On Tree-Inhibition in Polyethylene

During a series of experiments aimed at developing a better understanding of the basic mechanisms of electrical treeing in polymer insulations, the dependence nce of the dc conductivity of polyethyle ( with and without additives) on temperature and field strength was examined. It was discovered that all additives, and especially acetophenone, which is well known to resist electrical treeing, substantially increase the conductivity. Due to the low conductivity of insulators, in the case of ac voltage excitation capacitive voltage division ion generally occurs. This leads to enormous stress concentrations at the sites of impurities, which finally may lead to the start of trees. Computations using the results of the dc conductivity measurements showed that these local stress concentrations ions can be diminished by the field-depen conconductivity of the polymer even for 50 or 60 Hz ac voltage, thus leading to an improved resistance to treeing. The experimental proof of 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 cross linked polyethylene containing various amounts of etoacetophenone show a dependence on temperature and frequency as expected from the mutual capacitive-conductive lvoltage division by the additive.