Trap Distribution Dependent Charge Behavior of Fluorinated Oil-impregnated paper under the Harmonic Superimposed DC Voltages

The oil-impregnated paper in converter transformer, especially in valve side bushing, is suffered by the complicated electric field such as the composite electric field of AC and DC voltage or polarity reversal voltage. By reason of the power electronics and non-linear equipment, the oil-impregnated paper is also suffered by the harmonic voltage. In addition, the insulating aging is more prominent, which is attributed to the stray loss increasing caused by the DC magnetic bias. Then the degradation of cellulose chain occurs, which easily results in introducing new defects and forming the charge accumulation on the surface of oil-paper insulation. Consequently, the charges will lead to distorting the electric field and accelerating the probability of discharge. Surface fluorination is an effective technology of modifying the chemical components of polymer matrix. This paper is dedicated to investigate the surface charge dynamic behavior and trap distribution characteristics of fluorinated oil-impregnated paper under the harmonic superimposed DC voltages. And then the surface discharge tests were performed. Some conclusions can be drawn that the 3rd harmonic voltage has the most impact on the trap energy level of samples. The fluorination induces the deep trap to improve the charge dissipation.