Effects of reduced pressure and additives on streamers in white oil in long point-plane gap

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

“…The behaviour of the breakdown and acceleration voltages with increasing amount of low ionization potential or low excitation potential additives in cyclohexane and n-tridecane is in line with what has been observed in long (50 -80 mm) gaps, with pyrene added to cyclohexane [1] or DMA added to cyclohexane or to various low-aromatic mineral oils [5,6,7]. With increasing acceleration voltage and generally decreasing breakdown voltage, the ratio between them also increases as shown in Figure 10.…”

“…Usually 2 nd mode is not considered to be quite as fast as what the figure shows. In long gaps, the velocity just before acceleration is 4.5 km/s or lower, even with 1 % (0.064M)DMA added [5,6,7]. Most likely part of the gap is crossed as 3 rd mode.…”

“…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]. In large gaps (typically 50 mm or more) above breakdown voltage (V b ) one has observed faster streamers of 3 rd mode (ill-defined in some liquids, but usually in the range 4 -20 km/s) and 4 th mode (usually significantly faster, often 70 -200 km/s for positive streamers) bridging the final part of or the whole gap in a "fast event" [4,5,6,7,8].…”

“…This has consequences for a practical insulation system's response to overvoltages of limited duration: a fairly slow streamer may not bridge the available gap in the duration of the overvoltage, while a 4 th mode streamer often will. 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].…”

“…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].…”