Measurement of pressure wave from AC tree in polymeric insulators and time-frequency analysis using wavelet transform

Degradation process of polyolefin materials subjected to an ac voltage was studied by wavelet analysis of the pressure wave associated with tree growth under a point-plane electrode configuration. 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.