Study on Degradation Diagnosis for Power Cable Using Wavelet Analysis of Pressure Wave Produced by Electrical Tree Propagation

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Treeing degradation diagnosis of electrical power cable was carried out using an AE method. Time-Frequency characteristics of pressure wave due to partial discharges in tree was calculated by continuous Wavelet transform. Results can be summarized as follows.Time-frequency analysis of pressure waves caused by an electrical tree from a point electrode in a low-density-polyethylene subjected to ac voltages. In the first case, the applied voltage was raised with 5 kV step at 5 minutes intervals. When a tree growth was started from the point electrode at 15kV, a high frequency pressure wave of about 8MHz was propagated from the tree. When the tree became large and more bushy, the high frequency pressure wave from the tree tip was followed by a lower frequency pressure wave from the tree stems. In the second case, a constant voltage of 25 kV was applied to the sample for 4 hours. Both the high frequency pressure wave and the following lower frequency pressure wave became larger at the first … More stage. However, the lower frequency pressure wave disappeared when the treeing degradation was attained to a critical level. This was because the carbonization of the tree wall was caused by the partial discharges. These results demonstrated that the time-frequency characteristics of the pressure waves were directly related to the partial discharge behavior in tree.Pressure waves from a block sample which was composed of low-density-polyethylene (LDPE) and ethylene-ethylacrylate (EEA) were measured by an AE sensor on the back of the plane electrode. Results of the time-frequency analysis of the pressure signal showed that a high frequency pressure wave due to partial discharge at the front area in the tree was followed by a lower frequency pressure wave due to partial discharge near the point electrode. In a sample of LDPE alone without EEA, only the lower frequency pressure wave disappeared when the treeing degradation proceeded to a critical level. The disappearance suggested that carbonization at the rear area in the tree was caused by repetitive discharges. However, the disappearance phenomenon did not observed for a LDPE/EEA blended sample. We confirmed that there was an optimum blend ratio between LDPE and EEA for the inhibition of carbonization.Pressure waves from a void in LDPE alone was measured at various positions. When the AE sensor was set just below the void, pressure waves was directly propagated to the sensor. However, the direct component was not detected when the position of the sensor was moved along the plane-electrode surface, so that the time-frequency characteristics was greatly changed. We concluded that the location of the void can be deduced from the result of the time-frequency analysis. Less