Impregnation Model and Experimental Study of Mineral Oil/Meta-Aramid Paper Combination Insulation

Meta-aramid paper is considered as an ideal insulation material for the next generation of oil-paper internal insulation for oil-impregnated-paper bushings (OIP bushings) due to its excellent insulation performance and far superior high-temperature resistance compared with the cellulose paper. In the production process of OIP bushings, in order to improve the insulation performance, the insulation paper is usually pre-impregnated with insulation oil in the manufacturing process to fully fill the internal air gaps and capillaries, greatly improving the thermal conductivity and insulation strength of the oil-paper insulation system. To explore the impregnation law of the new-type mineral oil/meta-aramid paper insulation system, this article analyzed the impregnation mechanism, where the internal pressure, insulation oil viscosity, and capillary gravity were regarded as the main factors affecting the impregnation process of oil-paper insulation. After obtaining the influencing factors, a study on the impregnation rate of single-layer insulation paper was first carried out, and experimental models for surfacewise and normal-direction impregnation were designed. Through experiments, the impregnation rates of mineral oil with different types of aramid paper at different temperatures were obtained, based on which a calculation formula was proposed to quantitatively calculate the time required for full impregnation of different types of insulation paper at different temperatures. Then, a miniaturized bushing capacitor core model was wound using Nomex T410 insulation paper. The multilayer insulation paper impregnation rate experiment was conducted, and the time required for the bushing model to fully impregnate along the surface and normal directions under different temperature conditions was calculated. The experimental results verified the correctness of the proposed time calculation model, and it was confirmed that 60 degrees C can be selected as the best impregnation temperature for Nomex T410, effectively increasing the impregnation speed by 51.406%, in order to achieve the goal of not increasing the production cost caused by excessive impregnation temperature while increasing the impregnation speed. The adequacy of impregnation and the correctness of the calculation model were further verified by measuring the dielectric loss and capacitance of the bushing model after impregnation. This study provides important data support for the production and application of a new-type oil-paper insulation system in high-voltage transformer bushings.