Partial discharges under DC voltage in paper-oil insulating system

Partial discharges proved to be important diagnostic parameter during technical lifetime of operated electrical machines. Until now PD phenomena was well described and understood in technical society, especially for characteristic arrangement, e.g. corona, tip to plane, sphere to plane, and others. Most of the studies were done under AC voltage. However as DC transmission is concerned, more attention must be paid to electrical field secondary effects. At DC voltage, space charge is formed and electrical field distribution is deformed. Hence description of PD behavior under AC can not be directly applied to DC voltage. Furthermore, DC PD phenomena is strongly dependent on voltage polarity, hence negative polarity is in particular arrangements similar to AC PD behavior, while DC positive polarity behaves differently. For deeper understanding of PD phenomena under DC voltage, laboratory experiments were performed and compared. Tested specimen were samples of paper-oil insulating system, which is candidate for HV DC applications for its easy designing for particular voltage level. Further the experimental arrangements were modeled in FEM software to support experimental conclusions. Space and volume charging of insulating materials deforms the electric field distribution which yields additional stresses on electrical insulation materials. Hence the end of life of insulating system (and the whole device even) is reached in shorter time.