Research On The Correlation Between Micro-structure And Macroscopic Performance Of XLPE Cable Insulation In Service

Cross-linked polyethylene（XLPE）electrical power cables have gradually replaced ethylene-propylene rubber（EPR）cables and oil-filled cables due to their excellent properties,simple installation and laying with no limitation of height drop.However,in the sevice of XLPE cable,such as thermal aging,electrical aging,electrochemical aging,water tree aging and other aging will lead to the deterioration of structure and performance,and even lead to breakdown accidents,which will seriously affect the safe and stable operation of the power system.Therefore,it is of great significance to study the deterioration law of the micro-structure and macro-performance of service cables,as well as the correlation between them,for the correct evaluation of cable operation state and the the reasonable analysis of cable fault causes.In this paper,the following researches are mainly carried out for 110k V XLPE cables in service for different times:Firstly,by using Fourier transform infrared spectroscopy,differential scanning calorimetry,scanning electron microscopy and other experimental methods,the deterioration law of micro-structure of XLPE was characterized.The results show that in the early stage of service,the micro-structure does not change significantly.With the increase of service time,the micro-structure begins to deteriorate rapidly,which is reflected in the increase of the number of C=O,C=C,-OH and other functional groups,the decrease of the crystallinity and melting temperature,the worse regularity of spherulite sturcture and the decrease of the size of spherulites.In addition,more serious deterioration can be observed on the inner layer insulation than in the outer layer insulation,which can be attributed to the higher temperature of the inner layer insulation during service.Then,the degradation law of macro-performance of XLPE is studied by thermogravimetric analysis,broadband dielectric spectrum and breakdown experiments at 50Hz.The results show that with the increase of service time,the thermal decomposition of XLPE begin to occur at lower temperature,the dielectric constant and low-frequency dielectric loss increase,and the breakdown field strength at 50 Hz decreases significantly.At the same time,the macro-performance of the inner layer insulation also showed a faster degradation rate.The decrease of thermal decomposition temperature can be attributed to the decrease of molecular weight caused by oxidation reaction and thermal electron bombardment.The increase in dielectric constant at low-frequency loss can be attributed to the increase of polar nolecules such as C=O,C=C,-OH.There are many factors affecting the breakdown field strength,which may include the increase of the number of carriers,the increase in the free path of carriers caused by the decrease of the free volume and the increase of the crystallinity.Finally,the macroscopic properties and microscopic properties of XLPE samples under different laying methods are studied.And the COMSOL simulation software is used to apply the same current-carrying capacity to the cables under the three laying methods to observe their temperature changes,and to verify their deterioration laws.The results show that:taking the applied current of 938A as an example,the maximum temperatures that the insulation withstands during direct burial,pipe row and cable channel laying are 89.98^oC,100.95^oC,and82.32^oC,respectively.Due to the large laying space and fast air convection speed,the cable channel laying method can dissipate heat faster when the temperature of the cable body rises,so the degree of deterioration is smaller than that of pipe laying and direct burial laying,which is reflected in the melting temperature.the crystallinity has been slightly improved,the spherulites are more complete,and the average size remains within a larger range.In addition,because the aging degree is the smallest,the chain scission inside the sample and the content of carbonyl polar groups are small,the increase in the dielectric constant and dielectric loss tangent is smaller than that of the pipe laying and direct burial laying,and the power frequency breakdown The field strength is the largest among the three.However,due to the difficulty of heat dissipation,the laying of the exhaust pipe has the deepest degree of deterioration during the service process,so the microstructure and macroscopic properties have the greatest changes.