Comparison of SF6/N2 and SF6/CO2 gas mixtures as alternatives to SF6 gas

The interest in SF₆ gas mixtures has been re-ignited in recent years by the issue of the greenhouse effect of the SF₆ gas, and most research work is now focused on a SF₆/N₂ gas mixture, which is suitable for application in electrical apparatus with slightly non-uniform fields. This paper presents a comparison of SF₆/N₂ and SF₆/CO₂ gas mixtures with a viewpoint of their possible applications to gas-insulated transformers, where both highly non-uniform field problems and partial discharges in gas/film insulation are inevitable. It is shown that in this case the dielectric strength of SF₆/CO₂ is superior to that of SF₆/N ₂ with a minor disadvantage related to the gas decomposition in SF₆/CO₂. However, this may not be a problem for the SF₆/CO₂ gas mixture to be used in gas-insulated transformers, where internal breakdown is not allowed     

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。     

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     

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。     

CF4/CO2混合气体工频击穿特性的试验研究

SF₆是目前电气设备中广泛使用的气体绝缘介质,但其产生的温室效应对环境影响极大,因此,研究能替代SF₆的环保型气体绝缘介质具有重要的意义。CF₄是一种具有低温室效应指数（global warming potential, GWP）和低液化温度的强电负性气体。通过工频击穿试验,研究了CF₄/CO₂混合气体的工频击穿特性,并对其协同效应和GWP值进行分析。结果表明：CF₄混合比为50%的CF₄/CO₂混合气体的绝缘强度能达到纯CF₄的90%左右,同时GWP值低,具有用于气体绝缘的潜力;CF₄/CO₂混合气体的协同效应值在0.12~0.38,表现出明显的协同效应。     

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     

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     

Fundamental insulation characteristic of high-pressure CO/sub 2/ gas under actual equipment conditions

SF₆ gas has been widely used in electrical power equipment such as circuit breakers and transformers due to its superior insulation and interruption characteristics. However since 1997, SF₆ gas has been designated a greenhouse gas subject to emission restrictions at COP3 (The 3rd session of the Conference Of the Parties to the United Nations Framework Convention on Climate Change) so a new insulating gas is needed as a substitute for SF₆ gas. This research considers the use of high-pressure CO₂ gas as an insulator while stressing the environment aspects. Fundamental insulation data for the insulating gas acquired supposing gas insulated switchgears (GIS) consists of; (1) insulation breakdown characteristics under clean conditions and, (2) insulation breakdown characteristics with metallic particle contamination. The parameters in this case were assumed from an actual apparatus viewpoint, to be a high gas pressure up to 2.0 MPa, an electrode size capable of determining the surface area effect, the electrode surface roughness, and metallic particle length, etc. at the base electrode of the 72 kV GIS. As a result, experiments using these parameters revealed insulation characteristics for high-pressure CO₂ gas and that negative lightning impulse decided the insulation design, as well as the present SF₆ GIS. The need for taking measures to suppress PD under AC voltage and also the need for restricting metallic foreign particles around the central conductor and insulating spacer were recognized     

Applicability of simple expressions for electrical breakdownprobability increase in vacuum and gas

Simple analytical expressions for the law of breakdown probability increase are suggested. They are tested by comparing with experimental data obtained by impulse and DC breakdown. The pressure 10^-4 Pa and interelectrode gaps d<0.1 mm are used for vacuum, and the pressure 1 bar-5 bars and the interelectrode gaps 1 mm-50 mm are used for gas breakdown. The insulation gas was a SF₆, N₂ and gas-mixture: SF₆-N₂. It is concluded that such a simplified approach is not appropriate for vacuum. It is also concluded that, for gas, the BPI (breakdown probability increase) law, in the form of a simple expression, is valid in the considered range of relevant experimental parameters     

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%     

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     

Characterization of Polyurethane Foam Dielectric Strength

Internal flashovers in hollow insulation systems can produce serious damage. Frequently, SF₆ or N₂ is used to eliminate this problem, but possible gas leakage may endanger the insulation. This paper proposes the use of polyurethane foams to fill the hollow spaces in insulation systems. Thus far, few publications deal with the dielectric properties of foams. This paper demonstrates the dielectric strength of three different foams which are investigated using ac and lightning impulse voltages under different humidity and temperature conditions. The results show that polyurethane foams have 2-3 times better dielectric strength than air. The breakdown strength decreases with the thickness of the foam; temperature and humidity have negligible effects on the breakdown voltage. The major parameter is the size of the voids in the foam. Reducing the size of the voids increases the breakdown strength. This can be achieved by improving the manufacturing technology.     

