Oil-paper insulation

as the main insulation of valve side winding

has long been affected by complex working conditions such as AC/DC composite voltage

harmonics

and polarity inversion. Under the action of electric field

temperature

moisture and other factors

it is easy to generate partial discharge（PD） and space charge/interface charge accumulation

inducing dielectric deterioration or breakdown. We summarized the research status

hotspots and achievements at home and abroad from the PD and charge distribution characteristics of oil-paper insulation. The effects of different voltage types

different aging factors and different insulation positions on PD and charge distribution characteristics are analyzed. Finally

the research trend of PD characteristics and mechanism in the future is prospected

and it is pointed out that PD characteristics under the action of extremely uneven electric field

harmonic and pulse voltage are the key research directions in the future

and the internal relationship between PD characteristics and mechanism should be explored at the same time from the macro and micro perspectives by combining charge distribution experiment/simulation/molecular simulation. The summary of these research results can provide references for on-line monitoring technology based on PD characteristics and improvement of insulation performance of converter transformer.