Oil impregnated pressboard is widely used as an electrical insulation medium in electrical equipment. Partial discharge（PD） caused by defects in an insulation medium can deteriorate insulation condition. Therefore

to study the electrical aging characteristic of oil impregnated pressboard under long-term PD

we developed a partial discharge model using needle-to-plane gap with oil-paper insulation system. Moreover

we investigated the variations in features of PD under long-term constant voltage during the entire electrical aging process. The surface damaging conditions near the tip of needle electrode were observed by microscope

with which the gas generating situation was also studied. After the electrical life of the pressband finished

its surface morphology was observed by scanning electron microscopy. The results show that the electrical aging process can be divided into three stages: initial stage

developing stage

and erupting stage. During the developing stage

PD will occasionally stop developing for a while. After both average PD magnitudes and pulse repetition rate have dropped to a minimum value in developing stage

the electrical aging of insulation material is close to breakdown. The variation of features of PD with the development of electrical aging is obtained in the end. The division criterion of three stages can lay the foundation for further studies on electrical aging properties of oil impregnated pressboard.