The increasing application of SF6 as an insulating gas has led to many studies on SF6 decomposition in gas-insulated equipment. In the presence-of an electric arc, spark or corona, SF6 decomposes to a wide variety of chemically active products which possess completely different properties from SF6. The accumulation of these decomposition products in the equipment has caused concerns regarding personnel safety and material compatibility problems. This paper reviews previous research in SF6 decomposition relating to the operation of gas-insulated switchgears, gas-insulated transmission lines, and electrostatic accelerators. Results on the qualitative and quantitative determination of the by-products and their formation ion rates in various modes of electrical discharges are summarized. The mechanisms leading to the formation of transient and stable products are described. In particular, the influence of discharge energies and impurities on the formation of SOF2 and SO2F2, the two dominant stable by-products, is discussed. The effects of the by-products on personnel safety and equipment ent dielectric integrity are presented. The application of SF6 gas analysis as a tool for diagnosing the internal condition of gas-insulated equipment is assessed. Based on the results of many phenomenological observations, future research activities are suggested to address the issues of safety, compatibility and equipment aging. More fundamental studies on electron, ion, and neutral reaction rates in an SF6 discharge are required to gain a better understanding of the decompositon mechanisms and the influence of the products on equipment operation.

SF6 Decomposition in Gas-Insulated Equipment

The widespread use of SF/sub 6/ by the electric power and other industries has led to increased concentrations of SF/sub 6/ in the atmosphere. This causes concern as to possible effects on global warming, because SF/sub 6/ is a potent greenhouse gas. This paper first touches on this issue and then documents the behavior of high pressure gases such as N/sub 2/ and SF/sub 6//N/sub 2/ mixtures that can be realistically considered as acceptable intermediate or long-term replacements for pure SF/sub 6/ in some HV applications. The possible use of dilute SF/sub 6//N/sub 2/ mixtures as an alternative to pure SF/sub 6/ for some of industry's insulation needs (albeit at higher pressure) is documented, and existing knowledge on these mixtures and on the individual components (N/sub 2/ and SF/sub 6/), both basic and applied, is compiled. A guide to existing literature is provided. >

SF/sub 6//N/sub 2/ mixtures: basic and HV insulation properties

Surface flashover in compressed-gas insulated systems is a much studied, but poorly explained phenomenon. In this paper we review the literature of surface flashover with primary emphasis on the understanding of physical processes leading to discharge initiation and insulator flashover under high voltage excitation. The flashover models presently in vogue will first be discussed, followed by the results of some recent experiments which are likely to have an impact on further modeling. Included in this context are phenomena such as ionization, surface charging, partial discharges, optical activity, and gas/dielectric interactions. Finally, the influence of system parameters such as insulator size, shape, surface condition, triple junction geometry, voltage waveform, gas formulation and particle contamination are discussed with regard to their effect on the flashover characteristics. Mechanisms are suggested in an effort to provide a physical explanation for the observed phenomena. Although the physics of the discharge initiation and propagation processes are presently not well understood, and the present models only account for a few of the mechanisms known to be important in the discharge development, all the work points to an interaction between the spacer and the various electron/photon processes in the surrounding gas volume. This interaction has not been accounted for in the discharge models proposed to date. Further modeling work should incorporate these interactions and the intrinsic properties of the dielectric ric which are related to these interactions. More basic research is suggested to provide a better understanding of the physics of the discharge initiation and breakdown phenomena.

Mechanisms of Surface Flashover Along Solid Dielectrics in Compressed Gases: a Review

It has been shown that it is possible to extend the validity of the Townsend breakdown criterion for evaluating the breakdown voltages in the complete pd range in which Paschen curves are available. Evaluation of the breakdown voltages for air (pd=0.0133 to 1400 kPa · cm), N2(pd=0.0313 to 1400 kPa · cm) and SF6 (pd=0.3000 to 1200 kPa · cm) has been done and in most cases the computed values are accurate to ±3% of the measured values. The computations show that it is also possible to estimate the secondary ionization coefficient ? in the pd ranges mentioned above.

