500-kV XLPE-insulated cable with an insulation thickness of 27 nim and its joint have been developed for long distance transmission lines. First, in the basic research made irom 1989 to 1992, elicited were the perfortnance-detertnining factors of XLPE cables and joints. Their respective insulation designs were inade in terms of these factors. On the basis of these designs, full-sized cables and joints were manufactured in the course of the development research performed from 1Y92 to 1993. The next step was to evaluate the initial clectrieal pcrforinance to estimate the suitability of our designs. As the final step of our series of researchcs so far carried out, a long-term loading cycle test was carried out by means of a model line with joints as insValled and using the same lechniques as employed in the actual commercial lines for full eight months. We could thus verify the long-term reliabilily for coniinercia1 use.

In order to improve the long-term reliability of XLPE cables, it is essential to understand the mechanism of deterioration and breakdown as a result of long-term voltage application. This paper presents the results of our study on the deterioration mechanism of polyethylene, which is the insulating material of XLPE cables, focusing mainly on the electrical deterioration luminescence phenomenon observed using model samples that simulate the regions of high electric field enhancement, e. g., impurities and protrusions. A report is also given on an evaluation of the long-life characteristics of the insulating material, based on electrical deterioration luminescence measurements that take into account the above deterioration mechanism.

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