The Study Of Insulation And Aging Status Of XLPE High Voltage Cable

Since the beginning of the new century,China has increased investment in the construction of urban and rural power grids,and the use of cables has become increasingly widespread.XLPE has good electrical properties and heat resistance.It is widely used in cable insulation of different voltage levels in electric power transmission and distribution systems.However,the aging problem of cable insulation produces a huge threat to the normal production and life of society.As the environmental conditions in rural are worse than in cities,and the humidity is higher,the aging problem of cable insulation is more serious.Therefore,in this thesis we have investigated the changes of accelerated aging of cable insulation,trap charge,dielectric and physical and chemical properties,summarized the aging mechanism and mode of insulation,which will provide a certain theoretical and data support for the upgrade and transformation of rural and urban power grids.Firstly,a multi-factor accelerated aging experiment was conducted.The results showed that the electrical tree growth characteristics of old and new cables under AC voltage were different.As the power frequency voltage level increased,the structure of the electrical tree branch changed significantly.At the same voltage level,the growth rate of the electric tree accelerates with the frequency increasing.Temperature can change the growth structure of electrical tree,and high voltage and temperature coupling can rapidly increase the growth rate of electrical tree.Electrical-heat-water combined aging will accelerate insulation degradation.After undergoing accelerated aging,the potential decay rate of the insulation is improved,the trap density is increased,the dielectric constant is significantly increased,molecular chain breaks,and the microstructure is significantly deteriorated.Secondly,through the experiments of surface potential test and space charge test,the influence of trap charge characteristics on insulation performance was analyzed,and the characterization results of physical and chemical characteristics were used to study the aging principle of cable insulation in actual working conditions.The results indicated that the surface charge of the cable insulation,which was faulty and had a long operating time,dissipated faster and the potential decay rate was higher.The charge density at the center of the deep trap in the new cable was higher than that at the center of the shallow trap.Aging cables had a higher charge density than new cables,and trap levels tended to become shallower.In addition,the heteropolar space charges accumulated in new cables,while the injection of homopolar space charges was dominated in the old cables;the dissipation speed of the heteropolar charges was significantly lower than that of the homopolar charges.As the cable operation time increases,the breakdown strength decreases,the resistivity changes within the same order of magnitude,the dielectric loss increases significantly,and the surface micro-morphology deteriorates significantly.Polar groups such as C=C bonds,carbonyl groups,and hydroxyl groups(-OH)appear,while the change of crystalline state is not very sure.Finally,combined with the isothermal relaxation current method,insulation dielectric and charge characteristics,the insulation status of active cables were comprehensively evaluated,and a mathematical evaluation model was established through multiple characteristic parameters to judge the remaining life of the active cables.The analysis found that the relaxation current of cables undergoing certain aging decays faster at high frequencies,the more severe the cable insulation ages,the slower the decay rate of the low frequency components of the current is,and the longer the relaxation current tends to a steady state value.The aging factor has an approximately linear relationship with the cable service time.There are correlations between some characteristic parameters of cable insulation.Based on the correlation analysis and the mathematical evaluation model established by multiple cable characteristic parameters,it can roughly predict the remaining service life of active cables.The results of this study showed that the generation of electrical trees and traps is an important sign of cable aging.In agricultural electrification,how to prevent the metal trace elements in the soil from being mixed into the cable insulation and the accumulation of charges is very important to improve the cable life.In addition,the multi-factor aging assessment and prediction can also be carried out in the cables used in agriculture through the mathematical evaluation model established in this thesis,so as to prevent potential safety risks caused by the aging of cables and to improve the service life of cables to some extent.