Performance evaluation of natural ester nanofluids under thermal ageing for transformer insulation applications

The present paper studies thermal ageing behaviour of natural ester-based nanofluids (NENF) with the addition of Aluminium oxide (Al 2 O 3 ) nanoparticles. NENF labelled as SNFa are prepared at different concentrations ranging from 0.001 vol% to 0.003 vol% by adding Al₂O₃ nanoparticles (30–50 nm in size) and are stabilized with a surfactant in a natural ester/vegetable oil coded as ISO. Based on the preliminary evaluation of dielectric performance, two optimal nanoparticle concentrations, 0.0015 vol% and 0.002 vol%, were selected. Before ageing, the selected optimal samples showed an AC breakdown voltage (BDV) that was 6–20% higher than that of silicone oil and 3–4% lower than that of mineral oil.The ageing procedure involves heating SNFa samples containing copper conductors and cellulose pressboard in air at 110 °C for 150 h, with and without contact with the solid and liquid media. After ageing, the BDV decreases just 3–6% compared to pre-ageing values. The dissipation factor and relative permittivity remain within acceptable levels. DF values were slightly more than 10^ {-1} at 50 Hz for selected SNFa samples before and after ageing. As the frequency increased from 250 Hz to 10 kHz, the DF decreased significantly, reaching values from 10^ {-1} to 10^ {-4}, which is better suited for use as dielectric liquids. The εᵣ values of the aged samples were in the range of 3.00–3.15. Thus, ageing simulations reveal excellent dielectric properties of the proposed Al₂O₃ NENF. This makes them promising candidates for use as sustainable insulating liquids in power transformer applications.