Phenomena of AC and DC Breakdown in Oxidized Polypropylene

To estimate the insulation performance of polypropylene (PP) material through its lifetime for cable application, thin PP films (~100 μm thick) were aged at the temperature of 150°C in a fan-assisted oven for up to 21 days. With the chosen ageing conditions, the polymer chains were attacked by heat energy and reacted with available oxygen, forming chemical products in undersurface region from the initial stage, notably carbonyl groups, whose concentration increases along with the ageing periods, as detected by the Fourier-Transform Infrared (FTIR) Spectroscopy in the present work. The breakdown performance was examined by applying ramp voltage at a rate of 500 V/s under AC and DC stress, respectively. It has been observed that after ageing, the AC breakdown strength (BDS) declines from 211.01 ± 2.71 to 197.43 ± 3.02 kV/mm, while the DC BDS initially increases from 506.15 ± 18.27 to 550.94 ± 19.39 kV/mm at 14 days of ageing, followed by a decrease with any further ageing. The observed difference is attributed to the different breakdown regions: AC breakdown is more likely to occur adjacent to the sample surface. The continuous decline versus ageing durations correlates with the higher concentration of carbonyl groups detected on the sample surface. On the other hand, DC breakdown tends to occur within the bulk of sample. The changes in BDS after ageing is related to the electric field distortion due to the space charge distribution induced by the chemical alterations.