Ageing indices and energy delivery for polymers undergoing PD activity under combined AC and DC stress

Samples of polypropylene and high density polyethylene have been exposed to partial discharge activity at elevated temperatures under the influence of combined AC and DC stresses in the range of 1 and 2.5 kHz. Based on FTIR-ATR and dielectric spectroscopy data values for the carbonyl index (CI), the low frequency conductivity (σ
₀
) and a proposed new index the susceptibility index (χI) have been calculated to characterize the degree of ageing in the samples. The susceptibility index reflects changes in the number of dipoles and the distribution of dipole moments in the system as a result of ageing. The rate of energy delivery from the partial discharge activity has also been calculated and has been found to be dependent on the frequency of the AC component of the stress. Strong correlations are observed between the behavior of the individual indices and the rate of energy delivery. Possible reasons for these observed correlations are discussed and the observed differences in the behavior of the ageing under different experimental conditions are considered.