Study On The Diffusion Of Externally Injected Additives In XLPE Cables And Their Inhibitory Effects On Electrical Tree

Cross-linked polyethylene（XLPE）insulated cables are widely used in power grids due to their excellent dielectric and mechanical properties.However,with the increase of operating years and the influence of harsh environments,the failure rate of cable insulation has gradually increased.At present,in order to combat the threat of insulation degradation,researches have been done to add functions such as water tree resistance and electrical tree self-healing to cables through material modification methods.Taking into account the advantages of external injection of additives such as high flexibility and small side effects on XLPE insulation,this article explores in detail the two core contents of the injection method:1)Efficient diffusion of additives.2)Enhancement of insulation performance by additives.It laid the foundation for the improvement of additives,on-site cable repair technology and the improvement of insulation reliability.The main research work and results of the thesis include:（1）According to the difference of material size and insulation enhancement mechanism,the following three representative materials were selected as additives:pure siloxane,siloxane+Ti O₂,siloxane+voltage stabilizer.In the laboratory environment,an external additive injection platform was built,and the test data of liquid additive diffusion under different pressures,temperatures and XLPE water-containing conditions were obtained through infrared spectroscopy technology,X-ray energy spectrum analysis（EDS）and precision weighing technology.At the same time,the XLPE electric tree accelerated aging test platform and measurement system were built.The test data of XLPE breakdown voltage,partial discharge and micro-morphological characteristics before and after the additive injection were obtained.（2）The diffusion law of the three additives in XLPE was explored,and the influence mechanism of structural differences on diffusion was analyzed.The results show that:1)Within a certain range,as the temperature and pressure of the diffusion environment increased,the diffusion efficiency of each additive also increased,and the diffusion efficiency reached the highest at 50℃and 0.1MPa.In addition,the diffusion effect of siloxane-based liquid and voltage stabilizer in XLPE was better,while the diffusion rate of nano-Ti O₂ additive in XLPE was obviously slower.2)When XLPE contains a lot of water,the additive reacted with water by hydrolysis and condensation,and the interfacial hindrance caused by the rapid reaction results in the greatly reduced diffusion rate of the additive.3)Increasing the concentration of voltage stabilizer can promote its diffusion in XLPE,while increasing the concentration of nano-Ti O₂ can seriously affect its diffusion rate in XLPE.4)By comparing the diffusion law of the three additives in XLPE,it was found that the geometric size of the group was the key factor affecting the diffusion efficiency.（3）The inhibitory effect of different additives on the growth of electrical trees and the mechanism of insulation enhancement were obtained.The results show that:1)After the injection of these three additives,the breakdown field strength had been significantly improved.The main reason is that the main diffusion paths of additives in XLPE are crystal defects and amorphous areas,which are the insulation weakness and the electrical tree initiation-growth area,and the introduction of additives can compensate for the performance of the weak areas.2)The voltage stabilizer had a strong inhibitory effect on the initiation of electrical trees,which was related to the uniform distribution of voltage stabilizer in XLPE.However,due to the nano-size effect and the scattering mechanism of nano-Ti O₂,its inhibitory effect on the growth of electrical trees was more significant.