Influence of molecular additives on positive streamer propagation in ester liquids

Molecular additives i.e. additives with specific molelcular propertie, has been shown to influence breakdown properties of dielectric liquids, in particular lightning impulse characteristics. In this study we investigate the propagation of streamers in a natural ester liquid with and without molecular additives in a needle plane electrode configuration with a 5 mm gap. In order to study streamer propagation, including fast events, in such a small gap, impulse voltages with 20-40 ns rise times are used. One molecular additive, dimethylaniline, has a low ionization potential and another, azobenzene, a low 1^st excitation energy in comparison with the natural ester. Both additives increase the acceleration voltage with approximately 40% at a concentration of 5 w%. Different processes may be involved in the breakdown event for the different additives, which is indicated by differences in the breakdown voltage data.