Polymeric HVDC cable design and space charge accumulation. Part 2: insulation/semicon interface

Non-homogeneous electrical insulating materials and interfaces between different insulating materials are frequently encountered in electric insulation systems. On one hand there are composite insulating materials obtained by a host material containing micro and/or nano fillers, which have been widely investigated with the aim of improving the properties of the base material. The presence of micro/nano scale interfaces may affect the physical and electrical properties of the composite material, improving or worsening them with respect to the base material, depending on the nature and dispersion of micro/nano filler inside the matrix. On the other hand, interfaces can be found on a macro-scale in several HV insulation systems, e.g., in cables. In particular, interfaces between cable insulation and semiconductive layers and/or between different insulating materials in cable accessories, e.g., XLPE and EPR used for cable joints and/or terminations, are very common in transmission and distribution networks. It must be remembered that interfaces are often claimed to be critical within the complete insulation system, affecting electrical, mechanical and thermal properties. Indeed, the effects of electro-thermal and mechanical stresses can be enhanced in the presence of interfaces which may, thus, become the weakest points of the insulation system, both in AC and DC. Interfaces can act as a trigger for partial discharges (PD), when the contact between surfaces is not well made and such activity should be strictly avoided for cable and accessories, because organic insulation (e.g., XLPE and EPR) might not be able to withstand PD activity even for short times. It is likely that PD activity will be a second-order problem under DC, due to the smaller repetition rate with respect to AC. However, under a DC field space charge may accumulate in the insulation bulk, especially if interfaces are present. These latter, in fact, can behave as favoured sites for charge build up, particularly for low-mobility materials (e.g., polymeric insulation). Space charge can modify the electric field in such a way that the actual field in the insulation can differ significantly from the geometric field, thus causing accelerated degradation and, at last, premature breakdown of insulation systems. These are the reasons for the concerns in the application of polymeric materials to HVDC insulation systems, even if the use of polymeric insulation in place of paper-oil (still used) would lead to great environmental and economic advantages. Therefore, the investigation of the behaviour of interfaces between different insulating polymeric materials in the presence of thermal and electric stresses is a crucial research topic for the improvement of the design of polymeric cables and accessories, which could benefit by a more accurate knowledge of the actual field applied to the insulation system under design. In this paper, the second part of a three-paper series, interface and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be exploited to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.