More than fifty years after the publication of the early work on conduction and dielectric breakdown of solids, we are still unable to describe quantitatively the electrical response of these materials. During this period of time, concepts derived from semiconductor physics have been transposed to the case of insulating solids, and among them, to polymers. Alternative descriptions have been proposed as well. In spite of this, there is still no agreement on how to describe charge transport and there is still some controversy as regards the applicability of semiconductors physics to the case of disordered insulating materials and in particular to polymers used in electrical engineering applications. The last twenty years have been marked by the publication of excellent review papers summarizing the physical concepts available to describe charge transport. Enormous steps forward have been achieved as regards computing facilities and our ability to spatially map the space charge, quantitatively, inside dielectric materials. We consider these two factors as fundamental in providing possibilities for developing sound models of charge transport, by using the basis of fundamental knowledge that has been accumulated in the previous years, and by coupling up-to-date techniques in experiments and in simulation. In this paper, which is not a review of either the published work on modeling or of new concepts in dielectric physics, we emphasize recent progress in the field of atomistic and macroscopic modeling and we discuss challenges based on such approaches that, we think, constitute a direction for future research.

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

The aim of this article is to investigate the extent of space charge accumulation due to a temperature gradient in comparison with other charge-supply mechanisms, particularly injection from the electrodes. For this purpose, space-charge measurements were carried out on HVDC cable models under application of different temperature gradients across the cable insulation, above and close to the threshold field for space-charge accumulation. The main results, consisting of space-charge patterns and extracted quantities, are discussed here.