By studying the effect of DC electric field on space charge characteristics of oil-paper insulation in the different aging processes

we can provide experiment references for space charge characteristics of oil-paper insulation of converter transformers while it is running for a long time under high electric field intensity. Therefore

we studied the variations of space charge characteristics of oil-paper insulation with electrical aging for 1 000 hours under electric field of 20 k V/mm by using the pulsed electro-acoustic method. The results show that the variation rules of space charge characteristics are similar at different aging status. As the space charge is caught by the trap

homo-charge injection and accumulation are easy to take place. As the degree of aging deepens

the amount of charge accumulation inside the sample becomes larger

and the dissipation velocity of charge during depolarization becomes slower

since both trap density and trap level are increasing. Throughout the electrical aging process

the amount of fast moving charge does not remarkably change

while slow moving charge grows gradually with the trap density increasing

which decreases the mobility rate. Charge injection caused by electrical aging leads to electric field distortion inside the samples during polarization

which easily promotes the generation of large current and lowers the breakdown voltage.