Effects of Electron-attaching and eEectron-releasing Additives on Streamers in Liquid Cyclohexane

Positive Breakdown Streamers and Acceleration in a Small Point-Plane Liquid Gap and Their Variation with Liquid Properties.

“…The ratios observed in the short gap are much higher than in the long gaps, though: in a long gap, cyclohexane without additive has a ratio about 1.1, increasing to 2.5 with 1 % (0.064M, mole fraction 0.007) DMA, or to 2.1 with 0.1M pyrene). The reason probably is that 2 nd mode streamers seem to be considerably more branched in short gaps [3] than in long gaps [5], and extensive branching seems to suppress the transition to faster modes, Nordic Insulation Symposium -Nord-IS 13 -Trondheim, Norway, June 9 -12, 2013 presumably because of the mutual shielding keeping the tip fields comparatively low.…”

“…Point to plane gaps are usually used, mainly because it is possible to induce streamers with moderate voltages, but also because a more homogeneous gap will suppress many streamer phenomena which may take place if there are protrusions or more or less sharp particles present in an insulation system. Small gaps, typically 5 -10 mm but sometimes up to about 25 mm have been much used in such studies [1,2,3]. It is usually non-breakdown streamers that are studied in small gaps, and nonbreakdown streamers are usually of the slow type (20 m/s up to a few hundred meters per second) often designated 1 st mode streamers or a somewhat faster type (1-3 km/s) designated 2 nd mode [4].…”

“…Thus the study of which aspects of the liquid influences the difference between V a and V b has become of practical importance, and this has usually been done in long gaps, also in our laboratories [5,6,7,8]. It has been observed that a high degree of branching of 2 nd mode streamers increases V a [1,5,6,7] , and so far it seems that a small concentration of constituents or additives with significantly lower ionisation potential than the bulk of the liquid leads to increased branching [1,3,5,6,7].The liquid dependent branching tendency of typically nonbreakdown streamers can be studied in small gaps [1,3]. Since one wants to vary electronic properties of the liquid these studies often involve additives that are expensive, toxic or both and which are therefore more conveniently handled in a small test cell.…”

“…Azobenzene (AZO) was selected to investigate if the low lowest excitation level could have an effect, although it too has an ionization potential that is lower than for cyclohexane and n-tridecane, but not as low as DMA. Trichloroethene is included because it shows a significant effect on negative streamers in long gaps, presumably because of its effect as an electron scavenger [3,5,6] , and usually little effect on positive streamers although some reduction in acceleration voltage has been observed [6,7].…”

Modeling the transition to fast mode streamers in dielectric liquids

“…The value of Q c is typical 18 for hydrocarbon gases [7], while values ranging from 5 to 23 have been used for liquids [6], [8], [9].…”