Differential Low-Temperature AC Breakdown Between Synthetic Ester and Mineral Oils: Insights From Both Molecular Dynamics and Quantum Mechanics

Synthetic ester oil (SEO) is promising for bolstering sustainable development and enhancing the operational reliability of transformers at low temperatures. The intention of this study is to acquire an understanding of differential low-temperature ac breakdown characteristics exhibited by SEO and mineral oil (MO), while delving into the microscopic influencing factors. The results unveil that the breakdown voltage between −20 °C and 20 °C illustrates a “V”-shaped trend, with SEO consistently higher than MO. SEO presents its maximum breakdown voltage at −20 °C and its minimum at 0 °C, whereas MO demonstrates its maximum breakdown voltage at 20 °C and its minimum at −10 °C. Molecular dynamics (MD) and quantum mechanics (QM) calculations reveal that hydrogen bond and interaction energy (IE) associated with the state of water, along with the fraction of free volume (FFV), mean square displacement, and diffusion coefficient (DC) associated with particle transport property, collectively exert considerable influence on the breakdown voltage. Compared to MO, SEO exhibits a higher number of hydrogen bonds and interaction energies, while displaying lower FFVs, mean square displacements, and DCs. Furthermore, the presence of electron trap, in conjunction with these combined factors, leads to a substantially higher breakdown voltage of SEO (31.4 kV) than that of MO at sub-zero temperatures. The wider energy gap of MO compared to SEO leads to a slightly higher breakdown voltage for MO (19.9 kV) compared to SEO at above-zero temperatures. This study provides experimental data and theoretical guidance for the promotion and stable operation of SEO-immersed transformers in UHVDC systems deployed in cold regions.