanoporous materials exhibit a higher gravimetric fluid uptake and thermodynamic separation selectivity. The SIFSIX-2-Cu material exhibits a slightly higher kinetic selectivity at ambient and high pressures.

Entropy generation in concentric annuli of 400 kV gas-insulated transmission line

The object of this study is entropy generation analysis in the concentric annuli of a gas insulation transmission line enclosure (GIL) loaded with air and SF6-N2 mixture gas. The inner and outer cylinders of the cavity are hot and cold, respectively. Because of symmetric conditions the ½ cylinder is considered for modeling. The standard k–ɛ turbulence model, the equation of entropy generation, and the conservation equations are transformed from dimensional form to non-dimensional form utilizing the definition of dimensionless parameters, stream function, and vorticity. Then dimensionless governing equations are solved by a finite volume method (FVM) using an innovative ANSYS Fluent non-dimensionalization scheme. Inflated layers applied to the fine grids to improve the simulation capability for turbulent modeling. The effects of the Rayleigh number at Ra = 2.5 × 106, 1.7 × 109, 2.1 × 109, 4.2 × 109 and 5.7 × 109 are investigated. The results demonstrate that the Nusselt number grows as the Rayleigh number grows and entropy generation number and the Bejan number decreases as the Rayleigh number increases. The GILs have a proper morphology in geometry that reducing the high irreversibility in higher Rayleigh numbers conditions.

Dielectric breakdown properties of CF3I-N2 mixtures containing a small amount of SF6

CF3I-N2 mixtures have garnered much attention as potential SF6-substitutes due to their excellent dielectric strengths, and the dielectric strength of SF6-N2 mixtures has a good synergy effect. This paper attempts to propose an approach of adding a small amount of SF6 to improve the dielectric strength of the CF3I-N2 mixture, by making an all-around consideration of dielectric strength, impact on the environment and the limitations of engineering pressure and the lowest ambient temperature. The influences of adding different amounts of SF6 on the electron energy distribution function, the reduced ionization, attachment and effective ionization coefficients, the critical reduced electric field strength and critical electron temperature of ternary SF6-CF3I-N2 mixtures are investigated by Boltzmann analysis. Then the relations of the critical electric fiend strength, mixed ratio and saturated vapor pressure of the ternary SF6-CF3I-N2 mixture are built and analyzed. The results demonstrate that an effective improvement of its dielectric strength can be obtained by adding a small amount of SF6 into the CF3I-N2 mixture, and that a proper choice of the mixed ratio and the gas pressure of the ternary SF6-CF3I-N2 mixture can provide a comparable dielectric strength to pure SF6.

Suppression of Electron Avalanches in Ultra-Dilute SF6-N2 Mixtures Subjected to Time-Invariant Crossed Fields

Ultra-dilute SF6 + N2 mixtures are subjected to crossed fields with the applied magnetic field normal to the electric field in order to investigate critical magnetic field values for suppression of electron avalanche growth resulting in increased dielectric strength. Electron avalanche growth inhibition together with the related electron mean energy variation and electron energy distribution functions are reported in the binary mixtures subjected to the combined fields. Even if a very small amount of greenhouse gas (SF6) is used, high dielectric strength can be achieved in the binary mixtures with 0.25%, 0.50%, 0.75%, 3%, and 7% SF6 contents subjected to the evaluated combined fields. The magnitude of the crossed magnetic field, which results in avalanche growth inhibition, decreases as the electronegativity of the mixture is increased.

Effects of vibrationally excited N2 molecules on dielectric breakdown properties of SF6-N2 mixtures

This paper aims at studying the effects of the vibrationally excited N2 molecules on the dielectric breakdown properties of the SF6-N2 mixtures. The population of the vibrationally excited N2 molecules is determined from the Boltzmann distribution. For the cold SF6-N2 mixtures, the critical reduced electric field strength (E/N)cr decreases with the increasing vibrational temperature. For the hot SF6-N2 mixtures, the equilibrium compositions are calculated by the minimizing the Gibbs free energy before determining the vibrational distribution function by assuming the vibrational temperature equals to the gas temperature. The calculation results show that when the influence of the vibrationally excited N2 molecules is considered, the critical reduced electric field (E/N)cr of the SF6-N2 mixtures is reduced, and the deviation may be enhanced with a further increase in gas pressure and N2 concentration.

