Mineral oil has been used for many years in various electrical equipment, including transformers, as a cooling and insulation medium. However, its low biodegradability and poor performance in terms of fire protection have prompted the search for fluids to replace it, with vegetable oils being prominently considered. In this study, the dielectric, chemical, and physical properties of four vegetable oils obtained from different seeds (sunflower, rapeseed, soybean, and palm) and a biodegradable synthetic fluid are analysed throughout their lifespan in transformers. Their performances are compared with a traditional mineral oil to assess which one is more suitable for use in transformers employing this type of paper. To achieve this, the fluids were subjected to thermal ageing in combination with copper and a thermally upgraded kraft (TUK) paper, with its degradation controlled by measuring the degree of polymerisation. The results demonstrate that the origin of the vegetable oils affects their properties and degradation rates. It was found that most of the alternative fluids are suitable for use in transformers with the TUK paper and that they can increase their lifespan.

This letter presents a novel hybrid dielectric system for current transformers (CTs) designed to reduce dielectric fluid volume while maintaining thermal and dielectric performance. The proposed approach incorporates a metallic joint bellow with a hybrid insulation scheme that employs synthetic ester in the upper section and silicone gel in the bushing section of the CT. Numerical Finite Elements (FEM) simulations are conducted to evaluate the dielectric field distribution and thermal behavior under rated operation conditions. Results indicate that the bellow modification alone reduces the dielectric fluid volume by 16.4%, while the hybrid configuration achieves a total reduction of 36.5% when compared to the original design. Electric field simulations reveal a decrease of about 10% in maximum field intensity in critical regions, attributed to the dielectric properties of the alternative materials. Thermal simulations confirm that the proposed configuration maintains equivalent heat dissipation to the original design (same hot spot value). The solution is technically viable, allows material volume savings, and improves operational reliability without any loss of performance.

The dielectric performance of power transformer insulation systems is strongly influenced by the type of cellulose paper, the impregnating dielectric liquid, and the paper moisture content. This study evaluates the moisture-dependent dielectric response of three cellulose insulating papers, Kraft, Thermally Upgraded Kraft, and Diamond Printed Enhanced, impregnated with three dielectric liquids: mineral oil, a rapeseed-based natural ester, and a Palm Fatty Acid Ester. Paper samples were first conditioned at controlled moisture contents ranging from 1% to 5%, then impregnated under stabilized conditions, and finally characterized by Frequency Domain Spectroscopy. The results show that increasing the moisture content produces a systematic rise in the dielectric dissipation factor, indicating higher dielectric losses and reduced insulation performance. The relative dielectric benefit of the ester liquids was strongly dependent on paper type, moisture content, and frequency. This paper-dependent behavior was particularly pronounced for Kraft and Thermally Upgraded Kraft papers. Finally, laboratory-derived moisture-dependent 50 Hz tan δ crossover points were identified as dielectric indicators for retrofilling assessment. These findings provide comparative dielectric information for the investigated paper–liquid insulation systems and may support preliminary retrofilling assessment under controlled laboratory conditions.

The dielectric performance of current transformer insulation systems is influenced by the temperature-dependent properties of solid and liquid insulating materials. This study analyzes the dielectric and electric field behavior of advanced paper–liquid insulation systems by characterizing Kraft paper impregnated with mineral oil, natural ester, and synthetic ester to assess the influence of the liquid and mineral oil-impregnated Kraft paper, Thermally Upgraded Kraft, and Diamond Printed Enhanced papers to assess the influence of the paper. Frequency Domain Spectroscopy measurements were performed from 20 to 65 °C to evaluate the influence of temperature, impregnation liquid, and paper type on the dielectric response and field distribution. Temperature-dependent complex permittivity functions at 50 Hz were subsequently coupled with a thermo-hydraulic model and an electrostatic model. At 20 °C, replacing mineral oil with natural and synthetic esters reduced the maximum electric field in Kraft insulation by approximately 9.1% and 4.2%, respectively. However, accounting for the non-uniform operating temperature increased the calculated peak electric field by approximately 3.1–4.6% for all investigated insulation systems. These results demonstrate that temperature-dependent dielectric properties should be considered for a more representative electrical assessment of advanced current transformer insulation systems.