Temperature-Dependent Electric Field Distribution in Current Transformers with Advanced Paper–Liquid Insulation Systems

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.