Abstract
Liquid insulation is used in high voltage equipment such as power transformers as both dielectric medium and coolant. Breakdown in liquid insulation tends to be governed either by streamer initiation under more uniform fields, or by streamer propagation under more non-uniform fields. A model streamer propagation study, which screens the effectiveness of additives based on cyclohexane and mixtures with diethylaniline (DEA) and toluene, is presented in this paper. The effect of additives, at different concentrations, on streamer propagation velocity in cyclohexane under an applied lightning impulse voltage of positive polarity is studied. Cyclohexane (ionisation potential 9.88 eV) was chosen because, being a hydrocarbon, it shares similarities with the constituents of common insulating liquids. Previous studies have also shown how, in general, the addition of additives of lower ionization potential than the bulk liquid can slow down streamer propagation in insulating liquids. A point-plane electrode configuration of 70 mm gap with a 5 μm tip radius is used and subjected to an applied positive polarity impulse of 1.2/50 μs. A high velocity imaging system is also used to capture streamer images to validate a Time-To-Breakdown (TTB) measurement approach used in inferring approximate streamer velocity. The DEA (ionisation potential 6.98 eV) was found to be an effective additive to slow down positive polarity streamers in cyclohexane in the applied voltage range (≈220−280 kV peak) in concentrations above approximately 0.33% (by volume). Toluene (ionisation potential 8.82 eV) was found not to significantly slow down streamers in cyclohexane, even at 10% concentration, for the same voltage range. This is postulated to be due to the fact that toluene does not have a low enough ionisation potential (with respect to that of the cyclohexane) to change the streamer branching characteristics sufficiently during propagation.