Evaluation of relationship between residual charge signal and AC breakdown strength of water-tree degraded 22 to 77 kV classes XLPE cables removed from service using pulsed voltages

The authors have been developing a diagnostic method for water-tree degraded cross-linked polyethylene (XLPE) cables, in which pulsed voltages are used rather than the DC and AC voltages used in the conventional residual charge method. In this study, we first applied the proposed method to the diagnosis of water-tree degraded XLPE cables in classes from 22 to 77 kV, and successfully detected current signals related to water-tree degradation due to residual charges in the XLPE cables removed from service. Next, we investigated the relationship between the residual charge signal and the AC breakdown strength, and our results show that the correlation between the residual charge magnitude and the AC breakdown strength was weak, but the removed cables with a longer duration of the current signal related to water-tree degradation had a lower AC breakdown strength. Also, the ratio of the duration to the rise time of the applied pulsed voltage was strongly correlated with the AC breakdown strength. Therefore, we determined that this ratio can serve as a water-tree degradation index. Thirdly, we demonstrated the reason for this strong correlation using a simplified water tree model. Finally, to obtain greater sensitivity in current signal detection, we developed positive and negative pulse generators and confirmed that an increase in the applied voltage would increase the current signal detection sensitivity.