| 110 kV cross-linked polyethylene(XLPE)cables are subject to electrical,thermal,mechanical,and environmental stress during actual operation,and their insulation properties deteriorate to varying degrees.Among them,electrical and thermal factors play a decisive role in the change of cable insulation performance.Therefore,studying the degradation characteristics of cable insulation under the effect of electricity and heat during actual operation is of great significance for the evaluation of high-voltage cables.This paper starts with consideration of the different operating conditions encountered in the actual operation of high-voltage cables to obtain the degradation changing characteristics of XLPE within the high-voltage cross-linked polyethylene cables.This paper studies the changing characteristics of cable insulation degradation mainly from the changes of insulation microstructure,aggregate structure and electrical external characteristics.Among them,the change of microstructure can be detected by Fourier transform infrared spectroscopy(FTIR)and space charge measurement.The change of aggregate structure can be analyzed by differential scanning calorimetry(DSC)and X-ray diffraction(XRD)tests.The change of electrical external characteristics can be obtained through electrical current measurement,dielectric spectroscopy measurement,and breakdown field strength measurement.Analysis of these three aspects of cable insulation characteristics will reliably improve the accuracy of insulation degradation assessment.In order to highlight the characteristics of high-voltage cable insulation degradation,this paper adopts two accelerated aging methods to treat high-voltage cross-linked polyethylene cables with different operating years,namely the accelerated thermal aging test for cable insulation slices and the combined electrical and thermal accelerated aging test for cables.Relative physicochemical and electrical experiments are conducted to analyze the changes in the insulation properties of the cable insulation before and after the accelerated aging tests.This paper will start from these two parts to study the influence of single thermal factor,and dual factor of eletricity and heat on the degradation of XLPE.During its actual operation,the thermal factor analysis of the cable shows that the insulation degradation is closely related to the regular working temperature of the cable at 90℃,the overheating temperature under fault or overload,and the temperature change of the laying environment of the cable.Based on the above three situations,a spare cable and the cables with severice years of 15 and 32 were selected to intercept into slices.Relative accelerated thermal aging tests were conducted on these samples.Correspondingly,it is divided into the following three sections.The first section applied 85,90,95,and 100℃ as the temperature points of the accelerated thermal aging test.The samples were thermally cycled 20 times,and the obtained samples before and after the heat treatment were measured by DSC,electrical conductivity,and dielectric spectroscopy tests.The experimental results show that the insulation performance of each sample increases first and then decreases with temperature rising,and the insulation performance is relatively improved at 95℃.In the second section,130℃ was used as the temperature point of the accelerated thermal aging test.Thermal cycling was performed on XLPE-0 and XLPE-30 for 5,10,and 20 times.The samples before and after the heat treatment were measured by DSC,FTIR,and breakdown field strength tests..The experimental results show that the insulation performance of the cable insulation with different operating years varies greatly under different overheating times.The spare cable can withstand 20 times of overheating,while the 32 service years cable insulation has been deteriorate after 10 times of overheating.In the third section,different heating and cooling processes of rground,tunnel and pipeline laying modes were used as conditions for accelerated thermal aging tests.The samples were thermally cycled 80 times,and DSC,FTIR,XRD,and breakdown field strength measurement were adopted to analyze the degradation of the samples.The experimental results show that the insulation properties of the ground mode samples perform the best,and the pipeline mode of the samples are the worst.In order to further explore the influence of high-voltage cables on the changing degradation characteristics of XLPE under the combined effect of electric and thermal factors during actual operation,180-day electric-heat combined aging tests were conducted on two retired cables that have been operated for 15 and 32 years.Each cable insulation was sliced to obtain the inner,middle and outer layers.The relevant physicochemical and electrical measurements were executed to analyze the changes in the insulation properties of XLPE before and after the accelerated aging test.The experimental results show that the performance changes of the insulation layers of the two retired cables after the accelerated aging test are not obvious,and still have the potential for long-term operation in the practice.In this paper,two different accelerated aging methods are adopted to study the insulation degradation changing characteristics of cables with different operating years.It is found that the process of cable insulation degradation is closely related to changes in cable operating temperature conditions and the laying environment.The degradation changing characteristics can improve the cable insulation status under specific conditions somehow,which will provide a certain reference for the power operation and maintenance department,and provide a basis and theoretical basis for extending the service life of the cable.In addition,a correlation between morphology and thermal parameters on cross-linked polyethylene(XLPE)cable with different insulating states has been revealed.Several cables were selected to detect the physicochemical and thermal parameters of the XLPE.The results show that the cable ampacity is determined by the thermal parameters,which are deeply subjected to the morphology of the XLPE.The molecular chain and crystal structure of the XLPE have a close connection with the thermal resistivity.The physicochemical parameters of carbonyl index(CI)and unsaturated band index(UBI)from Fourier transform infrared spectrum(FTIR)and melting range(Rm)from differential scanning calorimetry(DSC)can be the indicator to evaluate the diversity of the thermal resistivity.The change of thermal capacity is governed by the crystal distribution of the XLPE.The physicochemical parameters of crystallinity(χ)and lamellar thickness(L)from DSC can be the indicator to evaluate the change of the thermal capacity.In addition,FWHM of the crystallization peak ΔW,crystalline rate index(TO-TP)and cross-linking degree(G)can also be the indicator of the thermal parameters.Finally,this paper proposes a more accurate on-line monitoring method for electric power industry by detecting thermal parameters to diagnose the operating cables in the practical application. |
PDF Full Text Request