Aging of Two Natural Esters in the Presence of Transformer Construction Materials

Although they are widely used, further laboratory investigations are essential to better understand their behaviour in the presence of transformer construction materials under conditions of elevated temperature, which is important for their practical application.

This study extends previous research on the behaviour of natural esters and investigates properties of two readily biodegradable, rapeseed base natural ester liquids (NE1 and NE2).

Experiments were conducted at 150 °C in stainless steel vessels under nitrogen blanket during 49 days. Each system included ester liquid combined with construction material, aluminium or core, without paper involved and with non-thermally upgrade paper, in defined proportions. Prior to testing, all materials were prepared, liquid was dried and paper was dried and impregnated. After each aging period, two vessels were analysed: one cooled under ambient conditions and the other subjected to 100 °C for additional 40 hours. This approach provided insight into water content in both liquid and paper, as well as dissolved gas concentration at elevated temperatures, enabling comparison with samples taken at room temperature.

Gas formation was monitored using Dissolved gas analysis (DGA) together with methanol quantification, while liquid properties such as acidity number, interfacial tension, viscosity, antioxidant concentration, were also measured. Paper properties as water content, degree of polymerization and tensile strength were tested. Although both ester liquids shared the same base, they exhibited different gas production behaviours mostly regarding ethane (C2H6), with variable concentrations over the set of experimental conditions. NE1, which contained a lower concentration of the phenolic antioxidant, showed a greater tendency to produce ethane compared to NE2, which had nearly three times higher concentration of same additive. Type of construction material (aluminium, core) hasn’t affected significantly gas formation for each individual liquid under applied experimental conditions. However, elevated aging temperature led to increased acidity, more prominently in NE1, while majority of the other properties remained stable. Differences in the aging patterns of the two esters were observed, but no significant influence from the type of metallic material was identified. Additionally, measured methanol concentrations together with paper degradation parameters (degree of polymerization and tensile strength) provide a deeper understanding of their interrelations.

The result highlight that liquids with the same base and similar initial properties, differing in additive concentration, can exhibit different gas formation patterns and aging rate of insulating paper immersed in those liquids. The findings from these aging experiments, including absolute values, trends and correlations, contribute to a deeper understanding of the impact of construction materials on ester degradation process and provide valuable insights for their use in service environments.