Effect of Aromatics and Polar contaminant of Natural Ester Oil on Dielectric Strength and Breakdown Time in Mineral Oil under Lightning Impulse Voltage Excitations

Mineral oil forms a backbone in high voltage oil filled power apparatus (transformer, reactors, instrument transformer etc.,) having the main role of dielectric, coolant and diagnostic due to its good physical, thermal and chemical performances with low cost and wide availability. The chemical composition of insulating oil is a main key factor in impulse breakdown voltage and time to breakdown. Therefore, an effort has been made to investigate the effect of aromatics and polar contaminants on impulse withstand voltage and time to breakdown of mineral oil. This effect is studied under positive and negative polarity of impulse in the extremely nonuniform field for electrode gap between 1mm to 10mm. In addition, to analyze the effect of aromatics, two different types of mineral oils, paraffinic inhibited mineral oil (contains predominantly paraffinic chain structure) and naphthenic inhibited mineral oil (contains a higher proportion of cyclic structures and aromatics compared to paraffinic oils). Further, to examine the effect of polar contamination in mineral oil, natural ester oil (highly polar in nature) is mixed with mineral oil which is highly non-polar in nature. The lightning impulse withstand voltages of natural ester oil and mixture oil are deduced and compared with mineral oil to analyze the effect of aromatics and polar contaminants.