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

采用声发射方法对电力电缆的树枝退化进行了诊断。利用连续小波变换计算了树木局部放电压力波的时频特性。主要研究结果如下：1.低密度聚乙烯点电极电树枝在交流电压作用下产生的压力波的时频分析。在第一种情况下，以5千伏的阶跃每隔5分钟升高施加的电压。当点电极在15KV下启动树木生长时，从树上传播出约8 MHz的高频压力波。当树变得更大、更茂密时，树尖发出的高频压力波之后是树干发出的低频压力波。在第二种情况下，对样品施加25千伏的恒定电压4小时。在第一个…，高频压力波和随后的低频压力波都变大了更多的舞台。然而，当树木退化达到临界值时，低频压力波消失。这是因为树墙的碳化是由局部放电引起的。利用平面电极背面的声发射传感器测量了由低密度聚乙烯(LDPE)和丙烯酸乙酯(EEA)组成的块状样品的压力波。压力信号的时频分析结果表明，树体前部局部放电产生高频压力波后，点电极附近出现局部放电产生的低频压力波。在未加EEA的LDPE样品中，当树枝降解达到临界值时，只有低频压力波消失。这种消失表明，树后部的碳化是由重复放电引起的。然而，在LDPE/EEA共混物样品中没有观察到消失现象。我们证实了LDPE和EEA之间存在一个抑制碳化的最佳共混比例。在不同位置测量了LDPE中空隙产生的压力波。当声发射传感器设置在空隙正下方时，压力波直接传播到传感器。然而，当传感器的位置沿平面电极表面移动时，没有检测到直接分量，因此时频特性发生了很大的变化。从时频分析的结果可以推断出空洞的位置。较少

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