Electrical treeing is among the main mechanisms responsible for electric breakdown in polymeric high voltage cables. In this paper, electrical treeing in electro-thermally aged low density polyethylene (LDPE) was evaluated by means of a real-time microscopic detection system. A wire -plane electrode test object with semiconducting tab was used in the study. The ageing was performed at a constant temperature of 80°C and an applied DC voltage of 10 kV of both polarities and lasted from 100 up to 800 hours. For testing the resistances to electrical treeing, tests were performed by applying 50 Hz AC voltage at a ramping speed of 0.5 kV/s. The obtained experimental results showed that mainly the thermal component of the ageing had an influence on electrical treeing parameters, namely on the initiation voltage as well as the shape and occurrence frequency of the trees. With an increase of the ageing time, the tree inception voltage gradually decreased.
This paper presents electrical treeing behavior in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) after exposure to thermal and DC electro-thermal ageing. Both the ageing and the treeing tests were performed by means of two different types of test objects with wire-plane electrode geometry. One type of the tested objects contained only wire electrode of 10 µm diameter, whereas in the other type the wire electrode was attached through a semiconducting tab. The ageing was performed at 80°C with and without 10 kV DC voltage of both polarities connected to the wire and lasted up to 800 hours. The AC electrical treeing tests were applied afterwards for detecting changes of material properties after the ageing. The results showed that the electrical tree inception voltage consistently decreased with increasing time of thermal exposure, whereas the applied DC electric stress had a negligible effect on the observed behavior. Similar effects were found in both the tested materials (LDPE and XLPE) though the object type also influenced the results. For the objects with semiconducting tab, a higher level of the scale parameter was registered because of shielding effect of the tab on the electric field strength at the wire electrode. It also yielded less number of trees growing in parallel at the electrode. The dominant effect of thermal stress on the ageing of LDPE was elucidated by using various analytical techniques, like differential scanning calorimetry, infrared spectroscopy, inductively coupled plasma optical emission spectrometry and oxidation induction time, and it is believed to mainly affect antioxidant content in the test objects.
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