The high-voltage lead-out device is the key component connecting the transformer winding leads and bushings. The oil-paper insulation system tends to accumulate moisture under multi-stress effects

impairing its insulating property. For the purpose of assessing the discharge risk of lead-out device under moisture

the discharge characteristics of the oil-paper insulation system of the device are studied

and the core function of moisture in surface discharge process is revealed. The typical insulation structure for lead-out device is a multi-layer composite insulation consisting of overlapping pressboard and crepe paper. A multi-layer insulation structure model is constructed

and its discharge characteristics are statistically analyzed. The results show that the increase in the moisture content of the pressboard leads to a decrease in the partial discharge inception voltage and a segmented decrease in the breakdown voltage. At low moisture level(＜6%)

overlapping regions are the weakest insulation points

whereas at high moisture level(＞6%)

the moistened pressboard becomes the critical weak zone. As the overlap length increases

partial discharge inception voltage remains unchanged

and breakdown voltage increases linearly and slowly. It is found that the “white marks” on the pressboard surface and the bubble accumulation between the crepe paper layers are typical characteristics of surface discharge of the lead-out device under moisture conditions. The research results are of great significance for optimizing the structural design of the lead-out device and evaluating the degree of insulation moisture in high-voltage lead-out devices.