Space charge properties of EPDM under different electric field and thermal ageing

Ethylene Propylene Diene Monomer (EPDM) has been widely used as the insulating material of power cables and accessories. Due to the formation and injection of space charge under DC electric field, the distortion of electric field strength will influence the dielectric properties of EPDM. In the paper, the space charge profiles at different electric field and ageing temperature were carried out. The total charge quantity and trap distribution of aged EPDM were analysed. The results show that the charge injection from electrodes was intensified with the increase of electric field. The behaviour characteristics of the space charge in EPDM during polarization can be divided into four stages: (I) no charge injection, (II) bipolar injection, (III) charge transformation and recombination, and (IV) anode injection. Space charge properties of EPDM aged at different temperature are also different. With the ageing time increasing, homocharge injected from anode increases, and heterocharge accumulates in the vicinity of cathode. When EPDM is aged at 120 °C, the total charge quantity at the initial ageing stage increases with ageing time. Then it decreases. However, the total charge quantity of EPDM aged 2 days at 160 °C reaches to a higher value. The space charge properties of aged EPDM can be explained by degassing process and the theory of trap filling.