Temporal Change of Time-Frequency Characteristics of Pressure Waves Due to Growth of an Electrical Tree in Polyethylene

Abstract

This paper describes the results of time-frequency analysis of pressure waves caused by an electrical tree from a point electrode in a polyethylene subjected to ac voltages. Experiments were done in two cases. 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 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.