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     

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     

AC particle-triggered corona discharge in low pressure SF/sub 6/ gas

This paper deals with AC particle-triggered corona discharge as a follow-up to our previous research with DC voltage to clarify the particle-triggered corona discharge process in SF₆ gas. Corona current pulses, charges associated with a corona current pulse, and corona light pulses were observed with an aluminum ellipsoidal particle suspended in a parallel plane electrode system under a SF₆ gas pressure range of 30 kPalesPles50 kPa by changing the particle position. Corona mode, phase (Phi)-charge (q) characteristics as well as corona discharge processes were discussed and the following results were obtained. AC corona mode depended on the instantaneous applied voltage, voltage gradient as well as the particle position and then, the Phi-q characteristics were also affected by those parameters. Charges flow into the floating particle due to coronas on the both tips of particle and excite field fluctuations around the opposite side of particle in addition to the applied AC field. The field fluctuation in SF₆ gas by the corona charges was about 4% of the applied field and much lower than that in air gap which was about 70%. That is, the corona development was suppressed effectively by high electron affinity of SF₆ gas even in the case of floating particle. The less effective interference between coronas on the both side of particle in SF₆ gas results in an obscure local minimum in the breakdown voltage characteristics as the particle is in the vicinity of electrode as contrasted with a drastic fall in the breakdown voltage by the particle in air     

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%     

Mechanisms for formation of inorganic byproducts in plasma chemicalprocessing of hazardous air pollutants

Plasma chemical behavior of hazardous air pollutants (HAPs) (Cl ₂C=CCl₂, Cl₂C=CHCl, Cl₃C-CH ₃, Cl₂CH-CH₂Cl, CH₃Cl, CH ₃Br and benzene), their molecular probes (CH₄, CH ₃-CH₃, and CH₂=CH₂), and carbon oxides (CO_x) was investigated with a ferroelectric packed-bed plasma reactor to obtain information on the formation of CO _x and N₂O. It has been shown that the oxidation of CO to CO₂ is a slow reaction in plasma, and that CO and CO ₂ mainly result from different precursors. Simultaneous achievement of complete oxidative decomposition of HAPs in plasma and recovery of CO as a chemical feedstock could be favorable. The process of N₂O formation is affected by HAP structures and oxygen concentration. In the decomposition of olefinic HAPs, such as Cl₂ C=CCl₂ and Cl₂C=CHCl, high-power short-residence-time operations are effective in suppressing N₂ O formation. In the cases of CH₃Cl and CH₃Br, low specific energy density operations could be necessary to reduce N₂O concentrations. The yields and selectivities of CO, CO₂ and N₂O change drastically by adding only 2% of oxygen to N₂, and oxygen concentration is not a good factor to control these inorganic oxides     

Dielectric breakdown characteristics of an insulator with a sharp edge in SF6 gas under lightning impulse voltage

This article discusses the dielectric breakdown characteristics of an insulator with a sharp edge that forms inhomogeneous gaps in SF₆ under a lightning impulse voltage. The high voltage electrode is a plane electrode, and the grounded electrode is a plane electrode with a column insulator, which has an inside electrode. The column insulator has a height of 50 mm and a diameter of 50 mm. The inside electrode in the column insulator has a height of 20 mm or 30 mm or 40 mm. The inside electrode changes the electric field on the surface of the column insulator. The SF₆ gas pressure is 0.225 MPa. Dielectric breakdown does not occur at the edge of the column insulator, but rather at the top plane of the column insulator. The dielectric breakdown voltage is defined by the electric field at the center of the column insulator