The Influence of Chemical Composition on Streamer Initiation and Propagation in Dielectric Liquids
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Effects of electron-attaching and electron-releasing additives in cyclohexane on the initiation and propagation of positive and negative non-breakdown streamers have been studied qualitatively and quantitatively. Fast impulses (<20 ns, <40 kV) were applied to the plane electrode of a 10 mm point-to-plane gap and studied by shadowgraphic imaging and a 0.1 pC sensitive differential charge measurement technique. The additives studied are 1,1-difluorocyclopentane, perylene, perfluoromethylcyclohexane, perfluoro-1-heptene, trichloroethene, perfluoro-n-hexane, 1,4-benzoquinone, 1,2-dichloroethane, 1-methylnaphthalene,di-n-propylether, toluene, 2,3-dimethyl-2-butene, indole, N,N-dimethylaniline,tetramethyl-p-phenylenediamine and tetrakis-dimethylamino-ethylene. The characteristic of the additives. Electron-attaching additives facilitate the propagation of negative streamers, whereas the most effective electronreleasing additives reduce initiation voltages and facilitate the propagation of positive streamers. Depending on the reactivity and concentration of the additives, streamer filaments become thinner and fewer while propagating faster and further. 1,1-difluorocyclopentane is the only additive without a measurable effect in either point polarity. The results follow expectations, considering point cathode streamers to be governed mainly by injection of epithermal electrons from a gaseous phase, and point anode streamers to be governed mainly by more energetic (hot) electrons, extracted from the liquid at higher electric fields. The Townsend–Meek theory for streamer inception in gases has been adapted to a solution and applied to analyze the voltage dependence of the positive streamer propagation. Results show a quantitative dependency on the ionization potential and additive concentration in agreement with experimental trends. This implies that electron avalanches in the liquid phase are responsible for a particularly fast propagation mode. The fast mode extends from initiation until terminated by a slower mode which is not affected by the additives. Transient conduction currents of measurable magnitudes in the nano- to microampere range are observed for voltages around initiation for both point polarities, and may be induced by ions left behind by the avalanches that are not sufficiently developed to initiate streamers. In another part of this thesis work, the time-averaged optical emission from fast, filamentary, and luminous positive and negative streamers in chlorocarbons liquids under pulsed divergent field conditions have been studied. The liquids were dichloromethane, 1,2-dichloroethane, tetrachloromethane, trichloroethene, and tetrachloroethene. Light emitted from the first 10–15 μm trail of a few thousand streamers were accumulated. Atomic lines of hydrogen, chlorine and carbon as well as excited states of C2 radicals (Swan bands) have been observed, and with sufficient resolution for evaluating line and band-shapes. The characteristic broadening, shift and asymmetry of atomic lines varied significantly between the liquids. Differences between the two streamer polarities were comparatively small. Densities of electrons and neutrals in the illuminated phase have been deduced from broadening of atomic lines, atomic excitation temperatures from absolute line intensities, and rotational and vibrational temperatures from the Swan bands. The gas densities of the propagating streamers were generally very high (~10% of critical) and with a high degree of ionization (~1‰). Dichloromethane and 1,2-dichloroethane produced re-illuminating streamers with densities close to atmospheric conditions, in agreement with a rapid pressure relaxation. Rotational temperatures were high and in the range 2 x 103 – 6 x 103 K for the different liquids. Results can be interpreted to suggest a partial local thermodynamic equilibrium in the streamer plasmas. It is believed that the high pressure and the high electron density help equilibrate the temperatures of the various particles. The energy-consumption and qualitative chemical composition of the illuminated phase are similar to that of an equivalent system in complete thermodynamic equilibrium.