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We reported that AC breakdown voltage of dry-cured XLPE cables decreases to about half of the initial breakdown voltage by the immersion treatment. In this paper, the causes of this decrease are investigated and are. reported in detail. From some experiments, the decrease is found to be mainly caused by the moisture existing in the insulation. Our results show that the value of the breakdown voltage is not related to the moisture above ten ppm order. However, even little moisture of ten ppm order may reduce the breakdown voltage. To clarify this, we investigated the state of water in the insulation using the FT-IR and the optical microscope. The results show that the moisture is mainly composed of a pure liquid water, and exists in two regions; (1) dissolved in the amorphous region and (2) condensed in the form of microvoids, in amounts exceeding its solubility limit in the insulation. Also, we assume that the moisture in the amorphous region causes the reduction of the AC breakdown voltage.

Influence of Moisture on the Initial AC Breakdown Voltage of Dry-cured XLPE Power Cables