Thermal ageing and its impact on charge trap density and breakdown strength in polyethylene

With the growing interest in HVDC transmission systems, the impact of space charge on insulation has attracted more attention recently and the traps in the materials are considered as one of the dominated factors that affect the electrical performance of the insulation. It has been found that a small amount of deep traps, introduced by slightly thermal ageing, suppress the space charge injection and lead to the increase of DC breakdown strength. In this paper, further experimental works were carried out to explain this particular phenomenon and utilize the change of trapping characteristic to interpret the change of electric breakdown strength in thermally aged LDPE. Differential scanning calorimetry (DSC) was used to investigate the impact of thermal ageing on the internal structure and crystallinity of polyethylene. DC electrical conductivity was measured to verify the hypothesis about the substantial growth of conductivity in seriously aged LDPE is the reason why rapid charge decay rate were observed in these samples. It has been found that the change in DC breakdown strength is related to the change in charge trap density. The results suggest that the change in charge trap density of LDPE samples can be used to characterize electrical performance of the material, therefore, the ageing status.