Evaluation Of Water Treeing Aging Status Of XLPE Cable Based On Loss Factor And Aging Factor

Cross-linked polyethylene(XLPE)cables have excellent electrical properties,and as the main carrier of power transmission in urban power grids,they have become an important component of the power system.Water-tree aging is the main aging form of medium and low voltage insulated XLPE cable.Water-tree will lead to deterioration of insulation performance,reduced service life,and even insulation failure,thus affecting the reliability of power grid operation.For this reason,this paper takes the XLPE cable used in 8.7k V nuclear power plant as the research object,and applies the test of loss factor and aging factor as the experimental method to study the water-tree aging state assessment technology of insulated XLPE cable.In order to achieve the preparation of insulated XLPE cable samples with different water tree aging degrees,referring to the requirements in the DL/T1070-2007 standard,a water tree aging test scheme under water bath heating is proposed.The three-dimensional model of the water tree aging test device was designed by using Solid Works software,and the water bath circulation pipeline was built with ABS and PPR pipes;then the power consumption of electromagnetic induction and water bath heating methods were analyzed respectively.Water bath heating is better than electromagnetic induction heating from the perspective of power consumption,and water bath heating is easier to achieve precise temperature control;finally,the test power supply and temperature recording part are designed.The rationality of the design of the water bath aging test device and the test scheme is verified by monitoring the temperature of the water tree aging test.In order to establish the correlation between the loss factor and the water-tree aging state of the insulated XLPE,the variation law of the ultra-low and power frequency dielectric loss of the insulated XLPE cable with different aging degrees was experimentally studied.The aging microstructure and physicochemical properties of water trees were characterized by using polarized light microscopy and infrared spectroscopy results with methylene blue staining.The experimental results show that: with the increase of aging time,water trees are formed in the insulation,and the dielectric loss increases;the dielectric loss at 0.1Hz is more obvious than the dielectric loss at power frequency,and the dielectric loss at 0.1Hz can reflect the damp condition of the cable.In this paper,a test platform for isothermal relaxation current of XLPE cable is built,which consists of high-voltage DC power supply,switch control and depolarization current acquisition parts.The test platform is driven by the computer through the GPIB interface to drive the output of the Keithley2290 DC control voltage,and obtains the current information through the Keithley6517 B high-resistance meter.Using the basic theory of isothermal relaxation current method,the test current is fitted and analyzed,and the aging factor A is obtained by calculation.The change rule of aging factor of insulated XLPE cable under different aging degrees is studied through experiments,and the correlation between aging factor and water-tree aging state of insulated XLPE is established.The experimental results show that: with the increase of aging time,the aging factor becomes larger.The above test results show that the XLPE cable used in nuclear power plants has water-tree aging,but it has not reached the end of its life.The loss factor and aging factor can reflect the water tree aging state of the insulated XLPE cable.