The Insulation Aging State Assessment Of High Voltage Cable Based On The Isothermal Relaxation Current

As the cross-linked polyethylene (XLPE) cable has excellent performance, itwas widely used in the power grid around the world when it came out. Now it hasbecome the main carrier of the power transmission and one of the importantcomponent in the power system. However, XLPE cable will be effected by manyfactors in the running, such as electricity and heat, they will cause aging occur. Agingmakes the cable insulation performance and life expectancy reduce, and made a greatimpact to the safety and reliability of the power grid. Therefore, to assess the cableinsulation state is very important for the power grid’s safety and reliable operation.For the subject of the cable insulation state assessment, this paper looked uplarge number of domestic and foreign relevant literature, more comprehensivelyanalysis and summary of the assessment method on cable insulation state anddeveloping trends. On the basis of a variety of measurement analyzing andcomparing, the paper decides to research the subject based on isothermal relaxationcurrent method.This paper introduced some theory related the isothermal relaxation currentproduce or form, such as the dielectric response and polarization. Combining withthe cable aging equivalent model and according to the isothermal relaxation currentdecay characteristics, the paper proposes a fitting peak separation curves method thatshows isothermal relaxation current different relaxation process characteristics, anduses the known data to verify the method feasibility.To meet the need of subject, the XLPE cable’s isothermal relaxation current testexperimental platform was constructed in the laboratory. The experimental platformincludes high voltage DC power supply, Switch control circuit and relaxation current collection system. The high voltage DC power supply was consisted of regulationcircuit, rectifier circuit, intermediate frequency inverter circuit and boost circuit.Switch takes high voltage reed relay as the core, through the external ship typeswitch to control different relay on and off in different time. Data acquisition partwas completed by keithley6517B electrometer and Labview software via a serial portcommunication.On the basis of the experimental platform, the paper carried out the study ofcable isothermal relaxation current. A long up to5m8.7/15kV three-phase XLPEcable as test sample was selected in the laboratory. After studying the influence ofpolarization time, polarization voltage amplitude and temperature about the cableisothermal relaxation current cable, choosing the appropriate conditions torespectively testing each phase of the cable in the isothermal relaxation current. Theresults show that the isothermal relaxation current measurement method can betterassess the cable insulation aging state.