J. Casanovas

Photoconductivity studies in some nonpolar liquids

Measurements of the photoconductivity of liquid n‐pentane, n‐hexane, cyclopentane, cyclohexane, neopentane, neohexane, and tetramethylsilane induced by far UV radiation (7⩽hν⩽10 eV) have been performed. We have studied the influence of the incident photon energy, of the applied electric field strength, and of the concentration of two electron scavengers (carbon tetrachloride and perfluoromethylcyclohexane). Our results show that in going from the gas phase to the liquid phase, the ionization energy threshold values of these low dielectric constant liquids are lowered by about 1.6 eV. The introduction of an electron scavenger reduces drastically the solvent photocurrent. Our results are consistent with the quenching, by the scavenger, of a solvent expanded excited state, precursor of the geminate ion pair, and not with an epithermal electron scavenging as in high energy photon irradiation.

Study of the decomposition of SF6 under dc negative polarity corona discharges (point‐to‐plane geometry): Influence of the metal constituting the plane electrode

SF6 samples (PSF6=100 or 200 kPa) were submitted to point‐to‐plane dc negative polarity corona discharges in the presence of water [concentration=2000 ppmv (parts per million by volume)] or without the addition of water. The stable gaseous byproducts formed, (SO2F2, SOF2, and S2F10) were assayed by gas‐phase chromatography. The variation of their yields against the charge transported (up to 10 C) was studied for two metals (aluminum and stainless steel) constituting the plane electrode, at various values of the SF6 pressure, the water content, the gap spacing (2.5 and 8 mm), and the discharge current [12≤I (μA)≤25]. The results indicate an important effect of the metal constituting the plane electrode and of the moisture conditions, particularly on the production of SOF2 and S2F10.

Study of the decomposition of wet SF6, subjected to 50‐Hz ac corona discharges

Mixtures of SF6 (100 kPa≤PSF6≤400 kPa) and water (concentrations ranging from 240 to 2000 vpm) have been submitted to point‐plane 50‐Hz ac corona discharges. The only stable gaseous by‐products detected, either by gas‐phase chromatography or gas chromatography–mass spectrometry techniques, were SOF2 and SO2F2. The variation of their concentrations as a function of the discharge current value (3–25 μA rms), the charge transported (0.15–1.5 C), the water content, the SF6 pressure, and the gap spacing (1.5–5 mm) have been studied. The results indicate that, at least in our experimental conditions, the 50‐Hz ac corona discharges behave more like negative than like positive dc corona.

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.

Spark decomposition of SF/sub 6//H/sub 2/O mixtures

The spark decomposition of mixtures of SF/sub 6/ (partial pressure from 100 to 500 kPa) and water (concentrations between 600 and 2000 VPM) has been studied. The sparks were generated between a point (stainless steel) and a plane (aluminum) either under 50-Hz AC voltage or by discharging a capacitor. Owing to the high water content the only stable gaseous oxyfluorides detected were, in order of decreasing importance, SOF/sub 2/ and SO/sub 2/F/sub 2/. The yields of these two byproducts have been measured as a function of the number of sparks, the SF/sub 6/ pressure, and the H/sub 2/O pressure. When the sparks additionally struck a solid insulator (polytetrafluoroethylene or biphenolic resin), CF/sub 4/ was detected. Its appearance was paralleled by a sharp increase in the yield of SOF/sub 2/, whereas that of SO/sub 2/F/sub 2/ remained unchanged. >

Study of the decomposition of SF/sub 6/ in the presence of water, subjected to gamma irradiation or corona discharges

The influence of water on the degradation of SF/sub 6/ was studied by subjecting mixtures of SF/sub 6/ and H/sub 2/O either to irradiation by /sup 60/Co gamma -rays or to corona discharges (point-plane electrodes; voltage: DC negative or positive polarity). The stable-reaction products formed were analyzed and assayed with various analytical techniques including gas-phase chromatography, gas chromatography-mass spectrometry, specific electrode, colorimetry, atomic absorption, and EDAX (energy-dispersive analysis by X-rays). The influence of the energy input, the water content, the pressure of SF/sub 6/ and, for the corona discharges, the value of the discharge current and the polarity of the applied voltage on the formation of SOF/sub 2/ and SO/sub 2/F/sub 2/ which were the only two stable gaseous compounds detected, were all studied. The results obtained were compared with data from the literature and with the reaction schemes proposed to date, to explain the decomposition of SF/sub 6/ under the effect of similar or different input energies. >

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.

Optical detection of corona discharges in SF6, CF4, and SO2 under dc and 50‐Hz ac voltages

The emission spectra resulting from corona discharges generated under dc and 50‐Hz ac voltages in SF6, N2, SOF2, SO2F2, S2F10, CF4, SO2, and C2F3Cl3, and mixtures of these gases with SF6 were investigated in the 220–900‐nm wavelength range for gas pressures between 30 and 400 kPa. Characteristic emissions were ruled out only for N2, SF6, CF4, SO2, and C2F3Cl3. We also observed that small concentrations of N2, CF4, or SO2 can be detected in SF6 from the analysis of light emitted between 250 and 420 nm.