Analysis of Paschen Curves for air, N2 and SF6 Using the Townsend Breakdown Equation

The electric power Industry's preferred gaseous dielectric (besides air), sulfur hexafluoride (SFg), has been shown to be a greenhouse gas. In this report we provide information that is useful In Identifying possible replacement gases, in the event that replacement gases are deemed a reasonable approach to reducing the use of SFg in high voltage electrical equipment. The report focuses on the properties of SFg as a dielectric gas and on the data available for possible alternatives to pure SFg (i.e., SFg alone). On the basis of published studies and consultation with experts in the field, it was attempted to identify alternative dielectric gases to pure SFg for possible immediate or future use in existing or modified electrical equipment. The possible alternative gases are discussed as three separate groups: (i) mixtures of SFg and nitrogen for which a large amount of research results are available; (ii) gases and mixtures (e.g., pure Nj, low concentrations of SFg in Ng, and SFg-He mixtures) for which a smaller yet significant amount of data are available; and (iii) potential gases for which little experimental data are available.

Gases for Electrical Insulation and Arc Interruption: Possible Present and Future Alternatives to Pure SF6

The outstanding electrical and arc-quenching characteristics of sulphur-hexafluoride (SF6) have already set new trends and standards for the construction of high voltage equipment. Intensive research is being carried out on the electrical properties of SF6, other electronegative gases and their mixtures with more common gases. The aims are to obtain an economical insulation with optimal electrical properties and to acquire a better understanding of their prebreakdown and breakdown behavior. Recently [1] we reviewed the breakdwon mechanisms operating in compressed gases and discussed the theoretical results in the light of experimental measurement in SF6. This paper deals with the breakdown and prebreakdown behavior of SF6 mixtures with other electron attaching and non-attaching gases. Uniform and non-uniform field breakdown under the applications of direct, alternating and impulse voltages is discussed. Improvement in properties such as higher breakdown voltages in non-uniform field gaps, less sensitivity to the electrode surface roughness and to the presence of contaminants and improved arc interruption characteristics of SF6-gas mixtures are discussed.

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

The sensitivity of compressed gases to free and fixed conducting and non-conducting particle contaminants is a limiting factor in the design of electrical apparatus. Conducting particles which may result from damage to a gas-filled system constitute a serious threat to its safe and reliable operation. This deterioration in the dielectric strength of the gas largely depends on the type and shape of the particles, the type and pressure of the gas insulation, and the electric field. This paper reviews the electrical performance of gases under particle contamination. Further areas of work are proposed in order to obtain a better understanding of the mechanism of breakdown and a reliable operation under particle contamination.

A Review of Particle-Contaminated Gas Breakdown

A method to calculate the uniform field breakdown gradients for SF6-N2, SF6-air and SF6-CO2 mixtures is discussed. Calculated values of the breakdown gradients in these mixtures are compared with experimentally measured values using plane-plane electrode configuration and direct applied voltages. The results show a reasonably good degree of agreement between the calculated and the measured values.

Breakdown Gradients in SF6-N2, SF6-Air and SF6-CO2 Mixtures

The well known breakdown theories, Townsend's generation mechanism and the streamer mechanism, are reviewed and applied to the results of breakdown in the strongly electronegative gas, sulphur-hexafluoride. Experimental results reported in the literature on the breakdown behavior of sulphur-hexafluoride are examined in the light of these theories. The breakdown theories are used for the estimation of the breakdown voltages in pure SF6. Other factors that may affect the breakdown characteristics of SF6 have been discussed. Further areas of work have been proposed in order to obtain a better understanding of the breakdown mechanism.

Breakdown Mechanisms in Sulphur-Hexafluoride

A simple method based on the streamer criterion [1] is proposed to calculate the discharge inception voltages in SF6-N2 mixtures with SF6 content between 1 and 100%. It is shown that the calculated values are in good agreement with experimental measurements for gaps having varying degrees of field non-uniformities.

A. H. Qureshi

Papers

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

A Review of Particle-Contaminated Gas Breakdown

Breakdown Gradients in SF6-N2, SF6-Air and SF6-CO2 Mixtures

Breakdown Mechanisms in Sulphur-Hexafluoride

Calculation of Discharge Inception Voltages in SF6-N2 Mixtures