Research On Water Tree Diagnosis Of XLPE Cable Based On PDC Method

Water tree aging is an important aging form of cross-linked polyethylene(XLPE)cable aging.When water branches appear in the cable insulation,it will do great harm to the insulation,affect the service life of the cable and threaten the safe operation of the power system.Because the micro water tree detection of XLPE cable is destructive,while the macro dielectric property detection can not directly obtain the growth status of water tree in the cable.Therefore,how to combine the micro morphology of XLPE cable water tree with the macro dielectric property to accurately and nondestructive judge the aging degree of water tree in XLPE cable is of great significance to the safe operation of power system.For the water tree aging problem of XLPE cables,this thesis builds a cable water tree aging platform based on the improved water needle electrode method,and accelerates the water tree aging of XLPE cables.Based on the polarization depolarization current method(PDC),the PDC circuit was designed and the PDC test experimental platform was built according to the circuit.The polarization /depolarization current of cables in different aging stages is tested under different test voltages,and a series of dielectric performance parameters of cables are deduced based on the test results of PDC.The cable slices were observed microscopically,including optical microscope observation,scanning electron microscope(SEM)observation,X-ray energy dispersion spectrum(EDS)test and infrared spectrum(FTIR)test.In terms of macro testing,the PDC test results of cable under different aging time and different test voltage are analyzed.It is found that the depolarization current of cable increases with the increase of aging time;Under different polarization voltages,the depolarization current of no aging cable increases in equal proportion with the multiple of test voltage,while the depolarization current of water tree aged cable increases more and more slowly,showing obvious nonlinear characteristics.According to the PDC test results,the aging factor,DC conductivity and 0.1Hz dielectric loss factor of the cable are calculated.It is found that the above parameters increase with the increase of aging time,and are not in positive proportion.In terms of microscopic testing,through optical microscope observation,the water tree growth model is established,the water tree density of the cable is calculated,the correlation between the macro parameters and micro water tree density of the cable obtained by PDC test is analyzed,and a new mathematical model representing the water tree growth density inside XLPE cable is established according to the correlation coefficient.The microstructure of the water tree inside the cable is successfully combined with the macro test parameters.In order to quantitatively analyze the EDS and FTIR test results of XLPE cables,a new parameter(carbon oxygen index)was proposed to quantitatively describe the changes of EDS results in the water tree area.Through the EDS test of the water tree area of the cable at different aging stages,it is found that the longer the aging time,the greater the carbon oxygen index,and the rate of increase is faster and faster.A new parameter(water content index)is proposed to quantitatively describe the variation of FTIR results in the water tree area.Through the FTIR test of the water tree area of the cable at different aging stages,it is found that the longer the aging time,the greater the moisture index.