Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. 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 since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.