| dc.description.abstract | Liquid insulation is widely used in various high voltage equipments crucial to a modern electrical grid. It is thus important to understand the behaviour and limits of performance for such liquids.
In these experiments, the emitted light from streamers in pure cyclohexane and pyrene dissolved in cyclohexane ($\SI{0.06}{\Molar}$) were studied using photomultipliers, optical filters, an impulse generator and a point to plane electrode geometry with gap distance of $\sim\SI{4.0}{\milli\meter}$. Needles with point radii of $\sim\!\SI{3}{\micro\meter}$ and $\sim\!\SI{15}{\micro\meter}$ were used as anodes, and were placed either in the bulk liquid or along a glass surface. The emitted ultraviolet light was studied qualitatively using a bandpass filter with peak transmittance at \SI{200}{\nano\meter}, and the experimental results indicate that the fraction of emitted ultraviolet light increases with applied voltage before reaching a constant level in all experiments except when the needle electrode was placed along a glass surface, where the fraction increased without reaching a constant level. The streamers observed in these experiments had an average propagation speed that corresponds to second mode streamers, but extrapolation of the experimental results support the hypothesis of photoionization being a major propagation mechanism of higher mode streamers.
Furthermore, a strong light impulse, hereby named "onset flares" in this thesis, at the onset of streamer formation has been observed in preliminary studies. These high energy photons are believed to be an important feed forward mechanism for fast streamers with propagation velocity $\sim\!\SI{100}{\milli\meter/\micro\second}$. This phenomenon has otherwise not been reported.
The relation between the amplitude and delay time defined as the time from the applied negative impulse voltage reaching 90\% of its minimum to the occurrence of an onset flare was studied. In some of the experiments, it was found that the amplitude decreased with delay time. Space charge limited fields has been proposed as a possible cause for this observation. | |
