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.
Although many researchers have investigated voltage life characteristics of XLPE cables, the voltage life curve of XLPE insulation has not been made clear because of large deviations in the obtained data. Moreover, the voltage life curve of a XLPE cable, which is significantly affected by defect size and shape, can not be applied to another cable having different defects. The authors obtained an intrinsic stress life curve of XLPE which included no defects. The intrinsic stress life curve demonstrates the existence of a threshold stress, which causes no degradation in the insulation. The application of the intrinsic stress life curve makes possible to estimate the voltage life curve of any cable with known defects. A method for calculating the defect size that would not initiate any degradation in XLPE insulation under a given electrical stress is proposed.
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