In order to study the partial discharge(PD) characteristics of aging oil-paper insulation in the actual operation condition of converter transformer valve-side, and to improve the engineering significance of the research on PD of oil-paper insulation under combined AC-DC voltage, by using the oil-paper samples from a 5-year-old converter transformer,we established a needle-plate electrode model to simulate the whole process of PD from the beginning to breakdown in ageing oil under the combined AC-DC voltage...