Recovery of SF6 from N2/SF6 gasmixtures by using a polymer membrane

The investigation of gas recovery from N₂/SF₆ gas mixtures using a polymer membrane has been reported. It has been shown that the purity of recovered gas, the recovery loss, and the gas handling speed depend on various parameters such as original gas mixing ratio, gas feeding pressure, gas flow rate, and temperature of the membrane. A gas recovering system with two membrane separators connected in cascade has proved to be very efficient in attaining high purity in a recovered gas, with negligibly small recovery loss and high handling speed for practical use. Furthermore, this system could be applied for CO₂/SF₆, He/SF ₆ and even for ternary mixtures such as N₂/CO₂/SF₆ without any modification in the system, where we expect much better performance in comparison with N ₂/SF₆     

Anomalous creeping flashover characteristics in N2/SF6 mixed gas under single pulse voltage

Creeping flashover characteristics for the needle electrode-plane electrode filled with N₂/SF₆ gas mixtures have been investigated. In the case of the needle-plane configuration, the flashover voltage was enhanced by an admixture of SF₆ into N ₂. On the other hand, by the insertion of a solid dielectric barrier between the needle and the plane, only the negative creeping flashover voltage was reduced drastically by the admixture of SF₆ gas into N₂ gas, and a highly emissive region at the top of a creeping corona was found in N₂/SF₆ gas mixture. This behavior is strongly affected by the creeping corona extension process, depending on the creeping distance and the gas pressure     

SF6/N2 mixtures: basic and HV insulationproperties

The widespread use of SF₆ by the electric power and other industries has led to increased concentrations of SF₆ in the atmosphere. This causes concern as to possible effects on global warming, because SF₆ is a potent greenhouse gas. This paper first touches on this issue and then documents the behavior of high pressure gases such as N₂ and SF₆/N₂ mixtures that can be realistically considered as acceptable intermediate or long-term replacements for pure SF₆ in some HV applications. The possible use of dilute SF₆/N₂ mixtures as an alternative to pure SF₆ for some of industry's insulation needs (albeit at higher pressure) is documented, and existing knowledge on these mixtures and on the individual components (N₂ and SF₆), both basic and applied, is compiled. A guide to existing literature is provided     

Very fast transient overvoltages in GIS with compressed SF6-N2 gas mixtures

This paper discusses the characteristics of very fast transient overvoltages (VFTO) in SF₆-N₂ gas mixtures at different percentages of SF₆. A comparison of the VFTO characteristics of pure SF₆ with those of pure N₂ is also presented. The investigations are performed using a laboratory model GIS bus duct having a test gap used for simulating a switching event leading to the generation of VFTO. A capacitive voltage sensor is used to measure the VFTO peak magnitude and temporal characteristics. Measurements were carried out at two different gap spacings (0.20 and 0.61 mm) over a pressure range of 100 to 500 kPa. VFTO characteristics for N₂, SF₆ and SF₆-N₂ mixtures obtained from the experiments show similar trends. The level of surge peak magnitude is <2.0 pu for all cases when the gap was 0.20 mm, but it reaches a maximum of 2.41 pu at 0.61 mm gap. At 0.20 mm gap, in SF₆-N₂ mixtures, the difference in peak magnitudes is not significant for 10% and 20% SF₆ mixtures (between 200 and 400 kPa) and also for pure SF₆ and 40% SF ₆ (between 200 and 300 kPa). The occurrence of corona stabilization during breakdown of the gap may be the cause for such a behavior. Unlike the above observations at 0.20 mm gap, at 0.61 mm gap, the peak magnitudes strictly increase with pressure for the pure gases and gas mixtures. At 0.20 mm gap, the time to breakdown of the gap is found to be almost constant in all cases. But at 0.61 mm gap, the time to breakdown is seen to be dependent on the mixture, pressure, and breakdown voltage, and this observation is in accordance with Toepler's spark law     

Partial discharge and breakdown mechanisms in ultra-dilute SF6/N2 gas mixtures

With the goal of reducing SF₆ gas usage, we investigated partial discharge (PD) and breakdown (BD) mechanisms in ultra-dilute (0 to 1%) SF₆/N₂ gas mixtures. The experimental results and discussions are given in this paper. In particular, we focused our research on the transition characteristics of the electrical insulation performance with an extremely small amount of SF₆ content, the PD behavior on the applied voltage, and the relation among prebreakdown streamer, return stroke and BD. Moreover, we discussed the PD and so mechanisms with reference of space charge behavior. From these results, the transition threshold at which the influence of SF₆ gas on the discharge characteristics began to appear, was clarified to be SF₆ content k=10 ppm for a total pressure of 0.1 MPa     

