A space-charge life model for ac electrical aging of polymers

The dc space-charge model, previously developed by the authors, here is modified to account for the contribution to electrical degradation provided by ac fields. First, the dc model is applied to both dc and ac multistress life test results relevant to a given material. The variations of model parameters from dc to ac data fitting provide indications about the modifications in the degradation mechanisms when passing from a dc to ac regime. Then, a description of aging under ac is achieved through proper assumptions about the space-charge buildup, injection mechanism, and dynamic condition of the polymer lattice. This approach enables the effect of frequency to be accounted for, in a framework where ac aging presents significant analogies with mechanical fatigue. The validity of the ac space-charge model is checked by applying it to the results of accelerated life tests performed on various insulating materials, at different values of voltage, temperature and frequency, on the whole finding very good agreement with experimental data.