Partial Discharge Patterns in Natural Esters From Different Vegetable Bases

Transformer insulation system is composed of solid and liquid insulation, working in conjunction for the safe, efficient, and optimal performance of the equipment. The condition of these two insulation components is closely related to the life expectancy of a transformer. Mineral oil (MO) has been widely used as transformer liquid insulation, although alternative liquids of vegetable base have begun to emerge. These alternative liquids offer advantageous physic-chemical properties, including dielectric performance evaluated through high-voltage tests. This article focuses on evaluating the partial discharge (PD) performance of four vegetable oils, comparing their properties with those of a transformer MO. The study was repeated for fresh samples and for samples with different aging degrees. PD tests were carried out in the lab using a needle-to-plane electrode system and two different test methods for applying the testing voltage. Four different interpretation criteria were used for determining the PD inception voltage (PDIV) value. Electrical parameters, such as PDIV, pulse repetition rate, and phase-resolved PD (PRPD), were measured for each oil sample. Additionally, other physic-chemical properties of the samples were measured. The results show that all the tested liquids presented a fair performance under PD, although significant differences were found between them. In some cases, as for the high-oleic sunflower-based natural ester (NE), the PDIV was found to be significantly larger than that of MO.