Partial Discharge Characteristics in SF6-N2 Mixture Gases Under High Voltage Direct Current

Partial Discharge Characteristics in SF₆-N₂ Mixture Gases Under High Voltage Direct Current

Analysis of the insulation characteristics of C3F8 gas mixtures with N2 and CO2 using boltzmann equation method

This paper studied the insulation characteristics of C3F8 gas mixtures with N2 and CO2 from the point view of electron swarm parameters. The density-normalized effective ionization coefficient (α-η)/N and electron drift velocity Ve in binary C3F8-N2 and C3F8- CO2 gas mixtures are calculated by solving the Boltzmann equation in the condition of steady-state Townsend discharge experiment. Values of the limiting electric field strength for which the electron ionization exactly balances the attachment are derived from the effective ionization coefficient data. It is found that the limiting field strength of C3F8-N2 gas mixture is greater than that of C3F8-CO2 for C3F8 gas content k varied from 0% to 100%, and the C3F8-N2 gas mixture containing 20% proportion C3F8 can achieve 60% dielectric strength of pure C3F8. Additionally, the synergistic effects of the insulation strength for C3F8 gas mixtures are also evaluated and compared with that of SF6-N2 mixture. The calculation results, together with the discussions on liquefaction temperature and global warming potential indicate that gas mixtures containing C3F8 are feasible for applying in electric power apparatus as insulation medium.

Two-temperature transport coefficients of SF6–N2 plasma

Sulfur hexafluoride (SF6) is widely adopted in electric power industry, especially in high-voltage circuit breakers and gas-insulated switchgear. However, the use of SF6 is limited by its high liquidation temperature and high global warming potential. Recently, research shows SF6–N2 mixture, which shows environmental friendliness and good electrical properties, may be a feasible substitute for pure SF6. This paper is devoted to the calculation of and transport coefficients of SF6–N2 mixture under both LTE (local thermodynamic equilibrium) and non-LTE condition. The two–temperature mass action law was used to determine the composition. The transport coefficients were calculated by classical Chapman–Enskog method simplified by Devoto. The thermophysical properties are presented for electron temperatures of 300–40 000 K, ratios of electron to heavy species temperature of 1–10 and N2 mole fraction of 0%–100% at atmospheric pressure. The ionization processes under both LTE and non-LTE have been discussed. The results show that deviations from local thermodynamic equilibrium significantly affect the properties of SF6–N2 plasma, especially before the plasma is fully ionized. The different influence of N2 on properties for SF6–N2 plasma in and out of LTE has been found. The results will serve as reliable reference data for computational simulation of the behavior of SF6–N2 plasmas.

Inception corona discharge voltages in SF6-N2 gas mixtures at high gas pressure

The use of sulphur hexafluoride (SF6) has led to increased concentration of this gas in the atmosphere. The purpose of this work is to investigate the behaviour of SF6-N2 mixtures that can be considered as a potential substitute of pure SF6. The onset corona discharge voltages have been determined from the measurements of the currentvoltage characteristics under high pressures and with highly inhomogeneous fields. The results show that the onset voltages increase with the increase of the gas pressure and are relatively higher for positive polarity. It can also be seen that for low concentrations of SF6 in the mixture the increase of the onset voltages is substantial, followed by a slow increase for elevated concentration of SF6 in the mixture, which, can be an advantage for the substitution of SF6.

Dielectric breakdown properties of SF6–N2 mixtures at 0.01–1.6 MPa and 300–3000 K

The dielectric breakdown properties of SF6–N2 mixtures were investigated at different concentrations of N2, 0.01–1.6 MPa, and 300–3000 K. The equilibrium compositions of different SF6–N2 mixtures at several gas pressures and temperatures up to 3000 K were first calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. The electron energy distribution function was then obtained using the composition data by Boltzmann equation analysis. It was found that adding N2 to SF6 gas can markedly reduce the kinetic energy of electrons at relatively high gas temperatures, which enhances the dielectric field strength. Finally, the critical reduced electric field (E/N)cr, defined as the value for which ionization is equal to attachment, of hot SF6–N2 mixtures was determined. The results indicate that in the gas temperature range around 2000–3000 K, increasing the concentration of N2 effectively enhances the (E/N)cr of SF6–N2 mixtures, and the (E/N)cr of SF6–N2 mixtures at a wide range of concentrations of N2 (5%, 50%, and 95%) are all higher than that of pure SF6 gas. Further, this trend exists at all the gas pressures considered (0.01–1.6 MPa).

