Research On Aging Assessment And Rejuvenation Method Of Xlpe Power Cable Insulation

As the power transmission medium, cross-linked polyethylene (XLPE) power cables are widely used in power systems. Water tree will occur in the insulation during long-term operation of the cable, which results in local electric field enhancement and insulation performance degradation. What’s worse, water tree may lead to sudden breakdown of the cable and the security and stability of power system operation will be threated. Consequently, recognizing the influence on dielectric properties of XLPE insulation because of water tree aging, based on which, developing of safe and effective non-destructive test methods for cables and assessing the condition of XLPE insulation to avoid accident are what the electricity power department needs urgently at present. Meanwhile, it is great significant to develop an insulation enhancement technique for cables aged seriously. Hopefully, the life of the cable will be extended, the running cost will be reduced and the economic and social benefits will be improved through the implementation of this rejuvenation technology.The mechanisms in changes of dielectric properties as well as the nonlinear behavior because of the water tree growth in XLPE insulation are studied. Dielectric analysis models of the overall insulation and water tree region are established, utilizing Maxwell-Wagner theory and Maxwell-Wagner-Sillars theory respectively. Meanwhile, finite element simulation models of water-treed XLPE insulation are established in Ansoft as well. Based on these models, changes of permittivity, conductivity, dielectric loss factor and internal electric field of the insulation with shape of aqueous micro-void and water tree length are analyzed. Through the analysis above, scientific basis are provided for well understanding the insulation performance change law and nonlinear properties of water-treed XLPE material while theoretical foundation for development of evaluation methods being laid.Based on the summary of diagnosis technologies for XLPE power cable insulation at home and abroad, two non-destructive test methods of aging assessment for cables based on isothermal relaxation current and harmonic distortion in loss current respectively are studied carefully. The principles of the two methods are introduced and analysis models are established as well. Furthermore, the relationships between the measured characteristics parameters and degree of water tree aging are discussed based on experiment and simulation analysis. Hence theoretical directions for the development of these two kinds of methods are provided by the research.The range of product reacted by rejuvenation fluid and water is proposed and a rejuvenation fluid which meets the requirements is prepared. In addition, a pressure injection system for rejuvenation fluid based on siloxane is designed and the components of this system are introduced in detail. Furthermore, a field aged cable is restored by this technology and the rejuvenation mechanism is analyzed in deep. It is quite evident that rejuvenation technology based on siloxane liquid can effectively extend the life of the cable through routine insulation test, accelerated water treeing tests and electric field simulation.