Defects and interfaces at DC voltage

In electrical insulating systems, breakdown is usually triggered by macroscopic defects or interfaces. The focus of this paper is on two phenomena that are of prime importance for understanding degradation and breakdown processes at DC voltage, i.e. partial discharge and space charge. An overview is given of the work that was performed in Delft, often in collaboration with other institutes, in the field of DC insulation systems containing such defects and/or interfaces. A significant amount of attention is paid to partial discharge and its dynamics under DC stress. In the absence of partial discharge, space charge is an important trigger for degradation. It is described how space charge affects the electric field in a complicated insulation system such as a cable joint. It is argued that space charge plays a major role in the degradation process. Finally, some challenges in the field of DC insulating materials are suggested.