In order to study the electrical characteristics of ingress of moisture on oil-immigrated paper（OIP） bushing

OIP bushing models were made and the moisture conditions were simulated by ultrasonic humidifier. The parameters of moisture in oil

partial discharge

and tanδ were measured after the bushing model damped at different time. It is indicated that the moisture in oil firstly increases and then decreases by the standing time of damped bushing

which is classified by the moisture in oil 20 mg/L as three stages: "low-moisture stage

high-moisture stage and moisture migration to paper stage". All of three stages

the curve of tanδ-U trends to increase. Therefore

the possibility of misjudgment for moisture bushing in the low-moisture stage may be obtained by traditional measurement. For the suspicious moisture bushing

the maximum value of Δtanδ under the voltage of 2Um/1.732（Um is the maximum opreation voltage of the equipment） should be lower than 0.2% for the high sensitivity of moisture bushing. Therefore

the insulation condition of OIP bushing can be judged by measuring the curve of tanδ-U and partial discharge so as to find out the moisture bushing earily.