Abstract
Characterization of the electrical, chemical, and microstructural properties of high voltage cables was the first step of the European project ARTEMIS, which has the aim of investigating degradation processes and constructing aging models for the diagnosis of cross-linked polyethylene (XLPE) cables. Cables produced by two different manufacturers were subjected to a large number of electrical, microstructural, and chemical characterizations, using cable peelings, instead of lengths of whole cables, as specimens for the measurements. Here the effect of surface deformation and roughness due to peeling and the relevance and significance of thermal pre-treatment prior to electrical and other measurements is discussed. Special emphasis is put on space charge, conduction current and luminescence measurements. We also consider the dependence of these properties on the spatial position of the specimen within the cable. It is shown that even though the two faces of the cable peel specimens have different roughness, the low-field electrical properties seem quite insensitive to surface roughness, while significant differences are detectable at high fields. Thermal pre-treatment is required to stabilize the insulating material to enable us to obtain reproducible results and reliable inter-comparisons throughout the whole project. The spatial position of the specimens along the cable radius can also have a non-negligible influence on the measured properties, due to differential microstructure and chemical composition.
Highlights
The two HV cables investigated were insulated by XLPE produced using extra-clean materials and processes
The surface quality does not, play a noticeable role especially at fields close to those used for cable design and accelerated life tests but it will have an influence at higher fields
Given that the purpose of the project is the investigation of diagnostic properties and the comprehension of the degradation mechanisms, the use of peeled films with appropriate conditioning seems the best compromise between practicality, economic constraints and relevant representation of the cable
Summary
The two HV cables investigated were insulated by XLPE produced using extra-clean materials and processes. The project started with the initial characterization of cable insulation, in order to constitute the reference for electrical, micro-structural, and chemical properties needed to single out aging markers on the basis of the property evolution with aging time. The first problems that were faced were the choice of the specimens to be used and the kind of pre-treatment necessary in order to have reproducible results. Extensive investigations of the effect of the surface quality on the electrical properties were carried out and are summarized in this paper. It was necessary to set up a description of the unaged material which would not change with storage time and would be repeatable and reproducible for each specimen taken from different lengths of the same cable (at a given distance from the inner or outer semiconductor screen). For the purpose of this paper data taken from different cables are reported, without referring to the relevant manufacturer, since the results for the different cables involved in the project did not in general vary significantly
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