Partial discharges at dc voltage: Their mechanism, detection and analysis

A concise review is given of the progress made in the field of partial discharges (PD) at dc voltage. Although ample reference will be made to work of other authors in this field, the paper will concentrate on the progress that was made at Delft University of Technology over a period of 14 years in three Ph.D. projects. In the first project, a start was made with the analysis of the physics of partial discharges at dc voltage and different types of PD were characterized based on parameters like time interval between PD and PD magnitude. In a second project, PD analysis was applied to HVDC apparatus and different means of classification of PD at dc voltage were proposed. In the third project, PD analysis was applied to HVDC mass-impregnated cables and test specifications were proposed. In this paper the work performed in the above three Ph.D. projects is summarized with ample reference to papers of other workers in this field. Attention is given to the mechanism of PD at dc voltage as compared to ac voltage and techniques for measurement and analysis of dc PD patterns. Examples of practical application of dc PD testing are given. Finally, some thoughts on future work are presented.