Electrical ageing of polyethylene during constant AC stress

The authors investigated the ageing of polyethylene under inhomogeneous electric fields well below the intrinsic breakdown field strength (>570 kV/mm RMS) but above a local critical field strength at which charge injection takes place. They observed partial discharge and the treeing behavior under different conditions of AC and DC stressing. Additional experiments with samples impregnated with an electronegative gas (SF₆) yielded further information on the process of polymer degradation. The results show that there is no deterioration during DC ageing; the polarity reversal that occurs under AC stressing is necessary for the ageing process. The observation of brittle walls of the tree structure indicates that the rate of impingement of hot electrons on the polymer is relatively low. Therefore the heat produced may be easily dissipated without appreciable rise of the temperature within the treed region. The experiments with the impregnated samples are in agreement with the assumption of a hot electron process. The SF₆ captures hot electrons and therefore reduces the probability of damage; in the present case no deterioration is observable up to 600 h