SF6/N2混合气体绝缘特性的实验研究

目前绝大多数气体绝缘开关设备采用SF₆气体绝缘,SF₆泄漏导致严重的环保问题,人们迫切希望少采用或不采用SF₆气体,以降低对环境的污染。为此,试验研究SF₆和SF₆/N₂混合气体在不同混合比、不同压力以及在不同电场结构下的击穿特性,并与SF₆气体的绝缘性能进行比较,试验结果表明：在N₂中注入20%~30%的SF₆气体后,SF₆/N₂混合气体绝缘性能指标可以达到纯SF₆气体的80%左右,但若继续增加SF₆气体的配比,则其耐电强度上升的幅度明显变慢;此外,试验研究还发现,极不均匀电场会大大降低气体的耐击穿电压强度。试验研究证明了采用SF₆/N₂混合气体代替纯SF₆气体的技术方案的可行性。     

Breakdown Characteristics of N2O Gas Mixtures for Quasiuniform Electric Field under Lightning Impulse Voltage

From the viewpoint of mitigating global warming by SF₆ gas, this paper discusses breakdown (BD) characteristics of different electronegative gas mixtures with N₂O gas as SF₆ gas substitutes for quasi-uniform electric field under lightning impulse voltage applications. Experimental results revealed the positive synergism in breakdown strength of binary N₂O / CO₂ and ternary N₂O / CO₂ / O₂ gas mixtures, respectively. Furthermore, N₂ gas as a retardant gas was also mixed with the electronegative gas mixtures in order to reduce the electron energy into the effective levels of electron attachment ability by the electronegative gas mixtures. As the result, ternary N₂O / CO₂ / N₂ and quaternary N₂O / CO₂ / O₂ / N₂ gas mixtures could exhibit the significant synergistic effect in breakdown strength. The optimum mixture rate of quaternary N₂O / CO₂ / O₂ / N₂ gas mixtures was consistent with that estimated by assuming the independent contribution of component gases to the improvement of impulse BD characteristics.     

Simulation of limiting field behavior in electron swarms inSF6+N2 gas mixtures

Limiting field behavior of the electron swarms in SF₆+N ₂ mixtures is investigated by a Monte Carlo simulation technique. (E/N)_lim values directly obtained from the simulation are given for 5, 10, 20, 40, 60 and 80 SF₆ fractional component in the mixture together with the corresponding electron drift velocities and mean energies. Furthermore, in the E/N range of 243⩽E/N⩽606T_d, effective ionization coefficients and mean energies in SF₆+N₂ mixtures are also evaluated with fractional SF₆ contents of 0, 20, 40, 60, 80 and 100%. The simulation results obtained are compared with the available data in the literature     

Spark decomposition of SF6: chemical and biologicalstudies

Because electric arcs, sparks or corona can decompose SF₆ insulators into byproducts having chemical properties different from SF₆, environmental concerns arise regarding inadvertent human exposures to electrically decomposed SF₆. Biological assays using mammalian cell culture systems have revealed that SF₆ , spark-decomposed under specific experimental conditions, can produce cell death. Chemical analysis of spark-decomposed SF₆ has identified the major decomposition pathways and byproducts. Biological testing of individual byproduct mixtures has indicated that these major decomposition products may not account for the majority of the cell-killing effects seen in the assays. Further experiments have suggested that S₂F₁₀ may be produced and accumulate under the specific decomposition conditions and that this compound may be a major contributor to the observed cell lethality. It is concluded that testing of samples from commercial facilities and assays of decomposed gas after ameliorative treatments would both be appropriate investigations     

Interruption capability of CF/sub 3/I Gas as a substitution candidate for SF/sub 6/ gas

We studied the interruption performance of CF₃I gas because its environmental effect is smaller than that of SF₆ gas with a model arc-extinguishing chamber. First, we measured the arc time constant and arc power loss coefficient using Mayr's equation. Comparing CF₃I with other gases, the arc time constants are SF₆3I2< H₂2. The arc power loss coefficient is H₂>SF₆ >CO₂>Air>N₂ >CF₃I. Next, we evaluated the short line fault (SLF) interruption capability by measuring the di/dt-dv/dt characteristic. Consequently, the SLF interruption performance of pure CF₃I was about 0.9 times that of SF₆. However, CF₃I application to gas insulated switchgear (GIS) and gas circuit breakers (GCB) is difficult because the liquefying temperature of the gas is high. Therefore, we adopted a countermeasure to obtain a lower liquefying temperature mixing CF₃I with CO₂. The result showed that the SLF interruption performance of the mixture approximated that of pure CF₃I when the ratio of CF₃ I exceeds 20%     

