Thermal Aging Dependent Surface Charge Characteristics of Fluorinated Oil-paper Insulation Under the Harmonic Superimposed DC Voltages

The oil-impregnated paper in the converter transformer, especially in the valve side bushing, is suffered by the complicated electric field. By reason of the power electronics and non-linear equipment, the oil-impregnated paper is subject to the harmonic voltages. The insulating aging is more prominent in contrast with the power transformer. Under this condition, the charges are capable to accumulate on the surface of oil-impregnated paper, which will lead to the distorting the electric field and accelerating the insulating degradation. Consequently, the performance deterioration of oil-paper insulation is inevitable. Surface fluorination is an effective approach of modifying the chemical components of polymer matrix. Hence, this paper is dedicated to research effects of thermal aging on the surface charge dynamic characteristics of fluorinated oil-paper insulation under the harmonic superimposed DC voltages. Some conclusions can be drawn that the comprehensive effect of harmonic voltage and thermal aging can significantly enhance the de-trapping of the surface charge in deep traps, the deep trap energy level density is greatly affected by the proportion of harmonic. As the thermal aging time is prolonged, the trapped charge becomes more difficult to detrap, the charge accumulation increases.