Polarization And Depolarization Characteristics Of XLPE Cables Under The Condition Of Sheath Is Wet For A Long Time

XLPE cable plays an irreplaceable role in modern power transmission.Although there are multilayer sheathing outside the cable insulation,insulation damp is still one of the main causes of cable failure.Therefore,it is of great meaning for the stable operation of power system to find and eliminate the fault earlier.Polarization and Depolarization Current(PDC)method based on dielectric response theory has been widely concerned in cable insulation detection,but there are few researches on the discrimination basis of cable moisture.Therefore,it is necessary to explore the polarization characteristics of the cable during the process of moisture,so as to provide reference for judging cable damp.In this thesis,a cable sample with the sheath wetted by water is designed,and a PDC test platform is built.Dampen new cables and periodically perform PDC tests.Check insulation resistance as a control.The polarization-depolarization current curves were fitted using the Debye model,and explore the changing law of the current curve and polarization characteristic parameters of the cable in the process of damping.Interpret and analyze the test results in combination with the damping mechanism of cable insulation,and the results were further verified by observing cable insulation slices.The research findings show that the first 15 s of the polarization and depolarization current curve has a good response to the cable moisture.When the cable is just immersed in water,the polarization and depolarization current continues to increase.After 40 to 60 days of dampness,the current begins to decrease,and gradually stabilizes after about 150 days of dampness.The initial current value after stabilization is about 3～4 times of that without moisture.The polarization and depolarization current keep good symmetry during the damp process,and there is no obvious effect of rated power frequency voltage on the polarization characteristics during 180 days of test.By analyzing the fitting parameters of the polarization and depolarization current,it is discovered that the change of the polarization and depolarization current is caused by the amplitude coefficient,and the time constant is basically unchanged during the test.The branch resistance of the Debye equivalent circuit first decreases and then increases and the branch capacitance first increases and then decreases.It is considered that the reason for the two trends is that when the cable is just soaked in water,the water diffuses through the amorphous area inside the insulation,resulting in the decrease of the equivalent resistance and the increase of the equivalent capacitance of the damp cable.As the water content of the micropores in the amorphous region increases,the resistance of the micropores decreases significantly,and the polarization time constant in some regions decreases significantly,which results in the decrease of the measured polarization depolarization current.The performance is that the Debye branch resistance increases and the branch capacitance decreases.