Current researches on the effect of overvoltage on oil-paper insulation focus on the voltage-number（U-N） characteristic of oil-paper insulation under impulse voltage. However

the researches about the change of physical and chemical characteristics of oil-paper insulation under accumulative effect of impulse voltage are insufficient and with emphasis on the effect of lightening impulse. In this study

the changes of electrical and mechanical characteristics of oil-paper insulation under accumulative effect of switching impulses are investigated. T

he results show that: 1)The DC conductivity of the oil-paper insulation samples increases under accumulative effect of switching impulse; 2)The dielectric characteristic and conductivity of the oil-paper insulation samples increase in 10-2

1

00Hz and keep steady in100

1

07Hz; 3)The dielectric loss factor of the oil-paper insulation samples increases in 10-2

1

03Hz and keeps steady in 103

1

07Hz;4)For the electrical characteristics of the oil-paper insulation at power frequency

the dielectric characteristic

conductivity and the breakdown voltage keep steady

but the inception and extinction voltages of partial discharge decrease

while the dielectric loss factor increases; 5)The degree of polymerization of the oil-paper insulation decreases slightly while its tensile strength keeps steady under the accumulative effect of switching impulses.