Moisture dependent surface charge behavior of fluorinated oil-impregnated paper under the harmonic superimposed DC voltages

Due to the power electronics and non-linear equipment, the oil-impregnated paper of valve side bushing is suffered by the harmonic voltage. Besides, moisture has a detrimental effect on oil-impregnated paper lifespan by lowering electrical breakdown strength and thermal endurance. Hence, 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-impregnated paper is inevitable. This paper is dedicated to research the surface charge behavior of fluorinated oil-impregnated paper with different moisture under the harmonic superimposed DC voltage. The insulating paper were put in the constant temperature and humidity chamber to moisture absorption for 15, 30 and 45 min respectively, of which the relative humidity was set to 40% and the temperature was set to 40 °C. Some conclusions can be drawn that the moisture induces the shallow trap. The comprehensive effect of harmonic voltage and moisture can significantly enhance the discrepancy appeared in the decay rate. Furthermore, the energy level of the traps on the specimen surface increases and is influenced by the different electric field.