Fundamental Study on Diagnostic Method of Water Tree Degraded XLPE Cables using Residual Charge Detection with Pulse Voltages

Abstract

One of the degradation diagnostic methods for water tree degradation of XLPE cables is the residual charge method. In this method, a DC voltage is applied to a water tree degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages. In this study, we first applied pulse voltages with a pulse width of about 1ms to an artificial water tree degraded XLPE cable, and found that the residual charge increased with the increase in the degradation degree. Second, we confirmed that residual charges were successfully detected in the removed 6.6kV XLPE cables that had water tree degradation. Finally, we investigated the relationship between the residual charge signal and the AC breakdown voltage of the removed 6.6kV XLPE cables. The results clarified that the removed cables with larger residual charge and longer duration of current signal related to water tree degradation had a lower AC breakdown voltage.