Thermal Aging Kinetics And Life Assessment Of Crosslinked Polyethylene Composite Insulation

Because power cable has much advantages such as small occupation area and high reliability.It is widely used in urban electric energy transportation and long-distance transmission.However,the power cable also has unfavorable laying,unfavorable maintenance and other shortcomings.With the gradual increase of the service life of the early cables,and the electrical,thermal and mechanical stresses and the erosion of environmental moisture in the operation process,the damage phenomena such as insulation breakdown and discharge gradually appear.In order to avoid the occurrence of such phenomenon and even bigger accidents,we need to monitor the running state of the cable in real time,find the situation and deal with it.The cable is usually composed of a shielding layer,a protective layer,an insulating layer and a conductor,among which the insulating layer is an important factor for the safe and reliable operation of the cable.XLPE has excellent insulation performance.It is the main insulation material for 10-220kV power grade cable.Studying the aging characteristics of crosslinked polyethylene materials and evaluating its status and remaining life can effectively help solve the above problems.Latent defects of insulating materials are difficult to be detected by macroscopic detection methods such as dielectric loss and leakage current.The aging of insulating materials is essentially a chemical reaction process,and activation energy is the intrinsic property of materials,which can characterize the degree of difficulty of chemical reaction of materials.Thermal stress is the main stress of insulation aging.In this paper,the XLPE sample was treated with accelerated thermal aging to test its microscopic characteristics,electrical properties and dynamic characteristics.The empirical formula of aging state evaluation and life prediction of XLPE insulation was established based on the dynamic characteristics of conductance activation energy.In this paper,accelerated thermal aging tests were carried out on XLPE insulation samples at 100? and 130? to obtain control groups of XLPE insulation samples in different aging states.Scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FITR)and differential scanning calorimetry(DSC)were performed to characterize the effects of thermal aging on the microstructure of samples.The experimental results show that as the aging time improving,the surface of the sample gradually appears lamellar and scale-like structures,and the precipitation of small solid particles increases.The aging of insulation can be divided into the early aging and the late aging:in the early aging,the reaction of internal groups is not intense,and the crystallinity has a slight rise;In the late aging stage,the molecular chain inside the material breaks,and thermal-oxygen reaction occurs,the number of carbonyl,ether group and other functional groups increases,and the crystallinity decreases.Conductive activation energy and thermogravimetric activation energy were measured for XLPE samples.The conduction mechanism of crosslinked polyethylene is ion jump conductance,and the relation between the activation energy of conductance and the thermogravimetric activation energy and temperature conforms to the Arrhenius equation.Through curve fitting and calculation,it is found that the mumerical trend of the two activation energies is the same,both of which show a trend of first rise and then decline,and the overall trend of zigzag decline.This is consistent with the results described in the previous article section.Among them,conductance activation energy is easier to measure and can be used in engineering applications.A breakdown voltage test platform was built to measure the dielectric loss and breakdown voltage of XLPE insulation samples under different aging time.The changing law of medium loss is different under the two aging temperatures,and the general trend of tortuous rise is presented.The breakdown field strength showed a tortuous downward trend.A method for evaluating the aging state and residual life of XLPE insulation based on the activation energy of conductance is established.Based on the Arrhenius equation,the temperature acceleration factor AF of accelerated aging test is 5.9653 and 37.9679,respectively.Taking the breakdown field strength decreased to 50%of the initial value as the insulation failure criterion,the remaining life of the XLPE insulation sample is 13.177a and 12.764a.The activation energy corresponding to insulation failure of XLPE insulation sample is 0.4623eV.An empirical formula for evaluating the aging state of XLPE insulation samples is established.When the activation energy of conductance is lower than 0.75eV,the insulation material has entered the late aging stage.At this moment,the remaining life of the sample can be obtained by using the empirical formula.The measured activation energies of conductance A,B and C of the retired cable insulation are 0.4593,0.5136 and 0.4824eV respectively,and the predicted residual life of insulation A,B and C is-0.0951a,1.4387a and 0.5843a.The results are consistent with the actual operation.