Aging State Diagnosis Of XLPE Cable Water Tree Based On Isothermal Relaxation Current And Return Voltage

Medium and low voltage cross-linked polyethylene(XLPE)cable is the main body of distribution network.Water tree aging is the main aging form of medium and low voltage XLPE cable.In order to ensure the safe and stable operation of distribution network,the research on XLPE cable insulation state diagnosis technology has attracted much attention.In this thesis,8.7/15 kV cable is taken as the research object,and the water tree aging state diagnosis technology of XLPE cable is studied by isothermal relaxation current and return voltage test.In this thesis,the XLPE cable electrothermal combined water tree aging test device was first built,and the nuclear power cable and ordinary power cable samples with different types and different aging degrees were prepared.According to the test principle of isothermal relaxation current method,an isothermal relaxation current test platform for XLPE cable is built.The test platform is driven and controlled by computer through GPIB interface to obtain current information by Keithley 6517 B high resistance meter.Using the above test platform,the isothermal relaxation current of unaged,thermal cycle,electrothermal combined aging and nuclear power cable with intermediate joint electrothermal combined aging were tested,and the aging factor A was calculated.The correlation between isothermal relaxation current,aging factor and water tree aging state of XLPE insulation was studied experimentally.The results show that the isothermal relaxation current and aging factor of XLPE cable are positively correlated with the applied polarization voltage.Due to the influence of intermediate joints,the relaxation current and aging factor of cable samples with intermediate joints are significantly higher than those of other types of samples.Combined with the power frequency dielectric loss,water tree microstructure and infrared spectrum test results,it is shown that the insulation has thermal oxygen aging and water tree,but the overall performance of the insulation is good.According to the test principle of the return voltage method,a XLPE cable return voltage test platform was built.The surface potential information was obtained from the TREK 341 B surface potentiometer by the computer through the data acquisition system.The built test platform was used to test the return voltage of nuclear power cables without aging,thermal cycle,electrothermal aging and electrothermal aging with intermediate joints,and the initial change slope Sr of recovery voltage was fitted and analyzed.The correlation between the return voltage and the water tree aging state of XLPE insulation was studied experimentally.The results show that the initial change slope of the return voltage curve of XLPE cable is positively correlated with the applied polarization voltage.Compared with ordinary power cables,the initial current,relaxation stable current,aging factor A and initial change slope of return voltage curve of isothermal relaxation current curve of power cables for nuclear power are larger.Finally,combined with the test characteristics of the two methods,a joint test of return voltage and isothermal relaxation current is proposed,and the feasibility of the method is verified by simulation.Through the analysis of the actual test results,the relaxation branch capacitanceΔC under power frequency is obtained.It is found that with the increase of aging time,the relaxation branch capacitance ΔC also increases.