Creeping discharges over pressboard immersed in jatropha curcas methyl ester and mineral oils

Due to their high biodegradability, non-toxicity and higher fire safety guarantee, vegetable-based oils are considered today as a potential substitute for mineral and synthetic liquids for electrical insulation and especially in high voltage power transformers. However most of known vegetable oils are derived from food materials (rape-seeds, sunflower, palm, olive). And one has to be vigilant to the fact that the replacement of mineral oil which is a petroleum-based product by natural ester fluids based on “renewably sourced” vegetable oils, does not create new problems as this could cause global food crisis due to diversion of food. An interesting product can be jatropha curcas oil extracted from jatropha curcas plants (“miracle tree”) which is non-food crops. This paper presents an experimental study of creeping discharges propagating over pressboard of different thicknesses immersed in jatropha curcas methyl ester oil (JMEO) and mineral oil (MO) under positive and negative lightning impulse voltages (1.2/50 μs), using two divergent electrode configurations (electrode point perpendicular and tangential to pressboard). It is shown that the thickness of pressboard significantly influences the characteristics of creeping discharge and especially the stopping (final) length L_f and the density of branches. For a given thickness, L_f increases with the voltage and decreases when the thickness increases. L_f is longer when the point is positive than with a negative point. For a given voltage and thickness of pressboard, the values of L_f in mineral oil and JMEO are very close.