Effect of Ageing on the Dielectric properties of EPDM for HVDC Cable Joints

Abstract:
Cable accessories represent critical components of cable networks due to the macroscopic material interfaces between the joint and cable insulation in conjunction with grease materials. In contradiction to AC stress, the electrical field distribution under DC stress in these interfaces is known to be dependent on local field strength and temperature at a given point in time, and might change due to aging of the materials. The main ageing mechanism in polymers is thermo-oxidation, which is attenuated inside of the cable joint due to the limited supply of oxygen. However, physico-chemical interactions between the joint insulation material and the lubricant might cause changes in material properties, thus affecting the long term field distribution at the interface. The experiments presented in this paper aim at investigating ageing phenomena in EPDM, which is used as insulation material for HVDC underground cable joints, due to thermal as well as electro-thermal stresses and interactions with silicone oil. The effect of variations in electrode materials as well as surrounding medium on the changes of the relative permittivity, loss factor and material resistivity due to ageing are evaluated. The results indicate a tendency of the silicone oil to prevent oxidative ageing of the EPDM samples, therefore changes in the measured electrical parameters are attenuated compared to the samples aged in contact with air. Furthermore, the measured change in loss factor is affected by an additional electrical stress imposed on the samples during the thermal ageing in case stainless steel electrodes are used. For the experiments using parameters which correspond most to practical service conditions, as a consequence of ageing only minor changes in the permittivity but no significant change in loss factor and resistivity can be concluded.