The effects of temperature on the tree growth phenomena and relevant PD

Electrical treeing tests at divergent field are a tool for the characterization of insulating materials which has been extensively studied in the recent years. The divergent field is generally obtained by means of point plane electrode geometries, where the point (needle) electrode can be either conductive or semiconductive. The main part of tests on this subject has been performed at ambient temperature, even though the service temperature of the insulation can be quite higher. In the paper it is shown how the temperature can affect both the different phases of the treeing phenomena (i.e. tree inception, tree growth and final breakdown) and the PD patterns, which are relevant to PDs acting inside the tree channels. Experiments have been performed on ethylen-vynilacetate copolymer (EVA) applying different constant AC voltage levels. Temperature effects on the time to tree inception, on the time to breakdown and on the tree growth are presented. Furthermore it is shown that PD patterns and their relevant statistical parameters can be significantly affected by a variation of temperature. In particular the variation versus time of some of the statistical parameters is temperature dependent.