Effect of Mechanical Tensile on Nonlinear Conductivity of Silicon Carbide/Silicone Rubber Composites for Prefabricated Joint of HVDC Cable

Field grading materials (FGMs) with nonlinear conductivity, which can uniform a local direct-current (dc) electric field, have potential applications in HVDC cable joints and terminals. For the prefabricated cable joint, an interface pressure between the main insulation (XLPE) and joint insulation (SiR) is mandated to avoid the insulation failure caused by surface flashover. Therefore, the nonlinear conductive insulation in prefabricated cable joints is maintained in an expanded state in practical application. This paper focuses on the effect of the mechanical tensile on the nonlinear conductivity characteristics in silicon carbide (SiC)/silicone rubber (SiR) composites. A three-electrode system is employed to measure the dc conductivity (σ) of SiC/SiR composites with a stretching ratio of 0 to 60% under 1-30 kV/mm, and the nonlinear coefficients are further calculated by the logE-logσ curves, respectively. The results show that when the SiC mass fraction is 90%, the conductivity and nonlinear coefficient of the sample increase with the increase of stretching ratio from 0% to 20%. However, when the stretching ratio increases further, the conductivity and nonlinear coefficient decrease. An electromechanical coupling model of SiC/SiR composites is established by using COMSOL Multiphysics. The effect of the mechanical tensile on the nonlinear conductivity is contributed to the varied conducting routes under the elastic strain of SiC/SiR composites. The doping content of SiC is related to the density of the conducting route in the composites. The higher the SiC content, the lower the sensitivity of the conducting paths to the strain, and the less sensitivity of nonlinear conductivity to the tensile strain.