The amount of literature on partial discharge (PD) and partial discharge induced degradation is vast. In the past 10 -20 years significant progress has been made on research within partial discharge induced aging of dielectrics. Researchers now agree on the main mechanisms pertaining to this topic. With the advent of a new generation of dielectrics of which many properties now can be affected by the introduction of small amounts of nano-sized particles it seems to be a good moment to review the progress on the understanding of PD induced aging. Focusing on internal partial discharge in solid polymeric insulation this paper tries to identify achievements and at the same time challenges still to be solved.

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( ) 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 massimpregnated 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.

HVDC cables start playing a more and more important role in interconnecting national grids. This paper deals with the calculation of electric fields in HVDC cables. The calculation of fields in an HVDC cable is far more complex than the equivalent case in HV ac cables. This is due to the fact that the conductivity of the cable insulation is temperature and field dependent and due to the fact that the electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive, intermediate (and time-dependent) or resistive nature. The kind of field depends on the stage the cable finds itself in: for instance, whether the voltage has just been applied, whether a polarity reversal has occurred or whether the field distribution has become stable. For each stage, the method of calculating, together with the computed results on a real HVDC cable are discussed. Usually, the effect of heating of the insulation by the leakage current may be disregarded. However, in certain cases, i.e. the cable temperature and applied voltage are high enough, the field distribution is influenced by these insulation losses. They even may lead to an instability that causes breakdown of the cable. A cable in service may be subjected to impulses superimposed on the dc voltage. The most severe case is that of an impulse superimposed on a dc voltage of opposite polarity. The calculation of the field distribution in this situation also is carried out.

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