Comparative evaluation of moisture, dielectric, and chemical characteristics of oil–paper insulation systems in mineral oil and rapeseed-based natural ester under accelerated thermal ageing

The reliability of power transformers is strongly influenced by the condition of their oil–paper insulation system. This study comparatively evaluates the moisture, dielectric, and chemical characteristics of oil–paper insulation systems consisting of kraft paper (KP) immersed in mineral oil (MO) and rapeseed-based natural ester (RBNE) under accelerated ageing conditions. Ageing experiments were carried out at 130 ± 2 °C for 1250 h to simulate long-term transformer operating conditions. Moisture content, dissipation factors (tan δ), and interfacial tension (IFT) were measured at selected ageing intervals to assess dielectric and ageing behavior. The results showed that the RBNE-based insulation system exhibited higher oil-phase moisture content and dissipation factor values than the MO-based insulation system, primarily due to the polar molecular structure and hygroscopic nature of natural ester fluids. The higher oil-phase moisture content of RBNE reflects its greater water affinity and moisture solubility compared with MO. In addition, RBNE exhibited a comparatively less fluctuating IFT profile throughout the ageing period than MO under the tested conditions. Conversely, the MO-based insulation system exhibited lower moisture solubility and greater fluctuations in IFT throughout the ageing period under the tested conditions. Overall, the findings demonstrate that rapeseed-based natural ester possesses higher oil-phase moisture affinity and favourable chemical stability characteristics despite exhibiting higher dielectric losses. These results highlight the potential of RBNE as a sustainable and environmentally friendly alternative insulating fluid for transformer oil–paper insulation systems.