As a non-destructive local gas breakdown phenomenon within a surface or bulk cavity surrounded by insulation

partial discharge (PD) contains important information which can be used to evaluate and diagnose electrical insulation systems. In this paper

distinctive characteristics of PD for DC versus AC are reviewed. A summary of experimental studies on DC PD in different insulation coordinations is presented

including PD in gaseous insulating media

PD in solid insulating media and PD in liquid insulating media. Conclusions and some thoughts based on existing studies are provided

and challenges and suggestions for future studies are given. This paper can serve as a guide reference for readers to learn the background and state-of-the-art of DC PD studies. More importantly

it is hopeful that this study can inspire novel ideas for further advancing fundamental research of DC PD mechanisms and PD mitigation methods.