| Silicone rubber is usually used as the insulation material for the termination of 10 k V cold-shrinkable power cable.The resistance produced by silicone rubber insulation during pre-expansion makes the silicone rubber insulation rebound,thus tightly wrapping the cable accessories on the cable body,which has the advantages of simple installation and operation and good insulation performance.Silicone rubber insulation material is under tension for a long time,and the force to resist deformation will gradually decrease with the passage of time,showing stress relaxation characteristics.Adjusting the crosslinking degree of silicone rubber insulation can change the entanglement between silicone rubber molecular chains,and then inhibit the stress relaxation phenomenon.At present,the effects of insulation stress relaxation of silicone rubber on the interfacial characteristics(interfacial pressure,interfacial field strength and interfacial temperature,etc.)of silicone rubber/crosslinked polyethylene composites,and the effects of crosslinking degree of silicone rubber on improving relaxation resistance are not completely clear,which needs further study.In this study,silicone rubber samples with different crosslinking degrees were prepared to study the influence of crosslinking degree on the insulation relaxation characteristics of silicone rubber,and the law of the insulation relaxation characteristics of silicone rubber on the interface characteristics of silicone rubber/crosslinked polyethylene was explored by combining experiments and simulations.The stress relaxation characteristics of silicone rubber insulation include mechanical relaxation and dielectric relaxation.The mechanical relaxation of silicone rubber was characterized by thermal elongation,tensile stress relaxation characteristics and thermal expansion coefficient.The dielectric relaxation characteristics of silicone rubber were tested by broadband dielectric spectrum.The results show that the higher the temperature,the greater the elongation of silicone rubber and the more obvious the relaxation of tensile stress,and the relaxation of molecular segments increases the thermal expansion coefficient and causes serious thermal deformation.There are dielectric relaxation processes δ caused by molecular chain relaxation and α caused by dipole segment relaxation in silicone rubber insulation at different frequency intervals.Increasing the degree of crosslinking can limit the rotational orientation of molecular chain and the steering motion of dipole segment,and reduce the polarization intensity of the two relaxation processes.Chemical stress relaxation reflects the molecular chain fracture of vulcanized silicone rubber network under thermal aging.Structural silicon oxidation occurs at the same time of molecular chain fracture of silicone rubber,which leads to further crosslinking reaction,increasing volume resistivity and breakdown strength.Based on the principle of time-temperature equivalence,the interface pressure of 10 k V cable terminal is predicted by finite element simulation,and the influence of defects on interface characteristics is studied considering that silicone rubber is easy to produce air gap cavities or impurities after stress relaxation.The results show that the stress relaxation phenomenon of silicone rubber will lead to the decrease of interface pressure.The higher the temperature,the faster the insulation stress relaxation of silicone rubber and the greater the interface pressure loss.When the cross-linking degree of silicone rubber insulation is high,its stress relaxation resistance is strong,and it is predicted that the interfacial pressure retention rate will still be as high as 90.7 % after 30 years,and it has a long storage period.When there is an air gap or impurity particles at the interface,the electrothermal force field of the interface is distorted easily. |
