Comparison of the Dielectric Properties of Soy‑Based Natural Ester Oil and Mineral Oil Subjected to Accelerated Thermal Ageing

Abstract

Even though natural ester oils are recognised as sustainable alternative dielectric liquids to mineral oil in oil-immersed power transformers, their long-term dielectric properties under realistic dielectric liquid–solid insulation–metal interactions remain poorly understood. Currently, there are no well-established dielectric property degradation models, posing uncertainty in the long-term insulation reliability of oil-immersed power transformers. In this study, accelerated thermal ageing experiment was conducted on soy-based natural ester oil (FR3 fluid) and conventional mineral oil (MO) with solid insulations (kraft paper and pressboard, thermally upgraded paper (TUP) and pressboard) and metal catalysts (copper, iron, zinc, and aluminium catalysts) in sealed glass bottles to determine their dielectric properties in response to  thermal ageing. Initially, these samples were pre-treated and tested in accordance with the IEC 60156 and IEC 60247 standards, where the samples were aged at 130 °C for 0, 250, 500, 750, 1000, 1250 and 1500 h. Periodic measurements were then conducted to determine the key diagnostic dielectric properties, namely, AC breakdown voltage (AC BDV), dissipation factor (tan δ), and volume resistivity. Despite the low dissipation factor and high volume resistivity, the mineral oil-based insulation systems demonstrated a significant decrease in AC BDV, suggesting low moisture tolerance. In contrast, the FR3-based insulation systems showed an improved and stable AC BDV despite their low volume resistivity and high dissipation factor, which aligned with the polar chemistry and better moisture scavenging of natural ester oils. Based on the results of the accelerated thermal ageing tests, the dielectric performance was most pronounced for the FR3–TUP system. The results showed that using FR3 fluid could improve the sustainability and long-term dielectric reliability of the transformer insulation. However, the results also indicated that the dissipation factor and volume resistivity should be interpreted by considering the fluid chemistry and moisture dynamics.