The Condition Assessment For 110kV XLPE Cable’s Insulation In Long Running Processes

The remaining life of running cable samples are assessed on the basis of aging mechanism of XLPE cable. The assessment is done from three angles which includes the physical, chemical and electrical methods. The research of 110 kV XLPE cable’s remaining life and aging state is divided into two stages. Three characteristics of aging test for cable are conducted in first stage which includes physical, chemical and electrical methods. Two characteristics of aging test for cable’s insulation are conducted in second stage which includes physical and chemical methods.The tests of first stage include partial discharge test, stepwise voltage-endurance method, isothermal relaxation current method, thermal aging method, thermogravimetry and differential scan calorimeter. The follow-up characteristics test of the physical, chemical and electrical methods for these samples could be carried out. The results of stepwise voltage-endurance method show that residual life of operational cable is 10-30 years and spare cable is 30-50 years. The results of isothermal relaxation current method show that the result conducted by aging factor is a kind of trend judgment. After electrical-thermal aging treatment, aging factor of the cables was increase. The results of thermal aging method show when the temperature of the cable core is between 80-90 ℃, the residual life of the cable is between 10-25 years. The results of thermogravimetry show that after thermal aging, the activation energy of four samples decreased and two samples increased. This illustrated that the mutual cooperation of material system, the use of antioxidant and process will affect the quality of the cable. The results of differential scan calorimeter show that after electrical-thermal aging treatment, the oxidative induction time of five cables reduced and one cable elevated.The physical and chemical characteristics of second stage for cable’s insulation include PEA, TG, FTIR and X-ray. The microscopic test results of PEA, TGA and FTIR show that compared with middle layer, space charge accumulation of inner layer increases by 14.86%, the activation energy drops by 3.68%, carbonyl index increases by 5.57% and methylene index drops by 66.78%. The aging state of inner and outer was more serious. The results of X-ray show that the distorted spherulites exist in insulating layer. In the early running, under anaerobic conditions the reaction of molecules of XLPE was dominated by crosslink which made the molecular structure more perfect. In the late running, under aerobic conditions the reaction is dominated by oxidative degradation which accelerates aging of insulation. Through the change of space charge accumulation, activation energy and carbonyl index can reasonable explain the aging law of inner, middle and outer layers.