ac aging and space charge characteristics in low-density polyethylene polymeric insulation

Abstract

Space charge characteristic in polymeric insulating materials under ac conditions is one of the areas receiving growing interests over the last few years. Following the establishment of several techniques that allow space charge under dc conditions to be characterised non-destructively, there is a continuous effort to apply the same techniques to ac conditions. Earlier results revealed that there was charge accumulation at very low frequency (<0.01 Hz) while the charge formation at high frequency was negligible especially at power frequency. These results gave the impression that space charge effect under ac conditions is not an issue to be concerned with despite new evidences emerging slowly indicate the effect of space charge on electrical deterioration of the insulating polymers. In the present work efforts have been made to investigate the influence of ac ageing on space charge dynamics in low-density polyethylene (LDPE). LDPE films with 200mm were aged under various electric stress levels at 50 Hz for various times at ambient temperature, however, the bulk work was carried out on the samples aged at 50 kV/mm. Space charge dynamics in the samples after ageing were monitored using the pulsed electroacoustic (PEA) technique. The results indicate that there is a significant amount of negative charge accumulation in the aged sample due to charge injection at high ageing stress. The amount of charge in the aged sample is related to the electric stress. Little amount of charge is present at low fields and accumulated charge increases rapidly once the applied field is greater than 10 kV/mm. Due to a very slow charge decay rate, it is believed that the injected charges are captured by the deep traps that may be formed during ac ageing. At 50 kV/mm, the total amount of charge increases with the ageing time initially and then levels off. It was also found that the amount of charge in aged samples is related to the electrode material. Little charge was observed when gold electrodes were sputtered on both side of the sample. The charge dynamics of the aged samples under dc bias differ from the sample without ac ageing, indicating changes brought in by ac ageing. Chemical analysis by infrared spectroscope (FTIR) and Raman microscope reveals no significant chemical changes taken place in the bulk of the material after ac ageing. Finally the consequence of the accumulation of space charge under ac conditions on the lifetime of the material has been discussed.

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