A M Casanovas

Chemical kinetics modelling of a decaying SF6 arc plasma in the presence of a solid organic insulator, copper, oxygen and water

The composition variations occurring in decaying SF6 arc plasmas in the presence of atoms released from the vaporization of organic insulators (e.g. Teflon, polyethylene, polypropylene, megelit, nylon), copper, oxygen and water were studied between 12 000 K and 300 K by means of a chemical kinetics model. From the results obtained at 300 K and a pressure of 101.3 kPa: (i) the role of the impurities on the formation of the SF6 decomposition products: SF4 , SOF2 , SO2 F2 and S2 F10 , was determined; (ii) it was confirmed that the vaporization of an organic insulator leads to the appearance of CF4 and an increase in the generation of the major byproduct (SF4 +SOF2 ) which is correlated to the production of CF4 ; (iii) it was seen that, for a given amount of vaporized insulator, insulators that contain fluorine atoms brought about less SF6 decomposition than those that did not.

Production of , , , , and in and (50 - 50) - mixtures exposed to negative coronas

This study concerns the production of , , , , and when is subjected to negative polarity corona discharges of varying durations (with transported charges of 1.5, 4 and 6 C) performed with a point-to-plane set-up using a stainless steel or aluminium plane electrode. During the experiments, the parameters varied were the way the measurement cell was prepared (clean and very clean), the pressure (50 to 400 kPa), the concentration of additives such as (0 to 50%) for total gas pressures of 200 kPa and 300 kPa, water (0 to 0.2%) and oxygen (0 to 1%) for a total pressure of 300 kPa. Analyses were carried out using gas phase chromatography. The mode of preparation of the cell proved to be representative of the action of impurities such as water and oxygen on each of the compounds studied. This effect was all the stronger when the pressure was low. In the very clean conditions (effect of and reduced to a minimum) we observed a decrease of the quantities of the main products formed as the pressure or the percentage of was increased. Concerning the effect of the small quantities of added water and oxygen studied in both pure and in the 50 - 50 - mixture, the results showed that, overall, the oxygen and the water enhance the production of all the sulphur oxyfluorides from the fragments (except for which is inhibited by oxygen to the benefit of ) and inhibit the production of . The presence of 50% , a fluorine source, inhibited the production of all the compounds studied independently of the transported charge, the metal used for the plane electrode and the percentages of impurities (, ) added.

Influence of discharge production conditions, gas pressure, current intensity and voltage type, on SF6 dissociation under point–plane corona discharges

The study of the formation of Sulfur Hexafluoride (SF6) dissociation products under point to plane corona discharges was carried out at PSF6=300 kPa using different discharges production conditions (50 Hz ac voltage, dc negative polarity voltage, mean discharge current intensity I varying between 2 and 45 μA for dc negative polarity voltage), for two plane electrode materials (aluminum and stainless steel), and moisture levels (200 and 2000 ppmv H2O). The stable gaseous by‐products formed (SO2F2, SOF4, SOF2, and S2F10) were assayed by gas‐phase chromatography. The results indicate an important effect of the metal constituting the plane electrode and of the moisture conditions whatever the SF6 pressure (100–300 kPa), discharges intensity (I) and voltage type studied. An effect of the increase of SF6 pressure up to 300 kPa was mainly observed for S2F10 and corresponds to a greater formation of this compound with PSF6. The influence of the mean discharge current intensity on SF6 by‐product formation carrie...

Decomposition products from negative and 50 Hz ac corona discharges in compressed SF6 and SF6/N2 (10:90) mixtures. Effect of water vapour added to the gas

SF6/N2 mixtures with a majority of nitrogen are currently highly recommended, at the international level, in gas insulated transmission lines as an alternative to pure SF6; indeed, these mixtures are much more friendly to the atmosphere and particularly cheap. Among the areas of investigation of such gas mixtures, their electrical decomposition as a function of impurity content and type of discharge must be studied. The present study concerns the decomposition rate of SF6 and SF6/N2 (10:90) mixtures at 400 kPa under negative and 50 Hz alternating current corona discharges carried out without and in the presence of 0.3% H2O added, up to 10 C of transported charge. The corona discharges were generated at 23 °C in a 340 cm3 experimental cell between a stainless steel point (radius of curvature 10 µm) connected to high voltage and a plane layer of aluminium (gap space 2.3 mm for pure SF6 and 3.4 mm for SF6/N2). The gaseous decomposition products SOF4, SO2F2, SF4+SOF2, SO2, S2OF10, S2F10, S2O2F10, S2O3F6, SF5NF2, NF3 and (SF5)2NF were assayed by gas chromatography at the end of each run. The comparison of the formation rates of the byproducts detected in SF6 and SF6/N2 mixtures led to the following conclusions. (i) The use of wet SF6/N2 presents an additional advantage with respect to pure wet SF6 besides those mentioned above; the production rates of all the usual SF6 decomposition products are much lower with the wet SF6/N2 (10:90) mixture than with wet SF6. (ii) More exactly the addition of 0.3% H2O to SF6/N2 greatly affects the production rates of all the compounds whose formation needs SF5 radicals as SOF4; so the lower quantity of products formed in SF6/N2 is in fact more due to the particularly inhibiting effect of water than to the low percentage of SF6 in the mixture. (iii) The production of NF3, SF5NF2 and (SF5)2NF remains very low compared to the other gaseous byproducts.

