Crosslinking dependence of trap distribution and breakdown performance of crosslinked polyethylene

The DC breakdown performance and trap distribution of crosslinked polyethylene (XLPE) with different crosslinking degrees were investigated in this paper. Results demonstrate that the breakdown strength initially rises from 276 to 352 kV/mm, followed by a decline to 300 kV/mm with the increase of crosslinking degree from 0 to 79.5%. The peak value occurs at the crosslinking degree of 52.1%. The deep trap density determined by isothermal surface potential decay (ISPD) displays a similar trend, sharing a same turning point with breakdown strength. In addition, an increasing crosslinking degree will lead to a lower crystallinity and an expanding amorphous region, while the content of by-products shows no obvious difference in XLPE samples due to the degassing process. It is therefore proposed that the crosslinking process can change the morphology of XLPE and influence the trap distribution. The formation of crosslinking network will increase the deep trap density, whereas the expanding amorphous region will result in the decrease of it. The increased deep trap density can cause homo-charges accumulation near the surface of XLPE and further suppress the charges injection from electrode. On the other hand, the charges in the bulk are easier to be captured and thus retard the charge transport. Dual dynamic processes contribute to the promotion of breakdown performance for XLPE.