Local ageing defects are the main causes of cross-linked polyethylene (XLPE) power cable faults. This research aims at assessing the severity level of typical cable local ageing defects based on the high-voltage frequency domain spectroscopy (FDS) non-linearity. Experimentally

typical XLPE cables ageing defect conditions are simulated

with different ageing types

degrees

and local proportions. Then

the high-voltage FDS detection is conducted and the results have been analysed. It has been found that the high-voltage FDS non-linearity is only related to the detection voltage and ageing degree. Especially

it is more sensitive to changes in the ageing severity. Thus

the local ageing defect severity assessment method based on non-linearity has been established. Additionally

the sensitivity has been compared between two diagnostic evaluation methods: the high-voltage FDS and the widely used very-low frequency method (VLF

0.1 Hz). It can be concluded from the comparison result that the proposed assessment method has higher sensitivity than that of the traditional methods. Finally

the effectiveness of the proposed method for evaluating the on-site long cables has been validated. The result demonstrates that the proposed method can assess on-site long cables and has the potential to be formed as quantitative standards.