SF6/N2流注放电仿真及协同效应研究

建立反映气体放电过程中粒子运动特性的二维流体模型,采用有限元和通量校正传输法对该模型进行数值求解,计算了50%SF₆+50%N₂在均匀电场下的放电规律,模拟了流注发展过程中粒子密度的分布情况,分析放电过程中带电粒子对均匀电场的影响。搭建气体放电实验平台,测量平板电极下绝缘间隙5 mm时SF₆/N₂混合气体的击穿电压,将SF₆/N₂击穿电压的实测值与折算值进行对比,研究不同混合比、气体压强对SF₆/N₂协同效应的影响。结果表明：随着流注向阳极运动,放电间隙内的电子数密度不断增大;在放电初始阶段,空间电荷对电场的影响很小,随着电荷数量不断增加,空间电场产生明显畸变现象。SF₆/N₂混合气体击穿电压的实验测量值大于折算值,且SF₆含量越高,实测值和折算值越接近。可以看出,SF₆/N₂的协同效应在含有少量SF₆时较明显,而当SF₆含量较高时,混合气体的协同效应减弱。     

Laser sensor for detection of SF6 leaks in high powerinsulated switchgear systems

A novel photoacoustic spectrometer (PA) has been developed for in-situ detection of SF₆ leaks in low concentrations. The developed system is equipped with a sound alarm system and has been tested in the laboratory for very minute SF₆ leaks. This newly developed SF₆ detection device utilizes a high quality factor resonant photoacoustic cell and continuous wave (CW) line tunable CO₂ laser at 10.55 μm wavelength. Whenever SF₆ is detected an acoustic signal is generated and no signal appears from ambient air if there is no leakage of SF₆. An electret microphone is used for the detection of these acoustic signals. The system is capable of detecting leaks of the order of 3.5 ppbv (parts per billion by volume) concentration. This device has been proved to have significant applications to industrial organizations that have electric power gas insulated systems (GIS). It could be also applied for other applications such as monitoring of environmental pollutants with minimal adjustments     

Ion mobility spectrometry for on-site sensing of SF6decomposition

Although gas-insulated system (GIS) and substations belong to the most reliable components of electric power networks, failures still occur in their long-term operation. In order to estimate the probability of malfunctions in advance, a system for monitoring the quality of the insulating gas SF₆ was established, which is based on the use of ion mobility spectrometry (IMS). In a set of experiments, partial discharge (PD) was generated inside a test GIS. The SF₆ was periodically analyzed using IMS. The results of this analysis is presented and the influence of impurities remaining inside the compartment is discussed. The analysis of gas contained in a circuit breaker in a hydroelectric power plant before and after operation is described as well as the structure of a system for automatic on-site sensing of SF₆ decomposition     

Calculation of the uniform breakdown field strength of SF6 gas

We calculated the uniform dielectric breakdown field strength of SF₆ gas over the temperature range of 300 to 3000 K. The local thermal equilibrium (ITE) composition of the dissociated gas is connected to the electron impact collision cross sections of the species SF₆, F₂, F and S. The critical reduced electric field strength of the composition is determined by a balancing electron generation and loss modeled by chemical reactions evaluated by the electron energy distribution function (EEDF) derived from the Boltzmann transport equation. At room temperature, pure SF₆ has a critical reduced electric field strength of 362 Td. With increasing temperature and decreasing density we found a small decrease of this value, whereas at temperatures higher than 1500 K, dissociation starts to decrease the dielectric strength of the composition. Furthermore, we found that generation of electrons by (associative) detachment from F ^- starts to play an important role at temperatures >2500 K, where the critical field strength still has a value of 118 Td. This value is found to decrease rapidly afterwards with increasing temperature to the value of 38 Td at 3000 K. The calculated results agree very well with independently predicted values and measured data     

Influence of reactive SFx gases on electrode surfacesafter electrical discharges under SF6 atmosphere

