| Since the 1960 s,cross-linked polyethylene(XLPE)has gradually been widely used as the insulation material for high-voltage cables(110k V and above level).In addition to its excellent electrical-thermal properties,it also has comprehensive characters such as environment friendly and economically.So that,there is still no alternative material.For its complete structure of high-voltage cable,deterioration of the insulation performance under different laying environments and operating conditions has always been a matter of eager concern for domestic-foreign scholars and grid operation departments.From the commercial operation of the XLPE high voltage cable on a large scale,the degradation mechanism of XLPE under various conditions,as well as the physical,chemical,and electrical performance degradation have attracted extensive research.After decades of research,the degradation mechanism of XLPE under different conditions and the changes in physical and chemical properties have been relatively revealed.On this point,various aging models and formulas for evaluating the degradation process of XLPE and other similar polymers have been proposed.From the initially single model to a multi-factor aging model gradually,one of the main purposes is to evaluate the insulation performance of actual running cables by using these theories and methods.However,the actual operating load and insulation temperature in highvoltage cables are not in a regular status,and the operating conditions do not have certain regularities.In this case,the application of current theories and models on the cable insulation for operating cables is short of accuracy,and the obtained remaining life expectancy is also not convincing.Much research on a large number of retired cables shows that the electrical performance of them is better than that of the new cables,the electrical performance of these cables that have been in operation for more than 20 years just mildly degraded,and some cables still meet the cable operating standards.Exactly,whether the cable could be put into operation for longer years has attracted extensive attentions.Tracing back to the cable production stage,the adjustment of the cable production technology is not just for improving the electrical performance of the cable insulation,but also the parameters must meet the relevant national standards could be guaranteed.These detected parameters are not totally at the best status of the material.Therefore,some literatures on the research of retired cables have found that the thermal process during the cable operation promoted the further changes in the microstructure and facilitated a better electrical performance of the cable insulation.Based on the existing theories related to cable insulation degradation,combining the possible temperature range in the cable operation and the relationship between the crystalline properties of XLPE and the temperature,this paper studied the changes in the crystal structure and the thermal-electrical properties of the retired cables under different thermal process.Taking three retired cables with different service year as the research object,the changes in the cable insulation during the thermal aging process were studied by four serial steps: 1)Using a conventional constant heat oven,the inner XLPE of the retired cable insulation were selected as the test samples.The heat treatment experiments were carried out at four temperature points(85,90,95,and 100 ° C),and the changes in the crystalline structure and thermal-electrical performance of the samples were analyzed.The results revealed that the thermal-electrical performance still have the feasibility of improvement;2)Compare the effects of four heat treatment methods on the crystalline structure and thermal-electrical performance of XLPE at the same temperatures same as 1).the effects of preset temperature and temperature holding time on the changes in structure and thermal-electrical performance were determined.3)Short cables were prepared and thermal simulation experiment of cable operation were did,the effects of heat treatments at different temperatures on the crystalline structure and thermal-electrical properties of XLPE from different positions of cable insulation layer were compared.It is determined that the short-term high-temperature thermal process would not cause the further deterioration on the cable insulation,and the crystal structure and the thermal-electrical performance could be improved if the heat treatment were done at optimum temperatures.Finally,the optimum heat treatment temperature for these cables were obtained.4)Two real cable circuits with long cables,joints,accessories and other components were built,to simulate the cable operation at the preset optimum temperature measured in step 3).The possibility of improving the thermal-electrical performance of cable was explored.In this step,it is obtained that the heat treatment performed at the optimum temperature would not cause the further degradation on the XLPE from any positions,and it can help to improve the crystal structure and improve the thermal-electrical performance.The results provided the first evidence of cable insulation performance can be improved by simulating the cable short-term operation at relatively high temperature. |
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