On tree-inhibition in polyethylene

For a better understanding of the basic mechanisms of electrical treeing in polymer insulations the dependence of the DC-conductivity of polyethylene (with and without additives) on temperature and field strength was examined. Here we found that all additives, and especially acetophenone which is a favorite additive against treeing, substantially increase the conductivity. Due to the low conductivity of insulators, in the case of an AC-voltage load generally there is a capacitative voltage division. This leads to enormous stress concentrations at the site of impurities, which finally may lead to the start of a tree. Considerations using the results of the DCconductivity measurements showed that these local stress concentrations can be diminished to a certain extent by the fielddependent conductivity of the polymer even for 50 or 60 IIz AC-voltage, thus leading to an improved resistance to treeing. The experimental proof for the relevance of this field-grading mechanism is given by the results of treeing-experiments with polyethylene containing specific additives. Furthermore tree-inception voltages of XLPE containing various amounts of acetophenone show a dependence on temperature and frequency as expected in case a mutual capacitativeconductive voltage division is taken into account.