Study On Dielectric And Aging Properties Of Nano-Modified Liquid Silicone Rubber For HVDC Cable Accessories Insulation

With the construction of HVDC transmission network and development of flexible DC transmission technology,XLPE cables have been widely used in asynchronous networks,underground power grids,and long distance submarine transmission over sea islands recently.The operational safety and reliability of DC cables is very important for HVDC transmission network,and the insulation damage of cable accessories is especially important.Under the DC voltage,the concentration and distortion of electric field distribution in the cable accessory was occurred easily,due to the accumulation of space charge and the mismatch of composite insulation conductivity.As a result,the local field strength is much higher than the normal working field strength,thus causing the electric field distortion or even insulation breakdown.It has become the main factor affecting the operational safety and reliability of HVDC cable accessories.In view of the above problems,a nanocomposite dielectric was prepared for cable accessories insulation in this study through the nano doping modification,from the perspective of accessory insulation materials development.The purpose is to expect to improve the dielectric properties of the cable accessories insulation,so as to achieve a conductivity matching with cable insulation XLPE and promoting uniform distribution of the electric field within the DC cable accessories to improve the safety and reliability of HVDC cable accessories.In this paper,the different certain doping concentrations of nanocomposites was prepared by using SiC and TiO2 nanoparticles as the filler,the liquid silicone rubber(LSR)as matrix.The effect of doping concentration,temperature and electric field strength on the conductivity properties of LSR nanocomposites was studied.The results show that compared with pure silicone rubber,the doping of nanoparticles both makes LSR nanocomposite have better nonlinear conductivity.With the increase of nano doping concentration,the conductivity of LSR nanocomposites both increases firstly and then decreases.Morever,the optimal doping concentration of nano SiC is 3wt%,and it mainly improves the sensitivity of LSR nanocomposites conductivity on the temperature.While the optimum doping concentration of nano TiO2 is 4wt%,it significantly improved the dependence of LSR nanocomposites conductivity on the electric field strength.The trap characteristics of LSR nanocomposites were tested by thermal stimulation current method(TSC).The results showed that the doping of nanoparticles introduced a large number of interfacial layers in the LSR matrix.The carriers can move easily along the conductive paths and percolation network formed by adjacent nanoparticles,resulting in an increase of migrating carriers number involved in the conduction and the LSR nanocomposites have a better nonlinear conductivity characteristics on the macroscopically.The conductivity of pure LSR is lower than nanocomposites,because the conductance is mainly caused by the carrier's jump over the potential barrier under the action of the external electric field.The nano doping reduces the trap level and depth of LSR nanocomposites,has an influence on the mobility of carrier and charge transport characteristics,and thus changed the macroscopic dielectric properties of the LSR nanocomposites.The dielectric spectrum and DC breakdown characteristics of LSR nanocomposites were measured.It was found that the relative permittivity of nano modification samples was higher than that of pure LSR,and the permittivity increase with the doping concentration increase.The DC breakdown strength of LSR nanocomposites was lower than that of pure LSR.The breakdown field strength was decreased with doping concentration increase,but the degree of declining is small,so the LSR nanocomposites still has good insulation margin.The space charge characteristics of LSR nanocomposites were tested by electroacoustic pulse method(PEA).It was found that the space charge amount accumulated in all concentrations LSR nanocomposites was lower than that of pure LSR at different temperatures and electric field,the 4wt%-TiO2/LSR sample has the most obvious inhibitory effect on space charge among them.In order to verify the effect of electric field homogenization of LSR nanocomposite within DC cable accessory,the simulation of electric field distribution in DC cable accessories was analyzed by Comsol-Multiphysics.The results show that LSR nanocomposites with nonlinear conductivity applied to stress cone can realize the homogenization of electric field,reduced the difference between XLPE and accessories insulation caused by temperature and electric field,improved the degree of electric field concentration.The thermal oxygen aging properties of LSR and SiC/LSR nanocomposites were studied.It was found that the dielectric properties of pure LSR after thermal oxygen aging has little effect,but the mechanical tensile properties were greatly decline.With the high elasticity gradually missing,the stress relaxation characteristics of LSR was obviously presented,so that it could not continue to provide sufficient interface holding force for the cable accessories,resulting in discharge easily at the composite insulation interface.The life prediction analysis of pure LSR and SiC/LSR nanocomposites was calculated by using Arrhenius model with stress relaxation characteristics as the aging performance index.It was found that the mechanical tensile properties of SiC/LSR nanocomposites were improved compared with pure LSR.Further,the decline in performance after aging slowed down and the predicted lifetime is also higher than that of pure silicon rubber.This provides some reference information for improving the service life of XLPE DC cable accessories.