Dielectric breakdown properties of SF6–N2 mixtures in the temperature range 300–3000 K

Reduced critical electric field strength (E/N)cr is an important indicator for the evaluation of dielectric breakdown of SF6 and its mixtures. This paper aims to analytically investigate the dielectric breakdown properties of SF6–N2 mixtures. First, (α − η)/N and (E/N)cr of SF6–N2 mixtures at room temperature, and then (E/N)cr of hot SF6 at several pressures are calculated and compared with other studies. The results confirmed the validity of the present calculation method and parameters. In addition, the electron energy distribution function, reduced ionization coefficient α/N and attachment coefficient η/N of SF6–N2 mixtures are obtained at different proportions and under 1 atm in the gas temperature range 300–3000 K. Then (E/N)cr can be determined when the effective ionization coefficient (α − η)/N = 0. The results show that in a certain temperature range, due to the effect of N2 on reduction in the proportion of high-energy electrons, (E/N)cr of SF6–N2 mixtures will be enhanced by increasing N2.

Multichannel Discharge Characteristics of Gas Switch Gap in SF6-N2 or SF6-Ar Gas Mixtures Under Nanosecond Triggering Pulses

Experiments were carried out to ascertain multichannel discharge characteristics in a self-designed coaxial field-distortion gas switch filled with SF6-N2 or SF6-Ar gas mixtures of different mixing ratios. In these experiments, the pressure varied from 0.1 MPa to 0.2 MPa, the voltage pulse peak applied to the switch was in the range from 40 kV to 78 kV, and the pulse rise time was 11 ns. The static breakdown strength of the gas switch gap in the switch was also measured. The results show that in general the average number of discharge channels for SF6-Ar or SF6-N2 gas mixture which contains less SF6 is larger than that for gas mixture which contains more SF6, however, the average number of channels almost keeps constant as the gas mixing ratio varies when the pulse rise rate is high enough. The static breakdown strength of the gas switch gap decreases slightly as the content of argon or nitrogen increases.

Pure SF6 and SF6−N2 Mixture Gas Hydrates Equilibrium and Kinetic Characteristics

Sulfur hexafluoride (SF6), whether pure or mixed with inexpensive inert gas, has been widely used in a variety of industrial processes, but it is one of the most potent greenhouse gases. For this reason, it is necessary to separate and/or collect it from waste gas streams. In this study, we investigated the pure SF6 and SF6-N2 mixture gas hydrates formation equilibrium aswell asthe gas separation efficiency in the hydrate process. The equilibrium pressure of SF6-N2 mixture gas was higher than that of pure SF6 gas. Phase equilibrium data of SF6-N2 mixture gas was similar to SF6 rather than N2. The kinetics of SF6-N2 mixture gas was controlled by the amount of SF6 at the initial gas composition as well as N2 gas incorporation into the S-cage of structure-II hydrate preformed by the SF6 gas. Raman analysis confirmed the N2 gas incorporation into the S-cage of structure-II hydrate. The compositions in the hydrate phase were found to be 71, 79, 80, and 81% of SF6 when the feed gas compositions were 40, 65, 70, and 73% of SF6, respectively. The present study provides basic information for the separation and purification of SF6 from mixed SF6 gas containing inert gases.

Primary Investigation into the Laser Triggering Multi-Gap Multi-Channel Gas Switch in a Single Test Module Facility

A high precision laser trigger system is built up in the single test module of Primary Test Stand (PTS) facility. A fourth harmonic, with a wavelength λ of 266 nm, Q-switched Nd:YAG laser was used to trigger the 5 MV multi-gap multi-channel gas switch which was filled with high pressure SF6-N2 mixture gas. The maximum deviation and the standard deviation in the jitter time of the trigger system is ± 0.7 ns and 0.3 ns respectively. The maximum deviation and the standard deviation in the jitter time for the multi-gap multi-channel laser triggering switch is ± 2.4 ns and 1.5 ns respectively. The curve of switch delay-time versus laser energy is obtained, which is helpful for the choice of fitting laser energy. The successful test with two lasers indicated that the design of using twenty-four lasers to trigger twenty-four switches respectively is feasible in ``PTS''.

Measurement of electron transport and effective ionization in SF6–air and SF6–O2 mixtures

The electron drift velocity ve, the density-normalized longitudinal diffusion coefficient NDL, the density-normalized effective ionization coefficient (α − η)/N and the limiting field strength E/Nlim have been measured for the binary mixtures SF6–air and SF6–O2 for SF6 concentrations ranging from 1% to 50%. The overall reduced electric field strength E/N was varied between 1.2 and 500 Td (1 Townsend = 10−17 V cm2). A pulsed Townsend technique was used to perform these measurements, which turned out to be sensitive enough to distinguish relatively small differences in the electron drift velocities and longitudinal diffusion coefficients over the mixture range 1 to 50% SF6 in SF6–air and SF6–O2. On the other hand, the effective ionization coefficients show a well-defined variation with the SF6 content in the mixture. The limiting value of E/N at which the effective ionization coefficient is zero, that is, the critical field strength, turned out to be smaller than that for the SF6–N2 mixture. However, the difference in behaviour between SF6–air and SF6–N2 is small. To the best of our knowledge, we are not aware of any previously published measurements of these parameters for these mixtures.