Spark decomposition of and mixtures

The spark decomposition of and of mixtures was studied principally at a gas pressure of 200 kPa. The sparks were generated between a point and a plane either under 50 Hz ac voltage (0.09 J per spark) or by discharging a capacitor (3.59 J per spark). Our attention was only focused on the gaseous by-products: and which were assayed by gas chromatography. The last three compounds were principally observed under the higher energy sparks. Their yields were studied varying the cell preparation technique, the metal constituting the plane electrode (aluminium, copper, stainless steel) and the concentrations of two additives, (between 0 and 1%) and (between 0 and 0.2%). The cell preparation procedure had a strong effect on the formation of all products except ; the yield was for example multiplied by when the cell was carefully dried and outgassed and with an aluminium electrode. The aluminium led, whatever the procedure used, to the highest levels of products. Under the high-energy sparks an increase of the oxygen content of or of the mixture led to a decrease of the and formation rates and to an increase of that of the other compounds. An increase of the content had very little effect on production and led to increased production of and and to a lowering of the formation of other compounds. Under the low-energy sparks the addition of to or to the mixture led to a lowering of the and yields like under high-energy sparks and to an increase of (which became observable) and of . Addition of water resulted in an increase of the and yields, in a lowering of and had no effect on which remained unobservable. Finally it should be noted that the addition of 50% of to the had very little effect on the rates of formation of the gaseous by-products except under low-energy sparks where the mixture led to lower production rates for and .

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.

Spark decomposition of SF6, SF6/N2 (10 : 90 and 5 : 95) mixtures in the presence of solid additives (polyethylene, polypropylene or Teflon), gaseous additives (methane, ethylene, octofluoropropane, carbon monoxide or dioxide), water or oxygen

The present paper is a continuation of the investigations on the sparking-induced decomposition of SF6 and SF6/N2 (10 : 90) mixtures which have already been carried out in our laboratory, both experimentally and numerically. It concerns the decomposition of SF6/N2 mixtures (100 kPa) containing 100%, 10% or 5% of SF6, under high-energy sparks (3.6 J spark−1) generated in a 340 cm3 experimental cell between a stainless steel point and a stainless steel plane. Our attention was focused on the following main by-products: (SF4+SOF2), (SOF4+SO2F2), S2F10, CF4, CO and CO2 which were studied by varying the concentration of the impurities added H2O, O2 (0–0.2%), in the presence of atoms such as H and C released from vaporized solid insulators (polyethylene [C2H4]n, polypropylene [C3H6]n, Teflon [CF2]n) or from gaseous additives (methane CH4 (0–4%), ethylene C2H4 (0–2%), octofluoropropane C3F8 (0–5%)), with the aim of simulating the occurrence of sparking in electrical devices, especially along spacers. As SF6/CO2 and SF6/N2/CO2 mixtures are reported to be able to constitute promising SF6 substitutes for industrial applications, we also studied the chemical stability of SF6 and SF6/N2 (5 : 95) mixtures in the presence of 0–20% CO2. The presence of additives CH4, C2H4, C3F8 or solid insulator (polyethylene, polypropylene, Teflon) leads to lower production of (SF4+SOF2) and S2F10 in dilute SF6 than in pure SF6 when the percentages of additives or the amounts of solid insulator vaporized are high. Concerning the additive CO2, we observe an increased production of (SOF4+SO2F2) and a formation of large quantities of CO, more pronounced in SF6/N2 (5 : 95) mixtures than in pure SF6. In contrast, the presence of CO leads to a lesser degree of decomposition of diluted than undiluted SF6.

Influence of a solid insulator on the spark decomposition of SF6 and 50% SF6 + 50% CF4 mixtures

It is now known that SF6/CF4 mixtures (50–50) make a suitable candidate to supplant pure SF6 in high voltage circuit breakers for low temperature environment applications. When a circuit breaker is operating, the arc plasma decays in the presence of metal, carbon, oxygen and water coming from partial vaporisation and desorption of the electrodes and nozzle (composed of an organic insulator) under the stress. So, in an effort to approach conditions occurring in practice and as a continuation of previous investigations1,2 in our laboratory, we studied the spark decomposition of SF6 and SF6+50%CF4 mixtures in the presence of a solid insulator struck by the electric arc.

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.

Chemical Decomposition of High Pressure SF6/N2 (5:95) Mixtures under Negative DC Corona Discharges

Sulfur hexafluoride (SF6) is the insulator gas used in most of equipments handling high and very high voltages.Owing to its high global warming potential, SF6 has been classified during the Kyoto conference on climate change among the greenhouse gases. Its emission in the atmosphere must therefore be reduced1. In order to comply with this aim, one of the solutions adopted by the manufacturers and the users of some of these equipments, consists in reducing the quantity of SF6 used by mixing it with zero (or very much lower) risk gases.For instance, as SF6/N2 mixtures with SF6 concentrations lying between 5 and 15% realise a satisfactory compromise between dielectric performances and cost, Electricite de France plans to use these mixtures to fill its future gas-insulated lines2. Under the effect of the corona discharges which can occur in these equipments, part of the SF6 and of N2 will dissociate and lead, in the presence of even low concentrations of H20 and/or 02, to the formation of a lot of other gaseous compounds: (SF4+SOF2), SOF4, S02F2, S2F10, NF3....Previous studies on the decomposition of SF6/N2 (10:90) mixtures under coronas3 showed that, with no impurity added, the total quantity of by-products formed is not very different from that measured in pure SF6 for the same charge transported value. This indicates that the percentage of SF6 consumed increases as its concentration in the gas phase decreases 3,4.This result led us to extend our experiments to SF6/N2 mixtures only containing 5% of SF6.