During the manufacture of cross-linked polyethylene cables the insulation material experiences temperature protocols which vary with the radial position. The time-temperature scheme has, however, a major influence on microstructure of the insulation and chemical composition of the peroxide by-products. Thus it could be expected that the electrical properties of the insulation would vary with the spatial position within the cable. The purpose of the present work is to identify if there is a difference in the DC-electric properties at different radial depths of an extruded high voltage direct current (HVDC) cable insulation. In this study, DC electrical resistivity and DC breakdown strength measurements were carried out on samples taken from the insulation regions close to the inner and outer the semi-conductive screens. The obtained results show no significant difference of the investigated electric properties between the two regions. Hence, the DCelectric properties of the bulk insulation are constant within the accuracy of measurements.

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chemical defects and impurities are thought to be responsible for the deep trapping of charge carriers in polymeric insulating materials such as peroxide crosslinked polyethylene (XLPE). Such excess charges, referred to as space charges, might cause localised high electric stress, possibly leading to dielectric breakdown under DC voltage application. Chemical defects and impurities found in electrical cables are inherent to the different compounds designed for the crosslinking process technology. In this study, the trap energy of a range of chemical species relevant to indwial electrical cables is computed. The trap depth is obtained using quantum mecbanical tools, namely the Density Functional Theory (DFT). The DFT is employed to compute the electron affinity of the compounds. Then, a relation between electron affinity and trap depth is employed. Calculations have been performed for isolated molecules as well as including the effect of a polymeric environment.

XLPE has been the dominant insulation material for medium voltage (MV) cables in Europe since end of the 1970's. The service experience over the years with these cables is excellent. Improved cable constructions (e.g. the use of tough MDPE jacketing) manufacturing processes (dry curing and triple extrusion) and quality control methods have ensured the high performance of the final cables. This means that within the standard formulation concept there is not much further room for practical improvements.However, physical damage caused by mishandling of the cable can allow water penetration into the insulation and may lead to water treeing. At present there is concern about the effect of water tieeing upon the life time of XLPEcables. This has called for efforts to develop improved materials and the paper will describe the development and testing of the resulting new water tree retardant (WTR) XLPE'insulation grade. The major points included are a description of a hypothesis relevant for the growth of water treeing, verification of the efficiency of the new material from laboratory to full scale compound manufacture, showing that it is possible to process the new grade on conventional cable production lines and finally the short and long term electrical properties of these cables.It is clearly shown that the resulting XLPE grade has significantly improved resistance to water treeing. This is achieved without sacrificing other important properties.