Fundamental Study on Detection Method of Residual Charge from Water Tree Degraded XLPE Cables with Pulse Voltage

摘要

One of the main degradation diagnostic methods used for the water tree degradation of cross-linked polyethylene (XLPE) cables is the residual charge method in Japan. 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, and after it is grounded, an AC voltage is applied to it to release deeply trapped space charges, i.e., residual charges. This method has been used for 6.6 to 77 kV XLPE cables. Recent studies have revealed that there is rapid charge accumulation and depolarization in water trees, and pulse voltages may be used instead of DC and AC voltages. If a diagnostic method using pulse voltages for water tree degraded XLPE cables is developed, the improvement of the detection sensitivity for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a measurement time that is shorter than that of the conventional method. Also, the diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. Furthermore, the method can be applied to the XLPE cables connected to gas-insulated switchgears when the pulse-width is smaller than the half period of an AC voltage with power frequency. In this study, instead of DC and AC voltages in the conventional residual charge method, the authors applied pulse voltages with pulse widths of about 1 ms to a removed 6.6 kV XLPE cable, and successfully detected current signals related to water tree degradation. Then, we investigated the relationship between the residual charge signal and the AC breakdown voltage of the removed 6.6 kV XLPE cables. The results verified that the removed cables having larger residual charge and longer duration of current signal related to water tree degradation had a lower AC breakdown voltage.