Decomposition of high-pressure (400 kPa) SF6–CO2, SF6–CO, SF6–N2–CO2 and SF6–N2–CO mixtures under negative dc coronas

Sulfur hexafluoride, SF6, is successfully employed by the electric power industry for gas insulated equipment. However, it is a potent greenhouse gas and mixtures of SF6 with more friendly environmental gases have recently been proposed. It was demonstrated that SF6 mixed with N2 and/or CO2 could be a good substitute for some insulation applications such as gas-insulated transmission lines (GILs).Considering the possible occurrence of corona discharges in GILs, the decomposition of these gas mixtures under such stress must be studied. In this paper, the chemical stability under negative corona of SF6 and SF6–N2 (10 : 90) or (5 : 95) mixtures was investigated at 400 kPa on adding percentages of CO2 up to 80%.The corona discharges were generated at 23°C in a 340 cm3 experimental cell between a stainless steel point connected to a high voltage source and a plane of aluminium. The gaseous decomposition products were assayed by gas chromatography at the end of each run carried out over a range of transported charges covering 8–17 C.The presence of CO2 in SF6 and SF6–N2 mixtures submitted to negative coronas leads to:a considerable increase in the formation of the major compound group (SOF4+SO2F2),formation of similar large quantities of COF2,abundant production of CO with diluted SF6 in nitrogen,decreased formation of S2F10.

Effect of the percentage of SF6 (100%–10%–5%) on the decomposition of SF6–N2 mixtures under negative dc coronas in the presence of water vapour or oxygen

Low SF6 content SF6–N2 mixtures have recently been proposed as a replacement for pure SF6 in the insulation of gas insulated lines (GIL). Among the areas of investigation of such gas mixtures, their electrical decomposition under corona discharges must be studied considering the possible occurrence of such stress in GIL.This paper presents data concerning the decomposition of high-pressure SF6–N2 (5 : 95) mixtures (400 kPa) submitted to negative dc coronas in the absence or presence of 0.3% H2O or 0.3% O2. The chemical stability of these mixtures is compared with that of SF6–N2 (10 : 90) mixtures or undiluted SF6 investigated in the same conditions in a previous paper.The corona discharges were generated with a point-to-plane set-up and the gaseous by-products were assayed by gas chromatography at the end of each run carried out over a range of transported charge covering 0–13 C. The following by-products were detected and assayed: SOF4, SO2F2, (SF4 + SOF2), SO2, S2F10, S2O2F10, S2O3F6, (SF5)2NF, NF3 and N2O.Whatever the type or the concentration of impurity added to the SF6–N2 mixtures the major compound groups, (SOF4 + SO2F2) and (SF4 + SOF2 + SO2), were formed with 5% SF6 in quantities very close to those observed with 10% SF6. However, the lesser production rates of S2F10, (SF5)2NF and NF3 measured with the most dilute SF6–N2 mixtures makes the use of SF6–N2 (5 : 95) more advantageous than that of SF6–N2 (10 : 90) in all cases.Considering the overall quantity of by-products formed in the presence of water or without any impurity added, it appears that SF6–N2 mixtures are, from this point of view, preferable to pure SF6.

Electron swarm and transport coefficients for the binary mixtures of SF6 with Ar and Xe

A pulsed Townsend technique was used to measure the electron drift velocity, the longitudinal diffusion, and the effective ionization coefficients, and the limiting field strength for the binary mixtures of SF6 with Ar and Xe. This paper covered a wide range of the density-reduced electric field strength E/N between 50 and 700 Td (1 Townsend (Td) = 10−17 V cm2). The content of SF6 in the gas mixtures was varied over the range 1–90%. For the SF6–Ar mixture, the electron drift velocities were found to be higher than those for pure SF6, and conversely for the SF6–Xe mixture. The above can be explained in terms of the larger momentum transfer cross-section for electrons in Xe than in Ar. The limiting field strength for the SF6–Xe mixture was found to be higher than that for the SF6–Ar one, but still lower than that for the SF6–N2 mixture.