A simulation of different electrical discharges under SF₆ atmosphere with subsequent surface and depth profile analysis is employed to investigate the effects that the formed corrosive byproducts have on gas-insulated system (GIS) electrodes. Cu, Al and Ag electrodes were treated with sparks (in the range of 0.15 J/spark) in an SF₆ discharge cell under various conditions at the Institut fur Spektrochemie und Angewandte Spektroskopie (ISAS) in Dortmund. The same materials were exposed to SF₆ in a GIS test compartment in which partial discharge (PD) occurred (Itaipu, Brazil). The treated materials were analyzed with energy dispersive X-ray (EDX) in a scanning electron microscope, X-ray photoelectron spectroscopy and high frequency (HF) plasma secondary neutral mass spectrometry (SNMS). The SNMS depth profiles were calibrated in order to quantify that part of the corrosive gaseous products having reacted with the solid material. The analysis of the electrodes exposed to the stressed SF₆ atmosphere show surface reaction only in the topmost nm, whereas with sparks, sulfur (S) and especially fluorine (F) compounds are detected to ~40 μm depths. A first comparison between surface analysis and Fourier transform infrared (FTIR) measurements of the reactive gaseous products indicates a correlation of SF₆ decomposition and the extent of electrode degradation     

Investigation of S2F10 production andmitigation in compressed SF6-insulated power systems

The Cooperative Research and Development Agreement (CRADA), established to study the production and mitigation of S₂F₁₀ (disulfur decafluoride), one of a number of toxic by-products formed in electric discharges in the insulating gas SF ₆, is described. The particular concern about S₂F ₁₀ is due to its highly toxic nature, the ceiling limit value being 10 parts per billion (ppb, or 1 part in 10⁸), and the need for development of sensitive detection techniques down to this level. In the presence of an electrical discharge such as an arc, spark, or corona, a portion of the SF₆ decomposes into lower fluorides of sulphur which can react to form a number of chemically active by-products including SOF₂ and SO₂F₂ . During the maintenance or repair of SF₆-insulated equipment, the handling of these gaseous is a matter of concern. Preliminary arc experiment results, reported health-related incidents caused by SF₆ by-products, and ongoing studies are discussed     

Breakdown behavior of SF6 gas-insulated systems at lowtemperature

The controversy surrounding low-temperature SF₆ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF₆ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24°C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43×10¹⁹ cm^-3 (equivalent to pressures of 105 and 624 kPa, respectively, at 24°C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF₆-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by ~10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30°C), take a constant value, and be fairly independent of density     

Corona characteristics for free conducting particles in various SF6-gas mixtures

This paper reports on experimental investigations concerning the behavior of pulsative corona discharges from free conducting wire particles of different sizes in parallel plane and concave electrode systems, insulated with SF₆ and its mixtures with nitrogen N ₂, triethylamine (C₂H₅)₃N and perfluorocarbon C₈F₁₆O gases. Corona inception and particle lift-off voltages as well as corona charge levels are measured for different experimental conditions. The results show that corona and particle lift-off characteristics are affected by particle parameters, electrode shape and gas composition. Generally, addition of triethylamine and perfluorocarbon to SF₆ reduces the corona charge level     

Reduction of CO2 from combustion gases by DC coronatorches

An experimental investigation has been conducted to reduce CO₂ from combustion gases by using DC corona torches. This plasma device, the corona torch, consists of two small-diameter hollow electrodes. The gas flow enters the upstream cylindrical hollow electrode and exits at a downstream cylindrical hollow electrode. Therefore, all the reactive gas passes through the active corona-induced plasma zone. High-speed gas flow near the exit of the electrode cools the electrodes; hence, the chemical reactions and the stability of discharge are enhanced. Various amounts of argon gas were added to the model combustion gas (N₂:O₂:CO₂=0.745:0.15:0.105). The results show that the CO₂ gas reduction rate increases with increasing corona current when the model gas is seeded with argon. The CO₂ gas reduction rate increases when the percentage of argon gas in the mixture increases up to 20%, and decreases when the percentage of argon gas in the mixture increases above 20%     

Partial discharge measurements in SF6 and air usingphase-resolved pulse-height analysis

Carries out a study on partial discharge (PD) phenomena in SF₆ gas and tries to construct a diagnostic system capable of separating noise from observed PD signals for assessing the condition of insulation performance in power apparatus. The authors have constructed a computer-aided PD measuring system which permits a time-sequential data acquisition and phase-resolved pulse height analysis. Using the PD measurement system, the authors measured 60 Hz AC PD in SF₆ gas and in air at atmospheric pressure for needle-plane electrodes with three different radii at the tip of the needle, 3, 30 and 500 μm. They proposed particular PD statistical parameters, unbalance ratios U _T and U_q of the pulse repetition rate and average of charge, respectively, between positive and negative PD pulses occurring per cycle; these parameters proved to be good indicators to distinguish PD occurring in SF₆ from that in air. The mechanism of PD inception was also discussed